Jan Blahůt

Ivanka Mitrovic-Woodell

Ivo Baroň

Miloš René

Matt Rowberry

Pierre-Henri Blard

Filip Hartvich

Jan Balek

Stavros Meletlidis

Dr Jan Blahut

; Miloš

Volcanic edifice slip events recorded on the fault plane of the San Andrés Landslide, El Hierro, Canary Islands

Keywords: volcanic collapse, frictionite, cataclasis metamorphism, silica layer, cosmogenic radionuclide dating, Canary Islands

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Dear Editor,

The comments of you and the reviewers are in black, while our responses are in red.

Editor:

Dear authors, At long last I have received two evaluations of your work -many potential reviewers have in fact turned down the review invitation.

While the manuscript has clear merits, there are still some significant issues:

-a more thorough and general introduction should be provided on fault slip and as to how peculiar volcanic edifices may be, and why we should care about the seismicity of landslides there. As Rev#1 points out, it is somewhat unclear "why being in a volcanic setting is noteworthy". This echoes one of your conclusions, ie "This is the first time that more than one slip event has been recognised from the fault plane of the San Andrés Landslide", which does not sound so ground breaking to me (if I may play with words). We have rewritten the introduction to capture the readers´ interest. We hope, that now the scope of the study is clearer.

-Some petrographic features at the dm-cm scale are missing to ensure a correct scale transfer of descriptions, say from Fig. 2 to say Fig. 5. We have added new photos of MSA1 and MSA2 samples to Fig. 2, followed by detailed description. We believe, that this might be sufficient to transfer from Fig. 2 to Figs. 5,6 and 7. -why are cosmogenic ages not discussed more thoroughly and reported on the surface? I find this really problematic. Table 1 is simply not enough and data should be scrutinized. We have carefully rewritten this part as we understand that the explanation presented was not enough. We explained the reasons why the cosmogenic ages yielded such different results.

-any hint as to the respective sizes of the landslides? as to a paleomagnitude?... We have added this information to chapter 2.1.

-You should pay attention to details. Care should be taken about statements such as: "strong ground shaking" (L493), which is not substantiated. We have corrected this statement -see answer to reviewer 2 (p. 18, l. 493). We also checked the manuscript and corrected such general statements where needed.

-the write-up could be improved. For example, conclusions should be made much more concise and straight to the point. The first part basically repeats the incentive of the study. We have rewritten the conclusions to be more concise.

-Take-home message: note that there is no scale to Fig. 9. This also does not seem to me like a very impactful take-home message figure and could probably be improved.

We agree with the reviewer, that the shaking induced from the debris avalanche itself wouldn´t be probably enough to cause re-activation of San Andrés Landslide. As more information about triggers of these giant debris avalanches is not known we changed the sentence to reflect this. p. 18, l. 494: should be 'a model' This was corrected. p. 18, l. 497: omit "only" This was corrected. The different colours of arrows represent distinct sets of striations and elongated bumps. We have added this explanation to the captions. We have added this to the figure captions as it is graphically nicer.

Highlights

 Landslide frictionite adorns the fault plane of the San Andrés Landslide on El Hierro  The frictionite comprises a dark cataclastic layer and a translucent silica layer  These cataclastic and silica layers formed during two separate fault slip events  Microstructural observations in good agreement with observations at the outcrop scale  It is suggested that the fault slip events occurred between 545-430 ka and 183-52 ka  First time that more than one slip event has ever been recognised on a single fault plane resulting from volcanic flank collapse on an oceanic island  First time a silica layer resulting from frictional melt has been described from a volcanic setting *Highlights

Introduction

Gravitational slope failures generateoften produce geological structures at a range of scales that are either similar or identical to those that result fromrelated to endogenous tectonic processes (Jaboyedoff et al., 2013).

Consequently, Ffaults generated that originate as a result of large slope failureslandslides are often analogous to their ose produced by tectonic counterpartsprocesses (Gomberg et al., 1995). Some of the largest slope failures on Earth are represented by volcanic flank collapses on oceanic islands -these events are so enormous that they are also comparable to the largest mass movements on Mars (Blahůt et al., 2019). The processes that lead to volcanic flank collapses on oceanic islands are not well understood despite their potential to generate catastrophic tsunamis (Paris et al., 2018;Walter et al., 2019). In part this situation reflects the dearth of events as volcanic flank collapses are estimated to have only occurred four times a century infor the past five hundred years (Siebert, 1992). Volcanic flank collapses on oceanic islands are usually represented by debris avalanches (sensu Ui et al., 2000), which displace huge volumes of rock and create amphitheatre shaped calderas (Siebert, 1984). Such debris avalanches localise shear deformation along thin sliding surfaces at the base of the displaced mass (De Blasio and Elverhøi, 2008). These events are triggered by a range of factors which can be broadly categorised as seismogenic or magmagenic (McGuire, 1996). Therefore, while huge debris avalanches may occur in other settings, volcanic flank collapses on oceanic islands are especially dangerous due to the interminable nature of the potential triggering events, i.e. volcanic tremors and related seismicity, coupled with the adjacency of an extremely high risk impact environment, i.e. seawater. The run-up heights of consequent tsunami waves may exceed 100 m (Karstens et al., 2019;Walter et al., 2019). Occasionally, volcanic collapses are represented by creeping or slumping but only a small number of studies have focused on these processes.

Examples come from Mount Etna in Sicily (Rasa et al., 1996), Pico Ridge in the Azores (Hildebrand et al., 2012), San Andrés in the Canary Islands (Blahůt et al., 2017), and Hilina Slump in the Hawaiian Islands (Liu et al. 2018).

Volcanic collapses usually occur as debris avalanches (sensu Ui et al., 2000), removing huge volumes of rock and generating amphitheatre shaped calderas (Siebert, 1984). These debris avalanches, as with other types of slope deformation, localise shear deformation along thin sliding surfaces at the base of the sliding mass (De Blasio and Elverhøi, 2008). In some instances, it possible to obtain information pertaining to creeping or slumping but such studies are rare. Examples come from Mount Etna in Sicily (Rasa et al., 1996), Pico Ridge in the Azores (Hildebrand et al., 2012), San Andrés in the Canary Islands (Blahůt et al., 2017), and Hilina Slump in the Hawaiian Islands (Liu et al. 2018).

In a volcanic setting it has been found that slow collapse of a Tertiary shield volcano in the Nicaraguan Highlands during the Holocene modified the local palaeostress field and generated normal, reverse, and strike slip faults [START_REF] Baroň | Palaeostress analysis of a giant Holocene rockslide near Boaco and Santa Lucia (Nicaragua, Central America)[END_REF]. However, volcanic collapses are somewhat difficult to study because they are comparatively rare events. Such failures have occurred at least four times a century for the past five centuries (Siebert, 1992). These events are triggered by a range of factors which can be broadly categorised as seismogenic or magmagenic (McGuire, 1996). Volcanic collapses usually occur as debris avalanches, (sensue Ui et al., (2000), removing huge volumes of rock and generating amphitheatre shaped calderas (Siebert, 1984).

These debris avalanches, as with other types of slope deformation, localise shear deformation along thin sliding surfaces at the base of the sliding mass (De Blasio and Elverhøi, 2008). In some instances, it has been possible to obtain information pertaining to creeping or slumping but such studies are rare. Examples come from Mount Etna in Sicily (Rasa et al., 1996), Pico Ridge in the Azores (Hildebrand et al., 2012), San Andres in the Canary Islands (Blahůt et al., 2017), and Hilina Slump in the Hawaiian Islands (Liu et al. 2018). It is, however, probable that more data will be collected in the future due to the increasingly widespread use of GPS and InSAR.

During landslide emplacement, flash heating may occur along thin shear layers to produce frictional melt, referred to as frictionite or landslide pseudotachylyite (Masch et al., 1985;Maddock, 1986;Legros et al., 2000;Lavallée et al., 2012;Mitchell et al., 2015). Frictionite is a subtype of pseudotachylyte, sensu lato, which is generated at the base of landslides, as opposed to pseudotachylyte, sensu stricto, which is generated in greater depths with higher pressure and temperature conditions (Maddock, 1986). Frictionite tends to have thicknesses of between one and three centimetres and develops parallel or subparallel to the dip of the landslide (Weidinger et al., 2014). Indicators of frictionite include significant grain size reduction, evidence for fluidised flow, and the presence of amorphous material., and evidence for fluidised flow. Other types of cohesive fault rocks, such as Mmicrobreccias and cataclasites, are common also form during the emplacement of large landslides emplacement. -typical indicators of cataclastic deformation are Cataclasites typically show grain size reduction, compared to the host rock, with and the development of foliation. To accurately assess the hazard relating to volcanic flank collapses it is crucial to be able to estimate the strength and velocity of the landslide. Fast slip along shallow faults is evidenced can be inferred by the presence of devitrified material observed in pseudotachylytes, clast-cortex aggregates (Smith et al., 2011;Han and Hirose, 2012;Rempe et al., 2014), and silica layers (Kirkpatrick et al., 2013;Faber et al. 2014), and other type of fault rock architecture.

Pseudotachylytes and frictionites are ultrafine-grained rocks, with the presence of partial melt. The Clast-cortex aggregates former are composedcomprise of a central clast enclosed within a layer, or concentric layers, of material akin to the matrix (e.g. Anders et al., 2010;Rowe et al., 2012). Silica layers while the latter are composed of translucent silica gel layer with microstructures that exhibit flow banding, armoured clasts, and extreme comminution compared to adjacent rocks (Kirkpatrick et al., 2013;Faber et al., 2014). These fault rock structures are not common -they are known to occur in a range of tectonic settings but have not been reported in relation to volcanic flank collapses. The first report of frictionite at the base of a landslide was described from the Köfels landslide in the Austrian Alps (Masch et al., 1985) while the first report of frictionite at the base of a landslide in a volcanic environment was described from the Arequipa volcanic landslide deposit in Peru (Legros et al., 2000). Furthermore, volcanoes tend to be basic in composition and silica layers are not expected to form in predominantly basic environment. Neither clast-cortex aggregates nor silica layers are thought to have ever been reported from a volcanic setting.

Reconstruction of past volcanico failures in their source areas is extremely rare as there is usually no material to study and/or the main scarp has since been covered by younger eruptions. For that reason, the majority of studies have focused on the sedimentological properties of these failures (e.g. Hunt et al., 2013). Until recently years, it has been assumed that the volcanic flank collapses occur as one major event that displaces hundreds of cubic kilometres of rock. However, new research suggests that at least some of these collapses occurred in multiple stages (Hunt et al., 2013(Hunt et al., , 2018;;León et al., 2017). Such findings may have serious implications for the calculated frequency of such hazards and reopens questions about the possible reactivation of apparently inactive structures. The San Andrés Landslide on El Hierro in the Canary Islands represents one of the few places in the world where it is possible to investigate the landslide mass and fault planes and landslide mass of a volcanic collapse structure. This study integrates a range of structural geological and geomorphological measurements, petrological and microstructural analyses, and cosmogenic nuclide dating in order to reconstruct the developmental history of this huge gravitational slope failure. Measurements Data have been obtained -directly or indirectly -from the surface of the fault plane while samples were collected from both the fault plane itself and an adjacent gully perpendicular to the footwall. Our hypothesis states that this landslide is not the result of a single event, as proposed by Day et al. (1997), but instead results from a number of successive events spanning a protracted period. This hypothesis is not straightforward to address as unambiguous evidence for multiple slip events along a single fault is difficult to elucidate due to problems associated with overprinting and weathering. Nonetheless, on the basis of previously published research and the results obtained during this study, it has been possible to propose a new developmental conceptual model is proposed for the development of the San Andrés Landslide., which is based on previously published research and the results obtained during this study.

Geological setting

The island of El Hierro

El Hierro is an active volcanic edifice that constitutes the smallest and youngest of the Canary Islands (Figure 1).

Its oldest subaerially exposed rocks are represented by the Tiñor Unit, with a maximum age of 1.12 Ma (Guillou et al., 1996), which form the northern and northeastern parts of the island. This unit is thought to have developed rapidly until around the time of the Tiñor debris avalanche at 0.88 Ma (Carracedo et al., 2001).

Thereafter, the rocks of the El Golfo-Las Playas Unit, with a maximum age of 0.55 Ma (Guillou et al., 1996), capped much of the Tiñor Unit and infilled the scarp area of the Tiñor debris avalanche (Carracedo et al., 2001).

ItsThe youngest subaerially exposed rocks are represented by the Rift Series, with hich has a maximum age of 0.16 Ma (Guillou et al., 1996). Over the past 33 000 years onshore eruptions have reoccurred approximately once every 1 000 years (Becerril et al., 2016a). The most recentlatest, ongoing, phase of volcanism began around 2.5 ka (Carracedo et al., 2001). Latterly aRecently a period of intense seismic activity commenced spanned fromin July 2011 (López et al., 2012) to 2014(Benito-Saz et al., 2017). During this period -prior to an offshore eruption startedthat began on 10 October 2011 and finishedended in March 2012 (Meletlidis et al., 2015). -and lasted until 2014 (Benito-Saz et al., 2017). A more detailed description of the geology of island has recently been presented elsewhere (Carracedo and Troll, 2016). Blahůt et al., 2018a).

The flanks of El Hierro have hosted a number of enormous gravitational slope failures which have contributed to the development of its characteristic three pointthree-point star morphology (Figure 1). Until now, seven debris avalanches have been identified: Tiñor (< 880 ka), Las Playas I (545-176 ka), Las Playas II (176-145 ka), El Julan (> 158 ka), El Golfo A (176-133 ka), El Golfo B (87-39 ka), and Punta del Norte with (of unknown age) (Masson, 1996;Urgeles et al., 1996Urgeles et al., , 1997;;Carracedo et al., 1999Carracedo et al., , 2001;;Masson et al., 2002;Longpré et al., 2011;Becerril et al., 2016b;Carracedo and Troll, 2016;León et al., 2017;Blahůt et al., 2018b2018a). The volumes of these debris avalanches vary between 50 km 3 and 234 km 3 , which makes them average from the known volcanic slope failures (for additional morphometric characteristics, see Blahůt et al., 2019). In addition, a large slump, sensu Moscardelli and Wood (2008), or a deep seated gravitational slope deformation, sensu Sorriso-Valvo et al. (1999) or Agliardi et al. (2001), is located on the southeast flank of the volcanic edifice. This feature, the San Andrés Landslide, is defined by a group of pronounced faults which represent landslide detachment planes. While Iits volume has not been determined however, the deformed toe is visible around 18

km off coast at a depth of more than 3 km (Becerril et al., 2016b). Similar features have been described from other volcanic islands including the Azores (Hildenbrand et al., 2012) and the Hawaiian Islands (Duffield, 1975).

It has previously been proposed that the San Andrés Landslide is an anchored block associated with the debris avalanche Las Playas I (Day et al., 1997).

San Andrés Landslide

The San Andrés Landslide has developed in the volcanic rocks of the Tiñor Unit. This unit comprises three distinct subunits (Gómez Sainz de Aja et al., 2010): the basal subunit is represented by relatively thin, 20-40 cm, steeply dipping lava flows; the intermediate subunit, which forms the majority of the unit, is represented by thicker, up to 4 m, shallow dipping lava flows; while the third uppermost subunit is represented by emission vents with well-preserved craters and associated lavas. The broad arcuate, and presumably listric, fault system which defines the boundaries of the slope deformation to the northeast is terminated to the southwest by an escarpment associated with the giant debris avalanche Las Playas II. Previously the San Andrés Landslide has been interpreted as either an aborted collapse structure (Day et al., 1997) or a deep seated gravitational slope deformation (Klimeš et al., 2016).

It has been reasoned that the fault system must have developed at some point between 545 ka and about 261-176 ka (Day et al., 1997). If correct, this implies that the collapsed landslide mass has remained anchored during all each of the subsequent giant debris avalanches on El Hierro (Carracedo and Troll, 2016). Support for the notion of protracted inactivity is provided by the presence of younger lavas crossing the fault without displacement and the presence of scree covering lava flows as old as 145 ka in the gully of Las Playas (Carracedo et al., 1997). However, recent research is starting to challenge these arguments. The step-like structure of the incised gullies in the collapsed mass, together with records of progressive creep along its main detachment plane, in the order of 0.5 mm/.yra -1

, suggests that the landslide mass may be moving steadily to the east and southeast (Klimeš et al., 2016;Blahůt et al., 2017.

The part of San Andrés Fault system investigated in this study is exposed along a narrow road which traverses the Barranco de Tiñor (27°47'18.82"N, 17°55'19.04"W). This outcrop is situated at c. 490 m asl, has a length of c. 70 m, and dips between 65° and 75° to the SE (Figure 2). The fault zone has a total thickness of between one and two metres. Both the footwall and hanging wall rocks comprise basaltic lavas of the Tiñor Unit with a few thin intercalated lapilli beds and soil horizons (Carracedo et al., 1997). No indurated breccia is present on the hanging wall. Its eastern section has only been uncovered relatively recently as a result of road construction and is distinguished by its whitish surface. This part of the fault surface hosts well-preserved slickenlines, which range in size from several millimetres to metre scale surface undulations. Detailed petrographic examinations of the fault rocks have been published previously (Day et al., 1997).

Methods

Field measurements data and structural analyses

Structural analyses of the surface of the San Andrés Fault plane incorporated structural geological field measurements alongside the interrogation of a high resolution digital terrain model (DTM). This DTM was constructed on the basis of photogrammetric imagery obtained from an unmanned aerial vehicle (UAV) which was thenand processed using the commercial software Agisoft PhotoScan (Agisoft LLC, 2014). The accuracy of terrain models constructed following this approach is directly comparable to those constructed on the basis of laser scanning (Balek and Blahůt, 2017). From the photogrammetric point cloud, it was possible to derive two complimentary slope gradient maps with grid cells of 2 x 2 cm. The first gives a ground plan view of the fault (Figure 3A) while the second gives a fault perpendicular view (Figure 3B). These maps have been analysed in detail using the ESRI® ArcGIS applications ArcMap and ArcScene. Slope gradient maps, in contrast to hillshade maps, are particularly suited for the identification of linear features irrespective of their spatial distribution and on the outcrop.

Schmidt hammer sampling can beis used in geomorphological research to estimate rates of weathering or and to obtain information relating to relative exposure ages for rock surfaces on the basis of rebound values, R (Aydin and Basu, 2005;Goudie, 2006). In A total of eighty eighteighty-eight test sites were sampled on the fault plane using a Proceq SilverSchmidt ST-PC. Each site was sampled twelve times in a direction perpendicular to the faultits surface. Of these twelve samples, the uppermost and lowermost values were disregarded, according to standard sampling protocol, and the remaining ten were used to calculate the rebound value.

These R values were processed in a GIS. Initially, an orthophotograph of the fault scarp had to be created from a set of photoshots. The position of each test site was recorded on a printed copy of the orthophotograph prior to image rectification in the GIS. The relative positions of several test sites were measured in the field and used as reference points during georeferencing. Each test site is represented by a point placed approximately in its centre and then the calculated R value was added to each test site. Interpolation of these values indicates changes in the relative surface hardness across the exposed part of the fault scarp. Several interpolation algorithms were tested but the character of the data led us to select the natural neighbour technique.

Petrographic and microstructural analyses

Petrographic analyses of the sampled rocks were performed at the Institute of Rock Structure & and Mechanics CAS and the Institute of Geology CAS in Prague. Optical microscopy was conducted using the polarisation microscope Leica DMR while chemical analysis of the primary and accessory minerals was conducteding using the electron microprobe CAMECA SX-100. Microstructural investigations have been made through the application of optical microscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). These investigations were made at the University of Vienna. Thin sections were cut perpendicular to the shear plane and parallel to the direction of sliding prior to mechanical polishing to thicknesses of c. 30 μm.

Selected samples were carbon coated for SEM. Optical microscopy was conducted using the polarisation microscope Leica DM4500 P with micrographs obtained under both plane polarised light (PPL) and crossed polarised light (XPL). Selected samples were carbon coated for SEM. The SEM imagining Scanning electron microscopy was conducted using a the SEM microscope FEI Inspect S with an accelerating voltage of 10-15 kV for spot sizes of 5-7. Images were obtained and presented in the back scattered mode (BSE). Element energy dispersive spectroscopy was also conducted to better understand the chemical effects of fault slip. The EDAX EDS systems were used as an add-on to the FEI Inspect S.SEM. Using the TEAM™ Software Suite and Smart

Phase Mapping it was possible to collect spectra and generate phase maps showing elemental distribution and associated spectra. The EDS operating conditions were 15 kV accelerating voltage for spot size of 5, at resolution of 514 x 514, dwell 200 µs.

Cosmogenic radionuclide dating

Cosmogenic radionuclide dating constrains the amount of time a rock has been situated at or near the surface of the Earth (e.g. Lal, 1991). Normal fault planes represent ideal objects for this dating technique as the incoming cosmic rays are only able to penetrate the progressively exposed parts of the fault (e.g. Palumbo et al., 2004). The basaltic lithologies of El Hierro determine that the most suitable cosmogenic nuclide with which to constrain periods of fault activity is 3 He (e.g. Kurz, 1986;Poujol et al., 2014). The sampling strategy aimed, first, to obtain samples from the least eroded parts of the fault and, second, to avoid obtaining samples from the whitish part of the fault plane as this was known to have been exposed during road construction. Five samples were collected from two profiles along the footwall (Figure 2). These samples range in thickness from two to five centimetres and belong to the same basaltic sequence which that erupted 1.050.02 Ma (Carracedo et al., 2001). Samples were crushed and sieved to isolate fractions of less than 0.25 mm, 0.25-0.5 mm, and 0.5-2.0 mm.0.2 mm to 2 mm and 0.25 mm to 0.5 mm. Several physical separation techniques including magnetic separation and density separation were used to isolate pure olivine and pyroxene phenocrysts.

All helium analyses were performed in the noble gas platform of the Centre de Recherches Pétrographiques et Géochimiques in Nancy. Samples were fused in vacuo at 1600°C for 15 minutes using the new custom designed metal induction furnace of CRPG (Zimmermann et al., 2018). Typical furnace blanks were ( He concentrations measured in the samples, respectively. Then the extracted gas then underwent standard gas purification (Blard et al., 2015). Helium was cryofucused at 8K before being released at 70K and introduced into the Split Flight Tube mass spectrometer, a Nier source static mass spectrometer optimised for the analysis of cosmogenic nuclides (Protin et al., 2016). The linearity of the mass spectrometer was carefully established by analysing standard splits with similar 4

He pressure to those of the samples. During the one monthone-month analytical period the observed reproducibility of the system was 2 % at 1σ, both for both 3 He and 4

He. Furthermore, during this one monthone-month period, two CRONUS-P pyroxene standards were analysed. These yielded 3 He concentrations of (4.860.14)  10 9 at.g -1 and (5.010.14)  10 9 at.g -1

. Both values stand within the internationally recognised error limits for this material (Blard et al., 2015). To determine the 3 He/ 4

He ratio of the magmatic component step crushing in vacuo was conducted on 0.5 mm to 2 mm olivines and pyroxenes obtained from Sample 5III (1 st step: 100 strokes, 1 min; 2 nd step: 500 strokes, 5 mins;) ( Table 1).

Crushing blanks were (32)  10 .a -1 has been used, computed using the equations of Andrews (1985) and the chemical composition measured in the phenocrysts and bulk rock samples (Supp. Table A). Combined with a value of 1.050.02 Ma for T eruption , (based on the K-Ar age of the San Andrés basalt presented in Carracedo et al. (2001)), this yielded a value of (63)  10 3 at.g -1 for 3 He nucleo .

In theory, the magmatic He production rate in the analysed olivines and pyroxenes. P 4rad is computed using the equations of Blard and Farley (2008) and the U, Th, and Sm concentrations measured in samples COS4-I and COS5-II (Supp. , for an average of (3.34.6)10 12 at.g -1

.

Thus the outlined approach for estimating 4 He rad represents the main source of uncertainty when computing the magmatic 3 He correction and, in turn, the main source of uncertainty when computing the final cosmogenic 3 He concentrations.

Instead, we decided to apply the following approach and compute a minimal and a maximal value for the cosmogenic 3 He concentrations, as follow ( He production rate, the standard atmosphere, and the Lal-Stone time dependent model (Martin et al., 2017). The production rate was computed taking into account the spatial characteristic of the samples, including self-shielding (Table 1).

Results

Field measurements data and structural analyses

The NE section of the San Andrés Fault outcrop is morphologically well preserved while its SW section has been smoothed as a result of weathering and incurredreceived some damage during road construction. In general, the exposed fault plane is straight with an orientation of approximately 170/70° (dip direction/dip angle). Only some irregularities can be observed such as two remarkable fault plane undulations protruding from the faultits surface, with widths of up 4 m, separated by a parallel depression (Figure 3A-B). These irregularities were also recognised identified during structural geological measurements. In greater detail, it has been possible to identify numerous linear features, which have been grouped into three families on the basis of their character and orientation (Figure 3A-C). First, a family of centimetre to metre scale striations were identified on the basis of structural measurements and from the slope gradient maps. These striations cover most of the outcrop but are best preserved on the fault plane undulations. Most commonly, the striations have plunge with directions of azimuths of 125° and 131°. Second, a family of smooth elongated undulations and strongly elongated protrusionsbumps were also identified on the basis of structural measurements and from the slope gradient maps. These features are best seen in the depressions and flats in the northeastern section of the fault plane. The undulations and bumps protrusions have a plunge direction ofwith an azimuth of 111°. Third, a family of very smooth, spatially limited, slightly elongated bumps protrusions were identified from the fault perpendicular slope gradient map. These are found in the lower SW and upper NE part of the 4 m wide fault plane undulation as well as at the crest of the other undulation protruding from surface to the NE. The bumps protrusionshave a plunge withdirection of an azimuth of 158°.

A photomosaic offering a perpendicular view of the fault outcrop in the background photomosaic overlain by a raster of interpolated R values is presented on Figure 4. Proportional plots have been used to illustrate the spatial variability in rock hardness across the outcrop. In general, higher R values have been measured on the lower and right side of the fault outcrop than have been measured on its upper and left side. However, notably low R values have been recorded in a wedge-shaped zone between 23 m and 26 m (R < 25), in an area where a rather soft gouge material is attached to the fault surface. However, as the vegetation cover descends much lower here than elsewhere, it is suggested that chemical and biological weathering processes are more intense hereabouts. The highest R values, indicating the least weathering, have been recorded in the lowermost part of the fault outcrop on its right side (R > 50). This finding is not surprising given that the hard whitish surface was exposed only recently during road construction of the road. In addition, highHigh R values have also been recorded on the far left side of the fault outcrop (R > 40). This finding is more surprising but it can be explained by several tens of centimetres of erosion on the fault surface and the presence of harder basalt blocks within the tectonic breccia. Conspicuously low R values have been recorded in a wedge-shaped zone between 23 m and 26 m (R < 25). Here a rather soft gouge material is attached to the fault surface and the vegetation cover descends much lower than elsewhere -this suggests that chemical and biological weathering processes are more intense in this area.

Petrographic analyses

The host rock basanites are composed of fine grained basic volcanic rocks (samples PET3 and PET4). These rocks contain are mainly pyroxene, plagioclase, olivine,, and amphibole and volcanic glass, together with the accessory minerals (apatite, chromite, magnetite, and Ti-magnetite). The pyroxene is represented by mainly comprises the monoclinic mineral diopside. Te while the plagioclases are usuallytends to be zonedal with an basic core (An 79-83acid rim) and acid rim (, An 45-70 ), and a basic core, An 79-83 . Olivine occurs only in basanites with subordinate amounts of volcanic glass. The olivine is enriched by has a predominant forsterite component, (75-80 mol. %)., T while the amphibole is represented byspans from magnesiohornblende to hornblende tschermakite. A high amount of volcanic glass is present while basanites with subordinate amounts of volcanic glass contain frequent phenocrysts of olivine. The texture of the groundmass is mostly ophitic and gives no indication of a pre-existing preferred fabric orientation. The tectonic breccias sampled from the fault plane are composed byofof fragments of volcanic rock (up to several centimetres), cemented by a very fine grained groundmass containing abundant fragments of the original volcanic rock (millimetres to centimetres on samples) ( PET1 and PET2). The higher proportion of fine grained Fe-oxides and Fe-hydroxides imparts a rusty colour of these tectonic brecciasto the rock. However, the volume of Fe-oxides and Fe-hydroxides is highly variable, with significant enrichment in the fine grained groundmass of the original volcanic rock. The tectonic breccias host cracks of highly variable orientation and sizes, filled,are cross-cut filled by zeolite veins that exhibitsin a range of orientations and sizes. Frictionite, or landslide pseudotachylyte, was found in samples obtained directly from the fault plane (Figure 2). ThisIt is composed of relict frictional melt and material similar to tectonic breccia (Figures 567).

Microstructural analyses

The NE part of the fault outcrop is covered by a whitish layer which attains a maximum thickness in the order of several centimetres. Figure 2 shows examples of the This whitish layer that usually overlies the greyish cataclasite and microbreccias. , and iIn some cases but it sometimes this whitish layer comes into direct contact with tectonic breccia. The tectonic breccia is highly oxidised (Figure 2D,F). Brecciation intensity increases from the undeformed basanite to the more deformed breccia, i.e. towards the fault surface. This This The tectonic breccia marks the transition from undeformed basanite, at greater depths, to more deformed breccia, at the fault surface. The breccia, and is highly oxidised (Figure 2D, F). It has not been analysed further butand it may be a the product of an earlier slip event. However, In this study, here we focus on the cataclasite and the whitish layer, i.e. Instead attention focuses here on the those layers that overlie the tectonic breccia. On the basis of Using optical microscopy it is possible to differentiate two distinct layers within the frictionite by their composition and microstructure: a structurally lower cataclasite and a structurally higher silica layer.

The cataclasite cuts the tectonic breccia while in places it is cut by the silica layer. Contact with the host rock is characterised by a sharp boundary that is observed in both hand samples and optical micrographs (Figure 6).

Although the cataclasite is grey in hand samples, it is dark brown in optical micrographs (Figure 6A A second cataclasite has a more lensoid shape which is apparently defined by Pand Y-shears (Figure 7). It exhibits strong P-foliation which is seen most readily in the elemental maps (Figure 7C,EC, E). Interestingly, In addition, cracks arewithin this layer are parallel to both the Y-shearing, and to the slip surface, and are filled with silica gel (Figure 7E).

The silica layer cuts both the tectonic breccia and the cataclastic layer. The boundary between the silica layer and the subjacent rocks is a sharp, single line, which crosscuts foliation in the cataclasite and truncates clasts in the tectonic breccia. The boundary between the cataclasite layer and the silica layer is sometimes marked by shiny sublayers that parallel the boundary (Figure 6B,HB, H) and exhibit a high interference colour when seen under cross polarised light with a gypsum tint plate (Figure 5C). There is no indication of mixing between the cataclasite and the silica layer across the boundary between separating these layers. The silica layer appears to be translucent, white to pale coloured under plane polarised light (Figures 567). It is composed of a microcrystalline to amorphous silica matrix with a thickness of c. 250 µm to 350 µm. The architecture varies:

the matrix consists of an inclusionless silica material that develops into complex structures with flow bands 5H). These sublayers appear to have a similar composition to the cataclasite layer but with a finer grain size and present evidence of material mixing with the silica layer. Some clasts embedded in the silica layer have a cortex, or armour, and these are referred to as clast-cortex aggregates. The clast-cortex aggregates vary in size, from c. 90 µm to 290 µm, complexity, and composition. Those clasts within the aggregates consist of fragments of adjacent rock (Figure 7G) or silica matrix (Figure 6I). A complex cortex

(Figures 5-7
texture with what appears to be several coatings has been observed (Figure 6G). Elemental maps show that the cortex composition varies as well (e.g. Figure 6L and Figure 7D,ED, E). It is believed that a significant amount of the cortex material was lost during preparation of the thin sections (e.g. Figure 6G-I).

Cosmogenic radionuclide dating

Cosmogenic 3 He exposure ages reveal significant differences between each of the two sampled profiles. Sample COS1 has yielded a maximum exposure age of 21±20 ka while samples COS2 and COS3 have yielded a maximum exposure age of 26±18 ka. Replicates of sample COS4 yielded exposure ages ranging from 21±19 to 61±19 ka while replicates of sample COS5 yielded exposure ages ranging from 52±17 to 183±17 ka (Table 1).

The older exposure ages obtained from the left hand profile may simply reflect more recent exhumation or more intensive erosion on the right hand part of the fault. These results indicate that movement across the currently exposed portion of the fault plane occurred over a short period of time as suggested by the fact that there is no gradual decrease in exposure ages from upper to lower part of the two profiles.

The right hand profile (samples COS1-3) revealed exposures ages that are below the detection limit i.e. these were exposed between 0 and 26 ka. These young exposure ages most probably reflect recent exhumation on the right hand part of the fault owing to the fact that the right hand profile is separated from the left hand profile by only 6-7 m. Intense erosion of the right hand profile can be excluded as the R values indicate much greater hardness, i.e. less weathering, here than on the left hand profile.

The left hand profile (samples COS4 and COS5) revealed exposure ages between 21 and 183 ka. The higher sample COS4 provides a younger age than the lower sample COS5. Under normal circumstances this should not be possible -the highest parts of the fault should be exposed before its lower parts -and it suggests that the higher sample has been subjected to weathering. Consequently sample COS5 offers the most reliable results as it has neither been affected by weathering nor by recent exhumation. The high variability of computed exposure ages between the replicate in theis sample is due to the considerable uncertainty associated with the estimate of the magmatic 3 He component as outlined in Section 3.3. Nonetheless it is believed that the most recent fault reactivation occurred between the calculated exposure ages of sample COS5 (52±17 to 183±17 ka).

Interpretation and discussion

Landslide displacement direction

Distinct families of linear and elongated features on at the fault plane evidence past behaviour of the rock mass (Sagy et al., 2007). Two major slip events are interpreted to have occurred on the studied fault plane On on the basis of the relative position and general morphology of suchits linear and elongated features, two major slip events are interpreted to have occurred on the studied fault plane. The older phase is represented by the family of smooth elongated undulations and strongly elongated bumps, best preserved in the depressions and flats in the northeastern section of the fault plane, which have a plunge direction of 111°. During this phase the rock mass moved towards the ESE. The younger phase is represented by the other two families of linear and elongated features. The onset of this phase is interpreted to have commenced with minor, most probably slow, displacement towards the SSE. Support for this slow displacement is gleaned from the smooth, spatially limited, elongated bumps which have a plunge direction of 158°. The displacement rate of displacement is then interpreted to have accelerated as the rock mass moved more towards the SE. Support for this is gleaned from the centimetre to metre scale striations which cover most of the fault outcrop and have plunge directions of 125° and 131°. The general appearance of these linear and elongated features is sufficiently similar to assume that they developed during a single slip phase. It is supposed that the this shift in the displacement direction, from the SSE to the SE, can be explained by either a sudden change in the geometry of the slip surface or a fundamental change in the morphology of the entire volcanic edifice. It is known that El Hierro underwent a clockwise rotation of 15°, beginning at around 440 ka (Széréméta et al., 1999), and it has been suggested that this rotation could be associated withrelate to activity on the San Andrés Landslide and the debris avalanche Las Playas I (Carracedo, 2008).

Fault related microstructures and dynamic rock reworking

It is contended that tThe microstructures seen at on the surface of the San Andrés Fault are believed to be a result of fast slip, associated with from frictional heating and partial melting during landslide emplacement.

and, therefore, this rock is a frictionite (or landslide pseudotachylyite). The first line of evidence for flash heating during flank collapse is observed in comes from both hand samples and polished slabs as energy is seen to haveheat diffused in the underlying material: . Tthe adjacent host rock and tectonic breccia exhibits increasing level of are only oxidisationed when in immediate contact with the frictionitetowards the fault surface. This is similar for pseudotachylyites reported from the Köfels landslide (Masch et al., 1985) and from the Arequipa volcanic landslide deposit in Peru (Legros et al., 2000). The second line of evidence comes from the crosscutting relationships between either the host rock or the tectonic breccia and the deformation layers.

All of the observed contacts are sharp and accentuated by multiple truncated clasts. This type of crosscutting relationship indicates fast displacement and is often considered a strong indication of a seismic slip (Passchier and Trouw, 2005;Price et al., 2012;Smeraglia et al., 2017). The third line of evidence comes from the boundary between the silica layer and the adjacent rocks. This boundary is characterised by, which has shiny bands that, exhibiting a high interference colour under cross polarised light and a positive optical sign under cross polarised light using a gypsum tint plate. These bands are tentatively interpreted as recrystallised material resulting from frictional melting but further analyses are needed in order to confirm this supposition.

The cataclastic layer exhibits P-foliation, grain size reduction, and the development of winged clasts in a heterogeneous matrix. The winged clast presented in Figure 5 resembles those of Grasemann & and Dabrowski (2015) and Grasemann et al. (2019). The foliation developed within this layer is typical of brittle deformation, with horizontal Y-shears, parallel to the slip surface, and inclined P-shears inclined relative to the slip surface (i.e. Figure 5I). This type of foliation cannot be used as an indicator for fast slip as it is recorded in association with both seismic and creep phenomena (Verberne et al., 2013;Smeraglia et al., 2017). However, unequivocal evidence for fast slip, instead ofrather than progressive creep, is provided by the fact that the contact between the cataclastic layer and the adjacent rock is sharp and accentuated by multiple truncated clasts. The cataclasite architecture is similar to those described in other volcanic pseudotachylyites (Lavallée et al., 2012(Lavallée et al., , 2014;;Kendrick et al., 2012Kendrick et al., , 2014) ) as well as to that described for in seismically deformed limestones (Smith et al., 2011). Therefore, it is necessary to define this cataclastic layer as frictionite sensu lato, as it was probably almost certainly emplaced during catastrophic landslide emplacement but there is no decisive proof of flash heating.

In contrast, the silica layer is defined as frictionite sensu stricto, due to the fact that as it is comparable to traditional pseudotachylyites (e.g. Passchier and Trouw, 2005;Price et al., 2012;Rowe and Griffith, 2015).).

Identification criteria of deformed pseudotachylyites are presented elsewhere (e.g.; Passchier and Trouw, 2005;Price et al., 2012). For example, Nonetheless the following the identification criteria of deformed pseudotachylytes (Passchier and Trouw, 2005;Price et al., 2012) we list featuresare Here a list of particularly pertinent features is presented with reference to the identification criteria for deformed pseudotachylytes (Passchier and Trouw, 2005;Price et al., 2012)in the context of this study. :

i.

(i) crosscutting Crosscutting relationships with sharp layer boundaries. The silica layer cuts the cataclasite foliation and the boundaries are accentuated by clast truncation and presence of shiny bands which have been interpreted as recrystallised material from frictional melting. Analogous microstructural features have been reported from numerous natural and experimental cases of seismic slip (e.g. Fondriest et al., 2013;Smeraglia et al., 2017;Tesei et al., 2017).

ii.

Second, the The presence of fragments of host rock. Fragments of adjacent rock appear as isolated islands and vary in size, shape, and complexity. The more complex clasts have an encasing cortex, or armour, and are referred to as clast-cortex aggregates. Typically, those fragments described here have a volcanic clast in the core, cataclastic material in the rim, and they parallel the foliation. In a number of senses, they are similar to "armoured clasts" or "accretionary rounded grains" (Anders et al., 2010;Rowe et al., 2012;Kirkpatrick et al., 2013) and clay-clast aggregates (Boutareaud et al., 2008;Han et al., 2011). The main differences are the ratio between the clast and the rim and their composition. The rims described here appear to be wider than previously reported while they are neither composed of clay nor any other mineral assemblage.

Consequently, the clast-cortex aggregates presenteddescribed here in this study are most readily comparable to the accretionary rounded grains described from the Corona Heights Fault (Kirkpatrick et al., 2013).

iii.

Third, the The presence of an ultra-fine grained matrix. The silica layer comprises a homogenous silica matrix, ultra-fine grained to amorphous, which is very similar to silica gel (Kirkpatrick et al., 2013;Faber et al., 2014) and amorphous nanopowder (Rowe et al., 2019). More detailed studies are needed in order to measure the amount of water and crystalline material that occurs in this matrix. The presence of new grains that have grown in the voids or bubbles suggests indicates that at least some of this matrix must be crystalline. Further analyses are needed to fully understand their origin. Similar to these flow bands are the cataclastic sublayers, which are also structurally controlled, but with evidence for the reworking of cataclastic material. This feature, together with presence of clast-cortex aggregates is, can be interpreted as fluidised granular flow which has been reported in many geologic settings (Boutareaud et al., 2008;Anders et al., 2010;Ujiie et al., 2011;Kirkpatrick et al., 2013).

Furthermore, fluidised cataclasites have been suggested as a record of seismic slip rates (Brodsky et al., 2009;Rowe et al., 2005).

i.v. Fifth, differences Differences between the matrix and host rock mineral assemblages. The most conspicuous difference between the silica layer and the adjacent rocks is in the mineral assemblage. The homogenous silica layer contrasts greatly with the polymineralic cataclasite and the host rock. Even though the silica layer can be assumed to be a product of hydrothermal fluid deposition, the presence of kinematic indicators such as shears and flow bands consistent with observations at the outcrop scale, indicate that the silica layer was formed as a product of frictional slip. Silica has been reproduced in fast slip experiments (e.g. Di Toro et al., 2011;Kirkpatrick et al., 2013;Rowe et al., 2019) and . It is considered to be a product of frictional wear thatwhich weakens the fault, due to the fact that it actings as a lubricant (Rowe et al., 2019).

Developmental Conceptual model for the San Andrés Landslide

A conceptualdevelopmental model for the development of the San Andrés Landslide is proposed here on the basis of the findings presented in this study and age estimates presented elsewhere in the published literature (Figure 8). The first slip event as evidenced by the cataclastic layer is proposed to have occurred between 545 ka and 430 ka. The lower limit for fault activity is defined by the onset of volcanism associated with the El Golfo Series (Guillou et al., 1996) while the upper limit for fault activity is defined by the onset of the clockwise rotation of El Hierro (Széréméta et al., 19971999). The second slip event as evidenced by the silica layer is proposed to have occurred between 183 ± 17 and 52 ± 17 ka based on the presented cosmogenic 3 He exposure dates. This age range spans termination of volcanism associated with the El Golfo-Las Playas Unit and the onset of volcanism associated with the Rift Series at 145 ka (Guillou et al., 1996). The lower limit for fault activity broadly relates to the probable timing of the debris avalanches El Golfo A, between 176 ka and 133 ka, and Las

Playas II, between 176 ka and 145 ka. Moreover, the upper limit for fault activity broadly relates to the probable timing of the debris avalanche El Golfo B, between 87 ka and 39 ka. One of these giant debris avalanches, accompanied by strong ground shaking, Thus, it is believed that the factor, or factors, which triggered one of these giant debris avalanches may have triggered the second slip event described in this studyhere but . no other data have yet been found to support this hypothesis. In addition, a conceptual model is presented for the development of the surface of the San Andrés Fault (Figure 9). The developmental model outlined here is broadly consistent with the findings of Day et al. (1997). In that study, it was suggested that the minimum age of the San Andrés Fault should be between 250 ka and 150 ka and that the fault could only have been active only for a geologically short period of less than 400 ka. Their findings assertions are based on a younger undisturbed lava flow which covers the southern part of the fault.

Conclusions

In this studyThe the fault plane of the San Andrés Landslide on El Hierro in the Canary Islands has been investigated in this study through a combination of geological and geomorphological measurements, petrological and mineralogical analyses, and cosmogenic 3 He exposure dating.

 The steeply dipping fault surface is composed of a striated frictionite, with a thickness of up to 5 cm, which is in contact with a highly oxidised tectonic breccia, with a thickness of up to 1.5 m. The tectonic breccia transitions into the underlying undeformed basanite host rock. Microstructural analysis of the frictionite has revealed two distinct layers: a dark heterogeneous cataclastic layer and a translucent silica layer with a thickness of up to 1 cm.

 The architecture of the samples and the observed crosscutting relationships between the layers and adjacent host rock suggest that the cataclasite layer and the silica layer represent two separate slip events associated with seismic rupture during landslide emplacement. The older cataclastic layer cuts the tectonic breccia and presents evidence for P-fabrics and grain size reduction. The younger silica layer cuts the tectonic breccia and the cataclasite and preserves clast-cortex aggregates, flow bands, and reworked cataclastic material in an ultra-fine grained to amorphous matrix. Moreover, the notion of These two distinct slip phases is supported by are in accordance with the kinematic markers etched on the fault surface.

 ThisCoupling this information -coupled with the cosmogenic This is the first time that more than one slip event has ever been recognised on a single fault plane resulting from volcanic flank collapse on an oceanic island. It suggests that the possibility of reactivating supposedly inactive fault structures may be far greater than hitherto assumed. This is a key finding, which may lead to reevaluation of the threats related to volcanic islands collapses. Furthermore, it is believed that this is the first time a silica layer resulting from frictional melt has been described from a volcanic setting. from the fault plane of the San Andrés Landslide. It is also believed to be the first time a silica layer resulting from frictional melt has been described in a volcanic setting.

Figure captions

Figure 1 A shaded topographic and bathymetric relief map of El Hierro, Canary Islands, overlain by a geological map simplified from Ancochea et al. (2004). The inset shows an aerial image of the studied fault outcrop. SAL: San Andrés Landslide.

Figure 4

The interpolated R-values obtained through Schmidt hammer sampling on the San Andrés Fault.

The size of the proportional plotsblack dots illustrates the rock hardness at each of the test sites. (2017), and the findings presented herein. The uppermost labels denote the main geological events to have taken place on El Hierro, i.e. rotation and volcanism, while those beneath denote the supposed age ranges for each of the giant landslides. The proposed timing of activity on the San Andrés Fault is presented underneath the timeline: the first slip event is associated with the formation of the cataclasite while the second slip event is associated with the formation of the silica layer. The time axis depictsshows thousands of years before present.

Figure 9

A proposed conceptual model for the development of the surface of the San Andrés Fault. Prior to slip a tectonic breccia, of basaltic origin, hosted zeolite veins, as shown in green. During the first slip event the tectonic breccia was sheared and a foliated cataclasite formed, as shown in red.

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Table captions

Table 1 Results of the cosmogenic 3 He exposure dating for five samples obtained from the San Andrés Fault.

Supp. Tab. A

Chemical composition of the five samples obtained from the San Andrés Fault for cosmogenic

Introduction

Gravitational slope failures generate geological structures at a range of scales that are similar or identical to those that result from tectonic processes (Jaboyedoff et al., 2013). Consequently, faults generated as a result of large slope failures are often analogous to their tectonic counterparts (Gomberg et al., 1995). Some of the largest slope failures on Earth are represented by volcanic flank collapses on oceanic islands -these events are so enormous that they are also comparable to the largest mass movements on Mars (Blahůt et al., 2019). The processes that lead to volcanic flank collapses on oceanic islands are not well understood despite their potential to generate catastrophic tsunamis (Paris et al., 2018;Walter et al., 2019). In part this situation reflects the dearth of events as volcanic flank collapses are estimated to have only occurred four times a century for the past five hundred years (Siebert, 1992). Volcanic flank collapses on oceanic islands are usually represented by debris avalanches (sensu Ui et al., 2000), which displace huge volumes of rock and create amphitheatre shaped calderas (Siebert, 1984). Such debris avalanches localise shear deformation along thin sliding surfaces at the base of the displaced mass (De Blasio and Elverhøi, 2008). These events are triggered by a range of factors which can be broadly categorised as seismogenic or magmagenic (McGuire, 1996). Therefore, while huge debris avalanches may occur in other settings, volcanic flank collapses on oceanic islands are especially dangerous due to the interminable nature of the potential triggering events, i.e. volcanic tremors and related seismicity, coupled with the adjacency of an extremely high risk impact environment, i.e. seawater. The run-up heights of consequent tsunami waves may exceed 100 m (Karstens et al., 2019;Walter et al., 2019).

Occasionally, volcanic collapses are represented by creeping or slumping but only a small number of studies have focused on these processes. Examples come from Mount Etna in Sicily (Rasa et al., 1996), Pico Ridge in the Azores (Hildebrand et al., 2012), San Andrés in the Canary Islands (Blahůt et al., 2017), and Hilina Slump in the Hawaiian Islands (Liu et al. 2018).

During landslide emplacement, flash heating may occur along thin shear layers to produce frictional melt, referred to as frictionite or landslide pseudotachylyte (Masch et al., 1985;Maddock, 1986;Legros et al., 2000;Lavallée et al., 2012;Mitchell et al., 2015). Frictionite is a subtype of pseudotachylyte, sensu lato, which is generated at the base of landslides, as opposed to pseudotachylyte, sensu stricto, which is generated in greater depths with higher pressure and temperature conditions (Maddock, 1986). Frictionite tends to have thicknesses of between one and three centimetres and develops parallel or subparallel to the dip of the landslide (Weidinger et al., 2014). Indicators of frictionite include significant grain size reduction, evidence for fluidised flow, and the presence of amorphous material. Other types of cohesive fault rocks, such as microbreccias and cataclasites, are common during the emplacement of large landslides. Cataclasites typically show grain size reduction, compared to the host rock, with development of foliation. To accurately assess the hazard relating to volcanic flank collapses it is crucial to be able to estimate the strength and velocity of the landslide. Fast slip along shallow faults can be inferred by the presence of devitrified material observed in pseudotachylytes, clast-cortex aggregates (Smith et al., 2011;Han and Hirose, 2012;Rempe et al., 2014), silica layers (Kirkpatrick et al., 2013;Faber et al. 2014), and other type of fault rock architecture. Pseudotachylytes and frictionites are ultrafine-grained rocks, with the presence of partial melt. Clast-cortex aggregates comprise of a central clast enclosed within a layer, or concentric layers, of material akin to the matrix (e.g. Anders et al., 2010;Rowe et al., 2012). Silica layers are composed of translucent silica with microstructures that exhibit flow banding, armoured clasts, and extreme comminution compared to adjacent rocks (Kirkpatrick et al., 2013;Faber et al., 2014). These fault rock structures are not common -they are known to occur in a range of tectonic settings but have not been reported in relation to volcanic flank collapses. The first report of frictionite at the base of a landslide was described from the Köfels landslide in the Austrian Alps (Masch et al., 1985) while the first report of frictionite at the base of a landslide in a volcanic environment was described from the Arequipa volcanic landslide deposit in Peru (Legros et al., 2000). Furthermore, volcanoes tend to be basic in composition and silica layers are not expected to form in predominantly basic environment.

Reconstruction of past volcanic failures in their source areas is extremely rare as there is usually no material to study and/or the main scarp has since been covered by younger eruptions. For that reason, the majority of studies have focused on the sedimentological properties of these failures (e.g. Hunt et al., 2013). Until recently, it has been assumed that the volcanic flank collapses occur as one major event that displaces hundreds of cubic kilometres of rock. However, new research suggests that at least some of these collapses occurred in multiple stages (Hunt et al., 2013(Hunt et al., , 2018;;León et al., 2017). Such findings may have serious implications for the calculated frequency of such hazards and reopens questions about the possible reactivation of apparently inactive structures. The San Andrés Landslide on El Hierro in the Canary Islands represents one of the few places in the world where it is possible to investigate the fault planes and landslide mass of a volcanic collapse structure. This study integrates a range of structural geological and geomorphological measurements, petrological and microstructural analyses, and cosmogenic nuclide dating in order to reconstruct the developmental history of this huge slope failure. Data have been obtained -directly or indirectly -from the surface of the fault plane while samples were collected from both the fault plane itself and an adjacent gully perpendicular to the footwall. Our hypothesis states that this landslide is not the result of a single event, as proposed by Day et al. (1997), but instead results from a number of successive events spanning a protracted period. This hypothesis is not straightforward to address as unambiguous evidence for multiple slip events along a single fault is difficult to elucidate due to problems associated with overprinting and weathering.

Nonetheless, on the basis of previously published research and the results obtained during this study, it has been possible to propose a new conceptual model for the development of the San Andrés Landslide.

Geological setting

The island of El Hierro

El Hierro is an active volcanic edifice that constitutes the smallest and youngest of the Canary Islands (Figure 1).

Its oldest subaerially exposed rocks are represented by the Tiñor Unit, with a maximum age of 1.12 Ma (Guillou et al., 1996), which form the northern and northeastern parts of the island. This unit is thought to have developed rapidly until around the time of the Tiñor debris avalanche at 0.88 Ma (Carracedo et al., 2001).

Thereafter, the rocks of the El Golfo-Las Playas Unit, with a maximum age of 0.55 Ma (Guillou et al., 1996), capped much of the Tiñor Unit and infilled the scarp area of the Tiñor debris avalanche (Carracedo et al., 2001).

Its youngest subaerially exposed rocks are represented by the Rift Series, with a maximum age of 0.16 Ma (Guillou et al., 1996). Over the past 33 000 years onshore eruptions have reoccurred approximately once every 1 000 years (Becerril et al., 2016a). The latest, ongoing phase of volcanism began around 2.5 ka (Carracedo et al., 2001). Recently a period of intense seismic activity spanned from July 2011 (López et al., 2012) to 2014 (Benito-Saz et al., 2017). During this period an offshore eruption started on 10 October 2011 and finished in March 2012 (Meletlidis et al., 2015). A more detailed description of the geology of island has recently been presented elsewhere (Carracedo and Troll, 2016).

The flanks of El Hierro have hosted a number of enormous gravitational slope failures which have contributed to the development of its characteristic three-point star morphology (Figure 1). Until now, seven debris avalanches have been identified: Tiñor (< 880 ka), Las Playas I (545-176 ka), Las Playas II (176-145 ka), El Julan (> 158 ka), El Golfo A (176-133 ka), El Golfo B (87-39 ka), and Punta del Norte of unknown age (Masson, 1996;Urgeles et al., 1996Urgeles et al., , 1997;;Carracedo et al., 1999Carracedo et al., , 2001;;Masson et al., 2002;Longpré et al., 2011;Becerril et al., 2016b;Carracedo and Troll, 2016;León et al., 2017;Blahůt et al., 2018a). The volumes of these debris avalanches vary between 50 km 3 and 234 km 3 , which makes them average from the known volcanic slope failures (for additional morphometric characteristics, see Blahůt et al., 2019). In addition, a large slump, sensu Moscardelli and Wood (2008), or a deep seated gravitational slope deformation, sensu Sorriso-Valvo et al. (1999) or Agliardi et al. (2001), is located on the southeast flank of the volcanic edifice. This feature, the San Andrés Landslide, is defined by a group of pronounced faults which represent landslide detachment planes.

While its volume has not been determined, the deformed toe is visible around 18 km off coast at a depth of more than 3 km (Becerril et al., 2016b). Similar features have been described from other volcanic islands including the Azores (Hildenbrand et al., 2012) and the Hawaiian Islands (Duffield, 1975). It has previously been proposed that the San Andrés Landslide is an anchored block associated with the debris avalanche Las Playas I (Day et al., 1997).

San Andrés Landslide

The San Andrés Landslide has developed in the volcanic rocks of the Tiñor Unit. This unit comprises three distinct subunits (Gómez Sainz de Aja et al., 2010): the basal subunit is represented by relatively thin, 20-40 cm, steeply dipping lava flows; the intermediate subunit, which forms the majority of the unit, is represented by thicker, up to 4 m, shallow dipping lava flows; while the uppermost subunit is represented by emission vents with well-preserved craters and associated lavas. The broad arcuate and presumably listric fault system which defines the boundaries of the slope deformation to the northeast is terminated to the southwest by an escarpment associated with the giant debris avalanche Las Playas II. Previously the San Andrés Landslide has been interpreted as either an aborted collapse structure (Day et al., 1997) or a deep seated gravitational slope deformation (Klimeš et al., 2016).

It has been reasoned that the fault system must have developed at some point between 545 ka and about 261-176 ka (Day et al., 1997). If correct, this implies that the collapsed landslide mass has remained anchored during each of the subsequent giant debris avalanches on El Hierro (Carracedo and Troll, 2016). Support for the notion of protracted inactivity is provided by the presence of younger lavas crossing the fault without orientations. The obtained data were then compared to the structural geological field measurements recorded on the outcrop.

Schmidt hammer sampling is used in geomorphological research to estimate rates of weathering and to obtain information relating to relative exposure ages for rock surfaces on the basis of rebound values, R (Aydin and Basu, 2005;Goudie, 2006). A total of eighty-eight test sites were sampled on the fault plane using a Proceq

SilverSchmidt ST-PC. Each site was sampled twelve times in a direction perpendicular to its surface. Of these twelve samples, the uppermost and lowermost values were disregarded, according to standard sampling protocol, and the remaining ten were used to calculate the rebound value. These R values were processed in a GIS. Initially, an orthophotograph of the fault scarp had to be created from a set of photoshots. The position of each test site was recorded on a printed copy of the orthophotograph prior to image rectification in the GIS.

The relative positions of several test sites were measured in the field and used as reference points during georeferencing. Each test site is represented by a point placed approximately in its centre and then the calculated R value was added to each test site. Interpolation of these values indicates changes in the relative surface hardness across the exposed part of the fault scarp. Several interpolation algorithms were tested but the character of the data led us to select the natural neighbour technique.

Petrographic and microstructural analyses

Petrographic analyses of the sampled rocks were performed at the Institute of Rock Structure and Mechanics CAS and the Institute of Geology CAS in Prague. Optical microscopy was conducted using the polarisation microscope Leica DMR while chemical analysis of the primary and accessory minerals was conducted using the electron microprobe CAMECA SX-100. Microstructural investigations have been made through the application of optical microscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). These investigations were made at the University of Vienna. Thin sections were cut perpendicular to the shear plane and parallel to the direction of sliding prior to mechanical polishing to thicknesses of c. 30 μm. Optical microscopy was conducted using the polarisation microscope Leica DM4500 P with micrographs obtained under both plane polarised light (PPL) and crossed polarised light (XPL). Selected samples were carbon coated for SEM. The SEM imagining was conducted using a FEI Inspect S with an accelerating voltage of 10-15 kV for spot sizes of 5-7. Images were obtained and presented in the back scattered mode (BSE). Element energy dispersive spectroscopy was also conducted to better understand the chemical effects of fault slip. The EDAX EDS systems were used as an add-on to the FEI Inspect S. Using the TEAM™ Software Suite and Smart Phase

Mapping it was possible to collect spectra and generate phase maps showing elemental distribution and associated spectra. The EDS operating conditions were 15 kV accelerating voltage for spot size of 5, at resolution of 514 x 514, dwell 200 µs.

Cosmogenic radionuclide dating

Cosmogenic radionuclide dating constrains the amount of time a rock has been situated at or near the surface of the Earth (e.g. Lal, 1991). Normal fault planes represent ideal objects for this dating technique as the incoming cosmic rays are only able to penetrate the progressively exposed parts of the fault (e.g. Palumbo et al., 2004). The basaltic lithologies of El Hierro determine that the most suitable cosmogenic nuclide with which to constrain periods of fault activity is 3 He (e.g. Kurz, 1986;Poujol et al., 2014). The sampling strategy aimed, first, to obtain samples from the least eroded parts of the fault and, second, to avoid obtaining samples from the whitish part of the fault plane as this was known to have been exposed during road construction. Five samples were collected from two profiles along the footwall (Figure 2). These samples range in thickness from two to five centimetres and belong to the same basaltic sequence that erupted 1.050.02 Ma (Carracedo et al., 2001). Samples were crushed and sieved to isolate fractions of less than 0.25 mm, 0.25-0.5 mm, and 0.5-2.0 mm. Several physical separation techniques including magnetic separation and density separation were used to isolate pure olivine and pyroxene phenocrysts.

All helium analyses were performed in the noble gas platform of the Centre de Recherches Pétrographiques et Géochimiques in Nancy. Samples were fused in vacuo at 1600°C for 15 minutes using the new custom designed metal induction furnace of CRPG (Zimmermann et al., 2018). Typical furnace blanks were (1.90.6)  10 -19 and (54)  10 -15 mol.g He concentrations measured in the samples, respectively. The extracted gas then underwent standard gas purification (Blard et al., 2015). Helium was cryofucused at 8K before being released at 70K and introduced into the Split Flight Tube mass spectrometer, a Nier source static mass spectrometer optimised for the analysis of cosmogenic nuclides (Protin et al., 2016). The linearity of the mass spectrometer was carefully established by analysing standard splits with similar 4 He pressure to those of the samples. During the one-month analytical Finally, 3 He cos exposure ages were computed using the CREp calculator (crep.crpg.cnrs-nancy.fr), with the world averaged 3 He production rate, the standard atmosphere, and the Lal-Stone time dependent model (Martin et al., 2017). The production rate was computed taking into account the spatial characteristic of the samples, including self-shielding (Table 1).

Results

Field data and structural analyses

The NE section of the San Andrés Fault outcrop is morphologically well preserved while its SW section has been smoothed as a result of weathering and incurred some damage during road construction. In general, the exposed fault plane is straight with an orientation of approximately 170/70° (dip direction/dip angle). Only some irregularities can be observed such as two remarkable fault plane undulations protruding from its surface, with widths of up 4 m, separated by a parallel depression (Figure 3A-B). These irregularities were also recognised during structural geological measurements. In greater detail, it has been possible to identify numerous linear features, which have been grouped into three families on the basis of their character and orientation (Figure 3A-C). First, a family of centimetre to metre scale striations were identified on the basis of structural measurements and from the slope gradient maps. These striations cover most of the outcrop but are best preserved on the fault plane undulations. Most commonly, the striations plunge with azimuths of 125° and 131°. Second, a family of smooth elongated undulations and strongly elongated protrusions were also identified on the basis of structural measurements and from the slope gradient maps. These features are best seen in the northeastern section of the fault plane. The undulations and protrusions plunge with an azimuth of 111°. Third, a family of very smooth, spatially limited, slightly elongated protrusions were identified from the fault perpendicular slope gradient map. These are found in the lower SW and upper NE part of the 4 m wide fault plane undulation as well as at the crest of the other undulation protruding from surface to the NE. The protrusions plunge with an azimuth of 158°.

A photomosaic offering a perpendicular view of the fault outcrop overlain by a raster of interpolated R values is presented on Figure 4. Proportional plots illustrate the spatial variability in rock hardness across the outcrop. In general, higher R values have been measured on the lower and right side of the fault outcrop than have been measured on its upper and left side. The highest R values, indicating the least weathering, have been recorded

in the lowermost part of the fault outcrop on its right side (R > 50). This finding is not surprising given that the hard whitish surface was exposed only recently during construction of the road. High R values have also been recorded on the far left side of the fault outcrop (R > 40). This finding is more surprising but it can be explained by several tens of centimetres of erosion on the fault surface and the presence of harder basalt blocks within the tectonic breccia. Conspicuously low R values have been recorded in a wedge-shaped zone between 23 m and 26 m (R < 25). Here a rather soft gouge material is attached to the fault surface and the vegetation cover descends much lower than elsewhere -this suggests that chemical and biological weathering processes are more intense in this area.

Petrographic analyses

The host rock basanites are fine grained basic rocks (samples PET3 and PET4). These rocks contain mainly pyroxene, plagioclase, olivine, amphibole and volcanic glass, together with the accessory minerals (apatite, chromite, magnetite, and Ti-magnetite). The pyroxene is represented by diopside. The plagioclases are usually zoned with a basic core (An 79-83 ) and acid rim (An 45-70 ). Olivine occurs only in basanites with subordinate amounts of volcanic glass. The olivine is enriched by forsterite component (75-80 mol. %). The amphibole is represented by magnesiohornblende to hornblende tschermakite. The texture of the groundmass is mostly ophitic and gives no indication of a pre-existing preferred fabric orientation. The tectonic breccias sampled from the fault plane are composed of fragments of volcanic rock (up to several centimetres), cemented by a very fine grained groundmass containing abundant fragments of the original volcanic rock (millimetres to centimetres on samples PET1 and PET2). The higher proportion of fine grained Fe-oxides and Fe-hydroxides imparts a rusty colour of these tectonic breccias. However, the volume of Fe-oxides and Fe-hydroxides is highly variable, with significant enrichment in the fine grained groundmass of the original volcanic rock. The tectonic breccias are cross-cut by zeolite veins in a range of orientations and sizes. Frictionite, or landslide pseudotachylyte, was found in samples obtained directly from the fault plane (Figure 2). This is composed of relict frictional melt and material similar to tectonic breccia (Figures 567).

Microstructural analyses

The NE part of the fault outcrop is covered by a whitish layer which attains a maximum thickness in the order of several centimetres. Figure 2 shows examples of the whitish layer that overlies the grey cataclasite and microbreccias. In some cases this whitish layer comes into direct contact with tectonic breccia. The tectonic breccia is highly oxidised (Figure 2D, F). Brecciation intensity increases from the undeformed basanite to the more deformed breccia, i.e. towards the fault surface. This breccia may be the product of an earlier slip event.

However, here we focus on the cataclasite and the whitish layer, i.e. those layers that overlie the tectonic breccia. Using optical microscopy, it is possible to differentiate two distinct layers within the frictionite by their composition and microstructure: a structurally lower cataclasite and a structurally higher silica layer.

The cataclasite cuts the tectonic breccia while in places it is cut by the silica layer. Contact with the host rock is characterised by a sharp boundary that is observed in both hand samples and optical micrographs (Figure 6).

Although the cataclasite is grey in hand samples, it is dark brown in optical micrographs (Figure 6A, B) while the sharpness of the contact is accentuated by the truncation of several clasts (Figures 6 &7). The main characteristics of this layer are grain size reduction and P-foliation, terminology of Passchier and Trouw, 2005).

The zones of grain size reduction, in which grain sizes range from c. 10 μm to c. 100 μm, are defined by P-and Y-shears (Figure 6B, D). A dextral sense of shear is indicated by the P-and Y-shears along with a winged inclusion visible in the optical micrographs (Figure 6A,B). This finding is consistent with structural observations made at the outcrop scale. Elemental maps highlight the similarity between the composition of the host rock and the composition of the cataclasite (Figure 6E, F). Moreover, this similarity is accentuated by the apparent resemblance in composition of a truncated clast, itself in contact with the cataclasite (Figure 6E, F). A second cataclasite has a more lensoid shape which is apparently defined by P-and Y-shears (Figure 7). It exhibits strong P-foliation which is seen most readily in the elemental maps (Figure 7C,E). In addition, cracks within this layer are parallel to both the Y-shear, and the slip surface, and are filled with silica (Figure 7E).

The silica layer cuts both the tectonic breccia and the cataclastic layer. The boundary between the silica layer and the subjacent rocks is a sharp, single line, which crosscuts foliation in the cataclasite and truncates clasts in the tectonic breccia. The boundary between the cataclasite layer and the silica layer is sometimes marked by shiny sublayers that parallel the boundary (Figure 6B, H) and exhibit a high interference colour when seen under cross polarised light with a gypsum tint plate (Figure 5C). There is no indication of mixing between the cataclasite and the silica layer across the boundary separating these layers. The silica layer appears to be translucent, white to pale coloured under plane polarised light (Figures 567). It is composed of a microcrystalline to amorphous silica matrix with a thickness of c. 250 µm to 350 µm. The architecture varies:

the matrix consists of an inclusionless silica material that develops into complex structures with flow bands (Figures 5-7 5H). These sublayers appear to have a similar composition to the cataclasite layer but with a finer grain size and present evidence of material mixing with the silica layer. Some clasts embedded in the silica layer have a cortex, or armour, and these are referred to as clast-cortex aggregates. The clast-cortex aggregates vary in size, from c. 90 µm to 290 µm, complexity, and composition. Those clasts within the aggregates consist of fragments of adjacent rock (Figure 7G) or silica matrix (Figure 6I). A complex cortex

texture with what appears to be several coatings has been observed (Figure 6G). Elemental maps show that the cortex composition varies as well (e.g. Figure 6L and Figure 7D,E). It is believed that a significant amount of the cortex material was lost during preparation of the thin sections (e.g. Figure 6G-I).

4.4

Cosmogenic radionuclide dating Cosmogenic 3 He exposure ages reveal significant differences between each of the two sampled profiles. Sample COS1 has yielded a maximum exposure age of 21±20 ka while samples COS2 and COS3 have yielded a maximum exposure age of 26±18 ka. Replicates of sample COS4 yielded exposure ages ranging from 21±19 to 61±19 ka while replicates of sample COS5 yielded exposure ages ranging from 52±17 to 183±17 ka (Table 1).

These results indicate that movement across the currently exposed portion of the fault plane occurred over a short period of time as suggested by the fact that there is no gradual decrease in exposure ages from upper to lower part of the two profiles.

The right hand profile (samples COS1-3) revealed exposures ages that are below the detection limit i.e. these were exposed between 0 and 26 ka. These young exposure ages most probably reflect recent exhumation on the right hand part of the fault owing to the fact that the right hand profile is separated from the left hand profile by only 6-7 m. Intense erosion of the right hand profile can be excluded as the R values indicate much greater hardness, i.e. less weathering, here than on the left hand profile.

The left hand profile (samples COS4 and COS5) revealed exposure ages between 21 and 183 ka. The higher sample COS4 provides a younger age than the lower sample COS5. Under normal circumstances this should not be possible -the highest parts of the fault should be exposed before its lower parts -and it suggests that the higher sample has been subjected to weathering. Consequently sample COS5 offers the most reliable results as it has neither been affected by weathering nor by recent exhumation. The high variability of computed exposure ages between the replicate in this sample is due to the considerable uncertainty associated with the estimate of the magmatic 3 He component as outlined in Section 3.3. Nonetheless it is believed that the most recent fault reactivation occurred between the calculated exposure ages of sample COS5 (52±17 to 183±17 ka).

Interpretation and discussion

Landslide displacement direction

Distinct families of linear and elongated features at the fault plane evidence past behaviour of the rock mass (Sagy et al., 2007). Two major slip events are interpreted to have occurred on the studied fault plane on the basis of the relative position and general morphology of such features. The older phase is represented by the family of smooth elongated undulations and strongly elongated bumps, best preserved in the depressions and flats in the northeastern section of the fault plane, which have a plunge direction of 111°. During this phase the rock mass moved towards the ESE. The younger phase is represented by the other two families of linear and elongated features. The onset of this phase is interpreted to have commenced with minor, most probably slow displacement towards the SSE. Support for this is gleaned from the smooth, spatially limited, elongated bumps which have a plunge direction of 158°. The displacement rate is then interpreted to have accelerated as the rock mass moved more towards the SE. Support for this is gleaned from the centimetre to metre scale striations which cover most of the fault outcrop and have plunge directions of 125° and 131°. The general appearance of these features is sufficiently similar to assume that they developed during a single slip phase. It is supposed that this shift in the displacement direction, from the SSE to the SE, can be explained by either a sudden change in the geometry of the slip surface or a fundamental change in the morphology of the entire volcanic edifice. It is known that El Hierro underwent a clockwise rotation of 15°, beginning around 440 ka (Széréméta et al., 1999), and it has been suggested that this rotation could be associated with activity on the San Andrés Landslide and the debris avalanche Las Playas I (Carracedo, 2008).

Fault related microstructures and dynamic rock reworking

The microstructures seen on the surface of the San Andrés Fault are believed to be a result of fast slip, associated with frictional heating and partial melting during landslide emplacement. The first line of evidence for flash heating during flank collapse is observed in both hand samples and polished slabs as heat diffused in the underlying material: the adjacent host rock and tectonic breccia exhibits increasing level of oxidisation towards the fault surface. This is similar for pseudotachylytes reported from the Köfels landslide (Masch et al., 1985) and from the Arequipa volcanic landslide deposit in Peru (Legros et al., 2000). The second line of evidence comes from the crosscutting relationships between either the host rock or the tectonic breccia and the deformation layers. All of the observed contacts are sharp and accentuated by multiple truncated clasts.

This type of crosscutting relationship indicates fast displacement and is often considered a strong indication of a seismic slip (Passchier and Trouw, 2005;Price et al., 2012;Smeraglia et al., 2017). The third line of evidence comes from the boundary between the silica layer and the adjacent rocks. This boundary is characterised by shiny bands that exhibit a high interference colour under cross polarised light and a positive optical sign under cross polarised light using a gypsum tint plate. These bands are tentatively interpreted as recrystallised material resulting from frictional melting but further analyses are needed to confirm this supposition.

The cataclastic layer exhibits P-foliation, grain size reduction, and the development of winged clasts in a heterogeneous matrix. The winged clast presented in Figure 5 resembles those of Grasemann and Dabrowski (2015) and Grasemann et al. (2019). The foliation developed within this layer is typical of brittle deformation, with Y-shears parallel to the slip surface, and P-shears inclined relative to the slip surface (i.e. Figure 5I). This type of foliation cannot be used as an indicator for fast slip as it is recorded in association with both seismic and creep phenomena (Verberne et al., 2013;Smeraglia et al., 2017). However, unequivocal evidence for fast slip, instead of progressive creep, is provided by the fact that the contact between the cataclastic layer and the adjacent rock is sharp and accentuated by multiple truncated clasts. The cataclasite architecture is similar to those described in other volcanic pseudotachylytes (Lavallée et al., 2012(Lavallée et al., , 2014;;Kendrick et al., 2012Kendrick et al., , 2014) ) as well as to that described in seismically deformed limestones (Smith et al., 2011). Therefore, it is necessary to define this cataclastic layer as frictionite sensu lato, as it was almost certainly emplaced during catastrophic landslide emplacement but there is no decisive proof of flash heating.

In contrast, the silica layer is defined as frictionite sensu stricto, as it is comparable to traditional pseudotachylytes (e.g. Passchier and Trouw, 2005;Price et al., 2012;Rowe and Griffith, 2015). Here a list of particularly pertinent features is presented with reference to the identification criteria for deformed pseudotachylytes (Passchier and Trouw, 2005;Price et al., 2012):

i.

Crosscutting relationships with sharp layer boundaries. The silica layer cuts the cataclasite foliation and the boundaries are accentuated by clast truncation and presence of shiny bands which have been interpreted as recrystallised material from frictional melting. Analogous microstructural features have been reported from numerous natural and experimental cases of seismic slip (e.g. Fondriest et al., 2013;Smeraglia et al., 2017;Tesei et al., 2017).

ii.

The presence of fragments of host rock. Fragments of adjacent rock appear as isolated islands and vary in size, shape, and complexity. The more complex clasts have an encasing cortex, or armour, and are referred to as clast-cortex aggregates. Typically, those fragments described here have a volcanic clast in the core, cataclastic material in the rim, and they parallel the foliation. In a number of senses, they are similar to "armoured clasts" or "accretionary rounded grains" (Anders et al., 2010;Rowe et al., 2012;Kirkpatrick et al., 2013) and clay-clast aggregates (Boutareaud et al., 2008;Han et al., 2011). The main differences are the ratio between the clast and the rim and their composition. The rims described here appear to be wider than previously reported while they are neither composed of clay nor any other mineral assemblage. Consequently, the clastcortex aggregates presented in this study are most readily comparable to the accretionary rounded grains described from the Corona Heights Fault (Kirkpatrick et al., 2013).

iii.

The presence of an ultra-fine grained matrix. The silica layer comprises a homogenous silica matrix, ultra-fine grained to amorphous, which is very similar to silica gel (Kirkpatrick et al., 2013;Faber et al., 2014) and amorphous nanopowder (Rowe et al., 2019). More detailed studies are needed in order to measure the amount of water and crystalline material that occurs in this matrix. The presence of new grains that have grown in the voids or bubbles indicates that at least some of this matrix must be crystalline.

iv.

Inherited compositional variations from flow structures. Flow bands in the silica layer are defined by optical and compositional variations. Optical variations are evident as shiny bands and they can be traced along the whole length of the layer. The flow bands are structurally controlled, as they parallel the P-and Y-shears, and they localise clasts and clast-cortex aggregates as well as Mgenriched material. Further analyses are needed to fully understand their origin. Similar to these flow bands are the cataclastic sublayers, which are also structurally controlled, but with evidence for the reworking of cataclastic material. This feature, together with presence of clast-cortex aggregates is interpreted as fluidised granular flow reported in many geologic settings (Boutareaud et al., 2008;Anders et al., 2010;Ujiie et al., 2011;Kirkpatrick et al., 2013).

Furthermore, fluidised cataclasites have been suggested as a record of seismic slip rates (Brodsky et al., 2009;Rowe et al., 2005).

v.

Differences between the matrix and host rock mineral assemblages. The most conspicuous difference between the silica layer and the adjacent rocks is in the mineral assemblage. The homogenous silica layer contrasts greatly with the polymineralic cataclasite and the host rock.

Even though the silica layer can be assumed to be a product of hydrothermal fluid deposition, the presence of kinematic indicators such as shears and flow bands consistent with observations at the outcrop scale, indicate that the silica layer was formed as a product of frictional slip. Silica has been reproduced in fast slip experiments (e.g. Di Toro et al., 2011;Kirkpatrick et al., 2013;Rowe et al., 2019) and is considered to be a product of frictional wear that weakens the fault, acting as a lubricant (Rowe et al., 2019).

Conceptual model for the San Andrés Landslide

A conceptual model for the development of the San Andrés Landslide is proposed here on the basis of the findings presented in this study and age estimates presented elsewhere in the published literature (Figure 8).

The first slip event as evidenced by the cataclastic layer is proposed to have occurred between 545 ka and 430 ka. The lower limit for fault activity is defined by the onset of volcanism associated with the El Golfo Series (Guillou et al., 1996) while the upper limit for fault activity is defined by the onset of the clockwise rotation of El Hierro (Széréméta et al., 1999). The second slip event as evidenced by the silica layer is proposed to have occurred between 183 ± 17 and 52 ± 17 ka based on the presented cosmogenic 3 He exposure dates. This age range spans termination of volcanism associated with the El Golfo-Las Playas Unit and the onset of volcanism associated with the Rift Series at 145 ka (Guillou et al., 1996). The lower limit for fault activity broadly relates to the probable timing of the debris avalanches El Golfo A, between 176 ka and 133 ka, and Las Playas II, between 176 ka and 145 ka. Moreover, the upper limit for fault activity broadly relates to the probable timing of the debris avalanche El Golfo B, between 87 ka and 39 ka. Thus, it is believed that the factor, or factors, which triggered one of these giant debris avalanches may have triggered the second slip event described in this study but no other data have yet been found to support this hypothesis. In addition, a conceptual model is presented for the development of the surface of the San Andrés Fault (Figure 9). The developmental model outlined here is broadly consistent with the findings of Day et al. (1997). In that study, it was suggested that the minimum age of the San Andrés Fault should be between 250 ka and 150 ka and that the fault could have been active

only for a geologically short period of less than 400 ka. Their assertions are based on a younger undisturbed lava flow which covers the southern part of the fault.

Conclusions

In this study the fault plane of the San Andrés Landslide on El Hierro in the Canary Islands has been investigated through a combination of geological and geomorphological measurements, petrological and mineralogical analyses, and cosmogenic 3 He exposure dating.

 The fault surface is composed of a striated frictionite which is in contact with a highly oxidised tectonic breccia. The tectonic breccia transitions into the underlying undeformed basanite host rock.

Microstructural analysis of the frictionite has revealed two distinct layers: a dark heterogeneous cataclastic layer and a translucent silica layer with a thickness of up to 1 cm.

 The architecture of the samples and the observed crosscutting relationships suggest that the cataclasite layer and the silica layer represent two separate slip events associated with seismic rupture during landslide emplacement. Moreover, the notion of two distinct slip phases is supported by kinematic markers etched on the fault surface.

 This information -coupled with the cosmogenic exposure dating and knowledge about the geological history of the island -forms basis of a conceptual model for the development of the San Andrés Landslide. The first slip event, represented by the cataclastic layer, is thought to have occurred between 545 ka and 430 ka while the second slip event, represented by the silica layer, is thought to have occurred between 183 ± 17 and 52 ± 17 ka. This is the first time that more than one slip event has ever been recognised on a single fault plane resulting from volcanic flank collapse on an oceanic island. It suggests that the possibility of reactivating supposedly inactive fault structures may be far greater than hitherto assumed. This is a key finding, which may lead to reevaluation of the threats related to volcanic islands collapses. Furthermore, it is believed that this is the first time a silica layer resulting from frictional melt has been described from a volcanic setting.

Figure captions

Figure 1 A shaded topographic and bathymetric relief map of El Hierro, Canary Islands, overlain by a geological map simplified from Ancochea et al. (2004). The inset shows an aerial image of the studied fault outcrop. SAL: San Andrés Landslide.

Figure 4

The interpolated R-values obtained through Schmidt hammer sampling on the San Andrés Fault.

The size of the black dots illustrates the rock hardness at each of the test sites. The interpolated R-values obtained through Schmidt hammer sampling on the San Andrés Fault.

The size of the black dots illustrates the rock hardness at each of the test sites.

Figure 3 :

 3 Figure 3: What do different colors represent?The different colours of arrows represent distinct sets of striations and elongated bumps. We have added this explanation to the captions.

Figure 4 :

 4 Figure4: should be 'black circles' or 'black dots'? This was corrected.

Figure 8 :

 8 Figure 8: Add 'Age before present' to indicate what these numbers are? We have added this to the figure captions as it is graphically nicer.

  ,BA, B) while the sharpness of the contact is accentuated by the truncation of several clasts(Figures 6 & 7). The main characteristics of this layer are grain size reduction and P-foliation, (according to terminology ofafterPasschier and Trouw, (2005)). The zones of grain size reduction, in which grain sizes range from c. 10 μm to c. 100 μm, are defined by P-and Y-shears (Figure6B,DB, D). A dextral sense of shear is indicated by the P-and Y-shears along with a winged inclusion visible in the optical micrographs (Figure6A,BA, B). This finding is consistent with structural observations made at the outcrop scale. Elemental maps highlight the similarity between the composition of the host rock and the composition of the cataclasite (Figure6E,FE, F). Moreover, this similarity is accentuated by the apparent resemblance in composition of a truncated clast, itself in contact with the cataclasite (Figure6E,FE, F).

  ), cataclastic sublayers (Figure 5H), and clast-cortex aggregates (Figure 6G,IG, I). Flow bands, which vary in thickness from c. 25 µm to c. 110 µm, correspond to P-and Y-shears in the cataclastic layer fabric and are characterised by a change in the amount of Mg (Figure 5L and Figure 6C,DC, D). Cataclasite sublayers form lenticular shapes (Figure

  compositional variations from flow structures. Flow bands in the silica layer are defined by optical and compositional variations. Optical variations are evident as shiny bands and they can be traced along the whole length of the layer. More detailed analyses are needed to fully Formatted: Font: Not Italic Formatted: Font: Italic Formatted: List Paragraph, Numbered + Level: 1 + Numbering Style: i, ii, iii, … + Start at: 1 + Alignment: Left + Aligned at: 0.63 cm + Indent at: 1.9 cm Formatted: Font: Italic Formatted: List Paragraph, Numbered + Level: 1 + Numbering Style: i, ii, iii, … + Start at: 1 + Alignment: Left + Aligned at: 0.63 cm + Indent at: 1.9 cm Formatted: Font: Italic Formatted: List Paragraph, Numbered + Level: 1 + Numbering Style: i, ii, iii, … + Start at: 1 + Alignment: Left + Aligned at: 0.63 cm + Indent at: 1.9 cm Formatted: Font: Italic understand their origin. The flow bands are structurally controlled, as they parallel the P-and Yshears, and they localise clasts and clast-cortex aggregates as well as Mg-enriched material.

3

  He exposure ages dating and current knowledge about the geological history of the island -forms basis of a conceptual model for the development of the fault surface, it has been possible to propose a development model for of the San Andrés Landslide. The first slip event, represented as evidenced by the cataclastic layer, is proposed thought to have occurred between 545 ka and 430 ka while the second slip event, represented as evidenced by the silica layer, is thought proposed to have occurred between 183 ± 17 and 52 ± 17 ka.based on the presented cosmogenic 3 He exposure dates.

Figure 2 (

 2 Figure 2 (A) General view of the San Andrés Fault outcrop showing the location of each sampling site. Thenumbers in parentheses following after COS samples 1-5 indicate the estimated exposure ages in ka. (B) Detailed view of the fault outcrop which shows a series of conspicuous vertical or subvertical slickenlines and the whitish layer adorning the recently exposed parts of its surface; (C) Contact between the footwall (left) and the hanging wall (right); (D) Detailed view of the oxidised rock visible in the footwall of the fault which reflects the presence of fine grained Fe-oxides and Fehydroxides. (E, F) Samples used for microstructural analysis: (E1) hand sample MSA1 shows slickenlines composed of the whitish layer -note the dashed line parallel to the slickenlines; (E2) closeup view of the slickenlines, (E3) rock slab used to extract thin sections shows the whitish layer that covers the cataclasite and breccia. The; black rectangles mark the positions of Figures 5-6; (F1) core sample MSA2 drilled perpendicular to the fault surface shows large clasts in the breccia and white layer -red dashed line highlights the orientation of the slickenlines; (F2) rock slab shows the gradual increase in oxidation of the breccia towards the fault surface; (F3) thin section shows the sharpness of the slip surface and the barely-visible white silica layer. The black rectangle marks the location of Figure 7.

Figure 3

 3 Figure 3 Structural analysis of the surface of the San Andrés Fault outcrop conducted on the basis of a remotely sensed UAV DTM. (A) Slope gradient map giving a ground plan view presented in inverted greyscale and showing distinct sets of striations and elongated bumps. (B) Slope gradient map giving a fault perpendicular view presented in inverted greyscale and visualised in three dimensions (for reference see the right hand part of the fault in (A)). (C) Orientation of the fault surfaceStereoplot of the structural geological measurements and the striations revealed by the

Figure 5

 5 Figure 5 Photomicrographs and BSE images of rocks from the San Andrés Fault, sample MSA1. First area presented (aA) under plane polarised light, (bB) under cross polarised light, (cC) as a sketch, (dD) in BSEbackscattered mode, and (e-fE-F) in elemental maps. These show tectonic breccia (TB) covered by a cataclastic layer (CL) which is, in turn, covered by a silica layer (SL). The cataclasite cuts both the tectonic breccia and a zeolite vein (Z) and has preserved P-foliation and a winged inclusion. The boundary between the silica layer and the adjacent cataclastic layer is sharp with high interference bands, characteristic of recrystallisation. The area is rotated during analysis and the sense of shear is marked on the sketch. Second area presented (gG) under plane polarised light, (hH) under cross polarised light, (iI) as a sketch, (jJ) in BSEbackscattered mode, and (k-lK-L) in elemental maps. These show tectonic breccia (TB) with zeolite veins (Z) covered by a thinner cataclastic layer (CL) and a thicker silica layer (SL). The fault slip surface is subhorizontal with a top-to-right shear direction. The boundary between the cataclastic layer and the silica layer is marked by high intensity bands of recrystallised material. The silica layer preserves clast-cortex aggregates (CCA) and flow bands (FB) along with parallel P-and Y-shears. The elongated void was most probably a clast-cortex aggregate lost during preparation of the sample.

Figure 6

 6 Figure 6Photomicrographs and BSE images of rocks from the San Andrés Fault, sample MSA1. Area presented in (aA) under plane polarised light, (bB) under cross polarised light, (cC) under cross polarised light using a gypsum tint plate, (dD) in BSEbackscattered mode, and (eE) as a sketch.These show tectonic breccia (TB) covered by a silica layer (SL). The tectonic breccia was cut during deformation which resulted in truncated clasts (TC) while the silica layer preserves clast-cortex aggregates (CCA), a cataclastic sublayer (CL), and flow bands (FB). These flow bands show a positive

Figure 7

 7 Figure 7 Photomicrographs and BSE images of rocks from the San Andrés Fault, sample MSA2. Area presented in (aA) under plane polarised light, (bB) under cross polarised light using a gypsum tint plate, and (cC) as a sketch. These show a large truncated clast (TC) in the tectonic breccia (TB) cut by cataclasite (CL) and covered by a silica layer (SL) which has flow bands (FB) and clast-cortex aggregates (CCA). The rectangle in (aA) indicates the region highlighted in (d-fD-F). In (d-fD-F) elemental maps show P-foliation in the cataclastic layer along with clast-cortex aggregates and flow bands in the silica layer. Dashed lines highlight characteristics presented in the sketch. The fault slip surface is horizontal with a top-to-right shear direction.

Figure 8 A

 8 Figure 8A proposed conceptual model for the development of the San Andrés Fault system over the last 550 ka based onDay et al. (1997),Carracedo et al. (1997),Széréméta et al. (1999),León et al.

  ), cataclastic sublayers (Figure 5H), and clast-cortex aggregates (Figure 6G, I). Flow bands, which vary in thickness from c. 25 µm to c. 110 µm, correspond to P-and Y-shears in the cataclastic layer fabric and are characterised by a change in the amount of Mg (Figure 5L and Figure 6C, D). Cataclasite sublayers form lenticular shapes (Figure

Figure 2 (

 2 Figure 2 (A) General view of the San Andrés Fault outcrop showing the location of each sampling site. The numbers in parentheses after COS samples 1-5 indicate the estimated exposure ages in ka. (B) Detailed view of the fault outcrop which shows a series of conspicuous vertical or subvertical slickenlines and the whitish layer adorning the recently exposed parts of its surface; (C) Contact between the footwall (left) and the hanging wall (right); (D) Detailed view of the oxidised rock visible in the footwall of the fault which reflects the presence of fine grained Fe-oxides and Fehydroxides. (E, F) Samples used for microstructural analysis: (E1) hand sample MSA1 shows slickenlines composed of the whitish layer -note the dashed line parallel to the slickenlines; (E2) closeup view of the slickenlines, (E3) rock slab used to extract thin sections shows the whitish layer that covers the cataclasite and breccia. The black rectangles mark the positions of Figures 5-6; (F1) core sample MSA2 drilled perpendicular to the fault surface shows large clasts in the breccia and white layer -red dashed line highlights the orientation of the slickenlines; (F2) rock slab shows the gradual increase in oxidation of the breccia towards the fault surface; (F3) thin section shows the sharpness of the slip surface and the barely-visible white silica layer. The black rectangle marks the location of Figure 7.

Figure 3

 3 Figure 3 Structural analysis of the surface of the San Andrés Fault outcrop conducted on the basis of a remotely sensed UAV DTM. (A) Slope gradient map giving a ground plan view presented in inverted greyscale and showing distinct sets of striations and elongated bumps. (B) Slope gradient map giving a fault perpendicular view presented in inverted greyscale and visualised in three dimensions (for reference see the right hand part of the fault in A). (C) Orientation of the fault surface and the striations revealed by the morphostructural analysis of the UAV DTM. The

Figure 5

 5 Figure 5 Photomicrographs and BSE images of rock from the San Andrés Fault, sample MSA1. First area presented (A) under plane polarised light, (B) under cross polarised light, (C) as a sketch, (D) in BSE, and (E-F) in elemental maps. These show tectonic breccia (TB) covered by a cataclastic layer (CL) which is, in turn, covered by a silica layer (SL). The cataclasite cuts both the tectonic breccia and a zeolite vein (Z) and has preserved P-foliation and a winged inclusion. The boundary between the silica layer and the adjacent cataclastic layer is sharp with high interference bands, characteristic of recrystallisation. The area is rotated during analysis and the sense of shear is marked on the sketch. Second area presented (G) under plane polarised light, (H) under cross polarised light, (I) as a sketch, (J) in BSE, and (K-L) in elemental maps. These show tectonic breccia (TB) with zeolite veins (Z) covered by a thinner cataclastic layer (CL) and a thicker silica layer (SL). The fault slip surface is subhorizontal with a top-to-right shear direction. The boundary between the cataclastic layer and the silica layer is marked by high intensity bands of recrystallised material. The silica layer preserves clast-cortex aggregates (CCA) and flow bands (FB) along with parallel P-and Y-shears. The elongated void was most probably a clast-cortex aggregate lost during preparation of the sample.

Figure 6

 6 Figure 6Photomicrographs and BSE images of rock from the San Andrés Fault, sample MSA1. Area presented in (A) under plane polarised light, (B) under cross polarised light, (C) under cross polarised light using a gypsum tint plate, (D) in BSE, and (E) as a sketch. These show tectonic breccia (TB) covered by a silica layer (SL). The tectonic breccia was cut during deformation which resulted in truncated clasts (TC) while the silica layer preserves clast-cortex aggregates (CCA), a cataclastic sublayer (CL), and flow bands (FB). These flow bands show a positive birefringence, in contrast to the rest of the silica layer, as evidenced by the second order blue interference colour in

Figure 1 A

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Figure 4 (Figure 5

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Figure 5 (Figure 6

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Figure 6 (Figure 7

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Table A

 A

		). Using the 1.050.02 Ma value for T eruption , this yields	4 He rad theoretical concentrations
	ranging from 1.210 12 at.g	-1 to 4.110 12 at.g	-1 .
	Such	4 He radio concentrations are similar to the total	4 He concentrations measured by fusing the samples, which
	range from 6.210 12 at.g	-1 to 1.3610 13 at.g	-1

Table 1

 1

	):

layer formed with clast-cortex aggregate and flow bands, as shown in blue.

He exposure dating (COS1 -COS5).

Acknowledgements

Bernhard Grasemann, Rostislav Melichar, Bernhard Grasemann, Gerlinde Habler, and Jackie Kendrick are thanked for stimulating and insightful discussions regarding the obtainedour results and their interpretation.

We are also grateful to the two reviewers and the editor for significantly improving the clarity of this manuscript.

Acknowledgements

Bernhard Grasemann, Rostislav Melichar, Gerlinde Habler, and Jackie Kendrick are thanked for stimulating and insightful discussions regarding our results and their interpretation. We are also grateful to the two reviewers and the editor for significantly improving the clarity of this manuscript.

Submission

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Funding sources Funding sources This study was supported by the Czech Science Foundation [Project GJ16-12227Y], by the OP VVV project CzechGeo EPOS-Sci No. CZ.02.1.01/0.0/0.0 /16_013 /0001800 and the conceptual development research organisation of the Institute of Rock Structure and Mechanics CAS [RVO:67985891].

Abstract: Volcanic flank collapses often result in giant debris avalanches that are capable of travelling tens of kilometres across the ocean floor and generating tsunamis that devastate distant communities. The San Andrés Landslide on El Hierro, Canary Islands, represents one of the few places in the world where it is possible to investigate the landslide mass and fault planes of a volcanic collapse structure. In this study, a new conceptual

displacement and the presence of scree covering lava flows as old as 145 ka in the gully of Las Playas (Carracedo et al., 1997). However, recent research is starting to challenge these arguments. The step-like structure of the incised gullies in the collapsed mass, together with records of progressive creep along its main detachment plane, in the order of 0.5 mm.a -1

, suggests that the landslide mass may be moving steadily to the east and southeast (Klimeš et al., 2016;Blahůt et al., 2017Blahůt et al., , 2018b)).

The part of San Andrés Fault system investigated in this study is exposed along a narrow road which traverses the Barranco de Tiñor (27°47'18.82"N, 17°55'19.04"W). This outcrop is situated at c. 490 m asl, has a length of c. 70 m, and dips between 65° and 75° to the SE (Figure 2). The fault zone has a total thickness of between one and two metres. Both the footwall and hanging wall rocks comprise basaltic lavas of the Tiñor Unit with a few thin intercalated lapilli beds and soil horizons (Carracedo et al., 1997). No indurated breccia is present on the hanging wall. Its eastern section has only been uncovered relatively recently as a result of road construction and is distinguished by its whitish surface. This part of the fault surface hosts well-preserved slickenlines, which range in size from several millimetres to metre scale surface undulations. Detailed petrographic examinations of the fault rocks have been published previously (Day et al., 1997).

Methods

Field data and structural analyses

Structural analyses of the surface of the San Andrés Fault plane incorporated structural geological field measurements alongside the interrogation of a high resolution digital terrain model (DTM). This DTM was constructed on the basis of photogrammetric imagery obtained from an unmanned aerial vehicle (UAV) and processed using the commercial software Agisoft PhotoScan (Agisoft LLC, 2014). The accuracy of terrain models constructed following this approach is directly comparable to those constructed on the basis of laser scanning (Balek and Blahůt, 2017). From the photogrammetric point cloud, it was possible to derive two complimentary slope gradient maps with grid cells of 2 x 2 cm. The first gives a ground plan view of the fault (Figure 3A) while the second gives a fault perpendicular view (Figure 3B). These maps have been analysed in detail using the ESRI® ArcGIS applications ArcMap and ArcScene. Slope gradient maps, in contrast to hillshade maps, are particularly suited for the identification of linear features irrespective of their spatial distribution and period the observed reproducibility of the system was 2 % at 1σ, for both 3 He and 4

He. Furthermore, during this one-month period, two CRONUS-P pyroxene standards were analysed. These yielded 3 He concentrations of (4.860.14)  10 9 at.g -1 and (5.010.14)  10 9 at.g -1

.

Both values stand within the internationally recognised error limits for this material (Blard et al., 2015). To determine the 3 He/ 4

He ratio of the magmatic component step crushing in vacuo was conducted on 0.5 mm to 2 mm olivines and pyroxenes obtained from Sample 5III

(1 st step: 100 strokes, 1 min; 2 nd step: 500 strokes, 5 mins; Table 1). Crushing blanks were (32)  10 4 and (72) .a -1 has been used, computed using the equations of Andrews (1985) and the chemical composition measured in the phenocrysts and bulk rock samples (Supp. Table A). Combined with a value of 1.050.02 Ma for T eruption , based on the K-Ar age of the San Andrés basalt presented in Carracedo et al. (2001), this yielded a value of (63)  10 3 at.g -1 for 3 He nucleo .

In theory, the magmatic , for an average of (3.34.6)10 12 at.g -1

.

Thus the outlined approach for estimating 4 He rad represents the main source of uncertainty when computing the magmatic 3 He correction and, in turn, the main source of uncertainty when computing the final cosmogenic 3 He concentrations.

Instead, we decided to apply the following approach and compute a minimal and a maximal value for the cosmogenic 3 He concentrations, as follow (Table 1):

Figure 8

A conceptual model for the development of the San Andrés Fault system over the last 550 ka based on Day et al. (1997), Carracedo et al. (1997), Széréméta et al. (1999), León et al. (2017), and the findings presented herein. The uppermost labels denote the main geological events to have taken place on El Hierro, i.e. rotation and volcanism, while those beneath denote the supposed age ranges for each of the giant landslides. The proposed timing of activity on the San Andrés Fault is presented underneath the timeline: the first slip event is associated with the formation of the cataclasite while the second slip event is associated with the formation of the silica layer. The time axis depicts thousands of years before present.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: