F O Marques
email: fomarques@fc.ul.pt

A Hildenbrand

A C G Costa

A L R Sibrant

The evolution of Santa Maria Island in the context of the Azores Triple Junction

Keywords: Santa Maria Island, Azores Triple Junction, large-scale mass wasting, flank collapse, island subsidence and uplift, age fossils

Santa Maria is the oldest island in the Azores, formerly belonging to the Eurasia plate and currently the only one sitting on the Nubia Plate, thus sharing a geodynamic evolution with the Azores Triple Junction. It is therefore important to evaluate the effects of active tectonics on the evolution of Santa Maria, for example on its vertical movements. We present new stratigraphic, geomorphologic, structural and geochronological data from Santa Maria which shed further light on how a volcanic ocean island evolves in a tectonically active setting. Santa Maria island started with a first shield volcano (Old Volcanic Complex) that emerged ca. 6.0 Ma ago and was active until ca. 5.3 Ma. The short time span between the first and second shield volcanoes (ca. 0.3 Ma) and the preservation of only the western flank of this first shield volcano indicate an initial flank collapse at ca. 5 Ma. The collapse scar was covered by an eastwards dipping sedimentary complex (Intermediate Sedimentary Complex), with a likely tsunami deposit at the base. A second shield volcano (Young Volcanic Complex) rapidly grew on these sediments from 4.8 to 3.8 Ma, and the island subsided by more than 100 m. At 3.7 Ma, a second flank collapse occurred, as inferred from the missing summit and eastern flank. Volcanism then resumed (3.6 to 2.8 Ma), giving rise to Strombolian cones lying unconformably on the collapse scar, and conformable parasitic cones on the unaffected flank. Submarine lavas occurring at up to 200 m altitude with a youngest age of 3.0 Ma indicate major uplift of the island since, at most, that time. Here we interpret uplift as the result of rift flank uplift on the southern shoulder of the nearby Terceira Rift in the last ca. 1.5 Ma.

Introduction

An outstanding problem in volcanic ocean islands close to major plate boundaries is that of evaluating the effect of active tectonics on their evolution. This is particularly so when the island is small and is located in a changing geodynamic setting where the stress field has undergone major changes (Luís ei al., 1994;[START_REF] Mitchell | Susceptibility of mid-ocean ridge volcanic islands and seamounts to largescale landsliding[END_REF][START_REF] Vogt | The Terceira Rift as hyper-slow, hotspot-dominated oblique spreading axis: A comparison with other slow-spreading plate boundaries[END_REF]Sibrant et al., 2015a). Additionally, although many volcanic islands around the world record evidence of important vertical displacements, the rates and mechanisms of the associated movements are still debated (e.g. [START_REF] Cas | Volcanic successions, modern and ancient[END_REF][START_REF] Jones | Pillow lavas as depth indicators[END_REF][START_REF] Jones | The flow of basalt lava from air into water, its structural expression and stratigraphic significance[END_REF][START_REF] Porebski | Lava-fed Gilbert-type delta in the Polonez Cove Formation (Lower Oligocene), King George Island, West Antarctica[END_REF]Ramalho et al., 2010aRamalho et al., , b, c, 2013[START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF][START_REF] Marques | The complex vertical motion of intraplate oceanic islands assessed in Santiago Island, Cape Verde[END_REF]. (Marques et al., 2013a). Azores islands from west to east: Flo -Flores; Cor -Corvo; Fai -Faial; Pic -Pico; SJo -S. Jorge; Gra -Graciosa; Ter -Terceira; SMi -S. Miguel; SMa -Santa Maria. For -Formigas islets. White arrows on opposite sides of the TR represent the plate velocities (ca. 3.5 ± 0.5mm/yr;Marques et al., 2013a). White half arrow indicates the dextral strike-slip motion in the Gloria Fault (GF). The East Azores Fracture Zone (EAFZ) is currently inactive, therefore Santa Maria belongs to the Nubia Plate. Black solid linestransform faults related to the MAR. Red solid line -transform fault related to the TR.

Background shaded relief from EMODnet portal (https://www.emodnet-bathymetry.eu/). (b) -Simplified geological map of Santa Maria Island adapted from Sibrant et al. (2015a), including previous relevant ages (Sibrant et al., 2015a;[START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF]. (c) -geological profile along X-Y in B showing the relationships between the main units, which includes the dip of the lava flows as measured in the field and inferred from the ages. Note the consistency between lava flow dip and younging to the west at sea level in the YVC on the southern coast.

Santa Maria is the easternmost island in the Azores, and lies close to the junction between the East Azores Fracture Zone, the Terceira Rift and the Gloria Fault (Fig. 1). Santa Maria has long been recognized as the oldest island in the Azores archipelago (e.g. [START_REF] Zbyszewski | Carta geológica de Portugal, Ilha de Santa Maria[END_REF][START_REF] Abdel-Monem | K-Ar ages from the eastern Azores group (Santa Maria, São Miguel and the Formigas Islands)[END_REF]Sibrant et al., 2015a;[START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF]. It is also the only island with clear evidence of submarine volcanism well above current sea level (e.g. [START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF][START_REF] Serralheiro | Stratigraphy and geochronology of Santa Maria Island (Azores)[END_REF][START_REF] Serralheiro | A Geologia da ilha de Santa Maria[END_REF], and is the island with the most complete marine fossil record [START_REF] Madeira | The marine fossils from Santa Maria Island (Azores, Portugal): an historical overview[END_REF][START_REF] Ávila | The Marine Fossil Record at Santa Maria Island (Azores). In Volcanoes of the Azores, Active Volcanoes of the World[END_REF]. Santa Maria can be viewed as a reference in the investigation of marine erosion shelves in the Azores and elsewhere [START_REF] Marques | Comment on "The insular shelves of the Faial-Pico Ridge (Azores archipelago): a morphological record of its evolution[END_REF][START_REF] Ricchi | Marine terrace development on reefless volcanic islands: New insights from high-resolution marine geophysical data offshore Santa Maria Island (Azores Archipelago)[END_REF], and the analysis of the shelves in the context of the long-term evolution of Santa Maria is critical to the present study. Constraining the age of marine fossils is very important to our study because they provide information on where the sea was at a given age (palaeoshoreline markers), and so help in the analysis of the vertical movements (mostly subsidence during YVC growth). We thus carried out our study in Santa Maria for three reasons: First, the island has grown in a tectonic setting that has changed over its > 6 Ma lifetime, thus possibly recording important steps in the evolution of the Azores Triple Junction (ATJ). Second, the evolution of the island is still not well understood, with gaps in our knowledge especially regarding stratigraphy, structure and tectonics, as well as strain and stress evolution. In this regard, geochronological data is used to constrain the age of the different volcanic complexes, the age of the inferred large-scale mass wasting (LSMW), and the age of the different fossil deposits. Third, it is a very small island, therefore seemingly not prone to LSMW [START_REF] Mitchell | Susceptibility of mid-ocean ridge volcanic islands and seamounts to largescale landsliding[END_REF][START_REF] Hildenbrand | Large-scale mass wasting on small volcanic islands revealed by the study of Flores Island (Azores)[END_REF], which is in apparent contradiction with the LSMW inferred in Santa Maria.

In order to answer the questions about the tectonic evolution of this volcanic island, we needed to: first establish a well-constrained volcanic stratigraphy, from which we could infer the evolution of the island and, in particular, the ages of the abundant fossil deposits In parallel with this, we needed to constrain the geometry of the major shield volcanoes and the position of the main feeding centres, from which we could infer the missing portions of the shield volcano, discuss the origin of the major unconformities in terms of marine/meteoric and LSMW types of erosion, and discuss ages and triggering mechanisms. Second, we needed to identify probable tsunami deposits, and young smaller-scale landslides that shaped the marine shelves to the east, south and west of Santa Maria, as well as to analyse Santa Maria's vertical movements and discuss their rates and mechanisms. Third, we needed to determine the orientation of the stress field from the observed deformation and dykes, and to discuss the relationship between the evolutions of Santa Maria and the ATJ. To accomplish these objectives, we analysed aerial images and digital elevation models for geomorphological and geological interpretations. This was accompanied by fieldwork to identify the major unconformities and volcanic units, and to find and measure faults and dykes. These data were used to unravel the geometry and position of the main volcanoes, and to find the critical palaeodepth markers (palaeoshorelines). Finally, sampling for high-precision K-Ar dating was carried out and used to calibrate the volcanic stratigraphy and to constrain fossil ages and critical palaeodepth markers. The present work builds on that of Marques et al. (2013b) and Sibrant et al. (2015a), by adding new field, geochronological and structural/tectonic data, which together made it possible to accomplish the objectives listed above.

Geological setting

Early geological mapping of Santa Maria [START_REF] Agostinho | Sobre a tectónica da ilha de Santa Maria[END_REF][START_REF] Zbyszewski | Carta geológica de Portugal, Ilha de Santa Maria[END_REF] distinguished two main volcanic units separated by a volcano-sedimentary complex. The abundant marine fossils allowed [START_REF] Zbyszewski | Carta geológica de Portugal, Ilha de Santa Maria[END_REF] to propose minimum and maximum ages for the older (pre-Vindobonian) and younger (post-Vindobonian) volcanic complexes, respectively (Vindobonian -ca. 16 to 7 Ma). [START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF] later introduced three additional units. As a result, the stratigraphy becomes, from the bottom up: [START_REF] Turcotte | Geodynamics[END_REF] two very small and poorly outcropping formations, one submarine (Cabrestantes Formation) and the other sub-aerial (Porto Formation); (2) the sub-aerial Anjos Volcanic Complex; (3) a mixed sub-aerial and submarine volcano-sedimentary complex, the Touril Complex; (4) a submarine volcanic complex, the Facho Complex; (5) the mixed submarine and sub-aerial Pico Alto Complex; and (6) the Feteiras Formation, which is composed of sub-aerial Strombolian cones and widespread pyroclasts.

Based on the recognition of major unconformities and K-Ar dating, Sibrant et al. (2015a) identified three main units: the Older Shield Volcano (ca. 5.7 to 5.3 Ma, which corresponds to the Anjos Complex and includes the Cabrestantes and Porto formations of [START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF], the Intermediate volcano-sedimentary Complex (5.3 to 4.3 Ma, far from Vindobonian, which corresponds to the Touril Complex of [START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF], and the Younger Shield Volcano (ca. 4.3 to 2.8 Ma, which encompasses the Facho and Pico Alto complexes of [START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF] with parasitic Strombolian cones on top (which correspond to the Feteiras Formation of [START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF]. Based on lava flow geometry, as well as the geometry and location of the two shield volcanoes, plus the nature and shape of the major unconformities, Marques et al. (2013b) and Sibrant et al. (2015a) inferred the existence of two large-scale flank collapses. The first occurred between ca. 5.3 and 4.3 Ma, and the second at ca. 3.6 Ma. [START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF] dated submarine rocks in SE Santa Maria at ca. 4.8 Ma (Fig. 1), which constrains the volcanic stratigraphy, since it is the oldest age of the Younger Shield Volcano. The 4.8 Ma age thus narrows the age interval for the first flank collapse, which lies between the youngest age of the Older Shield Volcano and the oldest age of the Younger Shield Volcano.

Methods

We started with a morphological analysis of the island using a digital elevation model (DEM) with a spatial resolution of 10 m, in order to identify the main morpho-structural units and better define areas of interest for fieldwork. The DEM was produced from a digital topographic map of Santa Maria Island made by the Centro de Informação Geoespacial do Exército (CIGeoE), and the full process is described in [START_REF] Costa | Largescale flank collapses in a steep volcanic ridge: Pico-Faial Ridge, Azores Triple Junction[END_REF]. Given that most of the outcrops are on the coast, we went around the island by boat to get a full picture of the main volcanic sequences and their first-order unconformities, faults and dykes. Field work was then carried out over the whole island on road cuts, quarries, and those sea cliffs accessible from the landward side. We systematically measured the attitude of lava flows, faults and dykes, and reconstructed the geometry of major unconformities (mostly landslide scars).

We measured 225 dykes, mostly on the coast, and plotted their dip and strike on graphs.

We also used previous geological maps where dykes had been mapped in detail inland. In many dykes we also measured thickness and noted the number of dykes in multiple intrusions. We measured the attitude of 41 faults and striations mostly on the coast, and plotted them on graphs.

When visible, we also measured fault striations and noted kinematic indicators for fault movement.

We took advantage of Santa Maria's high coastal cliffs to investigate and sample the base and top of lava flow piles. We collected many more samples than reported here and in Sibrant et al. (2015a). However, due to limited funding in our project only critical samples were dated to complement the K-Ar and 40 Ar/ 39 Ar ages acquired on separate phases in Santa Maria (Sibrant et al., 2015a;[START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF]. We focused on the oldest age and setting of the OVC (samples SMA16B, G and K; Fig. 2a for location) to better constrain the oldest stratigraphy and vertical displacements, as well as the age of the submarine YVC close to sedimentary deposits with marine fossils (sample SMA16O; Fig. 2a for location) to better constrain their age and the vertical displacements in Santa Maria. For a detailed description of the K-Ar method used in this study see Appendix 1 in the supplementary material.

Results

Topography/bathymetry

Morphologically, Santa Maria can be divided into two distinct parts (Fig. 2). The first is the low-lying western half, which is gently sloping to the west. This surface is most likely structural, as it generally follows the westward gentle dip of the OVC and YVC lava flows (Fig. S-1). The flat shelf on which the airport lies, close to the western coast, could be a palaeoshelf, as proposed by [START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF]. (2) The second part is the higher and more rugged eastern half of the island, whose highest peak is at an altitude of ca. 590 m and comprises a deeply eroded portion of the YVC. The divide between the two parts is rather sharp and arcuate, and concave to the east (Fig. 2).

In detail, there are other curved, concave seaward topographies that shape the coast line (Fig. 2b). These could well correspond to eroded scars of smaller scale landslides. Offshore, there is also a topographic asymmetry, with the marine shelf being much wider in northern Santa Maria than elsewhere around the island. The shelf edge to the north lies between ca. -120 m and -200 m, and extends up to 7 km offshore (Fig. 2b). In contrast, elsewhere around the island the edge of the marine shelf is at < -100 m, locally even < -50 m, and is much narrower (< 1.5 km) (Fig. 2b). [START_REF] Ricchi | Marine terrace development on reefless volcanic islands: New insights from high-resolution marine geophysical data offshore Santa Maria Island (Azores Archipelago)[END_REF] analysed in detail the marine shelf, and concluded that the spatial distribution of marine terraces around Santa Maria depends on the complex interplay between sea level fluctuations, vertical displacements, and the intensity of marine erosion. In an abandoned quarry NE of the north end of the airport runway (site of sample SM16B, Fig. 2a), the sub-aerial OVC overlies steeply eastward-dipping submarine flows intruded by dykes, which might correspond to an OVC passage zone (palaeoshoreline) or to the basement complex. The OVC occupies the western half of Santa Maria, and the lava flows dip gently to the W or WSW (Fig. 2 and Fig. S-1). In contrast to the westerly dip in the OVC, the dip of the unconformity above the OVC is to the east. This unconformity is covered by the Intermediate Sedimentary Complex (ISC), which, at its base, shows chaotic conglomerates composed of polygenetic, large, poorly sorted and poorly rounded boulders [START_REF] Serralheiro | A Geologia da ilha de Santa Maria[END_REF] this study). To the east, the ISC may pass into marine conglomerates. Towards the top, the sediments become finer and the carbonate component increases. Filling the space removed from the original OVC (east flank), and lying unconformably on the ISC, is the Young Volcanic Complex (YVC), which is initially composed of submarine lava flows, and later of sub-aerial lavas as the volcano emerged out of the ocean. Submarine and sub-aerial lava flows dip to the west all along the southern and northern Santa Maria coasts. Eastern Santa Maria has a great number of Strombolian cones, the lava flows of which dip gently to the east and west. On the eastern coast, passage zones (palaeoshorelines) occur where gently dipping sub-aerial flows pass into steeply dipping submarine flows (Fig. 3).

At Pedreira do Campo (site of sample SMA16O in Fig. 2a), the submarine YVC is seen to lie conformably on the fossiliferous marine ISC through hyaloclastites (Fig. 4), or perhaps with "slight erosion prior to the deposition of the volcanic sequences above" as recognised by [START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF]. From the nature of the contacts between marine sediments and underlying sub-aerial OVC (major unconformity covered with thick chaotic debris deposit), and the overlying submarine YVC (conformable with apparently similar marine environment), we infer that the age of the fossils is close to the age of the pillow lavas dated here (ca. 3.12 Ma). Dykes show a heterogeneous distribution around the island, with the greatest density of dykes in the SW quadrant, and the least dykes in the NW quadrant. There is an overall dominant ENE-WSW trend to their orientation within the OVC, which is largely due to the dominant SW quadrant. Dykes older than the YVC are radial (cf. Fig. 5), and there is a convergence of these older dykes towards a triangular area (inside red circle in Fig. 5), which we infer to have been the approximate position of a major feeding centre for the OVC. A high concentration of dykes with different orientations can be observed within a very small area in the YVC (1-2 km 2 , inside red circle in Fig. 5), which we interpret as a main feeder for the YVC, which is coincident with the inferred main feeding centre for the OVC. In the southern YVC (i.e. over most of the SE quadrant where there is only YVC), the dykes show a generally scattered trend, but a NW to N main trend is apparent. In the northern YVC (i.e. over part of the NE quadrant on the coast and inland where there is only YVC), the young dykes trend mostly N-S to NNE-SSW. Young dykes in eastern Santa Maria have a dominantly curved pattern (concave eastward) in the YVC, gradually changing from NW-SE in the south to NNE-SSW in the north (Fig. S-3).

Faults have a heterogeneous distribution around the island (Fig. 6), with most faults outcropping in the SE quadrant, although the number of faults in the NE quadrant might be significantly underestimated (due to the poor outcrop conditions, even on the coast). There are many more faults in the eastern half of the island, which is dominated by the YVC, implying a heterogeneous distribution in both space and time. Faults have an overall dominant NNW-SSE trend (Fig. 6). Most of the observed tectonic faults have only small displacements (< 10 m), but a few have significant (> 50 m) displacements (Figs. 7, 8, S-5). These higher displacement faults trend mostly around NNW-SSE, and dip either to the east or to the west. The striations are dipslip, or steeply plunging to north or south, thereby defining dominant normal faulting with a small strike-slip component (either dextral or sinistral). Two new pairs of conjugate faults have been identified that define, as named here, the Praia and Maia Grabens (Figs. 1c and7). The Praia Graben in central Santa Maria is bounded to the west by an eastward-dipping fault (Fig. 7), and to the east by a westward-dipping fault. This can be inferred from the different altitude of the base of the ISC in the graben (ca. 50 m) and on the eastern shoulder (ca. 85 m), where it should be at lower altitude given the east dip of the base of the ISC. The Maia Graben in eastern Santa Maria is bounded to the west by eastward-dipping faults (Fig. 8), and to the east by westwarddipping faults (Figs. S-4, 9 and 10). Note that the fault in Fig. 9 only affects the older YVC, but does not seem to cut through the ca. 3.6 Ma lava flows.

K-Ar dating

The new K-Ar ages obtained in this study range between 3.12 ± 0.05 Ma and 6.00 ± 0.09 Ma (Table 1 and Fig. 10). They are consistent with the available stratigraphic controls and with previous K-Ar and 40Ar/39Ar ages measured on separated groundmass (Sibrant et al., 2015a;[START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF]. The new age of 6.00 ± 0.09 Ma here obtained on sample SMA16K collected at Baía do Raposo (Fig. 10), just below the ISC, extends the age of the OVC up to ca. 6.0 Ma. Previous and new ages make it possible to better constrain the stratigraphic sequences in critical sections, and to better correlate well-exposed sections in different parts of the island (Fig. 10). The 6.0 Ma age in the OVC shows that the Cabrestantes (dated by [START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF] ca. 6.0 Ma) and Porto volcanic cones belong to the OVC. They are not independent units as defined by [START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF] (Marques et al., 2013b;Sibrant et al., 2015a;present study). The location of the first collapse is here deduced from fieldwork data, the position of the main feeding centre inferred from radial dykes, and the geological maps shown in Fig. 11. On the south coast (Fig. 11a), the sub-aerial OVC is in lateral contact with the ISC from sea level to ca. 100 m altitude (solid black line). Further inland, the sub-aerial OVC has a direct contact with the sub-aerial YVC at ca. 170 m altitude (dashed black line), or through an intervening ISC.

These relationships are summarized in the X-Y cross-section shown in Fig. 11c. On the northern coast (Fig. 11b), the geological configuration is similar to that mapped on the southern coast, but with a more complex structure, i.e. the landslide fault apparently has a graben-horst structure, instead of the two, simple east-dipping normal faults inferred for the southern coast. Here we better constrain the age of the collapse at ca. 5.0 Ma, using the youngest OVC age (ca. 5.2 Ma)

and the oldest YVC age (ca. 4.9 Ma). This allows us to infer a collapse of the summit and eastern flank of the first shield volcano, delimited by a concave fault running approximately N-S from the central/south coast to the NE coast of Santa Maria (Fig. 11).

Three pieces of evidence allow us to infer a second collapse. First, the position of younger (3.5-3.7 Ma) YVC cones on a surface that is concave to the east, which is the opposite of the general rule for well-preserved shield volcanoes, where parasitic cones sit on a convex conical surface. The adventive cones are therefore unconformable on the westward-dipping older (> 4.0 Ma) YVC, and their lava flows dip gently to the west and the east. Second, the summit crater or caldera is missing. Third, the passage zones in eastern Santa Maria (Fig. 3), which are made up of lavas flowing from the parasitic cones. The passage zone is here interpreted as the result of lavas flowing over a steep scarp (landslide scarp?), which must lie deeper to the west.

Given that on the south and north coasts lavas dip to the west, and unconformable east-dipping lavas only occur on the east coast, we conclude that the second landslide fault must lie between

Gravitational instability around Santa Maria Island

Given that LSMW in the area could be related to the tectonic evolution of the ATJ, as suggested for Santa Maria, it is important to have a complete view of LSMW around the island.

At least one submarine slide has been recognized near Santa Maria in the Monaco Bank [START_REF] Weiß | Submarine sedimentation processes in the southeastern Terceira Rift/São Miguel region (Azores)[END_REF] with the surrounding sub-aerial OVC is not clear. If it were to sit unconformably on the OVC and below the YVC, then it would have the same age as the deposit described above.

Discussion

Stratigraphy calibrated by geochronology

Given the similarity of the ages (ca. 6 Ma) of small and scattered outcrops of Porto, Cabrestantes (dated by Ramalho et al., 2017 at ca. 6 Ma) and Baía do Raposo (here dated at ca. 6

Ma), we consider the Porto and Cabrestantes units as OVC. The new age at Baía do (Bay of) Raposo (ca. 6 Ma) better constrains the oldest age of the OVC, and shows the consistency with the general dip of the OVC flows to the west.

In addition, the Touril Complex, considered by [START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF] to include subaerial and submarine lava flows and sediments, is here reduced to the sedimentary rocks, and is renamed Intermediate Sedimentary Complex (ISC). The section at Pedreira do Campo (Fig. 4)

shows the typical transition sequence from the OVC to the Young Volcanic Complex (YVC), through an unconformity between the OVC and sediments, which are conformably overlain by the YVC. Therefore, the sub-aerial lava flows belong to the OVC and the submarine flows belong to the YVC. We note that (Fig We also find that the submarine Facho Complex cannot be below the Pico Alto Complex Ma. The submarine lavas attributed to Facho actually lie below the Facho Volcano, and comprise the submarine volcanism of the YVC (4.8 to 3.1 Ma). Given the continuous (no unconformities recognized) age spread of submarine lava flows (4.8 to 3.9 Ma; Sibrant et al., 2015a;[START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF], sampled in the Touril, Facho and Pico Alto complexes, there is no argument for attributing them to three distinct volcanic units. We therefore group them together as the early submarine stage of the YVC.

Finally, we find that the Feteiras Formation ("feteiras" means "the place where fern grows"; cf. The "pyroclasts" attributed to Feteiras are actually not pyroclasts, but instead a soil resulting from deep in-situ weathering of lava flows (Fig. S-9). The Feteiras Formation also cannot be stratigraphically above the Facho Complex, as proposed by [START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF], because the cones attributed to this unit are much older (ca. 3.6 Ma; Sibrant et al., 2015a) than Facho (ca. 2.8

Ma; Sibrant et al., 2015a).

To summarize, the proposed new stratigraphy is, from bottom to top (Fig. 12): ( 1 From the stratigraphic logs shown in Fig. 10 and 12, and supported by the new ages (see also , it is apparent that the fossil deposits cannot all have the same age. In fact, there is a difference of ca. 1 Ma between the sediments at Malbusca Point (ca. 4.2 Ma) and at Pedreira do Campo (ca. 3.2 Ma). This has major consequences regarding the palaeontology of the marine fossils in Santa Maria (because the use of fossils for stratigraphy and correlation with similar fossils in other parts of the world depends critically on their age), and is vital to the interpretation of the evolution of the island and its vertical movements (especially subsidence during growth of the YVC).

Structure, tectonics, strain and stress

Comparison between Figs. 5 and6 shows that dykes of the OVC do not follow mapped faults, and the quadrant with the most dykes (SW) is not the one with the most faults. In the YVC, the SE quadrant is dominated by both dykes and faults, where many have similar orientations. From this we infer that this second population of dykes were instead injected into fractures sub-parallel to faults (only locally do dykes intrude actual faults).

Our interpretation of the geometrical, spatial and temporal relationships of dykes and faults is that the stress field changed over time and in space, especially during the transition from the OVC to the YVC, the time of the older flank collapse (ca. 5 Ma). In support of this, older dykes (> 5.2 Ma) show a bulk radial distribution, similar to what Marques et al. (2019) observed in Fogo Island, Cape Verde, from which we infer the location of the main feeding centre and a radial stress field imposed by the first shield volcano [START_REF] Marques | Topography as a major factor in the development of arcuate thrust belts: Insights from sandbox experiments[END_REF]. Then, there is a clear dominance of the WSW-ENE trend in the SW quadrant, which includes multipleinjection dykes (cf. Fig. S-2), attesting to preferential NNW-SSE opening (least compressive stress) due to the regional stress field. Next, the younger dykes (< 4.9 Ma) show a preferential trend along the NNW-SE to NNE-SSW direction, attesting to preferential ENE-WSW opening.

Finally, the faults are mostly normal, heterogeneously distributed in space (across central and eastern Santa Maria), younger than ca. 4.8 Ma, and show a preferential NNW-SSE to N-S trend (ENE-WSW to E-W opening).

Dykes older than ca. 5 Ma could be related to the similarly trending East Azores Fracture Zone, the old Nubia-Eurasia plate boundary. Instead, dykes younger than ca. 5 Ma could be related to ENE-WSW extension in the last 4.0 Ma (DeMets et al., 2010), and to the rift jump from the East Azores Fracture Zone to the Terceira Rift along several transient, short lived, grabens trending WNW-ESE [START_REF] Vogt | The Terceira Rift as hyper-slow, hotspot-dominated oblique spreading axis: A comparison with other slow-spreading plate boundaries[END_REF]. Building on the work of [START_REF] Luís | The Azores triple junction evolution since 10 Ma from an aeromagnetic survey of the Mid-Atlantic Ridge[END_REF], [START_REF] Vogt | The Terceira Rift as hyper-slow, hotspot-dominated oblique spreading axis: A comparison with other slow-spreading plate boundaries[END_REF] and Sibrant et al. (2015a), here we can specify that the change in stress field orientation occurred at ca. 5 Ma, as supported by our new geochronological data and volcanic stratigraphy.

The trend of the ca. 5 Ma LSMW scar is approximately N-S, at a high angle to the dominant WSW-ENE dyke trend in the OVC, thus excluding a relationship between collapse and dykes. The N-S trend is perpendicular to the far field extension in the eastern Azores [START_REF] Demets | Geologically current plate motions[END_REF]. Therefore, we suggest that extension could have favoured the localization and the triggering of the two LSMW here proposed. It could also have produced the main grabens identified here. Finally, we propose that a transient Santa Maria Graben started opening at about 5 Ma, and was responsible for the flank collapse inferred for Santa Maria at that age. Note that both inferred LSMW occurred along faults striking NNW-SSE to N-S, consistent with extension and the trend of the dykes younger than 5 Ma.

LSMW events

Despite the small size of the Azorean islands, several LSMW events have been described

recently (Hildenbrand et al., 2012a;Marques et al., 2013b;[START_REF] Sibrant | Construction and destruction of a volcanic island developed inside an oceanic rift: Graciosa Island, Terceira Rift, Azores[END_REF]Sibrant et al., , 2015aSibrant et al., , b, 2016;;[START_REF] Costa | Largescale flank collapses in a steep volcanic ridge: Pico-Faial Ridge, Azores Triple Junction[END_REF][START_REF] Costa | Catastrophic flank collapses and slumping in Pico Island during the last 130 kyr (Pico-Faial ridge, Azores Triple Junction)[END_REF][START_REF] Marques | Evolution of a volcanic island on the shoulder of an oceanic rift and geodynamic implications: S. Jorge Island on the Terceira Rift, Azores Triple Junction[END_REF][START_REF] Hildenbrand | Large-scale mass wasting on small volcanic islands revealed by the study of Flores Island (Azores)[END_REF]. Exceptions in the eastern Azores are the islands of Faial (Hildenbrand et al., 2012b;Marques et al., 2014b;[START_REF] Trippanera | Relationships between tectonics and magmatism in a transtensive/transform setting: An example from Faial Island (Azores, Portugal)[END_REF] and Terceira (Marques et al., 2015;[START_REF] Quartau | The morphology of insular shelves as a key for understanding the geological evolution of volcanic islands: insights from Terceira Island (Azores)[END_REF], where LSMW has not been described [START_REF] Mitchell | Susceptibility of mid-ocean ridge volcanic islands and seamounts to largescale landsliding[END_REF][START_REF] Casalbore | Volcanic, tectonic and mass-wasting processes offshore Terceira Island (Azores) revealed by high-resolution seafloor mapping[END_REF][START_REF] Quartau | Morphology of the Faial Island shelf (Azores): the interplay between volcanic, erosional, depositional, tectonic and mass-wasting processes[END_REF][START_REF] Quartau | The insular shelves of the Faial-Pico Ridge: A morphological record of its geologic evolution (Azores archipelago)[END_REF].

From the bulk radial distribution of dykes at Santa Maria, we infer the position of coincident OVC and YVC main feeding centres on the divide between the eastern and western parts of the island (cf. Fig. 5). If this interpretation is correct, then the summits and eastern flanks of the two shield volcanoes are missing, as can also be deduced from the lava flow geometry (Fig. 12). One can propose an evolution of Santa Maria's morphology based solely on meteoric and marine erosion [START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF]. However, this view contains three inconsistencies.

First, field and geochronological data (lava flow dip and younging to the west) show that both OVC and YVC only have their western flanks preserved. Second, Santa Maria has undergone meteoric and marine erosion, and probably also small-scale landsliding over the last 3.6 Ma, if not longer. This is 10 times longer than the period between the OVC and YVC), but both are still mostly preserved. Third, the difference in age between the lavas of the older and younger shield volcanoes (ca. 0.3 Ma) is clearly not long enough for meteoric and wave erosion to remove the large amounts of lava flows missing in the older shield volcano (eastern flank and part of the summit). In addition, marine erosion would not have been selective in removing only one half of the shield volcano and preserving the other, especially the half facing the dominant winds (NW, W and SW). Therefore, the alternative is that the missing portion of the original OVC shield volcano has been removed by large-scale sector collapse. The chaotic debris deposit at the base of the ISC could, in part, represent a tsunami deposit associated with this collapse.

The youngest age measured for the OVC is ca. 5.3 Ma (but could be younger further to the west, in the sea) and the oldest age for the YVC is ca. 4.8 Ma (but could be older below current sea level). Therefore, the age of the first flank collapse is bracketed between <5.3 and >4.8 Ma, i.e. at around 5.0 Ma. We could not improve the accuracy of the age of the second collapse, but we could provide more detail on its location. The YVC lava flows dip to the west on the southern and northern coasts, but dip to the east on the eastern coast (Figs. 3,12,13), thus constraining the position of the collapse fault between the SE and NE corners of the island. Given the tectonic position of Santa Maria, its age and the inferred geometry of the landslide scars, the triggering mechanisms of the two inferred LSMW events may have been: [START_REF] Turcotte | Geodynamics[END_REF] high-magnitude earthquakes due to motion in the East Azores Fracture Zone, the Nubia-Eurasia plate boundary in the Azores prior to the jump to the Terceira Rift;

(2) NNW-SSE rifting in the ocean floor and dyke intrusion parallel to the inferred landslide scars.

Given that the onshore ISC deposits lie on an eastward-dipping unconformity atop the sub-aerial OVC, and that there is no evidence of a major palaeotopographic relief nearby, we interpret these deposits to be in part the result of a tsunami produced by the first flank collapse.

Vertical movements

The spatial and temporal distribution of sub-aerial and submarine lavas at Santa Maria is good evidence that: (1) either the island has moved vertically by hundreds of meters (stationary sea level), or (2) the sea level has changed by hundreds of meters (stationary island), or (3) both processes acted together. Hypothesis (2) does not seem viable because sea level estimates [START_REF] Miller | The Phanerozoic record of global sea-level change[END_REF] do not show such sea level highstands (ca. 200 m) in the period 6 to 0 Ma.

At Baía da (Bay of) Cré (Fig. S-10), young island uplift has exposed critical outcrops for the analysis of vertical movements of the island, especially the ones that pre-date younger uplift.

Sub-aerial lava flows of the OVC currently outcrop from sea level up to ca. 100 m (red ellipse in undetermined influence of sea level change until ca. 3-2.8 Ma. Ramalho et al. (2017, their Fig. 6) estimated subsidence and uplift rates based on a palaeoshoreline located at about 3. [START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF] is not justified, because the first submarine lavas post-dating the sub-aerial OVC lie above an east-dipping major unconformity and are off-set to the east relative to the OVC, which means that they would be submarine even without any subsidence of the island (Fig. 14).

According to the scenario proposed here, subsidence resulted from significant loading by the YVC growth filling the scar of the first collapse (Fig. 14). We can estimate the isostatic where ε is the topography, ρ l is the density of basaltic lavas, and ρ a is the density of the asthenosphere. The result of 112 m is an upper bound calculation of the isostatic response to constructional loading, therefore capable of explaining the inferred subsidence.

The new age (3.12 ± 0.05 Ma) obtained on YVC pillow lavas stratigraphically overlying marine carbonates of the ISC at Pedreira do Campo (Fig. 4), and underlying the sub-aerial ca. 2.8

Ma flows of the Facho volcano, indicates a progressive filling of the first collapse scar by the YVC, initially submarine and later sub-aerial, as the YVC shield volcano grew out of the ocean from east to west over a scarp dipping to the east (Fig. 14). Therefore, subsidence would have been active until, at least, ca. 2.8 Ma. Given the topographic position of 4.8 Ma old submarine YVC close to current sea level to the east (location in Fig. 10) and 3.1-2.8 Ma old submarine YVC to the west at ca. 170 m, we infer that, similarly to Baía da Cré, at least 200 m of subsidence occurred between 4.8 and 2.8 Ma. Subsidence rates at Santa Maria cannot be directly compared with the faster subsidence rates estimated for younger islands in the eastern Azores (Hildenbrand et al., 2012a;Marques et al., 2015), because they lie on younger lithosphere, and are thus subsiding faster due to faster lithospheric cooling [START_REF] Turcotte | Geodynamics[END_REF]. They also lie on rifting lithosphere, where Graciosa, Terceira and S. Miguel islands lie inside the Terceira Rift, and Pico, Faial and S. Jorge islands lie in the area of diffuse rifting (Marques et al., 2013a, 2014a[START_REF] Miranda | Distributed deformation close to the Azores Triple "Point[END_REF], and are thus currently undergoing additional vertical tectonic movements. [START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF] concluded that the only viable mechanism able to explain the uplift is crustal thickening by basal intrusions, and argued that uplift in response to the tectonic regime is unlikely because the area is under transtension. From a physical point of view, the argument of transtension is inconsistent with the known mechanics of rifting lithosphere, because rift shoulders are uplifted during rifting [START_REF] Weissel | Flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension[END_REF]. Although produced by lithospheric extension, rifts are known to have quite high shoulders, which are produced by uplift of rift flanks due to mechanical unloading of the lithosphere during extension and consequent isostatic rebound [START_REF] Weissel | Flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension[END_REF]. The Terceira Rift is no exception (Marques et al., 2015;[START_REF] Marques | Evolution of a volcanic island on the shoulder of an oceanic rift and geodynamic implications: S. Jorge Island on the Terceira Rift, Azores Triple Junction[END_REF], as well illustrated by the topographic profile shown in Figure 10.

Typically, magma ponding at the Moho (underplating) or higher (storing in magma chambers) is contemporaneous with volcanism at the Earth's surface, and differentiation during ponding can be responsible for the evolution of lavas from mafic to intermediate as observed in nearby S.

Miguel Island. Santa Maria does not show significant differentiation [START_REF] Beier | Geochemical evidence for melting of carbonated peridotite on Santa Maria Island, Azores[END_REF], which means that magma found easy ways to reach the surface without significant ponding. the age inferred for the Terceira Rift is correct [START_REF] Vogt | The Terceira Rift as hyper-slow, hotspot-dominated oblique spreading axis: A comparison with other slow-spreading plate boundaries[END_REF]Marques et al., 2015;[START_REF] Sibrant | Deformation in a hyperslow oceanic rift: Insights from the tectonics of the São Miguel Island (Terceira Rift, Azores)[END_REF][START_REF] Marques | Evolution of a volcanic island on the shoulder of an oceanic rift and geodynamic implications: S. Jorge Island on the Terceira Rift, Azores Triple Junction[END_REF], and Santa Maria's upward movement is (at least in part) due to rift flank uplift, then some (or all) of the 200 m of uplift is younger than ca. 1.5 Ma.

If we use this age and ca. 200 m altitude of the highest passage zone to calculate the uplift rate, we calculate a rate of ca. 0.133 mm/yr, which contrasts with the 0.057 mm/yr rate obtained using ca. 3.5 Ma [START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF].

Marine erosion shelves

Regarding the evolution of marine shelves, Santa Maria can be used as a reference in the Strombolian cones scattered on top of the island; therefore, most of the coastal area is made of rocks older than ca. 3 Ma, and the western coast is made of rocks older than 5 Ma. We look at the first-order topography of the shelves, because we are looking at their relationship with the main evolution of the island (volcanic construction/major landslides). In the absence of volcanic construction in the last 2.8 Ma, the explanation for the narrow insular shelves all around Santa Maria, except to the NE, cannot be found in regular marine abrasion and meteoric weathering.

We combine the observed short width of the shelf and age of rocks on the coast with the concave shape of the coast and/or shelf edge to propose that the shelf is narrow because of recent medium-scale landsliding. The slide to the south, centred on the village of Praia, could be responsible for the development of the large canyon on the headwall, the mouth of which is at Praia (due to sudden increase in potential energy), and for the slump currently creeping at Praia Background shaded relief from EMODnet portal (https://www.emodnet-bathymetry.eu/).

Conclusion

The revised volcanic stratigraphy proposed here for Santa Maria comprises four units, involving three main volcanic complexes (Basement, Old and Young) and a sedimentary complex. The Old and Young volcanic complexes are separated by a major unconformity on which the sedimentary complex lies. The lava flows of the Old and Young volcanic complexes dip gently to the west (consistent with the younging of radiometric ages to the west), and the major unconformity separating them dips gently in the opposite direction, i.e. to the east.

Prior to ca. 6.0 Ma, a seamount (Basement Complex -BC) began growing from the ocean floor to eventually reach the sea surface and form a proto-Santa Maria Island. At around 6.0 Ma, a shield volcano had emerged out of the ocean to form the original Santa Maria Island, which grew until ca. 5.2 Ma, the youngest age of the Old Volcanic Complex.

The time span between the youngest age of the Old Volcanic Complex (ca. 5.3 Ma or younger) and the oldest age of the Young Volcanic Complex (ca. 4.8 Ma or older) is < 0.5 Ma, inconsistent with marine and meteoric erosion being responsible for the selective removal of more than half of the first shield volcano. Therefore, we infer that, sometime between 5.2 and 4.9

Ma, a sector collapse removed the whole eastern flank and part of the summit of the first shield volcano (Fig. 14). The east-dipping collapse scar was first covered by a tsunami deposit made of chaotic conglomerates, and later by shallow marine sediments reflecting increasing water depth and age of fossils to the east.

At around 4.9 Ma, the Young Volcanic Complex started filling the collapse scar from east to west (Fig. 14), initially as submarine volcanism. The new shield volcano grew out of the ocean to the east until ca. 3.8 Ma, and the island subsided by more than 100 m due to loading.

Supported by the fact that more than half of the second shield volcano is missing, and that Strombolian cones aged ca. 3.6 Ma lie unconformably on the older Young Volcanic Complex, we infer that a second flank collapse at ca. 3.7 Ma removed the whole eastern flank and the summit of the second shield volcano. This second collapse was soon followed by the emplacement of conformable (on the undamaged western flank) and unconformable (on the collapse scar to the east) volcanic cones from ca. 3.6 to 2.8 Ma (Fig. 13). Based on the position of the main feeding centres inferred from the distribution of radial dykes here, we constrain the location of the faults produced by the two inferred flank collapses. Smaller landslides, together with marine and meteoric erosion have further shaped Santa Maria through to its present-day topography, and are responsible for the great variability in marine shelf width around the island.

The stress field within the island changed from NNW-SSE to ENE-WSW oriented least compressive stress at around 5 Ma, because the orientation of dykes intruding only the Old Volcanic Complex (i.e. older than 5.2 Ma) is mostly WSW-ENE, and dykes intruding only the Young Volcanic Complex (i.e. younger than 4.9 Ma) are mostly oriented NNW-SSE.

Coincidently, the inferred collapse faults have an orientation similar to the normal faults mapped at Santa Maria, which suggests a genetic relationship between extension/rifting and large-scale mass wasting on the island.

From the current altitude occupied by sub-aerial and submarine lavas, we can conclude that Santa Maria has undergone important vertical movements throughout its evolution.

Subsidence occurred during growth of the YVC, as attested by the diachronic deposition of marine sediments (which are younger to the west) and the distribution of marine and sub-aerial lavas of the YVC. Given that uplift postdates submarine volcanism (youngest at ca. 3.0 Ma), magma ponding as a mechanism to explain the inferred uplift is not consistent with the current data on Santa Maria. Instead, the major uplift experienced by Santa Maria, as indicated by submarine lavas outcropping at ca. 200 m asl, is here considered as being the result of rift flank uplift on the southern shoulder of the nearby Terceira Rift in the last ca. 1.5 Ma.

We conclude that one can identify large-scale sector collapses based on data collected onshore, mostly through the recognition of major unconformities and their interpretation, i.e. that they represent the removal of large portions (more than half) of the earlier edifice. Here we show that the inferred flank collapses may have been triggered by tectonic stresses and strain, i.e.

during a rifting stage active at around 5-3 Ma that produced the Santa Maria transient rift.

Therefore, Santa Maria can be used as a case-study for similar oceanic islands in tectonically active settings.

Figure 1 .

 1 Figure 1. (a) -Tectonic setting of the Azores Triple Junction. MAR and TR (thick green and cyan lines) are the Mid-Atlantic ridge axis and the Terceira Rift, respectively. The light blue shaded area delimited by cyan dashed line marks the diffuse Nubia/Eurasia plate boundary in the Azores(Marques et al., 2013a). Azores islands from west to east: Flo -Flores; Cor -Corvo;

Figure 2 .

 2 Figure 2. Shaded relief maps to show the main features of the onshore (a) and offshore (b) topography. (a) -the onshore shaded relief shows that the island is morphologically made up of a low, westward gently dipping slope in the western half, and a high (590 m maximal altitude), steep and deeply incised slope in the eastern half. Yellow dashed line marks the position of the divide. (b) -position of here inferred young (black lines, small-scale slides partly destroying the island and younger than 2 Ma), intermediate (red lines, large-scale slide partly destroying the

Figure 3 .

 3 Figure 3. Photograph showing the geometry of lava flows at Baía do Cura in eastern Santa Maria (see Fig. 2a for location). From the geometry, we deduce that flows cover an old sea cliff to the west (orange arrow).

Figure 4 .

 4 Figure 4. Images of the outcrop at Pedreira do Campo, where we collected sample SMA16O (see Fig. 2a for location). (a) -contact between pillow lavas, hyaloclastites and calcarenite with marine fossils, all showing a marine setting of deposition. No appreciable unconformity, e.g. erosion surface or angular unconformity, was observed in these outcrops. (b) -detail of collected pillow lavas. Red arrow in (a) points to sledgehammer (ca. 1 m long handle) for scale. Geologist on outcrop in (b) is 180 cm tall.

Figure 5 .

 5 Figure 5. Distribution and attitude of dykes in Santa Maria (blue short lines), and statistics of dyke attitude by quadrants (coloured stereoplots) and total (stereoplot at the bottom). Note that dykes in the OVC (SW, NW and NE quadrants) converge to a small triangular area, marked with a red circle.

Figure 6 .

 6 Figure 6. Heterogeneous distribution of faults in Santa Maria. Compare with Fig. 5 for relationships between dykes and faults.

Figure 7 .

 7 Figure 7. Image of outcrop of the fault bounding the Praia Graben in the west. The fault strikes N158 and dips 70º toward the observer (ENE), and is normal dextral given the plunge of the

Figure 8 .

 8 Figure 8. Normal faults dipping to the east (main fault) and west (minor), outcropping on the central southern coast and defining the west boundary of the Maia Graben. Red arrow points to geologist (180 cm tall) on the beach for scale.

Figure 9 .

 9 Figure 9. Photo of fault outcropping upslope the village of Maia (Fig. 10 for location), eastern Santa Maria, trending N170 and dipping 70º to the west. The striations (highlighted by white line on inset) and shear criteria indicate dominant normal faulting with a sinistral strike-slip component. Note that the fault only affects the Young Volcanic Complex (YVC), but does not seem to cut through the ca. 3.6 Ma lava flows.

Figure 10 .

 10 Figure 10. Proposed geological map with new stratigraphy calibrated by ages reported in Sibrant et al. (2015a -ages in blue on the logs, and also cyan on the map), Ramalho et al. (2017 -ages in orange) and this study (light blue stars). Also note the newly proposed position of the two large-scale landslide scars and of the two recognized main grabens. In the logs, vertical lines represent dykes, and oblique lines represent dip of lava flows.

Figure 11 .

 11 Figure 11. Panels (a) and (b) are extracts of the geological map of Serralheiro et al. (1987) with our interpretation of the location of the main landslide faults affecting the OVC, based on the distribution of the OVC, the ISC and the YVC, as represented in the X-Y cross-section in panel (c). Note that we have put LRCT and LRF together as the submarine part of the YVC. Also note that the old landslide faults are drawn where the OVC abruptly ends against the ISC and/or YVC (black line), and where the ISC abruptly ends against the submarine YVC (magenta line). Red lines plunging to the west in (c) represent lava flow dip.

  .[START_REF] Weiß | Submarine sedimentation processes in the southeastern Terceira Rift/São Miguel region (Azores)[END_REF] interpreted the slide using high-resolution bathymetry and seismic reflection, and concluded that it occurred as a slow LSMW event. In Fig.S-5bwe re-interpret the Monaco Bank slide regarding the faults and the original volume/shape of the slide block. We used shaded relief bathymetry and a bathymetric profile to interpret the landslide faults, mainly based on the scars observed on the bathymetry and the distribution of the debris.We reconstructed the original wedge, the volume of which compares very well with the volume of the debris deposit. From the ratio (10 km/2 km = 5) between the distance travelled by the debris (10 km) and the Monaco Bank height (2000 m), we infer that the slide was relatively slow, but maybe not as a slump as[START_REF] Weiß | Submarine sedimentation processes in the southeastern Terceira Rift/São Miguel region (Azores)[END_REF] suggested.4.6. Tsunami depositsSeveral conglomeratic deposits occur on Santa Maria, but here we only focus on two that are potential tsunami deposits (Figs. S-6 and S-7). The most widespread deposit is the one blanketing the unconformity atop the OVC and sealing the old dykes. This polygenetic chaotic debris deposit is very well exposed on the road into Praia from the west (Fig.S-6). It contains boulders from meters to centimetres in size, and comprises two main layers (Fig. S-6a): the bottom layer contains mostly well-rounded large boulders in a calcarenite matrix (Fig. S-6c), and the top layer contains smaller and more angular boulders in a grey clay matrix (Fig. S-6b).Moreover, the bottom layer comprises well-polished pebbles (Fig.S-6d), typical of a wind and sand polishing process in low-altitude coastal areas. From the nature of the matrix and pebble roundness and polish, we infer that the bottom deposit originated in the sea (tsunami runup), and the top deposit originated in land (tsunami backwash).A smaller deposit occurs at site SMA16B (see location in Fig.2) and is shown in Fig. S-7. This deposit is also polygenetic, chaotic, very poorly sorted with large boulders in a matrix of pebbles, gravel, sand and clay. This deposit includes large blocks of marine sediments (Fig. S-7b) and pillow lavas (Fig. S-7c) attesting to its marine origin. Unfortunately, the relationship

  . S-8) extrapolation of the age of dated pillow lavas at Pedreira do Campo (Fig. S-8c) to similar settings (Fig. S-8b and d) along the same yellow unit to the east (CT, Complexo do Touril of Serralheiro et al., 1987) leads to major inconsistencies. Furthermore, the submarine lavas (LRF) here dated at 3.1 Ma at Pedreira do Campo (Fig. S-8c) cannot be overlain by lava flows that are older (Fig. S-8b and d), and the CT sediments at Pedreira do Campo (ca. 3.2 Ma) cannot be the same as the CT sediments to the east atop Praia (Fig. S-8a) and at Malbusca (ca. 4.1 Ma) in Fig. S-8d. Therefore, there must also be a scarp where a thick black line marked F is drawn in Fig. S-8b, because the OVC (CA on the map) endsabruptly at sea level (whereas it reaches 150 m altitude immediately to the north). Likewise, the chaotic debris deposit (CT) also ends here, where the conglomeratic deposit passes into submarine lavas of the YVC (LRCT on the map in Fig.S-8d). This scarp/fault has to be older than the CT because it does not cross-cut it.

  (4.8 to 2.8 Ma), as proposed by[START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF], because the Facho Volcano is subaerial, and it is the youngest volcanism on the island, dated bySibrant et al. (2015a) at ca. 2.8

  Fig.S-9) is simply deposits from parasitic Strombolian cones that belong to the YVC, as indicated by the radiometric ages ofSibrant et al. (2015a) and[START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF].

Figure 12 .

 12 Figure12. Top panel -log with the new proposed stratigraphy calibrated with K-Ar data ofSibrant et al. (2015a) and present study, and 40 Ar/ 39 Ar data of[START_REF] Ramalho | Emergence and evolution of Santa Maria Island (Azores): The conundrum of uplifted islands revisited[END_REF]. Bottom panel -geological cross-section summarizing all field and geochronological data. Note the consistency between lava flow dip and younging to the west at similar altitude, and upwards, in both OVC and YVC. Also note that the young volcanic cones in the east lie unconformably on the basal YVC, through a LSMW scar, and in the west they lie conformably on the basal YVC as parasitic cones.

Figure 13 .

 13 Figure 13. E-W topographic cross-section with drawn geometry of lava flows and position of LSMW scars and young volcanic cones (< 3.7 Ma). Note that the cones in the west lie conformably on the western flank of the YVC, as parasitic cones, in great contrast to the cones in eastern YVC that lie at an altitude lower than the top of the west-dipping flows of the basal YVC, therefore unconformable on a surface here interpreted as a LSMW scar.

Fig

  Fig. S-10). This sub-aerial OVC is unconformably overlain by submarine lavas of the YVC from

  effect of constructional loading by assuming Airy isostasy and a maximum 900 m thickness of the YVC (current 600 m + 200 m missing summit + 100 m estimated below sea level): ε = 900 -(900*ρ l /ρ a ) = 900 -900*2800/3200 = 900 -788 = 112 m

Figure 14 .

 14 Figure 14. Sketch illustrating subsidence by loading induced by the growing YVC until 2.8 Ma, considering sea level stationary. Note the consistency between growth and subsidence using passage zones along an E-W section on the southern coast (the best constrained).

(

  Fig. S-11).

Figure 15 .

 15 Figure 15. Shaded relief bathymetry around Santa Maria (a), and X-Y topographic profile (b) across the East Azores Fracture Zone (EAFZ), the Santa Maria Island (SMa) and the Terceira Rift (TR). The cross-section shows that Santa Maria sits on the uplifted SSW shoulder of the TR.

Fig. S- 3 .

 3 Fig. S-3. Extract of the geological map of[START_REF] Zbyszewski | Carta geológica de Portugal, Ilha de Santa Maria[END_REF] in which dykes (orange lines) display a curved, concave to east, trend (highlighted in red), varying from NW-SE in the south, to NNE-SSW in the north.

Figure S- 4 .

 4 Figure S-4. Normal fault dipping to the west. This normal faulting attests to the change in the stress field from NNW-SSE σ 3 (dominant dykes of the OVC) to ENE-WSW σ 3 (dominant normal faulting cutting OVC and YVC).

Figure S- 6 .

 6 Figure S-6. Image of the polygenetic chaotic debris deposit outcropping on the road going down to Praia. Insets to show details described in the text. Green umbrella (ca. 0.5 m long) for scale in (a).

Figure S- 7 .

 7 Figure S-7. Image of chaotic debris deposit (a) containing large boulders originated in the sea: calcarenite with many marine fossils (b) and section of submarine pillow lava (c). The debris deposit lies unconformably on sub-aerial Old Volcanic Complex. Red arrow points to sledgehammer (ca. 1 m long handle) on pillow lava for scale.

Figure

  Figure S-8. (a) -full official geological map made by[START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF], surrounded by

Table 1 -

 1 Summary of the new ages obtained in this study, with uncertainty at 1σ confidence level. Final age in bold character, obtained by weighing by the amount of radiogenic Ar ( 40 Ar*).

  . The ca. 3.1 Ma age shows that the fossil deposits have different ages at different altitudes and from east to west (> 4.8 Ma to the east at sea level, and ca. 3.1 Ma to the west and at 100 m altitude), thus helping in reconstructing the growth of the

	Sample	Longitude	Latitude			40 Ar* (10	12	Uncertainty
	reference	(W)	(N)	K%	40 Ar* (%)	at/g)	Age (Ma)	1 σ (Ma)
	SMA16K 25.11723 37.00413	1.645	15.9	10.30	5.99	0.09
					15.0	10.35	6.02	0.09
						mean	6.00	0.09
	SMA16G 25.18262 36.99545	1.510	74.1	8.706	5.51	0.08
					76.5	8.689	5.50	0.08
						mean	5.51	0.08
	SMA16B 25.16627 36.98736	1.474	50.1	8.361	5.42	0.08
					51.0	8.383	5.44	0.08
						mean	5.43	0.08
	SMA16O 25.134978 36.946560	1.178	25.6	3.867	3.14	0.05
					24.2	3.818	3.10	0.05
						mean	3.12	0.05

  5 Ma, but these rates could be significantly different if the palaeoshoreline age is put at ca. 2.8 Ma, as indicated by the new age (ca. 3.1 and younger) of the pillows west of Facho (sub-aerial, ca. 2.8 Ma). Besides, 3.5 Ma is only an upper bound, because uplift could have started anytime between that age (or 2.8 Ma in the present study) and present day, in the absence of reliable markers younger than 2.8 Ma. On the other hand, subsidence starting at ca. 5.3 Ma

Acknowledgments

This is a contribution to Project MEGAHazards2 (PTDC/GEO-GEO/0946/2014) funded by FCT Portugal. A. Costa benefitted from a scholarship (SFRH/BD/68983/2010) funded by FCT Portugal, and A.L.R. Sibrant was supported by postdoctoral grant of the "Laboratoire d'Excellence" LabexMER. A special thanks is due to João Batista, who took us by boat all around the island to carry out critical field work. We thank three anonymous Reviewers and the Executive and Associate Editors Andrew Harris and Valerio Acocella, respectively, for their constructive and helpful reviews, and editorial work.

Figure S-11. (a) -3-D Google Earth image to show: (1) the position of the passage zone between sub-aerial (east) and submarine (west) lavas of the YVC; (2) the position of the Praia Graben, defined by an observed normal fault in the west (white solid line -see details in Fig. 7) and by an inferred fault (white dashed line) in the east (from different altitude of the base of the ISC); (3) the Praia Slump, currently creeping to the south. (b) -photo of house being torn apart by the internal creeping motion of the slump, and road showing small grabens in the foreground and background. Data from SIO, NOAA, U.S. Navy, NGA, GEBCO.

Supplementary material

Appendix 1 -K-Ar dating method

Careful microscopic examination of thin-sections was carried out to identify the textural and petrographic characteristics of the samples and ensure that they did not suffer any significant alteration. As all our new samples from Santa Maria are basic to slightly evolved lavas, the groundmass was selected. The samples were crushed and sieved to the chosen fraction (typically 125-250 µm, except sample SMa16O sieved at 125-250 µm). After ultrasonic cleaning in a dilute nitric acid solution (10%), followed by complete rinsing in distilled water and drying, heavy liquids were used to eliminate all the phenocrysts (e.g., plagioclase, pyroxene or olivine phenocrysts), which are K-poor and may carry potentially unsuitable inherited excess 40 Ar. The samples were then dated with the K-Ar Cassignol-Gillot technique, which has been shown wellsuited to date Quaternary volcanic products precisely, even low-K basalts and andesites (Gillot et al., 2006;[START_REF] Quidelleur | Causal link between Quaternary paleoclimatic changes and volcanic islands evolution[END_REF][START_REF] Germa | Volcanic evolution of the back-arc Pleistocene Payun Matru volcanic field (Argentina)[END_REF][START_REF] Germa | The K-Ar Cassignol-Gillot technique applied to western Martinique lavas: A record of Lesser Antilles arc activity from 2 Ma to Mount Pelée volcanism[END_REF][START_REF] Boulesteix | Eruptive response of oceanic islands to giant landslides: new insights from the geomorphologic evolution of the Teide-Pico Viejo volcanic complex (Tenerife, Canary)[END_REF]2013;[START_REF] Ricci | Volcanic evolution of central Basse-Terre Island revisited on the basis of new geochronology and geomorphology data[END_REF]2017). This technique has been extensively used in the last decade to constrain the age of volcanism and flank instabilities in the Azores (Hildenbrand et al., 2008;2012a[START_REF] Hildenbrand | Reconstructing the architectural evolution of volcanic islands from combined K/Ar, morphologic, tectonic, and magnetic data: the Faial Island example (Azores)[END_REF][START_REF] Costa | Largescale flank collapses in a steep volcanic ridge: Pico-Faial Ridge, Azores Triple Junction[END_REF]2015;[START_REF] Sibrant | Construction and destruction of a volcanic island developed inside an oceanic rift: Graciosa Island, Terceira Rift, Azores[END_REF]Sibrant et al., , 2015aSibrant et al., ,b, 2016)). K was measured by flame absorption spectrometry and compared with standards MDO-G [START_REF] Gillot | Two reference materials, trachytes MDO-G and ISH-G, for argon dating K/Ar and 40Ar/39Ar dating of Pleistocene and Holocene rocks[END_REF] and BCR-2 [START_REF] Wilson | The collection, preparation and testing of USGS reference material BCR-2[END_REF], both attacked and measured in the same conditions as the samples. Ar was measured by sector mass spectrometry [START_REF] Gillot | The Cassignol technique for potassium-argon dating, precision and accuracy: examples from late Pleistocene to recent volcanics from southern Italy[END_REF]. Details about the analytical procedure and calculation of uncertainties can be found elsewhere (e.g. [START_REF] Gillot | The K-Ar dating method: Principle, Analytical Techniques and Application to Holocene Volcanic Eruptions in the southern Italy[END_REF][START_REF] Germa | Volcanic evolution of the back-arc Pleistocene Payun Matru volcanic field (Argentina)[END_REF][START_REF] Boulesteix | Coeval giant landslides in the Canary Islands: implications for global, regional and local triggers of giant flank collapses on oceanic volcanoes[END_REF]Hildenbrand et al., 2014;2018). Decay constants and isotopic ratios of Steiger and Jäger (1977) were used. three extracts of the map to show inconsistencies between the stratigraphy established in 1987 and the stratigraphy constrained here and in Sibrant et al. (2015a) by K-Ar dating. Note that:

Supplementary figures

(1) extrapolation of the age of dated pillows at Pedreira do Campo (c) to similar settings (b and d) along the same yellow unit to the east (CT, Complexo do Touril of [START_REF] Serralheiro | Carta vulcanológica dos Açores: ilha de Santa Maria[END_REF] leads to major inconsistencies; ( 2


Dr Céline A S Burel

Dr Alexandre A Teolis

Dr Ahmed M Alsayed

Pr Christopher B Murray

Dr Bertrand Donnio

Rémi Dreyfus
email: rdreyfus@sas.upenn.edu

Dr R Alsayed

Dreyfus

Plasmonic Elastic Capsules as Colorimetric Reversible pH-Microsensors

Keywords: pH-sensors, plasmonics, microcapsules, nanoparticles, bacteria

Introduction

When corrosion occurs in a material, damages start from randomly located microscopic sites, where pH has already decreased. [1][2][3] When biofilms develop, bacteria change phenotypes and secrete substances that result in a local decrease in pH. [4][5][6] In the case of teeth, the more acidic environment causes local demineralization and is responsible for painful cavities. [7], [8] Although these three examples describe very different natural processes, they all carry one common feature: they describe heterogeneous processes, which start with local chemical changes at a microscopic level. Being able to track and detect these changes early open perspectives to prevent these processes and their subsequent adverse effects develop.

Monitoring these local alterations requires the fabrication of smart microsensors that are costeffective, easy to disperse, sense minute local pH changes rapidly, and able to translate these changes into a large, unambiguous signaling output. Optical signals have been used as output for microsensors because they are easy to read and visible to the naked eye without the need for a spectrophotometer. So far, the dominant optical pH sensors able to reach some of the microsensors goals relies on the use of dyes such as fluorophores [9][10][11][12][START_REF] Schreml | Proc. Natl. Acad. Sci[END_REF][START_REF] Wang | [END_REF][15] or chromophores [11], [16], [17] which light absorption or fluorescence emission depends on the pH. By matching the adsorption peak of dyes with the emission wavelengths of nanoparticles (NPs) such as quantum dots, the fluorescence of the pH indicators can be greatly enhanced. [18], [19] Organic indicator dye molecules can be dispersed, adsorbed, entrapped, covalently bonded, or encapsulated in a support matrix. [5], [11], [START_REF] Schreml | Proc. Natl. Acad. Sci[END_REF], [START_REF] Wang | [END_REF], [20], [21] Though this type of sensors has proved very efficient, it usually requires careful monitoring of the dye self-aggregation, large concentrations, complex chemistry for their synthesis or their chemical attachment onto the polymeric backbone or a perfect control of the leaking for dyes encapsulation. [5], [11], [20], [21] Moreover, most dyes are sensitive to photobleaching, which limits their long-term use. [11] Furthermore, depending on the type of matrix and indicator, each pH sensor has a different measurable pH range, which usually goes only over a couple of pH units. Metallic NPs such as gold (Au) and silver (Ag) NPs absorb light in the visible due to their plasmon resonance. [22], [23] Additionally, plasmonic NPs extinction is extremely sensitive depending on NP composition, size, shape, orientation, local dielectric environment, and interparticle distance. [22][23][24][25][26][27][28][29][30][31] Furthermore, contrasting with organic dyes, plasmonic nanoparticles do not photobleach, can be easily synthesized in one single step process [32] and produced in large quantities. Based on their very interesting optical properties, their resistance to light and their easy synthesis, plasmonic NPs constitute a very good alternative to organic dyes. Therefore, a new generation of optical pH sensors made of plasmonic NPs and actuating polymers has been developed in recent years. [33], [34] The working principles of these sensors are based on the chemical and mechanical properties of the polymers used such as N-isopropyl-acrylamide-terminated hyperbranched polyglycerols [34] and carboxyl group-terminated poly(2-vinylpyridine) [33] and, the optical properties of the NPs used such as Au or Ag NPs. The combined assembly of optically active plasmonic NPs and pH-responsive polymers leads to an alternative class of optical pH sensors able to transform chemical potentials into a strong and easily detectable optical response. Although these systems are efficient, they do not provide a dispersible micron size sensor and are not suitable for real-life applications as their incorporation in different matrices strongly impact their optical response.

Here, we present an original alternative for the fabrication of highly sensitive and dispersible pH plasmonic microsensors. Briefly, these microsensors are obtained from Pickering emulsions made of Au NPs closely packed at an oil/ water emulsion interface. [35], [36] Then, the Au NPs are locked within a polyelectrolyte polymer shell at the interface giving the final shape to the Au NPs-polymer microcapsules' sensors, thereafter referred to as plasmocapsules. We demonstrate how these microsensors show a significant optical response to pH changes due to their shift in the plasmonic extinction maximum. As schematically shown in Figure 1, as the polymeric matrix swells or shrinks upon pH change, the distance between neighbored Au NPs, and thus their interparticular coupling, changes, resulting in a different optical response. 33 In our emulsion templated method, billions of microsensors are fabricated simultaneously, each plasmocapsule acting as a single micron-size sensor. These colloidal microsensors are easy to fabricate, dispersible, robust, non-intrusive, reusable, reversibly sense minute local changes over a very large pH range (7 pH units), and able to translate these changes rapidly into a large, unambiguous signaling output. As proof of concept, the use of these colloidal microsensors for the detection of acid-producing bacteria is demonstrated. We anticipate that our approach enables the development of a large body of similar hybrid microcapsule sensors for tracking and sensing other stimuli in spatially homogeneous and heterogeneous systems.

Principle of the Synthesis

The precise synthetic protocol is described in details in the Methods Section and relies on the following methodology: Au NPs functionalized with poly(diallyldimethylammoniumnitrate-co-1-)vinylpyrrolidone (PVP DADMAN, full structural formula available in Figure S1 of the Supporting Information), are synthesized by reduction of Au salt with ascorbic acid [32], [37] . The Au NPs obtained are polydispersed, the mean diameter d is d = 30 nm ± 11 nm and the PVP-DADMAN capping layer is about 1.5 nm (TEM images, histogram of size distribution, and extinction spectrum of the Au NPs can be found in Figures S2 of the Supplementary information). In this size range, the polydispersity does not affect the position of the plasmonic peak but results in a small broadening of the peak. [22], [37] Then, a solution of toluene containing a mixture of monomers of methyl methacrylate (MMA) and butyl acrylate (BA) (50:50 w/w), is emulsified in a continuous aqueous phase of the previously synthesized Au NPs. By setting the pH of the solution to 1.2, electrostatic barriers are removed, and the NPs adsorb at the surface of the emulsion due to energy minimization, forming a so-called Pickering emulsion of Au NPs [37] (Figure 2a). The red color of the emulsion at the beginning of the process indicates Au NPs adsorption (Figure 2b). As synthesis proceeds, color change from red to blue acts as a good visual indicator of the NPs close-packed arrangement at the emulsion interface. Indeed, only a tight and dense packing results in the coupling of the plasmonic resonances of neighbored Au NPs, giving a strong blue color to the emulsion [22], [38] .

The polymerization of the encapsulated monomers occurs at the interface by raising the temperature above 60°C, giving rise to blue capsules (Figure 2c) made of an elastic shell in which gold nanoparticles are very closely embedded (Figures 2d). Mass spectroscopy is performed to quantify the amount of methyl methacrylate (MA), and butyl acrylate (BA) polymerized. Results show that essentially all of the two monomers are consumed (98.3% of MMA and 95.4% of BA, see Figure S4 in supplementary information).

Upon addition of sodium hydroxide, about 24 mol% of the acrylate and carboxylic acid moieties present on the polymer shell are transformed into carboxylate ions (see the Supplementary information section for more details on the characterization of the plasmocapsules' polymer shell). The pH sensitivity of the polymer [39], [40] in which the Au NPs are embedded, makes the collective assembly of nanoparticles sensitive to any pH-changing process. As pH is increased, the presence of numerous free carboxylic groups on the surface confers charges to the shell, resulting in osmotic swelling of the shells. The whole construct made of Au NPs embedded in a pH sensitive crust can then be used as a pH-microsensor, each capsule acting as an independent sensor.

Experimental Results and Discussion

The hypothesis that the synthesized microcapsules are de facto pH sensitive needed to be demonstrated first. To do so, we performed microfluidics experiments. We first introduced an acidic solution (pH ~ 1.2) containing blue microcapsules in a capillary, and then, a small droplet of a basic solution (pH ~ 13.8) was deposited gently at one end of the capillary. The droplet locally changed the pH, generating a front of increasing pH that propagated in the capillary. This front propagation could be easily followed by optical microscopy, the microcapsules changed colors as the pH solution was increased (Figure 3a, images 1-5, plasmocapsules in the capillary under different pH locally exhibit strong red and blue colors in the regions of high and low pH, respectively. The propagation of the pH front can be seen in movie 1 in the supplementary materials). At low pH, the polymeric backbone is uncharged, the nanoparticles are densely packed in the crust, and thus strongly coupled, and the microcapsules are blue. At higher pH, the polymer acquires charges (hydrolysis of acrylic acid -m-and acrylate ester -n and p-groups into carboxylate groups -m'-, see Figure S8 in supplementary information), the swelling crust increases the interparticle distance between the Au NPs, the plasmons are no more coupled and the microcapsules appear red. We also showed that this color change could even be monitored at the level of a single capsule (Figure 3b, images 1-10, and movie 2 in the supplementary materials).

To ensure that the change of color is solely due to the mechanical swelling of the polymeric crust embedding the Au NPs, we measured and plotted the color and size of the microcapsule as a function of time as the pH front propagation. It can be seen that as the propagation front moves with time over a microcapsule (Figure 4a), both the red intensity and the capsule diameter increase. When the red intensity was plotted as a function of capsule size (inset Figure 4a), a clear correlation between size and color could be found: a relative diameter change of about 10% is found sufficient to induce this color transition. To preclude the possibility that leaking or desorption of the Au NPs from the polymeric matrix might be responsible for the color change, we also check the reversibility of the color transition. We show that by changing the pH inside the microfluidics chamber back to acidic, the color of the microcapsules turns back to blue (Figure 4b). Moreover, the color of the capsules can be cycled, at least three times, from blue to red to blue, and so on, attesting of their flexibility and robustness (Figure 4c). This simple set of experiments evidences the pH-sensing potential of the microcapsules and how the coupling between the mechanical properties of the polymeric crust and the optical properties of the Au NPs allows us to obtain an easy read of local pH changes.

For the calibration of the colloidal microsensors, we performed a series of experiments in which the microcapsules were prepared and redispersed in different buffers of pH ranging from 2 to 12 (see the materials and methods section). They were first synthesized following the protocol described in the materials and methods section. Once they were formed, the capsules were redispersed in several solutions of different pH. The pH was measured using a microelectrode connected to a pH-meter. Direct observations of the microcapsules under the microscope confirmed that the color of the capsules was dependent on pH. Typical images of the plasmocapsules at five different pH values (i.e., pH = 2.4, 4.4, 7.4, 8.2, and 11.2) are showed in Figure 5. The color of the capsules is triggered by the change of pH in the surrounding suspension. To be more quantitative, for each pH sample, about 50 images were taken using a color camera, and the red, blue and green intensities were extracted from the camera channels and plotted as a function of pH (Figure 5a). A large red color transition in a narrow zone of pH, which extended from pH = 7.2 to pH = 8.2, was observed. The blue and green intensities remained quasi-constant at all pH values. The transition and the distinction between pH below 7.2 and pH above 8.2 are observable by the naked eye. In the pH range of 2 to 9, microcapsules color change due to pH increment as small as 0.5 pH unit were distinguished by eye under the microscope. As seen on the images of Figure 5 (a), a large range of colors is observed: blue, purple, pink, red, and strong red. The diffusion of hydronium ions over a 100 nm polymeric shell can be estimated by taking 10 -10 m 2 /s as typical value for the diffusion coefficient. [41], [42] The time it takes for a thin 100 nm pH-responsive polymer membrane to swell is then of the order of 0.1 ms: the membrane swelling of the plasmocapsules is almost instantaneous. This is consistent with the immediate change of color observed by eye under a microscope when a droplet of the base is added to an acidic droplet of plasmocapsules. This shows that the capsules can be indeed used to detect or monitor processes, which alter the pH of surrounding solutions in a large pH-range.

Finally, to demonstrate the applicability of these pH-sensitive plasmocapsules as efficient optical sensors, we used them to detect a natural process and replaced usual microelectrode measurements by simple in-situ straightforward optical measurements. As proof of concept, we used them to monitor pH change when a bacterial growth process occurred in a solution.

When certain bacteria strains grow in a solution with nutrients, they can metabolize surrounding sugars, and degrade them into acids. The increasing concentration of acid induces a decrease in the pH of the medium. Measuring a pH change is thus one way to diagnose the presence of a living and growing strain in a contaminated solution. A strain of Staphylococcus aureus was grown in a nutrient broth containing 1 wt% D-(+)-glucose. The bacteria were incubated for two days at 35°C. The broth alone (no bacteria) was also incubated and the pH was controlled during the entire duration of the experiment. S. aureus colony forming units (CFU)/mL was determined at different times by serial dilution, and plating. Simultaneously, the pH was measured both in the broth alone (without bacteria) and in the broth containing bacteria (see Figure S9). Over time, the S. aureus grew from 10 3 CFU/mL to 10 8 CFU/mL, the exponential growth phase, and the stationary phase respectively occurring in about 10 and 28 h after incubation. Concomitantly, due to the fermentation of glucose by S. aureus, the pH was found to decrease from 8 to 4.5. The larger pH drops occurred during the exponential phase when bacterial growth was the most important, the pH stabilized at 4.5 once the bacteria reached the stationary phase. As a reference, the pH of the broth alone containing no bacteria remained constant with time, showing that the observed pH decrease is only due to the fermentation of glucose by S. aureus. The plasmocapsules were then dispersed in the culture media at a different time of the incubation, and the different colors were analyzed. As the bacteria grew in solution and the pH subsequently decreased, the microcapsules changed color from red at pH 8 to purple at pH 4.5. The Red, Blue and Green intensities of the microcapsules were plotted versus the concentration of S. aureus (see Figure 5b). We observed that as the population of the bacteria increased, the red intensity decreased, and the blue and green intensities remained constant. A color shift in the red intensity could be measured from 10 6 bacteria in solution, setting the detection limit of this method. Therefore, the plasmocapsules can be used to detect the presence of acid-producing bacteria by sensing pH variations.

Conclusion

We have demonstrated the synthesis of pH-sensitive plasmonic microcapsules. They represent a new type of dispersible optical sensors made of a combination of a pH-sensitive polymeric shell, which swells or shrinks under pH changes, and embedded plasmonic nanoparticles, the optical response of which depends on the interparticle distance. Using regular colorimetric analysis, we have shown a direct correlation between the capsules color d the size of the plasmocapsules. We have calibrated the relation between the outside pH and color of the plasmocapsules. We have demonstrated that the optical response of the plasmocapsules is contrasted enough to detect different pHs, even though we use a path of synthesis that yields polydisperse nanoparticles. The plasmocapsules sensor allows for detection of pH in a relatively large range of 2 to 9. The plasmocapsules microsensors response time is of the order of 0.1ms, which is much smaller than competing organic dye technologies whose typical response time range from several tenths of seconds to a minute. [11] In the case where the microsensor would be a 10 microns particle of polyacrylic acid with Au NPs on its surface, the response time would be increased to about 1s. This validates the use of a thin shell of pH-responsive polymer instead of a large particle. We did not observe leaching of the NPs out of the polymeric shell, which was evidenced by the reversibility of the capsules over at least three cycles. Finally, we have shown that the plasmocapsules can be used to probe and follow the growth of bacteria, which change their environmental pH as they replicate. Because of all these characteristics, the pH-responsive plasmocapsules could be of interest to detect and monitor bacterial contamination in health care and the food industry.

The microsensors showed here were specially developed for pH measurement, however we believe that the effect and principle demonstrated here are general enough to be extended to other types of detectors. Swapping a pH-sensitive polymeric shell with, for instance, a fluoropolymer one, which, depending of the fluorination degree, would be able to selectively dissolve a wide range of gases (H 2 , CO, CO 2 , NO 2 , CH 4 ) and vapors' solvents, would allow the fabrication of very specific gas sensors. [43] Alternatively, a polymeric shell sensitive to e.g. polynitroaromatic compounds would be of interest in the area of explosive sensing. [44] Using a temperature sensitive polymeric shell, which would swell/shrink under small temperature variations, could also be used as a temperature sensor in the cold chain to ensure fresh and frozen aliment preservation, chemical hazards limitation, or drugs stabilities. Therefore, the nature of the shell provides the type of detection to be done while the optical reading would still lie on the plasmonic properties of the embedded nanoparticles. As such we anticipate that these microcapsules will likely represent a new class of cheap non-intrusive microsensors able to transform chemical potentials or external forces into a large optical absorption shift, and easily detectable by the naked eye.

Experimental Section

Materials: Gold (III) chloride trihydrate (99.9%, Sigma-Aldrich), L-ascorbic acid (99%, Sigma-Aldrich), hydrochloric acid (ACS reagent, 37%, Sigma-Aldrich), toluene (Chromasolv water emulsion. The formation of the polymer shell by polymerization of the monomer at the interface of the emulsion is carried out at 62°C for 2hrs.

Plasmocapsules in buffer solutions

  The plasmocapsules' spherical shells are observed by SEM. A high magnification image of the shell shows the dense packing of the nanoparticles (Figure2e). The thickness of the shell is measured from the SEM images to be approximately 100 nm +/-20 nm (see FigureS3in supplementary information).

:

  buffer solutions were prepared by mixing the following four solutions in different ratios: Citric acid (0.75 M), Na 2 HPO 4 (1.5 M), Na 2 CO 3 (0.75 M) and NaHCO 3 (0.75 M). Buffer solutions at a specific pH were mixed with the dispersion of microcapsules in a 50/50 v/v ratio, and the pH was controlled afterward. Each time the final pH value was close (within 0.5 unit of pH) to the pH of the buffer solution. For high (> 11) and low (< 3) pH, NaOH and HCl were respectively used to adjust the pH of the plasmocapsules dispersion. Electron microscopy: Scanning electron microscopy (SEM) was performed on JEOL 7500 HRSEM. The samples were prepared by air drying under ambient conditions by laying a drop of microcapsules dispersion in water on an electron microscopy science carbon-coated copper grid. Transmission electron microscopy (TEM) was carried out on a JEOL JEM-1400. The accelerating voltage was set at 120 kV. Optical characterizations: Optical microscopy images were acquired on an Olympus microscope in bright field and transmission mode. pH measurements: The pH of all solutions was measured with pH-meter (model 8015, VWR International) with a micro pH combination electrode (Z113425-1EA, Millipore Sigma). Bacteria cell culture and plate counting: A fresh culture of S. aureus was prepared by inoculating 9 mL Nutrient Broth with a single colony from a Tryptic Soy Agar plate streaked for less than two weeks. Then, culture was incubated overnight at 35 °C under shaking conditions (250 rpm) in an incubating mini shaker (12620-942, VWR International). Bacteria were harvested by centrifugation for 10 min at 2000×g, washed twice using the medium studied, before resuspending in the medium studied. The concentration of S. aureus was determined by serial dilution with subsequent plating on Tryptic Soy Agar plates and measurement of colony forming units (CFUs) after 24 h at 35 °C. pH-bacteria growth study in planktonic conditions: Two sterile culture media were prepared: a Nutrient Broth with 1 wt% D-(+)-glucose, 3 wt% sodium chloride; and a Nutrient Broth with just 3 wt% sodium chloride. Both solutions were adjusted to pH 8 with NaOH and filtered using 0.2 µm PES sterile syringe filters (VWR International). Bacteria were washed twice and resuspended in both solutions. Initial S. aureus concentration was adjusted by dilution at around 10 3 CFU/mL. Then, bacteria were cultivated in closed vials on a MaxQ4000 shaker (model 4342, Thermo Scientific) for 48 h at 35 °C with shaking (100 rpm). S. aureus concentration was determined by plating and pH was measured. The microcapsules were concentrated and washed several times with aliquot of the bacteria solution at a different time of incubation until the pH of the final dispersion matches the pH of the bacteria solutions. Then the microcapsules were imaged by optical microscopy. Supporting Information Structural formula of PVP-DADMAN; SEM image of a plasmocapsule polymer shell; HPLC chromatograms obtained for plasmocapsules at pH 1.2 and at pH 13.8; characterizations of the plasmocapsules polymer shell with FTIR spectra, GC and GC-MS chromatograms; schematic representation of the color change of the plasmocapsules upon swelling; variation of the pH and color of the plasmocapsules as a function of the concentration of S. aureus; additional materials and methods. Supporting Information is available from the Wiley Online Library or from the author.

Figure 1 :

 1 Figure 1: Schematic representation of the plasmonic response of the designed plasmocapsule

Figure 2 :

 2 Figure 2: (a) Schematic representation of the formation of plasmocapsules. (b) Optical

Figure 3 :

 3 Figure 3: (a) Plasmocapsules swelling. Initially in acidic solution (pH=1). The pH is slowly increased by flowing a solution of sodium hydroxide (pH=13.8, 10 -1 M) from left to right (1-

Figure 4 :

 4 Figure 4: (a) Plasmocapsules swelling and change of color over time (microcapsule diameter

Figure 5 :

 5 Figure 5: (a) Change of color of the plasmocapsules as a function of (a) the pH and (b) the

Acknowledgments

The authors thank the ANRT, GIE AIFOR, CNRS, and Solvay for financial support. The authors acknowledge the support of the French National Agency of Research (ANR) to the project REACT through the grant ANR15-PIRE-0001-06. SEM imaging was performed in facilities supported by the NSF MRSEC program under award No DMR-1720530. The authors also thank D. Bendejacq, R. Giordanengo, L. Gage, D. Radtke, C. Anderson, J. Hutchinson, and A.J. Yodh for fruitful discussions.

Received: ((will be filled in by the editorial staff))

Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))

Plus, 99.9%, Sigma-Aldrich), poly(diallyldimethyl-ammoniumnitrate-co-1-vinylpyrrolidone (PVP-DADMAN, Solvay®), Sodium Hydroxide (50/50 w/w Baker), methyl methacrylate (99%, Sigma-Aldrich), butyl acrylate (99%, Sigma-Aldrich), 2,2′-Azobis(2-methylpropionitrile) (AIBN) (98%, Sigma-Aldrich), citric acid (ACS reagent, EMD Chemicals), sodium phosphate dibasic (ACS reagent, VWR), sodium carbonate dibasic (99.5%, ACS reagent, Sigma-Aldrich), sodium bicarbonate (99.7%, ACS reagent, Sigma-Aldrich), D-(+)glucose (99.5%, Sigma Aldrich), sodium chloride BioXtra (99.5%, Sigma Aldrich), sodium hydroxide pellets (98%, Alfa Aesar), Nutrient Broth powder (NEOGEN Corporation), 9 mL Nutrient Broth tubes (General Laboratory Products), Tryptic Soy Agar plates (USP G60, Hardy Diagnostics). The bacterium used in the studies was Staphylococcus aureus subsp. aureus (ATCC 6538). Ultrapure water (type 1, 18.2 MΩ.cm) was used for all culture media preparation and deionized water was used otherwise.

Gold nanoparticles synthesis:

The Au NPs used in this procedure were synthesized by reduction of HAuCl 4 with ascorbic acid in presence of PVP-DADMAN. A solution of 5.7.10 -6 M PVP-DADMAN and 2.510 -3 M HAuCl 4 was brought to boil. Then, 12.5 mL of ascorbic acid (0.1 M) was added. The solution protected from light by aluminum foil was stirred for 1 h at 97°C. After synthesis, the Au NPs were left to rest for one day to remove the biggest nanoparticles. The rest of the dispersion was centrifuged and concentrated into a few milliliters' solution. MilliQ water was used throughout the entire synthesis process.

Plasmocapsules synthesis: An aqueous phase containing the Au NPs ([Au 0 ] = 0.02 M) and HCl (20 µL) is mixed with a toluene phase (0.5 mL) of the dissolved acrylate monomers (MMA 2.2 M and BA 1.7 M) and AIBN (0.12 M). The two phases are strongly agitated by sonication with a Brandson 3210 ultrasonic bath at 30°C for 15 minutes, forming an oil-in-