Jean-Marie Prival

Andrew J L Harris

Elena Zanella

Claudio Robustelli Test

Lucia Gurioli

Oryaëlle Chevrel

Jonas Biren

Emplacement dynamics of a crystal-rich, highly viscous trachytic flow of the Sancy stratovolcano, France

Emplacement dynamics of highly viscous, silicic lava flows remain poorly constrained due to a lack of consideration of crystal-rich cases. Emplacement models mostly apply to glassy or microlitic, vesiculated rhyolitic flows. However, crystalline, vesicle-free silicic lava can flow differently. We studied the Grande Cascade unit, which is a vesiclefree, phenocryst-rich, trachytic flow in the Monts Dore massif, France. Field work was carried out to define internal structures, and oriented samples were collected for chemical, petrological, and anisotropy of magnetic susceptibility analyses, allowing us to estimate emplacement temperature and viscosity. These data allow us to define a new silicic lava flow subtype that is low in temperature (800-900 °C), high in silica content (up to 66.8 wt%), high in viscosity (10 9 -10 11 Pa s), rich in phenocrysts (∼35%), and lacks vesicles. Brittle deformation of the lava occurs upon extrusion, generating a cataclasite basal layer and thin (3-m-thick) shear zone that accommodates all of the stress, allowing most of the flow's volume to slide over its base as a 40-m-thick plug in which there is no deformation. Blocks are rare, of a single size (10 ± 1 cm), and result from localized breakup of the basal shear zone. Emplacement dynamics are different from those of glassy, pumiceous lava flows. They are closer to glacier dynamics, where most of the volume slides over a thin basal shear zone and till is generated there by abrasion and milling of the underlying layer. For the Grande Cascade lava flow, abrasion means that the flow lacks its classical blocky crust and instead the flow base is marked by a layer rich in fine-grained material. The structures and emplacement dynamics of this crystalrich flow are consistent with ideal, gravitydriven shear flow. We thus argue for a global reassessment of silicic-rich lava emplacement based on crystal content and using a multidisciplinary approach focused on well-exposed examples in the rock record.

INTRODUCTION

Non-explosive silicic eruptions represent a ubiquitous style of volcanism that occurs in every geodynamic setting. Effusion of silicic lava, i.e., SiO 2 > 63 wt% [START_REF] Le Maitre | Igneous rocks: A classification and glossary of terms[END_REF], is characteristic of subduction zones and continental rift zones [START_REF] Wilson | Igneous Petrogenesis[END_REF] but is also observed on oceanic islands (e.g., [START_REF] Self | Petrology, volume and age relations of alkaline and saturated peralkaline volcanics from Terceira, Azores[END_REF][START_REF] Nielson | Geology of ascension Island[END_REF][START_REF] Wiesmaier | Timing, distribution and petrological evolution of the Teide-Pico Viejo volcanic complex[END_REF] and at some mid-oceanic ridges [START_REF] Wanless | Dacite petrogenesis on mid-ocean ridges: Evidence for oceanic crustal melting and assimilation[END_REF]. Extrusive activity results in the emplacement of voluminous domes, as well as flow lobes and silicic lava flows. Here we distinguish between flow and lobe using aspect ratio (i.e., unit length divided by width), where flows have aspect ratios of >3-5 [START_REF] Walker | Lengths of lava flows[END_REF][START_REF] Wadge | The lobes of lava flows on Earth and Olympus Mons[END_REF]. In addition, silicic lava flows typically have lengths greater than 1.5 km (e.g., De Silva et al., 1994;[START_REF] Harris | The thermal stealth flows of Santiaguito dome, Guatemala: Implications for the cooling and emplacement of dacitic block-lava flows[END_REF][START_REF] Lara | Rhyodacitic fissure eruption in Southern Andes (Cordón Caulle; 40.5°S) after the 1960 (Mw:9.5) Chilean earthquake: A structural interpretation[END_REF][START_REF] Tuffen | Exceptional mobility of an advancing rhyolitic obsidian flow at Cordón Caulle volcano in Chile: Nature Communications[END_REF]Auer et al., 2018;[START_REF] Bullock | Emplacement of the Rocche Rosse rhyolite lava flow (Lipari, Aeolian Islands)[END_REF]. Our focus here is on the emplacement dynamics and associated structures of silicic lava flows.

Previous work has found an array of structural and textural characteristics in silicic lava flows (e.g., [START_REF] Fink | Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain[END_REF][START_REF] Manley | Internal textures of rhyolite flows as revealed by research drilling[END_REF][START_REF] Anderson | Mount St. Helens and Santiaguito lava domes: The effect of short-term eruption rate on surface texture and degassing processes[END_REF][START_REF] Dadd | Structures within large volume rhyolite lava flows of the Devonian Comerong volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome[END_REF][START_REF] Smith | Folds produced by gravity spreading of a banded rhyolite lava flow[END_REF][START_REF] Smith | Ductile-brittle transition structures in the basal shear zone of a rhyolite lava flow, eastern Australia[END_REF][START_REF] Castro | Constraints on rheology of obsidian lavas based on mesoscopic folds[END_REF][START_REF] Castro | Dynamics of obsidian flows inferred from microstructures: Insights from microlite preferred orientations[END_REF][START_REF] Magnall | Emplacing a cooling-limited rhyolite lava flow: Similarities with basaltic lava flows[END_REF]. [START_REF] Manley | Internal textures of rhyolite flows as revealed by research drilling[END_REF] defined five textural facies that have since been found at most rhyolitic lava flows: coarsely vesicular pumice, finely vesicular pumice, obsidian, crystalline rhyolite, and breccia. Common structures include "crease structures" [START_REF] Anderson | Mount St. Helens and Santiaguito lava domes: The effect of short-term eruption rate on surface texture and degassing processes[END_REF], folds [START_REF] Smith | Folds produced by gravity spreading of a banded rhyolite lava flow[END_REF], and flow banding [START_REF] Tuffen | Repeated fracture and healing of silicic magma generate flow banding and earthquakes?[END_REF][START_REF] Gonnermann | Flow banding in obsidian: A record of evolving textural heterogeneity during magma deformation[END_REF]. However, these studies have focused on glassy lava flows mostly composed of obsidian and microlitic lava.

Studies of phenocryst-rich silicic lava flows include work on the crystal-rich andesitic lava flows of the Michoacán-Guanajuato monogenetic field (Chevrel et al., 2016a(Chevrel et al., , 2016b;;[START_REF] Reyes-Guzmán | Late Holocene Malpaís de Zacapu (Michoacán, Mexico) andesitic lava flows: Rheology and eruption properties based on LiDAR image[END_REF][START_REF] Ramírez-Uribe | Rancho Seco monogenetic volcano (Michoacán, Mexico): Petrogenesis and lava flow emplacement based on LiDAR images[END_REF] and Popocatepetl [START_REF] Ramírez-Uribe | The late Holocene Nealtican lavaflow field from Popocatépetl volcano: Emplacement dynamics and implications for future hazard scenarios and archaeology[END_REF]. Apparent viscosities for such flows are up to 10 8 -10 12 Pa s, which implies that these voluminous (up to 10 9 m 3 ) lava flow fields are emplaced very slowly over periods of months to years. Studies of crystal-rich, dacitic lava flows active at Santiaguito, Guatemala, have shown them to be slow moving (a few meters per day) but longlived (active for several years) and capable of extending several kilometers (e.g., [START_REF] Rose | Volcanic activity at Santiaguito volcano[END_REF][START_REF] Anderson | Mount St. Helens and Santiaguito lava domes: The effect of short-term eruption rate on surface texture and degassing processes[END_REF][START_REF] Harris | The thermal stealth flows of Santiaguito dome, Guatemala: Implications for the cooling and emplacement of dacitic block-lava flows[END_REF][START_REF] Escobar-Wolf | Geologic map of Santiaguito Volcano, Guatemala[END_REF]. Although active, Santiaguito's lava flows are not exposed, and comparison with a crystal-rich, trachytic flow exposed at the Puy de Cliergue in the Monts Dore massif of Central France by [START_REF] Latutrie | Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)[END_REF] revealed quite different structures and associated flow dynamics. For example, the lava flow emplaced in January 2000 at Santiaguito had a "caterpillartrack-type advance" similar to ʻaʻā flows [START_REF] Harris | The thermal stealth flows of Santiaguito dome, Guatemala: Implications for the cooling and emplacement of dacitic block-lava flows[END_REF]. But the Puy de Cliergue trachytic flow "slid" over a basal breccia and shear zone in a "glacier-like" motion [START_REF] Latutrie | Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)[END_REF]. Flow surfaces at Santiaguito, as well as at the 1980-1986 Mount St. Helens dome and Unzen [START_REF] Anderson | Deposits, petrology and mechanism of the 2010-2013 eruption of Kizimen volcano in Kamchatka, Russia[END_REF], were characterized by a blocky surface [START_REF] Rhodes | Textural insights into the evolving lava dome cycles at Santiaguito lava dome[END_REF]. But the Puy de Cliergue unit was characterized by † Corresponding author: andrew .harris@uca .fr. fine-grained matrix containing matrix-supported clasts, where rounding of clasts and matrix support was proposed to be consistent with clast abrasion, grinding, and crushing [START_REF] Latutrie | Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)[END_REF]. [START_REF] Siebe | Monogenetic volcanism of the Michoacán-Guanajuato volcanic field: Maar craters of the Zacapu basin and domes, shields, and scoria cones of the Tarascan highland (Paracho-Paricutin region)[END_REF] also observed that deposits exposed in lateral levees in thick, crystal-rich andesitic flows in Mexico resembled glacial moraines.

Hence, we appear to have two types of silicic lava flows (crystal-poor and crystal-rich), one well described, analyzed, and understood, and the other less so. To better define this second flow type, we studied a second crystal-rich, trachytic flow in the Monts Dore massif that is well exposed over a 300 m cross-flow distance. We defined internal structures, collected oriented samples for chemical, petrologic, textural, and anisotropy of magnetic susceptibility (AMS) analyses, and defined the grain-size distribution and particle morphology of the basal breccia. These data allow full definition of this new crystal-rich, silicic lava flow type and allow us to make inferences about the associated emplacement dynamics. We show that most stress is accommodated by brecciation, abrasion, and shearing of a thin basal layer, and most of the flow's volume slides over this basal layer as a plug with no deformation.

A Note on Terminology

We distinguish between crystal-rich and crystal-poor silicic lava flows. Crystal-rich is used for flows with phenocryst contents of >20-30% (cf. [START_REF] Latutrie | Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)[END_REF] and up to 50-60% (Chevrel et al., 2016b;[START_REF] Ramírez-Uribe | Rancho Seco monogenetic volcano (Michoacán, Mexico): Petrogenesis and lava flow emplacement based on LiDAR images[END_REF], 2022). Crystal-poor is used for obsidianbearing flows at classic case-types of locations such as Little Glass Mountain [START_REF] Fink | Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain[END_REF] and Obsidian Dome [START_REF] Castro | Dynamics of obsidian flows inferred from microstructures: Insights from microlite preferred orientations[END_REF] in the Western USA, or Rocche Rosse and Pietre Cotte [START_REF] Bullock | Emplacement of the Rocche Rosse rhyolite lava flow (Lipari, Aeolian Islands)[END_REF] in the Aeolian Islands of Italy. Our results, interpretations, and conclusions thus apply to crystal-rich, silicic lava flows and not to crystal-rich domes and lobes, or to crystal-poor cases.

Geological Setting

Located in the NW of the French Massif Central (Fig. 1), the Monts Dore volcanic complex comprises two stratovolcanoes: Guéry, active at 3.09-1.46 Ma [START_REF] Nomade | Tephrochronology of the Mont-Dore volcanic Massif (Massif Central, France): new 40 Ar/ 39 Ar constraints on the late Pliocene and early Pleistocene activity[END_REF], and Sancy, active at 1.10-0.23 Ma [START_REF] Nomade | Pyroclastic chronology of the Sancy stratovolcano (Mont-Dore, French Massif Central): New high-precision 40 Ar/ 39 Ar constraints[END_REF]. The Grande Cascade unit was emplaced at 0.38 ± 0.02 Ma [START_REF] Cantagrel | Chronologie des éruptions dans le massif volcanique des Monts Dore (Méthode potassium-argon)[END_REF]. The unit belongs to the "massif adventif" of Sancy, a group of ∼10 N-S-aligned trachytic domes and flows that are petrographically similar to but younger than the rest of the Sancy massif and represent its last activity [START_REF] Glangeaud | Le groupe volcanique adventif ou de superposition du Massif des Monts Dore[END_REF].

The Grande Cascade ("the Tall Waterfall") is a ∼35-m-high waterfall that drops from the Durbise plateau into Haute-Dordogne valley on its east side (Fig. 1). The Haute-Dordogne is a glacial valley carved during the last three glacial episodes [START_REF] Etlicher | La déglaciation würmienne dans le Massif Central français, le point des travaux récents / The Würmian déglaciation in the French Massif-Central[END_REF]. The waterfall is easily walked to from the Mont-Dore village (https://www .sancy .com /circuits /circuits-randonnee /randonnee-grandecascade/). It has long been an iconic tourist site, which is evidenced by numerous ancient postcards of the site digitized by the Archives Départementales du Puy de Dôme, e.g., Figure 2A (http://www .archivesdepartementales .puydedome .fr/). The Grande Cascade is listed in tourist and walking guides (e.g., Fédération française de la randonnée pédestre, 2018; Delabroy et al., 2019;[START_REF] Michelin | Guide Vert Auvergne[END_REF] and featured in geology field guides (e.g., [START_REF] Graveline | Known & Little-Known Volcanoes of the Massif Central[END_REF][START_REF] Chamina | Le volcanisme en Auvergne: Clermont-Ferrand[END_REF][START_REF] Richet | Guide des volcans de France: Paris, Belin[END_REF]. It is ranked number one among 400 waterfalls in the region [START_REF] Kalmar | Quatre cents cascades dans le Puy-de-Dôme: Le Mont-Dore[END_REF] and, since 2016, it has been listed in the National Inventory of the Geological Heritage (Inventaire National du Patrimoine Géologique). It is also within one of the 58 regional natural parks of France, the parc naturel régional des Volcans d'Auvergne (Fédération des Parcs, 2021).

The cliff behind the waterfall laterally exposes a massive, ∼40-m-thick trachyte lava flow ∼300 m wide, which offers continuous exposure of all components of a crystal-rich, silicic lava flow system (Fig. 2). The unit, mapped by [START_REF] Scrope | Memoir on the Geology of Central France, Including the Volcanic Formations of Auvergne, the Velay, and the Vivarais[END_REF] and [START_REF] Lecoq | Vues et coupes des principales formations géologiques du Département du Puy-de-Dome[END_REF], overlies a succession of older basaltic lava flows and pyroclastic deposits (Fig. 3).

METHODS

Different facies were defined by four criteria: geometry, lithology, structures, and movement patterns [START_REF] Cas | Volcanic Successions, Modern and Ancient[END_REF].

Flow Geometry and Structures

To infer the length, width, slope, and area of the Grande Cascade flow, we used the 1:50,000 geological map [START_REF] Brousse | Carte géologique de la France, feuille Bourg-Lastic (716)[END_REF][START_REF] Brousse | Notice explicative, carte géologique de la France, feuille Bourg-Lastic (716)[END_REF], 1:25,000 topographic map (Institut national de l'information géographique et forestière, 2016), 10-m resolution Digital Elevation Model (CRAIG-TopoGEODIS, 2009), and orthophotographs with a 0.25-m resolution (CRAIG-SINTEGRA, 2013). Field measurements allowed us to define variations in the thickness and geometry of the unit and its component layers, and we mapped and measured all flow structures, such as shear and foliation planes and striations.

Textural Analysis

Anisotropy of Magnetic Susceptibility

AMS determines the petrofabric of igneous rocks and reflects the spatial arrangement and relative abundance of para-and ferromagnetic minerals, which depend on emplacement mechanisms and deformation history [START_REF] Khan | The anisotropy of magnetic susceptibility of some igneous and metamorphic rocks[END_REF][START_REF] Hrouda | Magnetic anisotropy of rocks and its application in geology and geophysics[END_REF][START_REF] Cañón-Tapia | Anisotropy of magnetic susceptibility of lava flows and dikes: A historical account[END_REF]. Magnetic susceptibility (k) defines the linear relationship between an external magnetic field (H) applied to a material and the induced magnetization of the material (M):

M kH = (1)
M and H being in A m -1 , and k being dimensionless. If the material is magnetically anisotropic, M is not parallel to H, and the relationship becomes:

M k H i j i i j j = = ( ) , , , 1 2 3 (2)
in which k ij is the second-order susceptibility tensor. The three values k 11 ≥ k 22 ≥ k 33 , usually denoted as

k 1 ≥ k 2 ≥ k 3 (or k max ≥ k int ≥ k min )
, are the principal susceptibilities [START_REF] Tarling | The magnetic anisotropy of rocks[END_REF]. AMS is geometrically represented by an ellipsoid with axes parallel to the principal directions and semi-axis lengths equal to the principal susceptibilities. The k 3 direction defines the pole of the magnetic foliation plane, and the k 1 and k 2 directions lie on it, with k 1 giving the direction of magnetic lineation. Magnitudes of k 1 , k 2 , and k 3 are used to derive several descriptive parameters (as listed in table 1.1 of [START_REF] Tarling | The magnetic anisotropy of rocks[END_REF]. Of these, we use k 1 , k 2 , and k 3 to obtain mean magnetic susceptibility (k m ):

k k k k m = + + 1 2 3 3 (3)
corrected anisotropy degree (P j ),

P J m m m = - ( ) + - ( ) + - ( )         exp 2 1 2 2 2 3 2 η η η η η η (4)
shape parameter T (where i = 1, 2, or 3),

T = -- ( ) - ( ) 2 2 1 3 1 3 η η η η η / ( 5
)
where η i = ln k i , and η m = (η 1 + η 2 + η 3 )/3. Shape parameter T assesses the ellipticity of the AMS ellipsoid (T > 0 for oblate susceptibility ellipsoids; T < 0 for prolate). We collected oriented cores and oriented hand samples at seven sites (GC1 to GC7) (see Geological Society of America Bulletin, v. 130, no. XX/XX 3 Table S1 in the Supplemental Material1 ). Some sites were divided into sub-sites to check for spatial coherency over 1-2 m. All measurements were performed at the Alpine Laboratory of Paleomagnetism in Peveragno, Italy. Bulk magnetic susceptibility and anisotropy were measured using an AGICO Kappabridge KLY-3 for 280 specimens. AMS data were then cleaned to remove outliers [START_REF] Borradaile | Statistics of Earth Science Data: Berlin[END_REF].

Breccia Particle-Size Distribution, Componentry, and Particle Morphology

We analyzed particle-size distribution (PSD) of the basal breccia along with the componentry and morphology of breccia particles. A ∼4 kg sample of breccia was dried in an oven for 24 h at 70 °C, then sieved at a ϕ/2 interval. We calculated the corresponding grain-size parameters with Folk and Ward (1957) equations and the fractal dimension following [START_REF] Turcotte | Fractals and fragmentation[END_REF]. Assuming a spherical shape, PSD was converted from a weight to a number distribution using the particle density. The number (N) of particles larger than size L was then plotted in a log(N)-log(L) diagram on which data following a power law will plot as a straight-line segment, the slope of which defines fractal dimension D.

Componentry was determined by manually sorting particles under a stereo microscope. For each bin between 0ϕ (1000-1400 µm) and 4.5ϕ (44-63 µm), we analyzed a random selection of particles (a few hundred for the largest up to ∼20,000 for the finest) with a Malvern Panalytical Morphologi G3. The G3 software outputs particle area (A p ), the area of its convex hull (A ch ), and corresponding perimeters P p and P ch , which were used to calculate the morphological parameters of Table 1.

Chemical and Physical Properties of the Lava

Petrography, Chemistry, and Geothermometry

We performed petrographic and chemical analyses on the most representative samples of each facies. Bulk-rock major-element compo-sition was determined via inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Petrography was carried out on hand samples and two thin sections from the lava flow interior (the plug). Minimum phenocryst content was taken as the ratio between total crystal area and area of the scanned cut face of the sample and thin section (see Supplemental Material). Crystal aspect ratio was then determined as the ratio of the large and short axis of an equivalent ellipsoid, a value that approximates phenocryst volume fraction [START_REF] Heilbronner | Image Analysis in Earth Sciences[END_REF]. [START_REF] Gourgaud | Mélanges de magmas dans les séries alcalines et calco-alcalines: Leur rôle dans le genèse des laves intermédiaires et leur influence sur les mécanismes éruptifs: Clermont II[END_REF] determined emplacement temperature of the Grande Cascade flow as

A B

816-880 °C by applying magnetite-ilmenite [START_REF] Powell | Geothermometry and oxygen barometry using coexisting iron-titanium oxides: A reappraisal[END_REF] and plagioclase-liquid [START_REF] Kudo | An igneous plagioclase thermometer: Contributions to Mineralogy and Petrology[END_REF][START_REF] Drake | Partition of Sr, Ba, Ca, Y, Eu 2+ , Eu 3+ , and other REE between plagioclase feldspar and magmatic liquid: An experimental study[END_REF] geothermometers. We reused [START_REF] Gourgaud | Mélanges de magmas dans les séries alcalines et calco-alcalines: Leur rôle dans le genèse des laves intermédiaires et leur influence sur les mécanismes éruptifs: Clermont II[END_REF] crystal and groundmass chemical-composition data with the plagioclase-liquid thermometer, the alkali feldspar-liquid thermometer, and the two-feldspar thermometer in [START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF], along with the magnetite-ilmenite thermometer of [START_REF] Ghiorso | Thermodynamics of rhombohedral oxide solid solutions and a revision of the FE-TI two-oxide geothermometer and oxygen-barometer[END_REF].

Viscosity and Density

Lava viscosity was estimated considering a two-phase mixture of melt (i.e., the matrix) and crystals (for methodology, see [START_REF] Chevrel | Lava flow rheology: A comparison of morphological and petrological methods[END_REF]Chevrel et al., , 2016b;;[START_REF] Latutrie | Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)[END_REF][START_REF] Reyes-Guzmán | Late Holocene Malpaís de Zacapu (Michoacán, Mexico) andesitic lava flows: Rheology and eruption properties based on LiDAR image[END_REF]. This method involves first estimating the liquid phase viscosity (η liq ) using the major element composition and emplacement temperature in the model of [START_REF] Giordano | Viscosity of magmatic liquids: A model[END_REF]. Second, to account for the effect of crystals, we calculate the relative apparent viscosity (η r ) via the Krieger and Dougherty (1959) equation using the crystal volume fraction (see Supplemental Material for further details). The lava mixture (liquid + crystals) viscosity, η, is then calculated as η liq × η r . Lava dense-rock equivalent (DRE) density was obtained from the ratio between sample mass and volume of the powdered sample obtained using a Micromeritics AccuPyc II 1340 gas displacement pycnometer (for method, see [START_REF] Thivet | Variability of ash deposits at Piton de la Fournaise (La Reunion Island): Insights into fragmentation processes at basaltic shield volcanoes[END_REF].

Yield Strength, Strain Rate, and Velocity

Following [START_REF] Cigolini | Intra-crater activity, aa-block lava, viscosity and flow dynamics: Arenal Volcano[END_REF], we calculated the velocity gradient using:

v h z h h z h x = - + -       ρ α η gsin ( ) 2 2 0 2 1 2 (6)
where h, h 0 , ρ, α, and g are respectively flow height, plug height, lava bulk density, underlying slope, and acceleration due to gravity. Equation ( 6) applies between the base of the flow, where z = h and v x = 0, and the base of the plug, where z = h 0 and v x = v max . On top of this shear zone, all velocities are the same, and the whole plug thickness moves at the maximum velocity. For a Bingham model, yield strength (τ 0 ) is classically given by:

τ ρ α 0 0 = gh sin (7)
and strain rate (ε z ) by:

ε z / = dv dz x (8)
dv x /dz being the vertical velocity gradient.

RESULTS

We first consider flow geometry, geochemical, petrographic, and textural characteristics. Results are then presented facies by facies. Here, the four facies defined within the Grande Cascade flow based on macro-features are (1) basal breccia, (2) basal shear zone, (3) massive plug, and (4) the eroded surface (Fig. 2).

Flow Geometry

Although erosion has worked on the Grande Cascade lava flow for the last 0.38 Ma, the topography still allows us to infer its vent-proximal outline (Fig. 4A). A break in slope on the southwestern flank of Puy de Mareilh indicates the source of the flow (Fig. 4B), while two gullies define its lateral boundaries (Fig. 4A). The flow can be traced ∼700 m to the SW before being cut by the cliff at the Grande Cascade. We thus have a minimum length, as the medialdistal section has been removed by glacial erosion. However, other crystal-rich, trachytic lava flows in the vicinity are up to 2.5 km long [START_REF] Latutrie | Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)[END_REF]. To the south, the contact is lost under a more recent (0.23 ± 0.2 Ma) trachyandesite flow from Roc de Cuzeau (Fig. 1). The surface slope is ∼20° and is continuous up to the break in slope at the base of the Puy de Mareilh (Fig. 4).

The Basal Breccia

A breccia crops out discontinuously the flow base. Most of the breccia layer is covered by talus of angular slabs fallen from the plug above. Exposures are up to 3-5 m thick, but in no place is the entire thickness exposed. The breccia matrix is dominated by rare centimetric to decimetric clasts surrounded by a gray, powdery matrix (Fig. 5). We found pockets of fine-grained breccia in the shear zone between shear-band-defined zones of solid rock. In most locations, we found no blocks, and 97% of the particles are smaller than 3.2-4.8 cm (Fig. 6A). At the southern end of the exposure there are around 30 10-cm blocks, one of them 70 cm in diameter, which are matrix-supported. Some of these blocks have their long axes aligned with the orientation of the shear zone (Fig. 5).

We had an insufficient number of blocks to complete statistical analyses of block-size distribution (cf. [START_REF] Anderson | Deposits, petrology and mechanism of the 2010-2013 eruption of Kizimen volcano in Kamchatka, Russia[END_REF][START_REF] Golombek | Rock sizefrequency distributions on Mars and implications for Mars Exploration Rover landing safety and operations[END_REF][START_REF] Bulmer | Distinguishing between primary and secondary emplacement events of blocky volcanic deposits using rock size distributions[END_REF]. Of the 30 available, 29 had long-axis dimensions of 10 ± 1 cm. More than 6 wt% of the particles have diameters of <32 µm. The data are best fit by a Weibull distribution than a log-normal distribution, with residual square sums of 3.0 and 4.4, respectively (Fig. 6A). The PSD has a mean of 0ϕ, a standard deviation of 3.22ϕ (Table 2), and a fractal dimension of 3.36 ± 0.06 (Fig. 6B). In the 1ϕ fraction, particles have high values of convexity and solidity (medians of 0.915 and 0.942, respectively), meaning smooth, spherical particles (Fig. 7), with the ideal value being 1.0 for a circle. The values are similar for other grain The breccia bulk composition is slightly less evolved than that of the lower part of the plug (Table 3). This difference can be explained by the assimilation of basic enclaves in the breccia. The breccia is also slightly denser than the plug base (2.76 g/cm 3 versus 2.70 g/cm 3 ). This is also compatible with the assimilation of denser basic enclaves in the breccia. Componentry analysis shows only three categories of particles in the breccia: 70% are lava, 29% are lava with crystals attached, and 1% are free crystals.

The Shear Zone

The shear zone crops out discontinuously at the base of the plug. Where the contact with the underlying breccia can be seen, the shear zone is typically ∼3 m thick. Shear zone outcrops present sub-parallel shear planes (Figs. 8A and8C), with frequency decreasing with height. At site GC1, there is one plane every ∼7 cm at the base and one every ∼14 cm at the top of the shear zone (Fig. 8B). At site GC3, frequency decreases upwards from one every ∼12 cm to one every ∼24 cm (Fig. 8D). At some outcrops, shear planes are more closely spaced (one every 1 cm, Fig. 8E), which indicates local variations in stress.

In places, shear planes coalesce to form shear bands filled with powdery lava with occasional rotated centimetric blocks (Figs. 8C and8D), similar to a fault gouge (cf. [START_REF] Pallister | Petrology of the 2004-2006 Mount St. Helens lave dome-implications for magmatic plumbing and eruption triggering[END_REF]. These bands are lighter toned than the surrounding lava due to the presence of ground sanidine crystals. To the SW, where the plug is only ∼15 m thick, the shear zone is absent, and the plug lies directly on the basal breccia. At the center of the exposure where the plug is ∼40 m thick, the shear zone is thickest.

The Plug

The plug is the main flow facies; it is 15-40 m thick and constitutes ∼90% of the flow by thickness (Fig. 9A). Although massive, the plug presents few structures. Vertical fractures define crude (2-3-m-wide) columnar joints, which likely formed by thermal contraction of the lava upon cooling (cf. [START_REF] Mallet | XVI. On the origin and mechanism of production of the prismatic (or columnar) structure of basalt: The London[END_REF][START_REF] Tomkeieff | The basalt lavas of the Giant's Causeway district of Northern Ireland[END_REF][START_REF] Degraff | Surface morphology of columnar joints and its significance to mechanics and direction of joint growth[END_REF]. The main structure is subvertical foliation, which defines 5-20-cmthick slabs (Fig. 9B). These slabs provide an easy erosion surface. Slabs thus commonly detach from the cliff to contribute to a voluminous talus pile at the outcrop base (Fig. 2B). We found striated lava where the plug overlies the basal shear zone. Striations are ghost like, but parallel, and they extend ∼2 m E-W, an indicator of flow direction.

Plug lava is vesicle-free and porphyritic, with white tabular plagioclase and sanidine megacrysts (≤ 5 cm). This is typical of "sancyite," a local name for trachyte that contains large phenocrysts of sanidine and tridymite [START_REF] Le Maitre | Igneous rocks: A classification and glossary of terms[END_REF]. Millimetric phenocrysts include amphibole, clinopyroxene, and olivine, with occasional biotite. Olivine crystals are surrounded by pyroxene rims. Amphiboles commonly show a corona of oxide minerals, the result of a mixing process between two magmas that is a common phenomenon in the Monts-Dore massif [START_REF] Gourgaud | Magma mixing in alkaline series: An example from Sancy volcano (Mont-Dore[END_REF][START_REF] Gourgaud | Mélanges de magmas dans les séries alcalines et calco-alcalines: Leur rôle dans le genèse des laves intermédiaires et leur influence sur les mécanismes éruptifs: Clermont II[END_REF]. The trachyte matrix contains darker enclaves that are 2-30 cm in diameter. Enclaves have lobate shapes, glassy borders, and vesiculated cores that suggest a liquid state during incorporation in the host silicic magma. We interpret this as being associated with a more advanced stage of mixing 3). The mean phenocryst content is ∼35 vol%, and its DRE density is 2.70 g/cm 3 .

The Surface

The flow surface is covered by vegetation, and all surface breccia has been stripped away. Where exposure is available, it exposes the top of the plug zone. The Monts Dore massif was covered by ice during the last three glaciations: Mindel (ca. 480-420 ka), Riss (ca. 380-130 ka), and Würm (ca. 100-10 ka), in the Alpine nomenclature [START_REF] Jung | Géologie de l'Auvergne et de ses confins bourbonnais et limousins: Impr. Nationale[END_REF]. The Haute-Dordogne valley was filled with 200-300-m-thick ice down to 8 km downstream of the Grande Cascade [START_REF] Veyret | Modèles et formations d'origine glaciaire dans le Massif Central français: Problèmes de distribution et de limites dans un milieu de moyenne montagne[END_REF]. Considering the 0.38 Ma age of the Grande Cascade unit, it has endured the last two glacial episodes. Thus, any brecciated surface layer has been eroded. We found two outcrops on the northern side of the unit (at sites GC6 and GC7) and one in the streambed at the center of the unit (site GC5). At all three locations, the lava has similar foliation: 5-20-cmwide, sub-vertical bands (Fig. 10). At the center of the unit, AMS data give a foliation strike of 80°, but to the north strike increases to 130° (Fig. 11). The lava is lighter toned, less dense (2.63 g/cm 3 ), and more silica rich (Table 3) than in the underlying shear zone. The plug zone thus represents a more evolved magma, tapped from the top of the reservoir. It was thus emitted so as to be located at the top of the dome that then deformed SW to form the flow that we sample. Such a process is common in the Monts Dore massif [START_REF] Gourgaud | Mélanges de magmas dans les séries alcalines et calco-alcalines: Leur rôle dans le genèse des laves intermédiaires et leur influence sur les mécanismes éruptifs: Clermont II[END_REF][START_REF] Latutrie | Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)[END_REF].

Physical Properties of the Lava

Anisotropy of Magnetic Susceptibility

Mass susceptibility has a range of 546-12,587 × 10 -9 m 3 Kg -1 with a mean of 6746 × 10 -9 m 3 Kg -1 . Standard deviation are 50% due to low values of the surface facies (2241 ± 2068 × 10 -9 m 3 Kg -1 ), the high values in the plug (9805 ± 1727 × 10 -9 m 3 Kg -1 ), and highly dispersed values in the shear zone (Figs. 11C and11D). The mean value of degree of anisotropy is low (1.038 ± 0.019) but shows significant variation (1.007-1.129). The magnetic fabric varies from prolate (-0.778) to oblate (0.965) depending on position in the flow. Facies show systematic variations that can be summarized as follows.

• Mean mass susceptibility varies from 10 -2 -10 -3 SI at the base of the flow to 10 -3 -10 -4 SI at the relatively silica-rich surface. This means that magnetite controls AMS in the more basic facies at the flow base, while paramagnetic minerals (likely biotite) control AMS in the more silicic facies toward the surface (cf. [START_REF] Tarling | The magnetic anisotropy of rocks[END_REF]). • The basal shear zone has a mean mass-susceptibility of 7385 ± 2690 × 10 -9 m 3 Kg -1 . The AMS ellipsoid ranges from prolate to oblate, and there are two clusters of principal susceptibility axes (shear zone panels in Fig. 11). Thus, in fabric, two groups are observed in the shear zone:

(1) An oblate magnetic fabric (at sites GC2, GC2-S1, GC2-S4, and GC4-2; see shear zone stereoplots in Fig. 11) with a horizontal to slightly imbricated magnetic foliation. The mean degree of anisotropy varies from 1.027 to 1.048. The principal susceptibility axes are clustered (Tables S1 andS2; see footnote 1). The magnetic lineation is horizontal with a WNW-ESE trend.

(2) At sites GC2-S2, GC2-S3, GC3, and GC4-1 (Fig. 11), magnetic lineation trends WNW-ESE, but the magnetic foliation is steeper, with an inclination of 33-62°. The AMS ellipsoid is moderately oblate for GC2-S2 and GC2-S3 and strongly oblate at GC3 and GC4-1. The mean P j is lower (1.017-1.027). • At GC4 (Fig. 11A), we collected four samples from the bottom (GC4-1) to the top (GC4-5) of the shear zone. AMS ellipsoid changes progressively from oblate to neutral with height, and P j decreases. Magnetic foliation dips east at an inclination of 28-53°, which is consistent with lower degrees of shear with height. • Lowest mass-susceptibilities of all samples are at the surface. Magnetic foliation controls lineation, with slight (GC7, T = 0.229) to strong (GC5, T = 0.720; GC6, T = 0.588) oblate magnetic fabrics. Axes of k 1 and k 2 define a sub-vertical to vertical magnetic foliation, with an inclination of 65-87° (Fig. 11). Magnetic trends are consistent with a vertically oriented foliation.

Geothermometry

Temperature estimates range from 807 ± 30-970 ± 43 °C with a median of ∼850 °C (see Supplemental Material). Our interval is consistent with [START_REF] Gourgaud | Mélanges de magmas dans les séries alcalines et calco-alcalines: Leur rôle dans le genèse des laves intermédiaires et leur influence sur les mécanismes éruptifs: Clermont II[END_REF] estimate, as well as with the data of [START_REF] Latutrie | Eruption and emplacement dynamics of a thick trachytic lava flow of the Sancy volcano (France)[END_REF], who found emplacement temperatures of 873 ± 15-957 ± 15 °C for a similar trachytic flow on the western side of the Haute Dordogne valley (Fig. 1). The range is due to magma mixing, where many crystals show evidence of disequilibrium. Microlite crystallization may also have occurred in the conduit during the magma's slow ascent. Thus, temperatures should be considered maximum bounds for emplacement.

Viscosity

Viscosities calculated for the dry melt phase are 10 7 -10 9 Pa s for a temperature range of 807-970 °C (see Supplemental Material). Adding a 35% volume fraction of crystals increases the viscosity to 6.9 × 10 10 Pa s at 807 °C and to 2.2 × 10 8 Pa s for 970 °C. The lack of vesicles indicates that the magma was volatile poor when it arrived at the surface, which is consistent with silicic lava flows resulting from eruptions of highly degassed magma [START_REF] Eichelberger | Non-explosive silicic volcanism[END_REF][START_REF] Taylor | Hydrogen isotopic evidence of rhyolitic magma degassing during shallow intrusion and eruption[END_REF][START_REF] Anderson | Hydrogen isotopic evidence for extrusion mechanisms of the Mount St[END_REF][START_REF] Anderson | Mount St. Helens and Santiaguito lava domes: The effect of short-term eruption rate on surface texture and degassing processes[END_REF]. Following [START_REF] Anderson | Mount St. Helens and Santiaguito lava domes: The effect of short-term eruption rate on surface texture and degassing processes[END_REF], because the water content was likely low, we selected 0.1 wt% H 2 O as an upper limit. This reduces the lava viscosity to 10 9 Pa s at 807 °C and 10 8 Pa s at 970 °C. We thus estimate viscosities of 10 8 -10 10 Pa s for the silicic fraction of the flow, which represents the bulk of the flow by volume (Supplemental Material).

Yield Strength, Velocity, and Strain Rate

Yield strength calculated with Equation 7 is 3.2 × 10 5 Pa. This compares with silicic flows at Santiaguito (1.3-6. It is also consistent with Blake's (1990) conclusion that "low lava domes and coulees develop if 10 4 Pa ≤ τ 0 ≤ 10 6 Pa."

To account for the wide range of viscosities, we applied two velocity scenarios: (1) the highest possible viscosity for dry conditions at 800 °C, and (2) a most likely viscosity for a median temperature of 850 °C and 0.1 wt% H 2 O. For the first scenario (Fig. 12), the flow has a maximum velocity of 1.45 × 10 -5 m s -1 (∼1 m day -1 ). With the second scenario (Fig. 12), maximum velocity is 4.33 × 10 -5 m s -1 (∼4 m day -1 ). These values are of the same order of magnitude as lava flows with similar silica contents. [START_REF] Manley | Extended cooling and viscous flow of large, hot rhyolite lavas: Implications of numerical modeling results[END_REF] estimated a mean velocity of 2-7 m day -1 for lavas of the Bruneau-Jarbidge system. At Santiaguito, velocities of the 2000-2002 flows were 2-13 m day -1 [START_REF] Harris | The evolution of an active silicic lava flow field: An ETM+ perspective[END_REF]. At Cordón-Caulle, surface velocity estimates were 1.5-4 m day -1 [START_REF] Tuffen | Exceptional mobility of an advancing rhyolitic obsidian flow at Cordón Caulle volcano in Chile: Nature Communications[END_REF][START_REF] Magnall | Emplacing a cooling-limited rhyolite lava flow: Similarities with basaltic lava flows[END_REF]. At Sinabung, Nakada et al. ( 2019) measured velocities of 2-60 m day -1 , with the higher value corresponding to the initial phase, when slopes were steep (33°) and extrusion rates high. The breccia facies could be interpreted as a block-and-ash flow deposit. Rheomorphic ashflow tuffs can also be hard to distinguish from silicic lava units [START_REF] Henry | Distinguishing strongly rheomorphic tuffs from extensive silicic lavas[END_REF], and fragmentation of lava can occur during explosions at a flow base during passage over wet ground (e.g., Thorarinsson, 1953a;[START_REF] Greeley | Icelandic pseudocraters as analogs to some volcanic cones on Mars[END_REF][START_REF] Hamilton | Explosive lava-water interactions I: Architecture and emplacement chronology of volcanic rootless cone groups in the 1783-1784 Laki lava flow, Iceland[END_REF]. Here, we consider evidence for the breccia being generated by flow of the lava itself.

Fractal Dimension of the Particle-Size Distribution

PSD fractal dimension is linked to fragmentation energy and increases with the proportion of fine ash [START_REF] Kueppers | Explosive energy" during volcanic eruptions from fractal analysis of pyroclasts[END_REF]. For our basal breccia, the fractal dimension (3.36 ± 0.06) is similar to those of ignimbrite deposits, which range from 2.9 to 3.4 (table 2 in [START_REF] Kaminski | The size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions[END_REF]. [START_REF] Kaminski | The size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions[END_REF] obtained values of 4.2 that increased to 5.4 after a steel ball was added into the grinder. [START_REF] Jones | Primary and secondary fragmentation of crystal-bearing intermediate magma[END_REF] failed to produce D > 2.45 and concluded that high crystallinity prevents formation of fine ash because crystals are stripped of groundmass but are not broken. On the other hand, an increase in porosity favors an increase in D [START_REF] Jones | Primary and secondary fragmentation of crystal-bearing intermediate magma[END_REF]. Because the Grande Cascade trachyte has both high crystallinity and no porosity, it is difficult to determine their respective contributions to the fractal dimension. Milling is a kinetic process, and thus its duration influences D [START_REF] Kaminski | The size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions[END_REF]. Prolonged grinding of breccia beneath a slowly moving silicic lava flow could influence D. However, so too could explosive fragmentation [START_REF] Kueppers | Explosive energy" during volcanic eruptions from fractal analysis of pyroclasts[END_REF]. The fractal dimension alone is thus not sufficient to distinguish particles generated by magma decompression from particles generated by milling.

Morphology of the Particles

Our convexity (CVX: 0.899-0.919) and solidity (SLD: 0.926-0.950) shape-parameter values are too high for particles originating from magma fragmentation. Morphologi G3 data are scarce in the literature, but [START_REF] Thivet | Variability of ash deposits at Piton de la Fournaise (La Reunion Island): Insights into fragmentation processes at basaltic shield volcanoes[END_REF] analyzed basaltic ash from lava fountaining. They found rough particles, with median CVX and SLD of 0.798 and 0.874 for the 1ϕ fraction (Fig. 7). [START_REF] Buckland | Relating the physical properties of volcanic rocks to the characteristics of ash generated by experimental abrasion[END_REF] used the Morphologi G3 to analyze ash produced by abrasion of pyroclasts placed in a mill for four minutes. They obtained SLD and CVX in the 0.94-0.98 range. These data show that the Grande Cascade basal breccia likely results from a milling process. The lack of lithic fragments in our basal breccia also provides evidence that component particles do not have an explosive origin. Instead, the identical geochemical and petrological affinity to the overlying lava and its enclaves points to the lava body as being the source. Given a ∼40-m-thick lava body with an average density of 2650 kg/ m 3 , the base of the flow would be under a lithostatic pressure of ∼1 MPa. We conclude that the basal breccia results from milling of the base of the main lava body.

Non-Blocky Basal Breccia: Mode of Formation

The brecciation process generating the basal crust at the Grande Cascade is different than the classical "rolling over" process typical of 'a'ā flows [START_REF] Macdonald | Pahoehoe, aa, and block lava[END_REF], where blocks from the upper crust fall and are overridden by the flow front [START_REF] Harris | The thermal stealth flows of Santiaguito dome, Guatemala: Implications for the cooling and emplacement of dacitic block-lava flows[END_REF]. It also differs from a classic blocky crust that is typically associated with silicic lava flows (e.g., [START_REF] Finch | Block lava[END_REF][START_REF] Anderson | Deposits, petrology and mechanism of the 2010-2013 eruption of Kizimen volcano in Kamchatka, Russia[END_REF][START_REF] Rhodes | Textural insights into the evolving lava dome cycles at Santiaguito lava dome[END_REF]. [START_REF] Anderson | Deposits, petrology and mechanism of the 2010-2013 eruption of Kizimen volcano in Kamchatka, Russia[END_REF] showed that block-size distribution reflects stress conditions at the vent, where high extrusion rates result in small primary blocks and low extrusion rates lead to larger blocks. Primary blocks then fracture through mechanical and thermal processes as they move down flow [START_REF] Anderson | Deposits, petrology and mechanism of the 2010-2013 eruption of Kizimen volcano in Kamchatka, Russia[END_REF][START_REF] Bulmer | Distinguishing between primary and secondary emplacement events of blocky volcanic deposits using rock size distributions[END_REF]. The result is an exponential form to the blocksize distribution with a decrease in number with increasing diameter with distance [START_REF] Golombek | Rock sizefrequency distributions on Mars and implications for Mars Exploration Rover landing safety and operations[END_REF]. The blocks we found were all the same size (10 ± 1 cm) and match the same dimension as the fracture spacing in the shear zone at the Grande Cascade (Fig. 8). Blocks are also aligned with the horizontal fracture sets in the basal shear-zone. Thus, we suggest that the rare blocks present in the basal breccia are due to isolated break-up of the shear zone. Subsequently, blocks are not broken further. [START_REF] Anderson | Deposits, petrology and mechanism of the 2010-2013 eruption of Kizimen volcano in Kamchatka, Russia[END_REF] showed that blocks <12 cm do not accumulate on the flow surface because they slip toward the flow interior through void spaces between larger blocks. It seems that a reverse process occurs in the basal shear zone, where small blocks fall into the fine-grained matrix.

We thus see evidence of the basal shear zone breaking up into blocks in the same manner observed by [START_REF] Dadd | Structures within large volume rhyolite lava flows of the Devonian Comerong volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome[END_REF] and [START_REF] Smith | Ductile-brittle transition structures in the basal shear zone of a rhyolite lava flow, eastern Australia[END_REF] at the base of a silicic lava flow. [START_REF] Dadd | Structures within large volume rhyolite lava flows of the Devonian Comerong volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome[END_REF] observed that the basal breccia could be found as pockets in the lowermost portion of the massive lava and concluded that this was consistent with mechanical fracturing in a zone of high shear stress. [START_REF] Smith | Ductile-brittle transition structures in the basal shear zone of a rhyolite lava flow, eastern Australia[END_REF] also argued that in basal zones subject to high shear stress, brittle failure involves fracturing along horizontal shear planes as well as vertically. This fragments the basal shear zone into a blocky structure. In our case, we found these blocks to be rare and set in a fine-grained matrix more akin to a fault gouge.

Diamict Generation

The lithology of the basal breccia is that of a diamict, i.e., an unsorted to poorly sorted deposit with a wide range of grain sizes consisting of "floating" clasts in a fine-grained matrix Figure 12. Velocity (thick lines) and strain rate (thin lines) profiles through the flow thickness are plotted. Velocity is zero at the base of the flow and increases up to a maximum value at the base of the plug. Strain rate is inversely proportional, and the plug moves at v max with no deformation (riding along at the maximum velocity achieved at the top of the shear zone that carries the plug forward). Gray lines correspond to the scenario where the lava is at 800 °C and dry; black lines correspond to the scenario where the lava is at 850 °C and contains 0.1 wt% H 2 O. [START_REF] Menzies | Diamicton[END_REF]. Diamicts are found in a range of settings from glacial to sedimentary, and we found them at the sample site at the southern end of the exposure (Fig. 5). If lithified, such a deposit is called a diamictite [START_REF] Flint | Brazos River bar: A study in the significance of grain size parameters[END_REF], and this is the case toward the center of the exposure. Thus, the basal facies is an example of a diamict in a volcanic setting.

Magnetic Fabric: Flow-Dynamics, Emplacement Directions, and Paleosurface

AMS fabric reflects crystal orientation developed during molten state [START_REF] Hargraves | Distribution anisotropy: The cause of AMS in igneous rocks?[END_REF][START_REF] Rochette | Rock magnetism and the interpretation of anisotropy of magnetic susceptibility[END_REF][START_REF] Akkoyun | Using the anisotropy of magnetic susceptibility to infer flowinduced orientation of anisotropic particles: Feasibility and sensitivity[END_REF]. It is thus widely used as a proxy to infer flow dynamics (e.g., [START_REF] Tarling | The magnetic anisotropy of rocks[END_REF][START_REF] Cañón-Tapia | Anisotropy of magnetic susceptibility of lava flows and dikes: A historical account[END_REF][START_REF] Loock | Lava flow internal structure found from AMS and textural data: An example in methodology from the Chaîne des Puys[END_REF]. Magnetic lineation generally develops parallel to flow direction, so that k 1 coincides with flow and k 3 is perpendicular. This model is complicated by primary and secondary sources [START_REF] Bascou | Magnetic versus crystallographic fabrics in a basaltic lava flow[END_REF], so that the original fabric can show varying degrees of overprinting [START_REF] Schulmann | Some remarks on fabric overprints and constrictional AMS fabrics in igneous rocks[END_REF]. For imbricated fabrics, use of magnetic foliation is more accurate [START_REF] Hillhouse | Magnetic fabric, flow directions, and source area of the Lower Miocene Peach Springs Tuff in Arizona, California, and Nevada[END_REF][START_REF] Gurioli | Influences of urban fabric on pyroclastic density currents at Pompeii (Italy): 1. Flow direction and deposition[END_REF][START_REF] Boiron | Internal structure of basalt flows: Insights from magnetic and crystallographic fabrics of the La Palisse volcanics[END_REF]. Here, depending on the situation, we used magnetic lineation or magnetic foliation to define emplacement direction and local variations in flow direction, and magnetic foliation was well defined in all parts of the Grande Cascade lava flow (Fig. 13).

Low values of mass susceptibility at the surface are associated with silica enrichment and Fe 2 O 3tot depletion (Table 3). Inferred flow directions (Fig. 13) reveal movement toward the southwest and northwest. The sub-vertical magnetic foliation and oblate shape of the AMS is inherited from the conduit (cf. [START_REF] Závada | Internal fabric development in complex lava domes[END_REF], where foliation became aligned with shear around a vertically ascending plug. At the flow base, higher χ m relates to lower silica content and higher Fe 2 O 3tot . Westward to northwestward emplacement directions are the same as striation directions at the plug base (Fig. 9C). Positive, progressive imbrication in the basal shear zone suggests that the maximum velocity occurs above this zone (cf. [START_REF] Caballero-Miranda | Vertical AMS variation within basalt flow profiles from the Xitle volcano (Mexico) as indicator of heterogeneous strain in lava flows[END_REF]. A decrease in velocity and increase in strain with depth across the basal shear zone is also reflected in the orientations of AMS ellipsoid projections (Fig. 11). This is consistent with the velocity and strain profiles expected at the base of the flow (Fig. 12).

Presence of magnetic lineation slightly transverse to the main flow direction is due to a component of lateral spreading that was probably induced by irregularities in the paleoslope (cf. [START_REF] Caballero-Miranda | Vertical AMS variation within basalt flow profiles from the Xitle volcano (Mexico) as indicator of heterogeneous strain in lava flows[END_REF]. The southwestern flow direction of sample GC4-1 likely relates to the presence of a small gully beneath the flow. The flow would have moved down the gulley, but it would have spilled over the sides, consistent with thinning to the NE and SW.

Internal and surface structures are not complex and involve only vertically orientated foliation. There are no complex structures such as folds, ramps, or extensional fabrics that would be consistent with flow moving over a break in slope (cf. [START_REF] Bullock | Emplacement of the Rocche Rosse rhyolite lava flow (Lipari, Aeolian Islands)[END_REF]. The lack of internal structure is also consistent with modeling by [START_REF] Gregg | A laboratory investigation into the effects of slope on lava flow morphology[END_REF] that generated a lack of structures for slopes of 10-30°. The paleoslope for the gulley down which the flow moved thus seems similar to those of today's surface: uniform and around 20°.

The relative increment of Pj and clustered principal magnetic susceptibility axes for samples in the shear zone can be associated with shearing and variable degrees of vertical shortening caused by the flow weight and hence thickness [START_REF] Caballero-Miranda | Vertical AMS variation within basalt flow profiles from the Xitle volcano (Mexico) as indicator of heterogeneous strain in lava flows[END_REF]. Variation in T at the flow base can be related to interactions between the primary fabric and shear due to sliding of the plug over the shear zone (cf. [START_REF] Schulmann | Some remarks on fabric overprints and constrictional AMS fabrics in igneous rocks[END_REF]. Sliding is consistent with our source model for the basal breccia, i.e., breakup of the shear zone under intense shear stress and grinding of blocks into a powder due to the weight and slow motion of the overlying plug.

Emplacement Model of the Grande Cascade Lava Flow

Our structural and textural data, as well as the sequence of facies, allow us to build an emplacement model with three steps (Fig. 14).

(1) Genesis of Cataclasite by Magma Milling and Generation of Vertical Foliation

The out-gassed, crystal-rich, and water-poor magma that fed the Grande Cascade flow had a high viscosity (≥10 9 Pa s) (Fig. 14A). During ascent, magma underwent shearing and milling against the conduit walls. This generated a cataclasite layer around the central plug and involved abrasion of the annular shear zone to form a matrix-dominated deposit. Extruding silicic lava bodies are known to produce finegrained powders by grinding ascending, nearsolid magma against conduit walls (cf. [START_REF] Bluth | Observations of eruptive activity at Santiaguito volcano, Guatemala[END_REF][START_REF] Sahetapy-Engel | Thermal, seismic and infrasound observations of persistent explosive activity and conduit dynamics at Santiaguito lava dome[END_REF]. Cataclasite or fault gouge was observed in rhyolitic conduits by [START_REF] Tuffen | Fault textures in volcanic conduits: Evidence for seismic trigger mechanisms during silicic eruptions[END_REF], and a similar phenomenon was documented during 2004-2008 dome extrusion at Mount St. Helens [START_REF] Cashman | From dome to dust: Shallow crystallization and fragmentation of conduit magma during the 2004-2006 dome extrusion of Mount St[END_REF][START_REF] Kendrick | Extreme frictional processes in the volcanic conduit of Mount St. Helens (USA) during the 2004-2008 eruption[END_REF][START_REF] Pallister | Petrology of the 2004-2006 Mount St. Helens lave dome-implications for magmatic plumbing and eruption triggering[END_REF][START_REF] Pallister | Faulting within the Mount St. Helens conduit and implications for volcanic earthquakes[END_REF]. Foliation also developed during ascent. Foliation is a common feature of silicic lava flows as, for example, in the dacitic flows of Crater Lake [START_REF] Allen | Structures in the dacitic flows at Crater Lake, Oregon[END_REF], Comb Peak rhyolite [START_REF] Christiansen | Emplacement and thermal history of a rhyolite lava flow near Fortymile Canyon[END_REF], and Little Glass Mountain [START_REF] Fink | Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain[END_REF]. Foliation generally develops in the conduit through shearing [START_REF] De Lima | Feeder systems of acidic lava flows from the Paraná-Etendeka Igneous Province in southern Brazil and their implications for eruption style[END_REF]. At the Grande Cascade, sub-vertical foliation inherited from magma ascent was imprinted on the lava (Fig. 14).

(

2) Dome Extrusion

Extrusion built a low lava dome (senso [START_REF] Blake | Viscoplastic models of lava domes[END_REF]: the Puy de Mareilh (Fig. 1; Fig. 14B). Cataclasite formed the basal layer around the spreading dome and acted as lubricant to favor lateral deformation. The body began to spread downslope, following a gully to the west. Rare blocks were generated by brittle fracturing of the shear zone, but the basal breccia continued to be milled by the overlying plug. Shearing allowed

A B C

the solid plug to slide over the basal breccia, with the plug retaining its sub-vertical foliation inherited from the conduit.

(3) Formation of a Silicic Lava Flow If a dome extends onto a slope, it begins to deform preferentially in the downhill direction [START_REF] Blake | Viscoplastic models of lava domes[END_REF]) (Fig. 14C). [START_REF] Blake | Viscoplastic models of lava domes[END_REF] Equation 37 allows us to estimate the critical radius, R, at which a dome will spread as a function of slope and yield strength. With θ = 20° and τ 0 = 3.2 × 10 5 Pa, R is around 100 m. The length scale of the Grande Cascade unit, being greater than this threshold, means that spreading was possible, with deformation being accommodated by the basal shear zone and the plug sliding over this zone with no strain.

Fault gouge generated during ascent became the main component of the basal crust, and subvertical foliation inherited during ascent was retained in the plug. Minor, small, and uniformly sized blocks broke off from the basal shear zone to float in the fine-grained matrix of the breccia at the flow base as the plug slid downhill.

Spacing of flow-surface foliations (Fig. 10) is roughly the same as the fracture spacing in the basal shear zone (∼10 cm). Foliations could thus have broken up to form a scattering of small (10 cm) blocks at the surface. However, the lack of shearing at the plug top (Fig. 12) would have meant that the surface would likely have been largely breccia-free and not composed of angular blocks of meter-scale dimension (cf. [START_REF] Finch | Block lava[END_REF][START_REF] Anderson | Deposits, petrology and mechanism of the 2010-2013 eruption of Kizimen volcano in Kamchatka, Russia[END_REF][START_REF] Bulmer | Distinguishing between primary and secondary emplacement events of blocky volcanic deposits using rock size distributions[END_REF][START_REF] Rhodes | Textural insights into the evolving lava dome cycles at Santiaguito lava dome[END_REF]. Instead, the surface layer was likely composed of an easily erodible cataclasite similar to that remaining at the flow base, which formed around the ascending plug in the conduit (Fig. 12).

Analogies with Glacier-Flow Mechanics

Glacier motion is a combination of sliding of ice over its bed, plastic deformation, and/or deformation of the bed [START_REF] Benn | Glaciers and Glaciation[END_REF][START_REF] Cuffey | The Physics of Glaciers[END_REF]. The first two processes are analogous to viscous flow of crystalrich, silicic lava. Lava flow emplacement has long been compared to that of ice. On observing active lava at Vesuvius, Italy, Forbes (1846) mentioned that "the unexpected analogies presented by a torrent of fiery lava and the icy stream of a glacier." [START_REF] Forbes | Illustrations of the viscous theory of glacier motion.-Part I. Containing experiments on the flow of plastic bodies, and observations on the phenomena of lava streams[END_REF] summarized the resemblance between lava and ice flow: their velocities are equally slow, similar morphological features, including ogives (Thorarinsson, 1953b), and their parts can be rigid, while the body is fluid.

Basal slip rates at glaciers varies from 0.008 m day -1 to 1.1 m day -1 [START_REF] Cuffey | The Physics of Glaciers[END_REF]. Surface velocities are higher, but the basal-to-surface velocity ratio is as low as 0.03 [START_REF] Cuffey | The Physics of Glaciers[END_REF]. Thus, as we found, there is a velocity gradient between the flow base and the overlying plug. In addition, velocities are similar, and the mass slips over its bedrock. Viscosities and yield strengths for glaciers are ∼10 9 -10 13 Pa s and 5-6 × 10 4 Pa, respectively [START_REF] Barnes | The yield stress-a review or 'παντα ρει'-everything flows?[END_REF][START_REF] Fischer | Ploughing of subglacial sediment[END_REF]. These are identical to those estimated for the Grande Cascade, so that similar flow regimes, structures and emplacement styles are to be expected. Indeed, [START_REF] Iezzi | Kinematics of lava flows based on fold analysis[END_REF] found that strain patterns of the Porri lava flow on Salina (Italy) were similar to those described for surge-type glaciers by [START_REF] Lawson | The structural geology of a surge-type glacier[END_REF].

Formation of glacial till can be compared to the formation of the basal breccia at the Grande Cascade lava flow. In till, "grains evolve to finer sizes by abrasion and crushing," with a fractal dimension of 2.84-2.96 [START_REF] Cuffey | The Physics of Glaciers[END_REF]. This matches our results, where lithification of the basal breccia at the Grande Cascade resulted in a diamict, a rock that is generally associated with glacial settings and tills [START_REF] Menzies | Diamicton[END_REF]. Volcanology and glaciology are thus two disciplines that benefit from the understanding and modeling of emplacement dynamics; both involve high-viscosity bodies that slide and slip over a thin basal shear zone so as to mill its substrate into a fine, till-like powder.

Idealized Cross Section of a Crystal-Rich, Silicic Lava Flow: Implications for Flow Regime

Structures in outcrop at the Grande Cascade crystal-rich, silicic lava flow are less complex than those found in crystal-poor silicic lava flows (cf. [START_REF] Fink | Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain[END_REF][START_REF] Manley | Internal textures of rhyolite flows as revealed by research drilling[END_REF][START_REF] Anderson | Mount St. Helens and Santiaguito lava domes: The effect of short-term eruption rate on surface texture and degassing processes[END_REF][START_REF] Dadd | Structures within large volume rhyolite lava flows of the Devonian Comerong volcanics, southeastern Australia, and the Pleistocene Ngongotaha lava dome[END_REF][START_REF] Smith | Folds produced by gravity spreading of a banded rhyolite lava flow[END_REF][START_REF] Smith | Ductile-brittle transition structures in the basal shear zone of a rhyolite lava flow, eastern Australia[END_REF][START_REF] Castro | Constraints on rheology of obsidian lavas based on mesoscopic folds[END_REF][START_REF] Magnall | Emplacing a cooling-limited rhyolite lava flow: Similarities with basaltic lava flows[END_REF]. For the crystal-rich case, the plug is monotonously dominated by vertical foliation inherited from shearing during magma ascent (Fig. 14), and the entire unit contains none of the structures commonly found in crystal-poor flows (e.g., [START_REF] Fink | Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain[END_REF][START_REF] Castro | Dynamics of obsidian flows inferred from microstructures: Insights from microlite preferred orientations[END_REF][START_REF] Tuffen | Exceptional mobility of an advancing rhyolitic obsidian flow at Cordón Caulle volcano in Chile: Nature Communications[END_REF][START_REF] Bullock | Emplacement of the Rocche Rosse rhyolite lava flow (Lipari, Aeolian Islands)[END_REF].

We only found one exposure that is located 700 m from the source. However, this exposure cuts laterally across the entire flow width and reveals a simple cross-section involving a massive plug on a shear zone characterized by horizontal down-flow-oriented shear planes. The shear zone overlies a fine-grained deposit rather than the blocky crusts traditionally associated with silicic lava flows and domes (cf. [START_REF] Anderson | Deposits, petrology and mechanism of the 2010-2013 eruption of Kizimen volcano in Kamchatka, Russia[END_REF]. The unit also lacks both largeand small-scale folds and does not display the thrusts or ogives that are found in other crystalrich cases (as in the cases of, for example, [START_REF] Iezzi | Kinematics of lava flows based on fold analysis[END_REF][START_REF] Cioni | Structural geology of crystal-rich, silicic lava flows: A case study from San Pietro Island (Sardinia, Italy)[END_REF][START_REF] De Silva | Effusive silicic volcanism in the Central Andes: The Chao dacite and other young lavas of the Altiplano-Puna Volcanic Complex[END_REF]Chevrel et al., 2016b;[START_REF] Reyes-Guzmán | Late Holocene Malpaís de Zacapu (Michoacán, Mexico) andesitic lava flows: Rheology and eruption properties based on LiDAR image[END_REF][START_REF] Ramírez-Uribe | The late Holocene Nealtican lavaflow field from Popocatépetl volcano: Emplacement dynamics and implications for future hazard scenarios and archaeology[END_REF].

Shear structures that we do find (Fig. 8) are consistent with simple shear in a lava moving as an ideal shear flow (cf. [START_REF] Iezzi | Kinematics of lava flows based on fold analysis[END_REF]. Stabilization of a plug-dominated velocity gradient (Fig. 12) with deformation in a direction predominantly away from the vent is also consistent with gravity-driven flow (cf. [START_REF] Fink | Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain[END_REF][START_REF] Smith | Folds produced by gravity spreading of a banded rhyolite lava flow[END_REF][START_REF] Castruccio | Evolution of crust-and core-dominated lava flows using scaling analysis[END_REF]. Lack of pure shear implies confined flow of solid, or near-solid, lava on a uniform slope (cf. [START_REF] Manley | Physical volcanology of a voluminous rhyolite lava flow: The Badlands lava, Owyhee Plateau[END_REF][START_REF] Smith | Micro shear zones in a Miocene submarine dacite dome of southwest Japan[END_REF][START_REF] Fink | Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain[END_REF]. The resulting idealized cross-section through this plug-dominated, gravity-driven flow is thus simple, as shown in Figure 14C. Observations and modeling have suggested that "simple shear strain increases and accumulates with distance from the source" [START_REF] Iezzi | Kinematics of lava flows based on fold analysis[END_REF], and that near the source lateral extension, pure shear, and shear deformation dominate [START_REF] Merle | Internal strain within lava flows from analogue modelling[END_REF]. Our case is vent-proximal and indicates that simple shear can dominate even at the source and that such crystal-rich lava flows can thus be gravity-driven shear flows throughout their length.

CONCLUSION

By analyzing structures, textures, and physical properties of the Grande Cascade trachytic flow, we built a three-step model for emplacement of high viscosity, crystal-rich, silicic lava flows emplaced on a uniform slope. Our model, and the resulting flow dynamics, structures, and facies, are very different from those of the classical emplacement model of crystal-poor silicic lava flows, where ductile deformation occurs at a large scale. In our case, the presence of massive lava lacking vesicles means that there are no pumiceous parts that can rise by diapirism [START_REF] Fink | Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain[END_REF][START_REF] Fink | Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain[END_REF][START_REF] Manley | Internal textures of rhyolite flows as revealed by research drilling[END_REF]. Our model instead applies to plug-dominated, gravity-driven, dense and crystal-rich flow. The Grande Cascade flow's high crystal content and low porosity led to emplacement of a highly viscous lava, which underwent a brittle behavior like that of glacier ice. Abrasion and grinding of the basal breccia results in the generation of diamict, which extends the provenance of this lithology to silicic lava flow settings.

Our study demonstrates the different methods that need to be applied to older lava flows that are partially eroded and/or covered by vegetation to decipher emplacement style and their Geological Society of America Bulletin, v. 130, no. XX/XX association with resultant internal structures and facies. Applying such an array of analysis techniques to older flows provides a means of better understanding emplacement mechanisms that are applicable to those of similar flow types. The emplacement dynamics and associated structures of this crystal-rich, silicic lava flow are very different from those of their crystal-poor counterparts, which argues for a global reassessment of silicic lava flow emplacement based on crystal content.

Figure 1 .

 1 Figure 1. Orthophotograph of the Haute-Dordogne valley shows the location of the Grande Cascade and other sites mentioned in the text. Inset shows the topography of the four volcanic massifs of northwest Massif Central.

Figure

  Figure 2. (A) Postcard of the Grande Cascade from around 1930 is shown. The waterfall is ∼35 m high, and orientation of the cliff is N-S. Forest obscures this view today. Numbers of facies discussed in text: (1) basal breccia, (2) shear zone, (3) massive plug, and (4) surface. (B) Closer modern photograph looking northwest up to the Grande Cascade.

Figure 3 .

 3 Figure 3. Geological sketch viewed toward the southeast shows the full Grande Cascade sequence (after Scrope, 1827, plate XI). The Grande Cascade trachytic flow is unit 1. Units 2, 4, and 6 are pyroclastic deposits, and units 3 and 5 are basaltic lava flows.

Figure 5 .

 5 Figure 5. Breccia outcrop to the south of the Grande Cascade is shown. The sub-unit is at least 3 m thick, but its true thickness is greater, as the basal contact is covered by talus. Note the crude columnar jointing of the plug and shear bands between the plug and the breccia.

Figure 4 .

 4 Figure 4. (A) Orthophotograph shows the Grande Cascade lava flow (orange) extending from the southwestern flank of the Puy de Mareilh dome. The solid line is the cliff where the flow is exposed. The dashed line represents the flow's limits inferred from topography, while the dotted line is where the contact is lost under a more recent lava flow. The blue line is the eponymous stream that feeds the Grande Cascade. All names given are as on ING (Istitute Geographique National) map sheet 2432 ET (Massif du Sancy) on which the stream has no name but has its confluence at the Creux des Boeufs (Hollow of the Oxen). (B) Photograph taken from the Durbise plateau to the south shows the Puy de Mareilh dome and the bulge marking the source of the Grande Cascade flow on its southwestern flank.

  Figure 6. (A) Particle-size distribution of the breccia is plotted. Data fit with Weibull distribution (dashed line), and log-normal (dotted line) distributions are also shown. (B) Number, N, of particles larger than size L yield the fractal dimension of the particle-size distribution. Following Jones et al. (2016), we excluded the tail of the distribution (last data) from the fit.

Figure 7 .

 7 Figure 7. Diagrams show (A) convexity and (B) solidity of particles from the 1ϕ fraction of the Grande Cascade breccia (left, this study) and Piton de la Fournaise basaltic ash from the September 2016 eruption (right, from Thivet et al., 2020). In panel A, the 5-5.5ϕ bin includes all material finer than 5.5ϕ.

Figure 8 .

 8 Figure 8. Photographs and sketches of basal shear zone outcrops are shown. (A) Outcrop at site GC3. (B) Corresponding sketch shows subparallel shear planes with a frequency decreasing with distance from the base. (C) Outcrop at site GC1. (D) Corresponding sketch shows 5-15-cm-thick shear bands (in gray) containing powdery lava. (E) Outcrop at site GC2 shows closely packed shear planes (about one every 1 cm). Hammer handle is 28 cm long.

Figure

  Figure 9. (A) Plug exposure seen from the waterfall shows the slabby form due to vertical foliation. Cliff face is ∼40 m high. (B) Detail of the plug shows the sub-vertical foliation and slab formation. (C) Detail of the lave-etched striations under the plug; to show scale, a pen is parallel to the striations.

Figure 13 .

 13 Figure 13. map of the Grande Cascade flow, along with contour diagram of anisotropy of magnetic susceptibility (AMS) axes for each site. Directions of emplacement and relative error bars are marked at the perimeter of each hemispheric projection. Dark gray-direction defined using the magnetic lineation; light gray-direction defined through the foliation pole. In panel A, AMS ellipsoid projections along z-x planes (z-vertical, x-flow direction) are plotted on the calculated velocity and strain rate profiles and show alignment with these profiles.

Figure 14

 14 Figure 14. Three-step emplacement model of the Grande Cascade flow is shown. (A) Genesis of cataclasite by magma milling. (B) Dome extrusion and endogenous growth. (C) Formation and spreading of the flow downslope. This final panel serves as an idealized cross-section for such a flow case where the plug-dominated outcrop lacking deformation structures is consistent with simple shear in a gravity flow.

TABLE 1 .

 1 KEY SHAPE PARAMETERS USED IN THIS STUDY

	Shape parameter	Abbreviation	Formula	Sensitivity
	Form Factor	FF	4	2 p πA P p /	Form and roughness
	Convexity	CVX	P P ch p /	Roughness (textural)
	Solidity	SLD	A A p c h /	Roughness (morphological)

Note: See

[START_REF] Liu | Optimising shape analysis to quantify volcanic ash morphology[END_REF]

for a comprehensive summary of shape parameters.

TABLE 2 .

 2 MAIN PARAMETERS OF THE BRECCIA'S PARTICLE-SIZE DISTRIBUTION (PSD)

	Parameter	Formula*			Value
	Mean particle size	φ16 + φ	50 3	+	φ	84		0 ϕ
	Standard deviation (sorting)	φ84 -φ 16 4	+	φ 95 -φ 6 6 .	5	3.2 ϕ (very poorly sorted)
	Fractal dimension	N.A.				3.36 ± 0.06

*ϕX is the ϕ value corresponding to the Xth percentile of the PSD. Geological Society of America Bulletin, v. 130, no. XX/XX between two magmas. Plug lava is trachyte, and its silica content is ∼63 wt% (Table

TABLE 3

 3

	. MAJOR ELEMENT COMPOSITIONS
	OF THE GRANDE CASCADE LAVA
	Facies	Surface	Plug	Breccia
	Sample number	GC7-1	GC4-3	GCB
	SiO 2 Al 2 O 3 Fe 2 O 3 tot MgO	66.82 16.49 3.38 1.18	63.04 15.83 5.00 2.33	60.40 15.50 6.25 3.45
	CaO	2.40	3.79	4.65
	Na 2 O K 2 O TiO 2 MnO	4.29 4.30 0.76 0.08	4.28 3.97 1.22 0.13	3.97 3.59 1.56 0.14
	P 2 O 5 Ba	0.20 0.05	0.31 0.05	0.40 0.06
	Sr	0.04	0.04	0.05
	Note: Major elements are given in wt%. Sample
	coordinates are available in Table S1 (see text
	footnote 1).
	Following Le Maitre et al. (2002), analyses were
	normalized to 100% on an H 2 O-and CO 2 -free basis.

Supplemental Material. Full list of samples with geographic coordinates and orientations. Please visit https://doi .org /10 .1130 /GSAB .S.21075583 to access the supplemental material, and contact editing@ geosociety.org with any questions.

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ACKNOWLEDGMENTS

We thank Stefano Mannini, Benjamin Van Wyk de Vries, and Alexander Wilson for joining us in the field and sharing their thoughts about the lava flow. All textural, petrological, and chemical analyses were carried out at the Laboratoire Magmas et Volcans (LMV, Université Clermont-Auvergne, France). Analyses at LMV benefitted from the help of Mhammed Benbakkar, Christophe Constantin, Claire Fonquernie, and Krzysztof Suchorski. Hugh Tuffen and Jonathan Fink provided helpful comments on an early version of the manuscript. We thank the reviewers, Liam Bullock and Steve Anderson, as well as the associate editor for insightful, constructive, and helpful comments that improved the focus of this work. This research was financed by the Agence Nationale de la Recherche through project LAVA (program: DS0902 2016; project: ANR-16 CE39-0009). This is Laboratory of Excellence ClerVolc contribution no. 563 and ANR-LAVA contribution 24.