Endogenous and exogenous growth of the monogenetic Lemptégy volcano, Chaîne des Puys, France

Audray Delcamp1,*, Benjamin van Wyk de Vries2, Petit Stéphane3, and Matthieu Kervyn1 1Department of Geography, Earth System Science, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium 2Laboratoire Magmas et Volcans, UMR6524 CNRS, IRD R 163, Observatoire du Physique du Globe de Clermont, Université Blaise Pascal, Clermont-Université, 5 Rue Kessler, 63038 Clermont-Ferrand, France 3Véodis-3D, Hôtel d’Entreprises Pascalis, Parc Technologique de la Pardieu, 8 allée Evariste Galois, 63000 Clermont-Ferrand, France

ABSTRACT Vespermann and Schmincke, 2000; Valentine ment analysis, looking at the spatial and tem- and Gregg, 2008). Monogenetic eruptions are poral distribution of volcanoes in monogenetic The monogenetic Lemptégy volcano in common within volcanic cone fi elds, but also fi elds (e.g., Magill et al., 2005; Bebbington and the Chaîne des Puys (Auvergne, France) was on the fl anks of large shield or stratovolcanoes Cronin, 2011; Kiyosugi et al., 2010; Le Corvec quarried from 1946 to 2007 and offers the pos- (Davidson and De Silva, 2000; Hintz and Valen- et al., 2013), while other studies have been sibility to study scoria cone architecture and tine, 2012). Many of these mafi c monogenetic focused on the mechanisms of intrusive and evolution. This volcano was originally 50–80 m volcanoes host basement or mantle xenoliths that eruptive growth of monogenetic volcanoes, spe- high, but scoria excavation has resulted in are of great interest because they provide a win- cifi cally scoria cones (e.g., Valentine and Krogh, a 50-m-deep hole. Beginning in the 1980s, dow into the inaccessible mantle and deep crust, 2006; Rapprich et al., 2007; Valentine et al., extraction was carried out with the advice as well as providing information on magma ori- 2007; Keating et al., 2008; Brenna et al., 2011; of volcanologists so that Lemptégy’s shal- gin, ascent rates, and magma-crust interactions Kiyosugi et al., 2012). low plumbing system and three-dimensional (e.g., Rudnick et al., 1993; Jannot et al., 2005; The complexity of monogenetic volcano stratigraphy have been preserved. Detailed Deegan et al., 2010; Valentine, 2012). growth can be unraveled through the study of mapping enabled key stratigraphic units to be Monogenetic volcanoes are considered to be the shallow plumbing complex and deposits. distinguished and the constructional phases to formed during a single episode of volcanic activ- Access to and study of shallow intrusive com- be reconstructed. The emplacement and evo- ity, but fi eld investigations of excavated or eroded plexes can be gained indirectly through geo- lution of the shallow plumbing system have volcanoes illustrate that they are the theater of physical and experimental methods or directly also been unraveled. The growth of this mono- complex interactions with a range of concordant on old eroded systems and in quarries (e.g., genetic scoria cone included two temporally intrusive, effusive, and explosive activity (Con- Williams et al., 1987; Malengreau et al., 1999; well-separated eruptions from closely spaced nor and Conway, 2000; Valentine et al., 2007; Annen et al., 2001; Mathieu et al., 2008; Gal- vents. The activity included Hawaiian, Strom- Martin and Németh, 2006; Keating et al., 2008; land et al., 2009; Delcamp et al., 2012; Gailler bolian and Vulcanian explosions, lava effu- Riggs and Duffi eld, 2008; Hintz and Valentine, and Lénat, 2012; Hintz and Valentine, 2012; sion, cryptodome and dome formation, partial 2012; Valentine, 2012; Kereszturi et al., 2012). Valentine, 2012). collapse, satellite vent formation, eruptive Their structures and growth appear quite simple In this study, we perform a detailed survey pauses, and intrusion emplacement with con- at fi rst sight, but closer observations reveal many of the monogenetic mafi c scoria cone of Lemp- sequent uplift. The cone shape, structure, and complexities. They can provide evidence of tégy, Auvergne, France, which offers an almost hence the local stress fi eld, plumbing system, the interplay between regional, magmatic, and complete exposure of the edifi ce deposit and and thermal state were continuously chang- volcano-tectonic processes (Riggs and Duffi eld, its underlying shallow plumbing system (Fig. ing, which in turn infl uenced the eruptive style 2008; Valentine et al., 2007; Valentine, 2012). 1A). Lemptégy volcano is part of the monoge- and location. The plumbing system morphol- In active volcanic fi elds such as the Chaîne des netic fi eld of the Chaîne des Puys, and study- ogy and microtectonic structures both record Puys, future eruptions are likely to occur in loca- ing this edifi ce will help to understand the birth local stress fi eld and magmatic fl ow direction tions where no previous cone or dome existed. and growth of the numerous scoria cones of this changes. Lemptégy volcano’s internal archi- Historic eruptions forming monogenetic vol- world-famous volcanic fi eld. The overall study tecture, stratigraphy, and evolution show how canoes or small scoria cones (e.g., Monte Nuovo, shows that Lemptégy shares similarities in complex a monogenetic volcano can be. Italy; Cerro Negro, Nicaragua; Parícutin, Mex- terms of growth and structure with other small ico; cinder cones on Tolbachik, Kamchatka) active volcanoes, such as, e.g., Cerro Negro, INTRODUCTION have shown that volcanic cones can form over Nicaragua, and in terms of historic events, Parí- anything from a few days to several years. All cutin, Mexico, thus providing insights into the Monogenetic volcanoes are the most prevail- such eruptions have had large variations in erup- internal processes of active scoria cones. The ing volcanic landform on continents and are tive style, produced diverse deposits, and have complex interactions taking place within Lemp- mostly mafi c in composition (e.g., Wood, 1980; had shifting locations of eruptive vents. tégy can occur in the summit regions of larger Studies of monogenetic volcanoes have been stratovolcanoes, and so this study also provides *Email: [email protected]. done within the framework of hazard assess- information about such larger systems.

Geosphere; October 2014; v. 10; no. 5; p. 998–1019; doi:10.1130/GES01007.1; 15 fi gures; 3 tables; 1 supplemental fi le. Received 12 December 2013 ♦ Revision received 8 June 2014 ♦ Accepted 11 July 2014 ♦ Published online 18 August 2014

998 For permission to copy, contact [email protected] © 2014 Geological Society of America

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A Limagne fault simplified volcanological N France map of the Chaine Riom des Puys (modified from Boivin et al. , Puy de 2009a) Jume Puy de Chopine Puy de Coquille N45.81˚ Lemptégy Lemptégy

Cler- Clermont-Ferrand mont-Fer-

Key: N45.71˚ faults cones Chaîne lava flows volcanoclastites des tuffs trachytes Puys 1km sediments basement 1km E2.925˚ E3.125˚

B C N N Puy Chopine

Puy des Gouttes

Lemptegy before exploitation

Quarry Lemptégy L1 vulcania L2

exploitation 500 m equipment

Figure 1. (A) Shaded relied image and geological context of Lemptégy volcano in the Chaîne des Puys, France. (B) Lemptégy quarry and nearby Puy Chopine and Puy des Gouttes volcanoes (Google Earth). (C) Lemptégy quarry with Lemptégy 1 and Lemptégy 2 eruptive centers (photo: N. Vidal). A small inset sketch gives the structure of Lemptégy before exploitation.

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TABLE 1. MAJOR-ELEMENT ANALYSIS WITH OXIDE IN WT%

Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2OK2OP2O5 LOI Total Lemptégy1* 48.11 2.37 16.08 11.88 0.19 6.05 9.38 3.53 1.82 0.60 – 100 Lemptégy1† 48.41 2.38 16.18 11.95 0.19 6.09 9.44 3.55 1.83 0.60 – 100.66 Lemptégy2* 55.21 1.50 17.38 8.55 0.19 2.67 6.12 4.67 2.88 0.84 – 100 Lemptégy2† 55.57 1.51 17.48 8.40 0.21 2.75 6.24 4.64 2.93 – 100.71 S7.1 54.52 1.56 17.70 8.59 0.21 2.57 6.02 4.49 2.82 0.73 0.11 99.21 S9.7 54.57 1.54 17.41 8.49 0.21 2.58 6.01 4.52 2.88 0.73 0.08 98.95 S0.10 54.67 1.61 18.50 8.92 0.22 2.55 5.78 4.31 2.57 0.71 0.40 99.86 S4.6 54.26 1.52 17.40 8.45 0.21 2.55 5.98 4.51 2.83 0.73 0.04 98.46 S1.4 54.65 1.53 17.51 8.45 0.21 2.56 6.05 4.57 2.89 0.74 0.01 99.16 S2.11 54.83 1.54 17.60 8.51 0.21 2.56 6.01 4.58 2.89 0.74 0.06 99.48 S3.8 54.68 1.54 17.52 8.46 0.21 2.55 5.99 4.57 2.88 0.74 0.09 99.14 S6.3 55.05 1.53 17.51 8.45 0.21 2.56 6.02 4.43 2.91 0.74 0.10 99.41 R12 54.62 1.59 18.16 8.79 0.22 2.58 5.84 4.30 2.62 0.72 0.31 99.45 S8.9 54.23 1.58 18.20 8.76 0.22 2.55 5.79 4.29 2.70 0.72 0.33 99.04 Note: For reference, see Wallecan (2011). LOI—loss on ignition. *De Goër de Hervé et al. (1999). †Boivin et al. (2009a).

Lemptégy Quarry Ramond, 1815; Scrope, 1858). This volcano the neighboring volcano, Puy des Gouttes, are belongs to the Chaîne des Puys, a volcanic align- intercalated within the deposits of Lemptégy 1. According to de Ramond (1815), Lemptégy ment of around 80 monogenetic and small edi- Deposits from the nearby Puy de Côme (age was already being excavated for its scoria in the fi ces (Fig. 1A). The Chaîne des Puys has been bracketed between 13 ka and 16 ka; De Goër de early nineteenth century. He noted that Lemp- studied since the early 1800s and was one of the Hervé et al., 1999) and Puy Chopine (~8000 yr tégy was at fi rst considered as a trachyte hill due cradles of modern volcanology (Guettard, 1752; B.P.) mantle the Lemptégy 2 cone. to its mantling by a trachytic pyroclastic deposit Desmarest, 1771, 1773; de Montlosier, 1788; The Supplemental File1 gives a pictorial over- (the Puy Chopine deposit; Boivin et al. 2009a), de Dolomieu, 1794, 1798; von Buch, 1819; view of Lemptégy and is aimed to provide the but the early quarrying uncovered the basaltic Scrope, 1858; Lecoq, 1867; Lacroix, 1908). The reader with a clearer view of the overall context scoria interior. Immediately after WWII, exca- diversity of edifi ce morphologies, from scoria of the features described in this study. vation began in earnest to provide material for cones, lava domes and spines, to maars, makes reconstruction, and work continued until 2007. this volcanic chain a universally representative LEMPTÉGY 1 In the late 1980s, the quarrying became increas- monogenetic fi eld that has recently been nomi- ingly guided by the regular monitoring of vol- nated as a United Nations Educational, Scien- Early Lemptégy, referred to as Lemptégy 1, canologists from the Laboratoire Magmas et tifi c, and Cultural Organization (UNESCO) consists of a trachybasaltic cinder cone (Table 1; Volcans. The shallow plumbing system was pro- World Heritage site. The diversity of morpholo- De Goër de Hervé et al., 1999; Boivin et al., gressively uncovered, and the scoria layers were gies and eruptive styles refl ects the wide span 2009a) and formed a number of vents and lava cross sectioned, revealing information about the of magma compositions, ranging from basalt to fl ows (Figs. 2 and 3). While the topography volcano’s evolution. In 1993, Lemptégy became trachyte. prior to Lemptégy 1 emplacement has been cov- an educational and tourist attraction, and it now The north-south alignment of the volcanoes ered up by numerous eruptions, a drill core at receives over 100,000 visitors per year. is ~40 km long and runs parallel to the Limagne Vulcania Geoscience Park, 500 m to the south, This quarry is a unique place where it is rift, which is part of the Western European suggests that there was a 100-m-deep valley possible to study the fi rst stages of scoria cone rift (Ziegler, 1994; Merle and Michon, 2001). running just to the south of Lemptégy 1. construction and the related shallow plumbing Around Lemptégy, the Chaîne des Puys splits complex (Figs. 1B and 1C). into several NNE-trending parallel alignments Stratigraphy Detailed fi eld work, including stratigraphy, parallel to the Aigueperse fault, which forms petrography, and microstructure analyses across part of a the major Rhône-Saône transfer zone The Lemptégy 1 deposit is predominantly an Lemptégy quarry, was conducted to refi ne the between the Limagne and Rhine grabens (van agglomerate deposit, composed of welded or growth stages, to understand the interactions Wyk de Vries et al., 2012). The volcanism in compacted spatter, lapilli, and bombs that are between the phases of intrusive and extrusive the Chaîne des Puys began to at least 5 m.y. activity, and to document the deformation asso- ago, but the present volcanoes are all younger 1Supplemental File. Photographic overview of the ciated with the volcano growth. Magma fl ow than 200,000 yr, the last eruption being dated at Lemptégy Volcano with annotated images showing the patterns were also established through meticu- 7600 yr ago (Boivin et al., 2009b). various aspects described in the text. (A) General views lous detailing of microstructures, such as ten- Lemptégy was a breached scoria cone. The of Lemptégy. (B) Stratigraphic elements and textures sion gashes, striations, and vesicle orientations overall history of Lemptégy was established of deposits of Lemptégy 1. (C) Stratigraphic elements (Delcamp et al., 2007). These patterns have been by De Goër de Hervé et al. (1999), who iden- and textures of deposits of Lemptégy 2. (D) Collapse structures associated with Lemptégy 2. (E) Images of confi rmed by an anisotropy of magnetic suscep- tifi ed two main eruptive centers, Lemptégy 1 the Lemptégy 2 cryptodomes. (F) Images of dykes in tibility (AMS) study (Petronis et al., 2013). and Lemptégy 2 (Figs. 1B and 1C). Lemptégy 1 Lemptégy 1 and 2. (G) Lemptégy and Puys de Gouttes has been dated at 30 ± 4 ka and Lemptégy 2 stratigraphic and structural relationships. (H) Images Geological Context at 29.6 ±6 ka by thermoluminescence on lava of lava fl ows in Lemptégy. (I) Images of faulting in Lemptégy. If you are viewing the PDF of this paper or fl ows (Guérin, 1983). Stratigraphic relation- reading it offl ine, please visit http:// dx .doi .org /10 .1130 Lemptégy was fi rst described and identifi ed ships confi rm Lemptégy 1 as being the older /GES01007 .S1 or the full-text article on www .gsapubs as a volcano in the early nineteenth century (de center. Deposits from concurrent eruptions of .org to view Supplemental File.

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Puy Chopine deposit N Puy des Gouttes flower volcano termination Fig. 4B Puy Chopine deposit

Lemptégy 2 Puy des Gouttes Lemptégy 2 deposit deposit deposit fault mirror pa hoe Puy des Gouttes Puy de Come deposit N000 hoe flow deposit

Fig. 4A Fig. 4C dike fault mirror dikes N150 Lemptégy 2 intrusive center D4 Lemptégy 1 D3 fault mirror N150

D1 bombs Lemptégy 2 D2

vent Lemptégy 2

Lemptégy 1

Lemptégy 1 system

Figure 2. Panoramic overview of Lemptégy 1 extrusive and plumbing system (picture: S. Quattocchi; www.auvergne-volcan.com). D1 stand for dike 1 and so on. Locations of outcrops on Figure 4 are displayed as Figs. 4A, 4B, and 4C. Note that the thicknesses and orientations are biased due to picture stretching.

poorly layered or graded, with no clear units up. These latter are now being preferentially One small area at the south side of the Lemp- being distinguishable. The original height of eroded, and a conjugate pattern of faults can be tégy 1 deposits is a massive lava (Fig. 3), in Lemptégy 1 is estimated at ~30 m. The deposit observed (Fig. 2). The faults with largest throws which tension cracks and shear planes are fre- is dominated by bombs, which comprise ~70% originate from dikes that pushed and uplifted the quently visible. The distribution indicates fl ow of the deposit. The bombs are of various sizes, pyroclastic layers. down toward the southeast, and we interpret with a few being 2–3 m wide and many that are Two pahoehoe lava fl ows are seen in the this feature as a clastogenic lava fl ow within the meter sized. They are of various shapes, with northeast side of the Lemptégy 1 deposit (Figs. 2 agglomerates. cowpat, spindle, and bread-crust morphologies. and 4A). The zone below the lavas is a 4-m-high A clear unconformity is observed between The presence of lava ball bombs, welded facies, bulbous mass of highly vesiculated rock that nar- the Lemptégy 1 deposits and those associ- red-dominated deposits, and meter-sized bombs rows downward to a 40-cm-wide dike, oriented ated with Lemptégy 2 (Fig. 4B). The nature of indicates near-vent or within-vent deposits. N150° (dike D4). The sides of the bulbous mass the unconformity varies across the quarry and The lower 5 m section of the Lemptégy 1 are composed of intensely crushed agglomer- occurs as a centimeter-thick weathered horizon deposit is strongly compacted and cemented and ate. The dike has an intensively sheared margin or a centimeter-thick soil, or as slightly dis- is dark gray in color, while the upper part, which and connects southward into a strongly sheared colored Puy des Gouttes deposits between the forms the majority of the outcrop, is red. There fault plane, cutting the Lemptégy 1 agglomer- two eruptive deposits. is a fi ne, scoriaceous lapilli element within the ate (Fig. 4A). The pahoehoe lava fl ows, bulbous agglomerate that becomes dominant toward the mass, and the dike (dike 4) were seen to be con- Shallow Plumbing System top, where it replaces the agglomerates, in the nected during the excavation, and we interpret form of an overlying tephra layer. This tephra the ensemble as a feeding dike, vent, and lava The excavated Lemptégy 1 shallow plumbing blankets all the Lemptégy 1 deposits, except fl ows. The two pahoehoe units are intercalated system includes four main dikes exposed in the over an uplifted area, where it is has slumped with the Puy Des Gouttes scoria deposit, and quarry (Figs. 2 and 3). Dike 1 runs close to the off. Those scoriaceous lapilli are interpreted we thus correlate them temporally with the late later Lemptégy 2 center, and a depression can as being from the Puy de Gouttes volcano (De stage Puy de Gouttes activity after the initial be seen in the Puy de Gouttes and Lemptégy 1 Goër de Hervé et al., 1999). construction of Lemptégy 1. The lavas are not scoria above the dike tip, indicating that it was a The deposits are crisscrossed by faults that brecciated or fractured, so they must have been late-intruded dike. Dike 1 connects laterally and have either brecciated the clasts or have cre- erupted after the deformation of the adjacent upward with dike 2, into a central dike, dike 3 ated dilatant zones where voids have opened faulted zone. (Fig. 4C). The fourth dike is set apart from the

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L2 top contact N with younger tephras

L2 Unit 2 L2 unit 7 thin layer L2 unit 4

Lemptégy 2 / L2 unit 6 Gouttes contact

L2 lava flow 2

cryptodome 1 Gouttes / Lemptégy 1 contact cryptodome 2

cryptodome 3

Legend Small lava L1 cryptodome L2 lava flow 1 fed lavas

lava L2 trace of L2 dike L1 landslide

dike L2 vent fault

50 m

0 m 100 m Height difference

Figure 3. Geological and structural map of Lemptégy; light detection and ranging (LiDAR) image generated by S. Petit.

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Puy de Come A W E Lemptégy 2

Puy des Gouttes Lemptégy 1 dike 4 fault N150

Pa hoe hoe bulge

fault N N150

vegetation for scale

N B Puy des Gouttes Puy des Gouttes flank flank

SW NE P.d. Chopine P. d. C h o p i n e P.d. Come P.d. Gouttes Lemptégy 2 P.d. Gouttes Lemptégy 2

4m 4m Flower Lemptégy 1 Lemptégy 1 structure

Lemptégy 2 C NW SE Lemptégy 1 P. d. G o u t t e s

dyke 3 dike 3 dike 1 dike 2 fault dyke 3 mirror fault N000 mirror N N000

bag for scale

Figure 4. The small insets at each picture corner display the location of the outcrops; this will be used for all such fi gures. (A) Dike 4 is at the origin of the small pahoehoe lava fl ow extrusion, and of a small bulge formation. Dike 4 is associated with a sinistral strike-slip fault N150° (see Fig. 2). (B) Contacts and relationships between Lemptégy 1, Puy des Gouttes (P.d. Gouttes), Lemptégy 2, Puy de Côme (P.d. Côme), and Puy de Chopine (P.d. Chopine) deposits. Note the fl ower structure with grabens and horst within the deposits associated with the fault N000° and dike 3 in Figures 2 and 4C. (C) Dike connection in Lemptégy 1 plumbing system. Dikes 1 and 2 are oriented N045°, and dike 2 is connected to dike 3. Dike 3 was emplaced within a N000° sinistral strike-slip fault, which triggered a series of horst and grabens within the deposit of Puy des Gouttes (Figs. 4B and 5). The smaller picture at the right of the drawing shows a face view of the fault mirror. Picture is 20 m wide.

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others and (dike 4) can be traced to the two cating strike-slip movement (Figs. 2 and 4C). has fed the two late pahoehoe lavas. The dikes pahoehoe lavas mentioned above (Fig. 4A). The tension gashes and opening cracks along are thus associated with the uplift and deforma- Dike 3 shows shear features on its fl ank, such this fault and the associated dike show in gen- tion of the Lemptégy 1 deposit toward the end of as striae, indicating that it was injected later- eral that the movement was sinistral. In line with the Puy des Gouttes–Lemptégy 1 eruption. As ally into a fault and/or created a fault ahead of this fault, there are open fractures in the Lemp- previously described, Puy Des Gouttes tephras its propagating tip (e.g., Mathieu et al., 2008). tégy 1 and Lemptégy 2 lava fl ows in the Vul- cover all the Lemptégy 1 deposit, except over The fault, oriented N000°, branches and splays cania Geoscience Park (Fig. 1). one area, where the Puy Des Gouttes tephra has upward into a strike-slip fl ower structure. This Because the dikes cut, or cause deformation slumped off due to dike-triggered uplift. fl ower structure is one of the major features of, all the overlying Puy de Gouttes strata, they shown to visitors, as it provides an exceptional must have been intruded very late during the Lemptégy 1 and Puy Des Gouttes demonstration of a horst and graben system Lemptégy 1 history (Fig. 5). For example, dike 1 (Figs. 2, 4B, and 5). This major N000° fault has has a depression that is fi lled by Puy de Gouttes There is no sharp contact between the Lemp- a vertical dip, and it is intimately connected to tephra, dike 3 is connected via the strike-slip tégy 1 agglomerates and Puy des Gouttes depos- the sheared dike 3, which shares this orientation, fl ower and graben structure to grabens partly its, and Puy des Gouttes lapilli are found within cuts across the volcano, and has striations indi- fi lled by Puy des Gouttes deposits, and dike 4 the upper layers of the Lemptégy 1 deposit. The

facing NE N

Uplifted area

Flower termination Second uplift

N150 dike 4 dike 1 Main Fault dike 3 N000 Lemptégy 1 center Lemptégy 2 centre dike 2

Figure 5. Overview of Lemptégy 1 plumbing system with the links among the dike intrusion, faults, and uplift of the Puy des Gouttes and Lemptégy 1 deposit. The uplift demonstrates the late emplacement of dikes 3 and 4. Note the fl ower structure associated with dike 3 and the N000° fault. Trees in the background for scale.

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Puy des Gouttes scoria is distinguished only by Figure 6. Sequential maps of the evolu- A its constant lapilli grain size, and it becomes tion of Lemptégy 1. (A) Formation of Puy easier to identify as it increases in relative abun- de Gouttes, and initiation of a fracture line dance toward the top of the Lemptégy 1 agglom- at its base as the Lemptégy 1 feeder dike erate. The Puy des Gouttes tephra deposit is well arrives near the surface. (B) Eruption of sorted and layered. Lemptégy 1 formed two or more vents, with The two Lemptégy 1 pahoehoe lava fl ows are a lava fl ow directed southeastward (seen in found within Puy de Gouttes scoria. Thus, the the Vulcania Geoscience Park). (C) Growth two eruptions were coeval (De Goër de Hervé of the Lemptégy 1 uplift by intrusions, Puy de Gouttes et al., 1999; Boivin et al., 2009a; Figs. 2 and 4A). deformation of the original cone, and erup- The simultaneous activity is also confi rmed by tion of small pahoehoe lavas from vents the red color of the Puy de Gouttes deposits right above dike 4. 50m above the Lemptégy 1 agglomerates, which were N hot enough to heat the freshly deposited Puy de Gouttes tephra. A few centimeters above this B contact, the Puy de Gouttes pyroclasts become and bombs. Some zones contain unbroken progressively blacker due to decreasing infl u- bombs, while other zones within the same unit ence of the heat from Lemptégy 1 deposits. have dominantly broken bombs. Lemptégy 1 can thus be seen as a satellite system The two thin key layers, units 2 and 4, of the larger Puy de Gouttes cone. The history of are mainly made of <1 cm, highly vesicular Lemptégy 1 is schematized in Figure 6. pumice-like lapilli. The lowest, unit 2, is yellow- ish and composed of fragile, highly vesiculated, LEMPTÉGY 2 and low-bulk-density pumice (~77% vesicular- ity; bulk density of 0.62 g cm–3; data from Walle- small vents Lemptégy 2 is a basaltic trachyandesitic to can, 2011), while the base of the overlying unit trachyandesitic cone (Table 1; De Goër de Hervé 3 is made of angular, denser black scoriae (46% lava flow et al., 1999; Boivin et al., 2009a) that grew on vesicularity; bulk density of 1.45 g cm–3; data the west side of Lemptégy 1 (Figs. 1C and 7A; from Wallecan, 2011). The thickness of unit 2 Table 1). Even though Lemptégy 2 was thought is variable but does not exceed 10 cm. Unit 4 is C to have erupted shortly after Lemptégy 1, and also marked by a lighter color, and it is similar to radiometric dates for these two volcanic centers unit 2 in terms of density and vesicularity. The are within error, an intermittent centimeter- transitions between these two key layers and the thick weathered layer between the two forma- bounding units are not sharp, and trachybasaltic uplift tions indicates a gap in time. The climate then pumiceous lapilli are found in lesser amounts was subglacial and arid, so soil may have taken in the coarser surrounding layers, while a few decades or even centuries to develop. bombs are present within the pumiceous lapilli layers. Units 2 and 4 are only present on the pa hoe hoe Stratigraphy west side of the quarry. lava flow The other key layer, unit 6, is higher up in Lemptégy 2 deposits unconformably overlie the stratigraphic sequence of Lemptégy 2, and Lemptégy 1, Puy des Gouttes deposits, and/or it is characterized by massive, dense bombs the weathered layer. A full record of Lemptégy supported in a lapilli matrix, with some lateral 2 stratigraphy is accessible due to several quarry variation in vesicle content of the lapilli. Small benches (Fig. 7B). Although some units are not groupings of very large cowpat bombs are found continuous or are locally thinner than elsewhere locally. The lapilli are scoriaceous in the west Near to the Lemptégy 1 edifi ce, and the upper due to Lemptégy 1 relief and wind directions, side of the quarry, and they become angular and 10 m of the Lemptégy 2 deposits, lenses of angu- a correlation between units, and thus eruptive massive with very few vesicles toward the east. lar blocks and lapilli with coarse reverse grading phases, is possible throughout the quarry (Fig. 8). This unit can be found across most of the quarry. are common, indicating local avalanching as the Lemptégy 2 deposits are mostly composed Units 1, 3, and 5, which form the bulk of cone grew. Such avalanche layers dominate of scoriaceous lapilli and bombs of various Lemptégy 2, are made of red and/or black lapilli the breached area on the south side. shapes (fusiform, cowpat, bread crust) and vari- and bombs of various sizes, shapes, and vesicle Xenoliths are found at several locations, either able vesicularity. The layers are generally well contents. Unit 1 contains several meter-wide cow- as free clasts or within bombs as angular or fl u-

sorted (1 < σφ < 2), with little variation in chemi- pat bombs. Unit 7 is mostly composed of homo- idal and partially melted xenoliths. The xeno- cal composition, although there is a very slight geneous material, i.e., well sorted (sorting coeffi - liths are white and foamy and contain partially

decrease of SiO2 content from lower to upper cient varying from 1.33 to 1.44; Cas and Wright, melted mica, amphibole, and glass. Their min- units (Fig. 7A; Wallecan, 2011). Reverse and 1987), and bombs are scarce. However, this unit eralogy is similar to the surrounding basement normal grading are both observed, especially is also marked by discontinuous pumiceous observed a few kilometers away from Lemptégy within the upper part of the deposit. Lemptégy lapilli layers on the west side that become con- and on top of which the Chaîne des Puys is built. 2 deposits consist of seven main units (Fig. 7B). tinuous toward the east. The pumiceous lapilli However, their alteration and melting preclude The main units are composed of lapilli scoria layers are marked by a white line on Figure 8. easy assignment to a precise original rock type.

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O trachy-

2 10 phono-tephrite andesite 8 basaltic O + K (foid)ite trachy- Lemptégy 2

2 trachy- basanite basalt 6 andesite

(%) Na 4 Lemptégy 1 basalt andesite basaltic 2 picroba- salt andesite 0 35 40 45 50 55 60 (%) SiO2

S1.4

O 8 2 S9.7 S2.11 S7.1 S3.8 S6.3

O + K 7 2

6 (%) Na

5 54 55 56 (%) SiO2

B N S7.1 geochemistry sample density sample

S7.1 unit 7 20

unit 6 S9.7 W E

unit 4 unit 5 S0.10 unit 7 unit 5 unit 3 S4.6 S1.4 unit 2 unit 3 10 S2.11 thickness of layers (m) thickness of layers

S3.8 Lemptégy 2 unit 1 unit 1

Puy des Gouttes soil S6.3 Lemptégy 1

0 Key: scoria and lapilli bombs Note: the absolute clast sizes are not represented here, and the rounded cow-pat broken bombs of upper unit 1 log gives an idea on the size angular angular proportion for comparison.

Figure 7. (A) Total alkali-silica (TAS) diagram (Le Maitre et al., 2002). The data can be found in Table 1 (Wallecan, 2011). The arrow shows the slight change of chemistry from lower to upper units defi ned in B. (B) Log from where Lemptégy 2 stratigraphy has been established (7 units and 3 key layers). The samples collected for geochemistry and bulk density measurement are also shown by fi lled circles. Figure is modifi ed from Wallecan (2011).

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Puy de Chopine oward P.d.Gouttes Puy des Gouttes Lemptégy 2 Lemptégy 1 SW pgy Lemptégy 1 Lemptégy base of Puy de Côme deposit Lemptégy 2 Lemptégy P.d Gouttes P.d Lemptégy 2 key layers Lemptégy 1, and the small pictures are close-ups of the stratigraphic sequences. are Lemptégy 1, and the small pictures Figure 8. Stratigraphy reconstruction of Lemptégy 2. The largest picture shows an overview of the Lemptégy 2 deposits looking t The largest picture of Lemptégy 2. 8. Stratigraphy reconstruction Figure pumiceous lapilli layer / part of unit 7 angular and massive bombs and scoria/lapilli unit 6 pumiceous lapilli unit 2 pumiceous lapilli unit 4 base of Puy Chopine deposit KEY:

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The xenoliths were thus probably incorporated some fragments at this stage. These late-stage deformed layers are further covered by draped in the upper crust and partially remelted during deposits correlate with the presently exposed scoria layers, indicating that the intrusive activ- magma ascent. In Lemptégy 2, they are most vent textures and indicate an increasingly Vul- ity occurred during the growth of the cone. abundant and largest (up to 30 cm across) in canian aspect for the latter part of the eruption. The cryptodomes are seen feeding small lava the upper units, especially in unit 6. Rare xeno- Lemptégy 2 is covered by deposits of later fl ows (phase 3, Fig. 9): three small lava fl ows liths are also found within the Puy des Gouttes eruptions of the Puy de Côme (ca. 13–16 ka) originate from the southern cryptodome, and deposit, and they are abundant in Lemptégy 1. and Puy Chopine (ca. 8000 yr B.P.; De Goër de one from the northern one. The lower fl ows are We note that these xenoliths, being strongly Hervé et al., 1999; Boivin et al., 2009a). highly fractured, indicating that the dome was melted, are quite different from the fresh ones growing during the lava effusion. reported by Valentine (2012) in the San Fran- Lemptégy 2 Spatter Cones, Lava Flows, Other small lava fl ows are seen within the cisco volcanic fi eld. On the other hand, partially and Final Conduit southern breached area. These were progres- melted and foamy basement xenoliths can be sively uncovered during extraction between observed in other scoria cones, for example, Quarrying of the volcano revealed informa- 2004 and 2007. They were sourced from a at Parícutin (e.g., Rowe et al., 2011), at Mount tion about the early composite activity of Lemp- spatter vent within the breached zone. This is Goma (Denaeyer, 1975), and at the Beaunit tégy 2. An initial major lava fl ow that spread now destroyed, although the feeder dike is still Maar to the north of Lemptégy (Jannot et al., west and southwest occurred early in Lemptégy visible, and a remnant of a lava tube extend- 2005; Hardiagon et al., 2011). 2 history, as its base lies on just a few centimeters ing from the vent is preserved in the Lemptégy Red-black color variations within Lemp- of scoria above Lemptégy 1 deposits (phase 1, Exhibit hall. tégy 2 units are common, and we observed red- Fig. 9). This main lava fl ow is the one on which To the north of the early emplaced spatter dominant and black-dominant patches. The red the Vulcania Geoscience Park is now located. cones, we observed a mixture of angular brec- zones are adjacent to the larger intrusions and Several spatter-fed clastogenic fl ows were then cia, bread-crust bombs, prismatically jointed are likely to be related to heat-fl ux variations. emitted from 3 to 4 spatter cones, which are now blocks, and rounded abraded blocks <1 m We noted that red bombs could be found in excavated depressions a few meters deep (maxi- and larger (2–10 m) wide massive blocks of black scoria and black bombs in red scoria. mum 4 m) and ~10 m in diameter. Their rims glassy rock (Fig. 11). The breccias and blocks Some units are locally deformed due to later are usually made of massive, black, and glassy are strongly baked and striated, and clasts are intrusion emplacement. The deformation took layers containing rare elongated vesicles, minor mostly rounded. Many of the blocks and layers place as folding of layers above intrusions, or, scoriae, and fi amme-like structures that may be are dipping inward. This formation is ~20 m more rarely, as local faulting close to an intrusion fl attened spatter. These layers are sometimes thick, and we interpret it as the fi nal conduit, boundary. To the south, there is a well-exposed covered by a rough, elongated, vesicle-rich where the activity was concentrated toward the major discontinuity in the quarry cliff. The layers coating. Several small units show fl ow direc- end of the eruption (phase 4, Fig. 9). The larger are truncated, and the eastern side is composed of tions and dips toward the small vents (back- blocks within the conduit have sharp, spiny, blocks and a small lava fl ow. This area is a cross ward fl ows in Fig. 10). The ensemble is thus or pahoehoe-like sides and are prismatically section of the collapse breach that can be seen in interpreted as spatter-fed fl ows (Johnston et al., jointed. They are probably blocks or intrusions the old topography of Lemptégy (Fig. 1C). 1997). Below the rims, scoriae are nonwelded or of viscous, degassed magma that were fractured The Lemptégy 2 deposits indicate normal cemented by fi ne ash. Nonwelded scoriae, prob- during late-stage explosions and then were pas- Strombolian venting, with explosions produc- ably deposited later in Lemptégy 2 history, top sively deformed. The angular breccias are bro- ing vesicular bombs and scoria. The deposits the spatter cone rims. ken up versions of these. The very uppermost record mostly ballistic and jet emplacement The small spatter cones are bounded by par- layers of Lemptégy 2 contain similar rounded, (Riedel et al., 2003), with some tephra fall allel massive vertical walls mainly composed abraded scoria and angular bombs, as well as recorded in the fi ner lapilli fraction. Occasional of welded products. These vertical walls are the more characteristic Lemptégy scoria seen in large cowpat bombs may indicate that a small interpreted as eruptive fi ssures that propagated lower units. This indicates that Vulcanian explo- lava lake, or conduit fi lled by lava to the crater laterally from the spatter cones (eruptive fi s- sions dominated the terminal activity. Possibly, level, resided for short periods. There is a lack sure I, phase 2, Figs. 9 and 10). Friction struc- this was combined with or preceded by phreato- of rolled or composite bombs, which indicates tures along the inner walls are preserved (striae magmatic phases, as we observed xenoliths little vent recycling, or that recycled bombs did and tension cracks) and are interpreted as evi- along with massive, dense bombs below the top not generally exit the crater. The two layers con- dence of volcanic product fl ow-back related to unit of Lemptégy 2 (unit 6). taining higher concentrations of more vesicular a decrease in activity at the vent. Although the lapilli (units 2 and 4) may indicate short periods main activity was primarily concentrated around Lemptégy 2 Intrusions of increased fragmentation, possibly with more one spot (indicated by “1” in phase 2, Fig. 9), a sustained violent Strombolian activity. We ten- small shift in the eruption focus occurred during The Lemptégy 2 shallow plumbing system tatively correlate these with cone-breaching epi- the volcano’s growth, as shown by the forma- is less clustered and reveals more structural sodes similar to the one suggested by Németh tion of another eruptive fi ssure (eruptive fi s- and morphological features than that of Lemp- et al. (2011a) for the Los Morados cone. The sure II) at the north of previous active spot “1.” tégy 1. The plumbing system of Lemptégy 2 is dense lapilli in units 6 and 7, with the prismatic Fissure II gave birth to a channelized lava fl ow composed of bulges, cryptodomes, and dikes. blocks found at the very top of the Lemptégy that runs toward the northwest (indicated by “2” A summary description of intrusive morpholo- sequence, indicate that the vent was becom- in phase 2, Fig. 9). gies can be found in Petronis et al. (2013). We ing choked by material and that milling of vent The layers seen in the outer part of the south observed a continuum of widths, from thin dikes pyroclasts was occurring. The dense lapilli, and east quarry walls are tilted, faulted, and of 40 cm to 5 m to swollen bulges and crypto- associated with xenoliths in unit 6, may also fractured. This deformation was directly linked domes (near-surface bulges) that can reach up to indicate a minor phreatomagmatic quenching of to the underlying exposed cryptodomes. These ~10 m in diameter (Fig. 12).

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Macro- and microtectonic structures are PHASE 1 abundant along intrusion walls. The quarrying View facing North N has cut many intrusions, revealing their interi- ors, and these cross sections show the inner tex- tures and structures, including tension gashes, Lemptégy 1 shear zones, vesicles, and scoria incorporation early stage = Fig. 6 (Fig. 13). All those structures are indicators of spatter cones magma fl ow direction and emplacement mode. Cross-section The main fl ow direction deduced from the structures is inward and dipping slightly upward (Delcamp et al., 2007), which was confi rmed and complemented by AMS studies (Petronis 50m lava flowing south, where et al., 2013). Dike sections show that the thin- now Vulcania stands ner dikes usually have shearing along one of the PHASE 2 margins. The thicker dikes usually have shear- ing structures on opposite walls. Opposing shear 2 zones converge toward the dike center (Figs. 12A, 12B, and 12C). eruptive fissure II The bulges and cryptodomes show strongly 1/spatter cones evolve Lemptégy 1 sheared and brecciated zones and are asso- into eruptive fissures ciated with deformation of the overlying 2/lava flow at the NW. from an eruptive fissure layers (Figs. 12B, 12D, and 12E). A few lavas eruptive fissure I extruded out of these superfi cial bulges (see 1 Fig. 9 and “Lemptégy 2 Spatter Cones, Lava Fig.10 Flows and Final Conduit” section). The south- ern cryptodome in Figure 12E is responsible for at least one partial collapse, since a clear PHASE 3 discontinuity is associated with the deformed Cryptodomes and dyke emplacement, overlying layers. which fed lava flows The shallow plumbing system of Lemptégy 2 was strongly infl uenced by Lemptégy 1, which Fig.12D acted as a buttress during the intrusion of Lemp- Lemptégy 1 tégy 2 (Fig. 14A). The concentration of bulges is thus higher toward the east than toward the cryptodomes west, where magma could freely travel as dikes through the cone, in contrast to the east, where Fig.12E magma propagation was limited due to Lemp- The two southern tégy 1. Neither Lemptégy 2 dikes nor bulges flows are respon- sible for the have been observed in the area of Lemptégy 1. edifice collapse The structures of one dike in the south reveal magma fl ow from the intrusive center south- PHASE 4 ward. This dike is linked along strike to the wall final conduit of the southern breach (see Fig. 9). The breach Fig.11 was exploited during quarrying, providing easy access to the cone. The breach can still be deter- mined by the geometry of the dikes around the Lemptégy 1 main central intrusive system, which formed a half-cup–like shape open toward the south (Fig. 14B). The collapse scar is also seen clearly in the quarry wall deposits.

Faulting and new dikes emplaced mainly DISCUSSION along a NE-SW trend or following the collapse scar geometry Birth and Growth of Lemptégy thin dike eruptive fissure Eruptive activity began at Lemptégy 1 with thick dike and early spatter cone the construction of a 30-m-high coarse bomb bulge and late conduit and scoria sequence at the base of the Puy de cryptodome lava flow Gouttes, which was erupting simultaneously. Three main dikes were emplaced into Lemp- Figure 9. A sequential reconstruction of Lemptégy 2 phases (see text for details).

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WE more evolved system backward- flows

fiamme and scoria

spatter fed early system flow/rim spatter fed flows

5m spatter cone mature conduit (cf. Fig. 11) WE more evolved eruptive fissure system walls... backward- flows ... and associated eruptive fissure filling: welded scoria early system spatter cone (light gray) and spatter fed flows evolution of eruptive system of eruptive evolution scree (dark gray) track

Figure 10. Images of the early-formed structures of Lemptégy 2, with spatter cones and spatter-fed fl ows, to a more-evolved system, with eruptive fi ssure.

tégy 1, which converged toward the northeast, and evolution of Lemptégy 2’s shallow intrusive 6, and 7 could be traced across the quarry and toward the Puy de Gouttes (Fig. 15). At least one and extrusive systems. also why they are condensed to the NE and E, of these dikes (dike 4) reached the surface, pass- The stratigraphy of Lemptégy 2 was defi ned away from the main Lemptégy 2 center. ing through the coarse bomb sequence, to feed using the western part of the quarry, and cor- The eruption of Lemptégy 2 began with a proximal pahoehoe lava fl ows. The Lemptégy relation between layers across the quarry lava fl owing into a paleovalley to the south 1 eruptive activity stopped before the Puy des has allowed the eruptive phases to be recon- and east, at the base of the Puy de Gouttes and Gouttes volcano eruption ended, but intrusive structed as shown schematically in Figure 9. Lemptégy 1. A main central fi ssure (fi ssure I) activity continued, deforming erupted products, The sequence established for Lemptégy 2 is, developed, and activity continued with Strom- including those of the latter stages of the Puy de however, incomplete, and it is condensed in the bolian explosions from several vents that fed Gouttes eruption. The pahoehoe lavas from dike eastern part of the quarry due to the presence small lava fl ows. Activity subsequently concen- 4 were erupted after the bulging episode, as they of Lemptégy 1. While Lemptégy 2 was grow- trated into one central vent. As the cone grew, are not deformed. ing, the relief of Lemptégy 1 acted as a barrier intrusions extended outward, and cryptodomes A gap long enough to form a few centime- that impeded the eastward spread of the erupted formed in the lower fl anks. These breached the ters of poorly developed soil occurred before products. We cannot exclude the infl uence of cone and fed small lava fl ows. the renewal of eruptive activity. This new activ- the wind, which could have also played a role Layers of cowpat bombs, found mainly in ity was focused toward the west side of Lemp- in the dispersion of the fallout. The cone depos- units 1 and 6, attest to periods of probable small tégy 1. As indicated by the lack of Lemptégy 2 its, which are predominantly proximal ballistic lava lake activity and associated short lava foun- intrusions in Lemptégy 1, the latter acted as a clasts, had to surmount Lemptégy 1 in order to tain events. The two layers rich in pumiceous buttress that partially controlled the morphology spread north and east. This is why only units 5, lapilli (units 2 and 4) indicate an increase in

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N

SE NW

Lemptégy 2 early eruptive center

Lemptégy 2 dike

bombs inward dipping layers

inward dipping layers Lemptégy 2 lava flow Lemptégy 1 deposit

edges of the mature conduit

Lemptégy dike D1 tree and grass for scale

Figure 11. Mature Lemptégy 2 conduit system with an enlarged Vulcanian conduit and associated bombs. The large bombs are probably parts of the magma conduit cap, disrupted by explosions. Parts of these are found on the fl ank deposits as prismatic bombs, amongst a rounded scoria deposit.

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A B C W E SW NE thin dike inner shear zone pencil for scale

0.4 to 1.4m bulge/ cryptodome thick dike 2 to 5m

inflation in all directions > 5m hammer continuum for scale thin dikes thick dikes bulges and cryptodome D soil SW NE

deformed Lemptégy 2 fractures deposits

cryptodome

15m

feeder dike lava-flow anté-cryptodome

E SE NW soilssooilil carapace Dike feeding top lava fracture

lavas deformed Lemptégy 2 deposits cryptodome 10m

bulging thick dikes

Figure 12. Intrusion morphologies observed at Lemptégy 2. (A) Thin dike tapering to a point at the top of the image. The central part is vesiculated, while the margins are massive and contain sheared fabrics. (B) Thick dike cored by massive rock and covered by a breccia carapace. (C) Sketch showing the continuum from dike to bulges and cryptodomes (near-surface bulges). (D–E) Cryptodomes and associ- ated deformation (see Fig. 9 for location). Both of these intrusions sent dikes to the surface that fed short lava fl ows.

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Most of the magma fl ow directions inferred dike tips from the microstructures along the intrusion Riedel walls and within the dikes, i.e., tension gashes gauge zone shears and striations, indicate fl ow toward the main eruptive center. This is contrary to some other brecciated scoria tension studies that have suggested outward fl ow alone dike tip/branch gashes (e.g., Hintz and Valentine, 2012). At Lemptégy, vesiculated core the inward fl ow direction probably refl ects the primary fl ow rather than a late backward fl ow direction due to magma defl ection, because ductile-brittle structures showing inward fl ow, such as the shear zone shear zones, affect entire dikes, and some of these dikes formed early on. During the late mix of gray dike breccia stage of dike emplacement, the outer dike walls and crushed progressively cooled, and the deformation scoria induced by a decreasingly pressurized magma would probably not have deformed the solid shear zones walls, affecting instead only the inner parts. in breccia The fl ow toward the main eruptive center might indicate the presence of a deeper, wider vertical cut conduit at depth below Lemptégy that was feed- ing magma toward the central conduit and focus- horizontal cut ing the latter eruption phases. Similar feeding systems have been described, e.g., Keating et al. foliated (2008). The AMS data presented by Petronis shear zone et al. (2013) clearly suggest that this was the case. As Lemptégy volcano grew, its load Figure 13. Micro- and macrostructures that are commonly found along Lemptégy dikes. increased; at this stage, the volcano topogra- Those structures give indications of magma fl ow directions. Figure is adapted from Petronis phy and its own local stress fi eld would have et al. (2013). begun to dominate the internal volcano shallow plumbing, as suggested for monogenetic cones in general by Valentine and Gregg (2008) and explosivity. Because there is no sharp transi- with material, and a plug of magma formed. Valentine (2012). The crater and vent may also tion between pumiceous lapilli and surrounding This was the most violent episode, as attested by have been periodically choked with material, units, the changes in explosivity are inferred the fi nal upper layers of fi ner black ash, lapilli, and degassed magma may have accumulated in to have been progressive. We suggest that this and blocky bombs observed in the upper layers the deeper conduit. Consequently, this magma more intensive period corresponds to a phase of of Lemptégy 2. The breccias within the conduit had higher viscosity and low overpressure, and more lapilli- and ash-charged plumes, possibly and the uppermost layers of Lemptégy 2 sug- more work was required for it to propagate as a a short sustained venting, or violent Strombo- gest Vulcanian activity occurred during this last dike. Because of this, several dikes infl ated to lian event. This event may have been related to stage of Lemptégy growth. form thick dikes, with well-developed brittle- decompression linked to the progressive devel- During the cone growth, dikes were intruded ductile structures and intense shearing. Some opment of a landslide to the south, as suggested into the southern fl ank, and it was breached. A thick dikes received enough magma to approach for the Los Morados cone, Argentina (Néméth new vent was established within the scar, and a the surface, where they developed into crypto- et al., 2011a). small spatter cone and lava fl ow formed. This domes, which bulged the surface and occasion- The latter stages of Lemptégy 2 became was later covered by clasts avalanching from the ally erupted small lava fl ows. more explosive, as seen in unit 6, which has main vent, as the scar became partially infi lled. some very large bombs, and a dense angular The evolution of the cone’s shape changed the Implications for the Chaîne des Puys and fraction, which may indicate phreatomagmatic local stress fi eld, and the newly intruding dikes Relationship with Tectonics interaction. This is also the layer in which the were emplaced parallel to the collapse scar, i.e., largest granitic xenoliths are found, suggesting perpendicular to σ3, as half-cup dikes (Fig. The endogenous and exogenous growth of a link between these and the change in erup- 14B). As the eruptive activity continued, other Lemptégy volcano was far from being straight- tive style. The unresolved question is whether planar dikes were emplaced following a gen- forward and simple. Although it was a small the fragments were incorporated because of the erally NNE-SSW direction. The fi nal phase of system, barely 80 m high, this scoria cone increased explosivity, or if they caused it by activity was the development of a main central showed multiple stages of evolution. The pres- adding volatiles to the magma during melting conduit, which buried the previous vents (Fig. ence of a slightly weathered horizon between and ascension, as suggested for other volcanic 11). This conduit became progressively blocked the two Lemptégy volcanoes indicates that a systems (e.g., Hardiagon et al., 2011; Chadwick by recycled products and degassed magma. The pause in activity of at least a few years occurred. et al., 2013), or both. eruptive activity became sporadic, showing Vul- The lapilli unit of the Puy de Gouttes between The fi nal Lemptégy 2 stage was focused on canian behavior, with discrete debris-charged the Lemptégy 1 and 2 deposits also suggests the main conduit, which had become choked explosions. concurrent activity, possibly indicating a deep

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/5/998/3337196/998.pdf by guest on 27 September 2021 Delcamp et al. Lemptégy 1 Lemptégy eruptive fissure eruptive cone early spatter conduit and late flow lava bulges main eruptive centers ing at NE - SW planar dikes and half-cup like dikes thin dike thick dike and bulge cryptodome N bulges and crypto- domes planar dikes Lemptégy 1 Lemptégy final conduit half-cup like dikes N 50m N eruptive fissure I fissure eruptive view towards the E view towards concentration of bulges concentration early eruptive center early eruptive 10) (Fig. half-cup like dikes concentration of dikes concentration concentration of bulges concentration eruptive fissure II fissure eruptive Lemptégy 2. The half-cup–like dike results from the partial collapse of Lemptégy 2. Picture is about 50 m wide. the partial collapse of Lemptégy 2. Picture from The half-cup–like dike results Lemptégy 2. Figure 14. (A) Spatial and structural relationships between Lemptégy 1 and 2 systems. (B) Relationship collapse dik 14. (A) Spatial and structural relationships Figure view towards the NE view towards concentration of dikes concentration eruptive fissure II fissure eruptive B A

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Puy de la Coquille and de Jumes the general alignment of the Chaîne des Puys Puy Chopine (Fig. 1). Thus, both of the two main regional Puy des Gouttes trends appear in Lemptégy. There is a parallel- ism among the NE alignment of feeder dikes, Puy de Lemptégy the alignment of Lemptégy, the Puy de Gouttes NE and the volcanoes to the north, an area of base- ment shear zones to the northeast of the vol- canoes, and eventually the Aigueperse fault in the Limagne graben. This suggests a possible Pleistocene tectonic control on the activity in the Chaîne des Puys. Thus, it may be that the monogenetic nature of the eruptions is related deeper to the extensional tectonics and preexisting conduit of Lemptégy crustal structures, as suggested by van Wyk de Vries (1993) and van Wyk de Vries et al. (2007). The coincidence of extensional tectonics with a shallow, long-lived reservoir weak fracture zones would favor dispersed vol- canism. Such a link was observed at the 1999 Cerro Negro eruption, which was accompanied by local rifting (La Femina et al., 2004). Not to scale Implications for Monogenetic Volcanoes: Figure 15. Shallow and deeper intrusive system below Lemptégy–Puy des Gouttes–Puy de Plumbing Systems (Table 2) Chopine–Puy de la Coquille et Jumes alignment. Structures that are observed are in black, and the structures in gray are deduced from observations. The alignment of the volcanoes Studies of scoria cone plumbing systems is localized above a NE-SW–orientated long-lived shallow crustal reservoir that feeds sev- depend on the level to which the conduit is eral dikes at depth. At the surface, the dikes erupt and form volcanoes. Lemptégy quarry exposed. Most of the published papers concern exposes the upper tip of the dikes, which are infl uenced by both regional tectonics (NE-SW levels that are deeper than 10 m below the origi- orientation) and by volcano topography and volcano-induced stress fi eld. The dikes exposed nal surface. At this depth, the plumbing system at Lemptégy quarry are supposedly connected at depth through a deeper, larger conduit is seen to be reduced to a major conduit and a that connects to a hypothetical shallow crustal reservoir. few feeder dikes (e.g., Keating et al., 2008; Valen- tine, 2012; Hintz and Valentine, 2012). Lemptégy quarry exposes intrusive and eruptive products connection between the three volcanic centers, Such long-lived reservoirs feeding mono- from the fi nal volcano surface to a level around 30 similar to the Yucca Mountain range (Con- genetic eruptions have also been inferred in other m below the pre-edifi ce ground level. This repre- nor et al., 2000). This is also supported by the tectonic settings, such as in Central America and sents the missing link between intrusive plumb- similar morphologies of the other volcanoes Mexico. For example, Cerro Negro (Nicaragua) ing system at shallow depths and that in the cone further along the alignment to the northeast is also part of a group of cinder cone alignments itself. Similarly, it explains the morphological (Puy de Jumes, Puy de Coquille; Fig. 1). The (van Wyk de Vries, 1993; McKnight and Wil- differences between the larger and thicker dikes alignment of the Lemptégy-Gouttes-Chopine- liams, 1997; van Wyk de Vries et al., 2007). of exposed shallow plumbing systems observed Jumes-Coquille edifi ces might refl ect one over- Such cone fi elds host magmatic systems that are at greater depth below the surface. The dikes at all volcanic system with a common crustal mag- long-lived, active over thousands of years, and Lemptégy have a much greater range of thick- matic reservoir feeding multiple monogenetic evolve from basic to acid, similar to the Chaîne nesses and are more irregular than previously eruptions over a long time period (Fig. 15). The des Puys. Both in Central America and Mexico, described feeder dikes (Keating et al., 2008; Val- extended age range from Lemptégy 1 to the there is active extensional or transtensional entine, 2012; Hintz and Valentine, 2012). 8000-yr-old Chopine eruption could mean that faulting, and often a close relationship exists Depending on the active stress fi eld, the dikes the magmatic reservoir underlying this volcanic between the volcanoes and the structures (Con- in the upper crust usually follow existing faults alignment was active over at least 20,000 yr. nor et al., 2000; Valentine and Krogh, 2006; Val- (Connor and Conway, 2000; Valentine and Confi rmation of this hypothesis would require entine and Perry, 2007; de la Cruz Reyna and Krogh, 2006) until they reach the near surface or detailed petrological, geochemical, and isotopic Yokoyama, 2011). the developing cone. At this shallow level, dike studies. The longevity of the magmatic system A link between edifi ce alignment (with a propagation depends on the local stress fi eld has important hazard implications, as it extends main N-S trend and secondary NE-SW trend) induced by the topography (Hintz and Valen- the possible lifetime of each volcanic alignment and the local structure has been also highlighted tine, 2012). The intrusive system of Lemptégy 2 in the Chaîne des Puys. Many of these have had in Lemptégy and elsewhere in the Chaîne des shows both types of dikes (Fig. 15): It shows eruptions within the last 20,000 yr and could Puys (van Wyk de Vries et al., 2012). The dike the transition from dikes controlled by regional thus be considered as potentially active mag- orientations and the related fl ow trend at Lemp- and crustal factors to those relating to the edifi ce matic reservoirs. Such a conclusion is in agree- tégy may refl ect a local structural heritage, with structure and lithology. The feeder dikes show ment with recent work on the trachytic rocks of NE-trending Hercynian dikes and shear zones regional trends that guided the early orientation the Chaîne des Puys, which suggests long-lived (van Wyk de Vries et al., 2012) creating weak of eruptive fi ssures, and as they propagated up intrusive reservoirs (Martel et al., 2013). zones. In addition, the N-trending dikes refl ect into the growing cone, they caused structures

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TABLE 2. COMPARATIVE TABLE OF MONOGENETIC VOLCANO SHALLOW PLUMBING SYSTEMS IN EXCAVATED CONES OR ERODED Name Depth of s.i.s Orientation of dykes Flow pattern References Lunar Crater, USA Lower half of original scoria Radial; topography infl uenced Away from central conduit Hintz and Valentine (2012) cone Paiute Ridge, USA Upper crust below scoria cone Crustal structures; pre-existing NA Valentine and Krogh (2006) structure infl uenced Several volcanic fi elds, Upper crust below scoria cones Various; pre-existing structure NA Keating et al. (2008) USA (≤250m depth) infl uenced Lemptégy, Chaîne des Surface to about 30m depth Radial and crustal structures; topo and Toward central conduit This study; Petronis et al. Puys, France pre-existing structure infl uenced (2013) Note: s.i.s—Shallow intrusive system.

like small fl ank collapses and secondary vents. Parícutin (Erlund et al., 2010), and Schmincke absence of well-defi ned wall versus crater facies Other dikes were intruded to the west side of (2004) also described black, well-sorted lapilli as might be due to the simultaneous activity of sev- Lemptégy 2 from the conduit and formed a fan- a fi nal deposit of many scoria cones in the Eiffel eral spatter cones and cryptodomes. The fi nal shaped pattern of radial intrusions. As Lemp- volcanic fi eld, but related them to sub plinian Vulcanian conduit is quite wide (50 m) and has tégy 2 intrusions are not found on the Lemptégy activity. Lemptégy 2 exhibits a similar fi nal a well-defi ned funnel shape, which is infi lled 1 (east side of Lemptégy 2 conduit), it is likely phase to Parícutin, and conduit blocking with with blocks and bombs. Possibly this conduit that they were controlled by buttressing and the consequent Vulcanian activity may be a common widened enough to destroy the previous crater edifi ce stress regime of Lemptégy 1. feature of the fi nal stage of such eruptions. margin structure. The absence of outer and inner Lemptégy 1 dikes and structural orientations Violent Strombolian eruptions with sustained crater distinctions at Lemptégy 1 may be due roughly follow the regional trends; thus, the columns have been previously described from to the strong subsequent deformation that has intrusions that formed the uplift seen in Figure 5 the deposits of monogenetic scoria cones such obscured these features. were also regionally controlled and follow the as Lathrop Well (USA; Valentine et al., 2007), Although monogenetic cones share similar general trend of the Puy de Gouttes–Jumes and Irao (Japan; Kiyosugi et al., 2014), and Parícutin eruptive styles and activity, it is not yet possible Coquille ridge. (Luhr and Simkin, 1993). Similar activity is to develop a general model that would be valid Such breakouts at the bases of cones are com- suggested by units 2 and 4 at Lemptégy volcano for all the edifi ces. Indeed, while some started mon and have been observed at Cerro Negro (Table 3). with phreatomagmatic phases, other started with (1967 and 1999, for example: Hill et al., 1998; The growth of Lemptégy is complicated, with Strombolian/Hawaiian or effusive activity. La Femina et al., 2004), and at Paricutin, where numerous intrusive and extrusive phases, activ- Activities may be similar, but chronological the Salipichu vents are probably an equivalent to ity in many ephemeral vents, and shifts in erup- order differs, and it appears that each monoge- the Lemptégy 2 feeder dike vents. The Quitzocho tive style. Such diversity has been also observed netic volcano has its own development phases, ridge at Parícutin (Luhr and Simkin, 1993) could during historical eruptions and interpreted from and to generalize a simple generic model would be an equivalent to the Lemptégy 1 uplift. deposits in excavated or eroded cones such as be unrealistic. We propose that the local envi- Parícutin, Tolbachik, and Lathrop Wells (Valen- ronment (tectonic context, presence of crustal Implications for Monogenetic Volcanoes: tine et al., 2007; Erlund et al., 2010; Gordeev weak zones) plays an important initial role and Eruptive Dynamics et al., 2013). Another example is the shallow- that the volcano itself plays a major subsequent level emplacement of cryptodomes that has role in its own evolution: The eruptive dynam- Key comparison points between Lemptégy been observed within the crater in Cerro Negro ics and the intrusion and deformation phases and historical eruptions or other excavated in 1995 (e.g., in Petronis et al., 2013). In Lemp- interact with each other as the volcano grows cones are summarized in Table 3. Next, we dis- tégy, such shallow bulges have been observed in (Fig. 9). For example, the growth of an intru- cuss a few points, starting with the initial and the western fl ank as well as within the fi nal con- sion will change the local stress fi eld, which fi nal activity. duit. Such diversity of concurrent intrusive and will condition the geometry and style of the In many documented cases, the start of a extrusive activity in a basaltic volcano with a next intrusive and eruptive phases. Deforma- monogenetic eruption begins with a phreato- restricted range of composition can be attributed tion induced by intrusions can lead to small magmatic phase, followed by Strombolian activ- to the crystallization and degassing processes destabilization events that can infl uence or even ity (Table 3; e.g., Vespermann and Schmincke, that change the magma viscosity as suggested disrupt the shallow plumbing system. Similarly, 2000; Schmincke, 2004; Rapprich et al., 2007; for Lathrop Wells (USA; Valentine et al., 2007; the morphology of the cone itself plays a role Kiyosugi et al., 2014). Lemptégy 1 does con- Genareau et al., 2010). For both Lemptégy 1 and in the volcano’s evolution. Such complexi- tain abundant partially melted crustal xenoliths, 2, we suggest that fresh gas-rich magma was fed ties are refl ected in the various morphologies which have been associated with the early phre- in via feeder dikes, while magma was degassed of scoria cones and underline the fact that the atomagmatic stages of some Chaîne des Puys in the conduit and then intruded outward from variability in eruptive style strongly infl uences eruptions (Hardiagon et al., 2011). However, the center. When feeder dikes breached the sur- the fi nal edifi ce shape and subsequent erosion the deposits show no evidence of phreatomag- face, fl uid magma could then escape directly, as (Martin and Németh, 2006; Németh et al., matic quenching or fragmentation. This would in the Lemptégy 2 lava-fl ow–triggered breach- 2011b; Kereszturi et al., 2012; Kervyn et al., imply either that the quarry did not cut deeply ing, or the Lemptégy 1 pahoehoe fl ows. 2012; Kereszturi and Németh, 2012). The modi- enough into the volcano so the phreatomagma- In many scoria cones, the crater and fl ank fi cation of stress following the Lemptégy fl ank tism phase is not revealed or Lemptégy 1 did not facies are separated from each other by semi- collapse infl uenced the endogenous growth, but have such initial activity. circular faults or funnel-shaped surfaces on other scoria cones, such events have been For the fi nal stage of eruptions, a more explo- (Schmincke, 2004). This distinction is not so associated with a change in eruptive dynamics sive state has been suggested, for example, at clear in Lemptégy 1 and 2. In Lemptégy 2, the (Németh et al., 2011a).

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CONCLUSIONS reap great benefi ts and can raise the value of the H., and Stix, J., eds., Encyclopedia of Volcanoes: San Diego, California, Academic Press, p. 663–682. fi nal excavation. As quarrying is ongoing in most de Dolomieu, D., 1794, Distribution méthodique de toutes The Lemptégy volcano has been excavated monogenetic volcano fi elds worldwide, Lemp- les matières dont l’accumulation forme les Montagnes down to reveal the shallow feeder system of two tégy should serve as an example of both the ben- volcaniques ou Tableau systématique dans lequel peu- vent se placer toutes les substances qui ont des rela- scoria cones, Lemptégy 1 and 2. This provides a efi ts to be gained and of the way to achieve them. tions avec les feux souterrains: Journal de Physique, de remarkable opportunity to observe the growth of Chimie et d’Histoire Naturelle, Pluviose, p. 102–125. ACKNOWLEDGMENTS two monogenetic cones from their surface down de Dolomieu, D., 1798, Sur les volcans d’Auvergne et sur la volcanisation en général: Journal des Mines, v. 7, to their roots. The detailed analysis of the shallow We would like to warmly thank the Lemptégy staff p. 393–420. plumbing system and associated volcanic depos- for discussions and for giving us open access anytime Deegan, F.M., Troll, V.R., Freda, C., Misiti, V., Chadwick, to the quarry. We are also thankful to (many) volcalca- J.P., McLeod, C.L., and Davidson, J.P., 2010, Magma- its of Lemptégy leads to several key conclusions. carbonate interaction processes and associated CO nologists who came to discuss on the fi eld: J. Sumner, 2 (1) Lemptégy, and many recent works, shows release at Merapi volcano, Indonesia: Insights from T. Walter, O. Melnick, C. Seibe, S. Cronin, G. Val- that the simple morphology of a monogenetic experimental petrology: Journal of Petrology, v. 51, entine, S. Self, W.I. Rose, T. Thordarson, P. Grosse, no. 5, p. 1027–1051, doi: 10 .1093 /petrology /egq010 . scoria cone can hide signifi cant internal com- D. Rothery, J. Calvero, S. Carn, M. Petronis, J. Lind- De Goër de Hervé, A., Camus, G., Lavina, P., and Montel , plexities, resulting from many interactions that line, Teddy Wolf, N. Riggs, M. Ort, A. Marquez, J.L., 1999, Lemptégy: Volcan à Ciel Ouvert pour take place during the edifi ce growth. Certainly, O. Galland, G. Ernst, M. Giardino, B. Ward, E. Calder, Comprendre la Chaîne des Puys: Clermont-Ferrand, V. Troll, P. Byrne, E. Holohan, R. Herrera, and S.A.R.L. volcan de Lemptégy, 56 p. the early descriptions of Lemptégy as “insignifi - A. Foulks. Special thanks go to C. Olive Garcia and de la Cruz Reyna, S., and Yokoyama, I., 2011, A geophysical cant and simple” (e.g., Scrope, 1858) show that her team from the Conseil Géneral du Puy De Dôme. characterization of monogenetic volcanism: Geofi sica We also would like to thank Fran van Wyk de Vries for Internacional, v. 510, p. 465–484. the surface morphology gave little clue as to the Delcamp, A., van Wyk de Vries, B., and Troll, V.R., 2007, variety of events hidden below. effi cient proofreading of the manuscript. Finally, we Endogenous and exogenous evolution of Lemptegy (2) The shallow plumbing system at Lemp- would like to thank an anonymous reviewer, G. Valen- cinder cone, Chaîne des Puys, France: Geophysical tine, and Editors F. Mazzarini and S. de Silva for the Research Abstracts, v. 9, abstract 04948. tégy does not comprise a single, simple con- positive and constructive reviews provided. 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1018 Geosphere, October 2014

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Geosphere, October 2014 1019

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