Laboratoire Magmas et Volcans

The Pejo fault in the Italian Eastern Alps is a major sinistral transtensional fault. It marks the boundary between basement units displaying contrasting thermal histories, with Alpine (i.e., Mesozoic–Cenozoic) cooling ages preserved in the footwall juxtaposed against Variscan (i.e., Carboniferous– Permian) age in the hanging wall. Structural investigations, together with fission-track analysis, confirm a Late Cretaceous age for the Pejo fault, which excludes any direct kinematic contribution of the Pejo fault to the late Oligocene–Neogene evolution of the central-eastern segment of the Periadriatic fault. However, our results establish the importance of a major early Oligocene north-south to north-northwest–south-southeast shortening phase in the Central-Eastern Alps, which resulted in the development of new reverse shear zones, in the reactivation of the Pejo fault with a reverse motion, and in regionally important folding. The Pejo mylonites are folded on a kilometer scale around an east-northeast–trending axis. Field observations and fission-track analysis suggest a post-Oligocene age for the folding phase. Apatite fission-track data in the Pejo valley area reveal the base of a fossil apatite partial annealing zone exhumed to the surface. This finding argues for >4 km of exhumation since the Miocene, which was related to a major pulse of exhumation that began at ca. 15 Ma. This study suggests that the simple distinction between largely pre-Alpine fabrics of Variscan age in the hanging wall of the Pejo fault (Tonale nappe) and Alpine fabrics (Cretaceous) in the footwall (Campo-Ortler nappe) is not universally valid. Alpine overprinting is confined to the mylonitic shear zone itself. Deeper into the footwall, pre-Alpine structures are still well preserved. Earlier maps and interpretations based on a clear distinction between Tonale and Campo should be viewed with caution.

New structural, petrographic, and 40 Ar/ 39 Ar data constrain the kinematics of the ASRR (Ailao Shan‐Red River shear zone). In the XueLong Shan (XLS), geochronological data reveal Triassic, Early Tertiary, and Oligo‐Miocene thermal events. The latter event (33–26 Ma) corresponds to cooling during left‐lateral shear. In the FanSiPan (FSP) range, thrusting of the SaPa nappe, linked to left‐lateral deformation, and cooling of the FSP granite occurred at ≈35 Ma. Rapid cooling resumed at 25–29 Ma as a result of uplift within the transtensive ASRR. In the DayNuiConVoi (DNCV), foliation trends NW‐SE, but is deflected near large‐scale shear planes. Stretching lineation is nearly horizontal. On steep foliations, shear criteria indicate left‐lateral shear sense. Zones with flatter foliations show compatible shear senses. Petrographic data indicate decompression from ≈6.5 kbar during left‐lateral shear (temperatures >700°C). 40 Ar/ 39 Ar data imply rapid cooling from above 350°C to below 150°C between 25 and 22 Ma without diachronism along strike. Along the whole ASRR cooling histories show two main episodes: (1) rapid cooling from peak metamorphism during left‐lateral shear; (2) rapid cooling from greenschist conditions during right‐lateral reactivation of the ASRR. In the NW part of the ASRR (XLS, Diancang Shan), we link rapid cooling 1 to local denudations in a transpressive environment. In the SW part (Ailao Shan and DNCV), cooling 1 resulted from regional denudation by zipper‐like tectonics in a transtensive regime. The induced cooling diachronism observed in the Ailao Shan suggests left‐lateral rates of 4 to 5 cm/yr from 27 Ma until ≈17 Ma. DNCV rocks always stayed in a transtensive regime and do not show cooling diachronism. The similarities of deformation kinematics along the ASRR and in the South China Sea confirms the causal link between continental strike‐slip faulting and marginal basin opening.

Research Article| April 01, 2002 Secular changes in tonalite-trondhjemite-granodiorite composition as markers of the progressive cooling of Earth Hervé Martin; Hervé Martin 1Laboratoire Magmas et Volcans, Observatoire de Physique du Globe de Clermont-Ferrand, Université Blaise Pascal, Centre National de la Recherche Scientifique 5, rue Kessler 63038, Clermont-Ferrand cedex, France Search for other works by this author on: GSW Google Scholar Jean-François Moyen Jean-François Moyen 1Laboratoire Magmas et Volcans, Observatoire de Physique du Globe de Clermont-Ferrand, Université Blaise Pascal, Centre National de la Recherche Scientifique 5, rue Kessler 63038, Clermont-Ferrand cedex, France Search for other works by this author on: GSW Google Scholar Author and Article Information Hervé Martin 1Laboratoire Magmas et Volcans, Observatoire de Physique du Globe de Clermont-Ferrand, Université Blaise Pascal, Centre National de la Recherche Scientifique 5, rue Kessler 63038, Clermont-Ferrand cedex, France Jean-François Moyen 1Laboratoire Magmas et Volcans, Observatoire de Physique du Globe de Clermont-Ferrand, Université Blaise Pascal, Centre National de la Recherche Scientifique 5, rue Kessler 63038, Clermont-Ferrand cedex, France Publisher: Geological Society of America Received: 13 Sep 2001 Revision Received: 23 Nov 2001 Accepted: 27 Nov 2001 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2002) 30 (4): 319–322. https://doi.org/10.1130/0091-7613(2002)030<0319:SCITTG>2.0.CO;2 Article history Received: 13 Sep 2001 Revision Received: 23 Nov 2001 Accepted: 27 Nov 2001 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Hervé Martin, Jean-François Moyen; Secular changes in tonalite-trondhjemite-granodiorite composition as markers of the progressive cooling of Earth. Geology 2002;; 30 (4): 319–322. doi: https://doi.org/10.1130/0091-7613(2002)030<0319:SCITTG>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Archean tonalite-trondhjemite-granodiorite associations (TTG) are classically thought to generate through partial melting of hydrous metabasalts. However, the chemical composition of the least differentiated TTG parental magmas evolved from 4.0 to 2.5 Ga. During this interval, the Mg# as well as the Ni and Cr contents increased, which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite. Similarly, (CaO + Na2O) and Sr also increased over time, thus reflecting an increase in the abundance of plagioclase in the melt residue. The presence or absence of residual plagioclase is interpreted in terms of melting depth. The demonstrated interaction between TTG parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material. At 4.0 Ga, Earth's geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable. Consequently, due to the small thickness of the overlying mantle wedge, felsic magmas interacted little with the mantle. At 2.5 Ga, however, owing to lower geothermal gradients, the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab. As a result, felsic magmas reacted strongly with the mantle peridotite. The changes of TTG composition during Archean time can be thus interpreted as reflecting the progressive cooling of Earth. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Research Article| January 01, 2013 Deep Carbon Emissions from Volcanoes Michael R. Burton; Michael R. Burton Istituto Nazionale di Geofisica e Vulcanologia, Via della Faggiola, 32, 56123 Pisa, Italy, burton@pi.ingv.it Search for other works by this author on: GSW Google Scholar Georgina M. Sawyer; Georgina M. Sawyer Laboratoire Magmas et Volcans, Université Blaise Pascal, 5 rue Kessler, 63038 Clermont Ferrand, France and Istituto Nazionale di Geofisica e Vulcanologia, Via della Faggiola, 32, 56123 Pisa, Italy Search for other works by this author on: GSW Google Scholar Domenico Granieri Domenico Granieri Istituto Nazionale di Geofisica e Vulcanologia, Via della Faggiola, 32, 56123 Pisa, Italy Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2013) 75 (1): 323–354. https://doi.org/10.2138/rmg.2013.75.11 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Michael R. Burton, Georgina M. Sawyer, Domenico Granieri; Deep Carbon Emissions from Volcanoes. Reviews in Mineralogy and Geochemistry 2013;; 75 (1): 323–354. doi: https://doi.org/10.2138/rmg.2013.75.11 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Over long periods of time (~Ma), we may consider the oceans, atmosphere and biosphere as a single exospheric reservoir for CO2. The geological carbon cycle describes the inputs to this exosphere from mantle degassing, metamorphism of subducted carbonates and outputs from weathering of aluminosilicate rocks (Walker et al. 1981). A feedback mechanism relates the weathering rate with the amount of CO2 in the atmosphere via the greenhouse effect (e.g., Wang et al. 1976). An increase in atmospheric CO2 concentrations induces higher temperatures, leading to higher rates of weathering, which draw down atmospheric CO... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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Subduction drives plate tectonics and builds continental crust, and as such is one of the most important processes for shaping the present-day Earth. Here we review both theory and observations for the viability and style of Archean subduction. High Archean mantle temperature gave low mantle viscosity and affected plate strength and plate buoyancy. This resulted in slower or intermittent subduction, either of which resulted in Earth cooling profiles that fit available data. Some geological observations are interpreted as subduction related, including an “arc” signature in various igneous rocks (suggesting burial of surface material to depths of 50–100 km), structural thrust belts and dipping seismic reflectors, and high-pressure–low-temperature and low-pressure–high-temperature paired metamorphic belts. Combined geodynamical and geochemical evidence suggests that subduction operated in the Archean, although not, as often assumed, as shallow flat subduction. Instead, subduction was more episodic in nature, with more intermittent plate motion than in the Phanerozoic.

Scaled experiments have been carried out on caldera collapse mechanisms, using silicone as analogue magma and dry sand as analogue rock. Experiments were carried out in two and three dimensions using a range of roof aspect ratios (thickness/width 0.2 to 4.5) appropriate for caldera collapse. They reveal a general mechanism of collapse, only weakly dependent on the shape of the reservoir. For low roof aspect ratios (≤1), subsidence starts by flexure of the roof and the formation of outward dipping, reverse ring faults, which in turn trigger formation of peripheral inward dipping, normal ring faults. The subsidence always occurs asymmetrically. In cross section the reverse faults delimit a coherent piston, bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets. The surface depression consists of a nondeformed area (piston) surrounded by an annular extensional zone (tilted strata). For high aspect ratios (&gt;1), multiple reverse faults break up the roof into large pieces, and subsidence occurred as a series of nested wedges (2‐D) or cones (3‐D). The extensional zone dominates the surface depression. In the case where preexisting regional faults do not play a major role, the collapse mechanics of calderas probably depends strongly on the roof aspect ratio. Calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults. The annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias. Collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant.

Large volcanic eruptions on Earth commonly occur with a collapse of the roof of a crustal magma reservoir, forming a caldera. Only a few such collapses occur per century, and the lack of detailed observations has obscured insight into the mechanical interplay between collapse and eruption. We use multiparameter geophysical and geochemical data to show that the 110-square-kilometer and 65-meter-deep collapse of Bárdarbunga caldera in 2014-2015 was initiated through withdrawal of magma, and lateral migration through a 48-kilometers-long dike, from a 12-kilometers deep reservoir. Interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path explain the gradual, near-exponential decline of both collapse rate and the intensity of the 180-day-long eruption.

We describe laboratory experiments of granular material flowing over an inclined plane covered by an erodible bed, designed to mimic erosion processes of natural flows travelling over deposits built up by earlier events. Two controlling parameters are the inclination of the plane and the thickness of the erodible layer. We show that erosion processes can increase the flow mobility (i.e., runout) over slopes with inclination close to the repose angle of the grains θ r by up to 40%, even for very thin erodible beds. Erosion efficiency is shown to strongly depend on the slope of the topography. Entrainment begins to affect the flow at inclination angles exceeding a critical angle θ c ≃ θ r /2. Runout distance increases almost linearly as a function of the thickness of the erodible bed, suggesting that erosion is mainly supply‐dependent. Two regimes are observed during granular collapse: a first spreading phase with high velocity followed by a slow thin flow, provided either the slope or the thickness of the erodible bed is high enough. Surprisingly, erosion affects the flow mostly during the deceleration phase and the slow regime. The avalanche excavates the erodible layer immediately at the flow front. Waves are observed behind the front that help to remove grains from the erodible bed. Steep frontal surges are seen at high inclination angles over both rigid or erodible bed. Finally, simple scaling laws are proposed making it possible to obtain a first estimate of the deposit and emplacement time of a granular collapse over a rigid or erodible inclined bed.

Abstract Applying probabilistic methods to infrequent but devastating natural events is intrinsically challenging. For tsunami analyses, a suite of geophysical assessments should be in principle evaluated because of the different causes generating tsunamis (earthquakes, landslides, volcanic activity, meteorological events, and asteroid impacts) with varying mean recurrence rates. Probabilistic Tsunami Hazard Analyses (PTHAs) are conducted in different areas of the world at global, regional, and local scales with the aim of understanding tsunami hazard to inform tsunami risk reduction activities. PTHAs enhance knowledge of the potential tsunamigenic threat by estimating the probability of exceeding specific levels of tsunami intensity metrics (e.g., run‐up or maximum inundation heights) within a certain period of time (exposure time) at given locations (target sites); these estimates can be summarized in hazard maps or hazard curves. This discussion presents a broad overview of PTHA, including (i) sources and mechanisms of tsunami generation, emphasizing the variety and complexity of the tsunami sources and their generation mechanisms, (ii) developments in modeling the propagation and impact of tsunami waves, and (iii) statistical procedures for tsunami hazard estimates that include the associated epistemic and aleatoric uncertainties. Key elements in understanding the potential tsunami hazard are discussed, in light of the rapid development of PTHA methods during the last decade and the globally distributed applications, including the importance of considering multiple sources, their relative intensities, probabilities of occurrence, and uncertainties in an integrated and consistent probabilistic framework.

Because of a relatively low atomic packing density, (Cg) glasses experience significant densification under high hydrostatic pressure. Poisson's ratio (nu) is correlated to Cg and typically varies from 0.15 for glasses with low Cg such as amorphous silica to 0.38 for close-packed atomic networks such as in bulk metallic glasses. Pressure experiments were conducted up to 25 GPa at 293 K on silica, soda-lime-silica, chalcogenide, and bulk metallic glasses. We show from these high-pressure data that there is a direct correlation between nu and the maximum post-decompression density change.

Four samples from the metamorphic aureole around the Beni Bousera ultramafic massif were studied in detail for U–Th–Pb electron microprobe dating on monazite. The samples include three meta‐sedimentary granulites (kinzigites), collected at variable distance from the peridotites, and one kyanite‐bearing leucosome in the kinzigite. Two types of monazite were identified in thin section, using SEM. The main population consists of interstitial grains, 20–70 μ m in size, while the second population consists of small grains (&lt;20 μ m), included in garnet. A total of 64 U–Th–Pb electron microprobe measurements on 53 monazite crystals were undertaken. Most crystals have a Pb content lower than the Pb detection limit, indicating that they crystallized, or were reset, during a young event, probably Cainozoic in age. Few crystals, all entirely included in garnet, have Hercynian age, the best estimate of which is 284±27 Ma. This is a direct demonstration of the shielding effect of garnet for the U–Th–Pb system in monazite. The grains in inclusion in garnet are not reset by the post‐Hercynian events, despite the high temperature reached at this time (&gt;850 °C). Thus, the monazite closure temperature depends on its textural position in the host rock. The data also show that a Hercynian event occurred in the Beni Bousera granulitic metapelites, which equates with a high‐ P , high‐ T event. The emplacement of the peridotite in the Cainozoic may be linked a low‐ P , high‐ T event, followed by a low‐ P , low‐ T retrogression. These two events reset the U–Th–Pb system in almost all monazite grains, except for the few crystals shielded by garnet.

Abstract In 1997 Soufriére Hills Volcano on Montserrat produced 88 Vulcanian explosions: 13 between 4 and 12 August and 75 between 22 September and 21 October. Each episode was preceded by a large dome collapse that decompressed the conduit and led to the conditions for explosive fragmentation. The explosions, which occurred at intervals of 2.5 to 63 hours, with a mean of 10 hours, were transient events, with an initial high-intensity phase lasting a few tens of seconds and a lower-intensity, waning phase lasting 1 to 3 hours. In all but one explosion, fountain collapse during the first 10-20 seconds generated pyroclastic surges that swept out to 1-2 km before lofting, as well as high-concentration pumiceous pyroclastic flows that travelled up to 6 km down all major drainages around the dome. Buoyant plumes ascended 3-15 km into the atmosphere, where they spread out as umbrella clouds. Most umbrella clouds were blown to the north or NW by high-level (8-18 km) winds, whereas the lower, waning plumes were dispersed to the west or NW by low-level (&lt;5 km) winds. Exit velocities measured from videos ranged from 40 to 140 ms- 1 and ballistic blocks were thrown as far as 1.7 km from the dome. Each explosion discharged on average 3 x 10 5 m 3 of magma, about one-third forming fallout and two-thirds forming pyroclastic flows and surges, and emptied the conduit to a depth of 0.5-2 km or more. Two overlapping components were distinguished in the explosion seismic signals: a low-frequency (c. 1 Hz) one due to the explosion itself, and a high-frequency (&gt;2 Hz) one due to fountain collapse, ballistic impact and pyroclastic flow. In many explosions a delay between the explosion onset and start of the pyroclastic flow signal (typically 10-20 seconds) recorded the time necessary for ballistics and the collapsing fountain to hit the ground. The explosions in August were accompanied by cyclic patterns of seismicity and edifice deformation due to repeated pressurization of the upper conduit. The angular, tabular forms of many fallout pumices show that they preserve vesicularities and shapes acquired upon fragmentation, and suggest that the explosions were driven by brittle fragmentation of overpressured magmatic foam with at least 55 vol% bubbles present in the upper conduit prior to each event.

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The 7.5 ka Socompa sector collapse emplaced 25 km 3 of fragmented rock as a thin, but widespread (500 km 2 ), avalanche deposit, followed by late stage sliding of 11 km 3 as Toreva blocks. Most of the avalanche mass was emplaced dry, although saturation of a basal shear layer cannot be excluded. Modeling was carried out using the depth‐averaged granular flow equations in order to provide information on the flow behavior of this well‐preserved, long run‐out avalanche. Results were constrained using structures preserved on the surface of the deposit, as well as by deposit outline and run‐up (a proxy for velocity). Models assuming constant dynamic friction fail to produce realistic results because the low basal friction angles (1 to 3.5°) necessary to generate observed run‐out permit neither adequate deposition on slopes nor preservation of significant morphology on the deposit surface. A reasonable fit is obtained, however, if the avalanche is assumed simply to experience a constant retarding stress of 50–100 kPa during flow. This permits long run‐out as well as deposition on slopes and preservation of realistic depositional morphology. In particular the model explains a prominent topographic escarpment on the deposit surface as the frozen front of a huge wave of debris reflected off surrounding hills. The result that Socompa avalanche experienced a small, approximately constant retarding stress during emplacement is consistent with a previously published analysis of avalanche data.

ABSTRACT A specific type of granitoid, referred to as sanukitoid (Shirey &amp; Hanson 1984), was emplaced mainly across the Archaean–Proterozoic transition. The major and trace element composition of sanukitoids is intermediate between typical Archaean TTG and modern arc granitoids. However, among sanukitoids, two groups can be distinguished on the basis of the Ti content of the less differentiated rocks of the suite: high- and low-Ti sanukitoids. Melting experiments and petrogenetic modelling show that they may have formed by either (1) melting of mantle peridotite previously metasomatised by felsic melts of TTG composition, or (2) by reaction between TTG melts and mantle peridotite (assimilation). Rocks of the sanukitoid suite were emplaced at the Archaean–Proterozoic boundary, possibly marking the time when TTG-dominated granitoid magmatism changed to a more modern-style, arc-dominated magmatism. Consequently, the intermediate character of sanukitoids is not only compositional but chronological. The succession of granitoid magmatism with time is integrated in a plate tectonic model where it is linked to the thermal evolution of subduction zones, reflecting the progressive cooling of Earth: (1) the Archaean Earth’s heat production was high enough to allow the production of large amounts of TTG granitoids formed by partial melting of recycled basaltic crust (‘slab melting’); (2) at the end of the Archaean, due to the progressive cooling of the Earth, the extent of slab melting was reduced, resulting in lower melt:rock ratios. In such conditions the slab melts can be strongly contaminated by assimilation of mantle peridotite, thus giving rise to low-Ti sanukitoids. It is also possible that the slab melts were totally consumed in reactions with mantle peridotite, subsequent melting of this ‘melt-metasomatised mantle’ producing the high-Ti sanukitoid magmas; (3) after 2·5 Ga, Earth heat production was too low to allow slab melting, except in relatively rare geodynamic circumstances, and most modern arc magmas are produced by melting of the mantle wedge peridotite metasomatised by fluids from dehydration of the subducted slab. Of course, such changes did not take place exactly at the same time all over the world. The Archaean mechanisms coexisted with new processes over a relatively long time period, even if they were subordinate to the more modern processes.

A straightforward separation scheme is described for the separation of Sr, Pb, and Nd from silicate rocks.

This paper reports isotopic, major and minor element geochemistry of igneous and metamorphic rocks from the Kokoxili and Yushu regions of central and eastern Tibet. The first region lies along the Kunlun suture, which separates the Bayan Har‐Songpan Ganze (Songpan) terrane from the Tarim and Qaidam blocks. Two Kokoxili granitoids yield U‐Pb zircon dates of 217 ± 10 and 207 ± 3 Ma (Late Triassic), which represent the time of emplacement, and Rb‐Sr isochron dates of 195 ± 3 and 190 ± 3 Ma (Early Jurassic), which are interpreted as cooling ages. The geochemical signatures of these granitoids suggest that they are related to subduction continuing into the Late Triassic. In the Yushu area, three samples help constrain the age of the Jinsha suture, which separates the Songpan terranes from the Qiangtang blocks. A leucocratic granite and an orthogneiss in the suture zone yield U‐Pb zircon dates of 206 ± 7 and 204 ± 1 Ma, respectively, and a paragneiss south of it, a U‐Pb monazite date of 244 ± 4 Ma. The existence of coeval magmatism in both the Jinsha and Kunlun sutures suggests that the two subduction zones were simultaneously active. Combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions, we propose that the Kunlun and Qinling block boundaries, which were distinct in the Permian, subsequently formed a continuous, Late Triassic, northward subducting plate margin. Our data suggest that the Jinsha suture correlates with the Benzilan and Nan‐Uttaradit sutures, which together belong to a major Late Triassic subduction zone.

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