Géodynamique andine : résumés étendus = Andean geodynamics : extended abstracts = Geodinamica andina : resumenes expandidos

L'Holocène inférieur (900-4500 BP) du Cotopaxi est caractérisé par une activité fortement explosive dont les produits sont des ponces et des cendres rhyolitiques. Quatre retombées pliniennes accompagnées d'écoulements pyroclastiques et d'importants dépôts de cendres ont eu lieu vers 6000 BP, marquant la phase paroxysmale de cette activité. Une étude détaillée des dépôts permet de caractériser les divers événements. (Résumé d'auteur

Contrasting origin of two clay-rich debris flows at Cayambe Volcanic Complex, Ecuador

We investigate the sedimentological and mineralogical properties of a debris flow deposit west of Cayambe Volcanic Complex, an ice-clad edifice in Ecuador. The deposit exhibits a matrix facies containing up to 16 wt% of clays. However, the stratigraphic relationship of the deposit with respect to the Canguahua Formation, a widespread indurated volcaniclastic material in the Ecuadorian inter-Andean Valley, and the deposit alteration mineralogy differ depending on location. Thus, two different deposits are identified. The Río Granobles debris flow deposit (~1 km3) is characterised by the alteration mineral assemblage smectite + jarosite, and sulphur isotopic analyses point to a supergene hydrothermal alteration environment. This deposit probably derives from a debris avalanche initiated before 14–21 ka by collapse of a hydrothermally altered rock mass from the volcano summit. In contrast, the alteration mineralogy of the second debris flow deposit, which may itself comprise more than one unit, is dominated by halloysite + smectite and relates to a shallower and more recent (3200 m) volcanic soils. Our study reinforces the significance of hydrothermal alteration in weakening volcano flanks and in favouring rapid transformation of a volcanic debris avalanche into a clay-rich debris flow. It also demonstrates that mineralogical analysis provides crucial information for resolving the origin of a debris flow deposit in volcanic terrains. Finally, we posit that slope instability, promoted by ongoing subglacial hydrothermal alteration, remains a significant hazard at Cayambe Volcanic Complex

Unrest at Cayambe Volcano revealed by SAR imagery and seismic activity after the Pedernales subduction earthquake, Ecuador (2016)

International audienceFor the first time in decades, a sudden increase in seismicity has been observed and monitored at Cayambe volcano in Ecuador, in 2016. This seismic unrest, which occurred a few months after the April 2016, Mw 7.8 Pedernales subduction earthquake, has raised many questions, especially as there is no record of recent eruptions at Cayambe volcano. Here we analyze a time series of 104 images from Sentinel 1 (SAR) data spanning the period 2014–2018 using the NSBAS processing chain in order to quantify surface deformation around this potentially explosive and ice-covered volcano. We evidence a large-scale uplift reaching a maximum mean displacement rate of about 0.44 cm/yr in Line of Sight. This uplift is mainly due to a significant and sudden acceleration of the deformation pattern, focused on the SE flank starting in November 2016. We model this signal as related to magma emplacement at around 6 km depth below the summit, with a sudden volume influx of about 2.6 million m3. The inflation and surface deformation pattern is concomitant in time with two Mw 3 seismic events recorded at Cayambe volcano in November 2016 and is consistent with the location at the summit of the volcano-tectonic (VT) seismic swarm from September 2016 onwards. This VT swarm follows an earlier swarm in June 2016 located on the northern side of the volcano and closer to a western branch of the Chingual-Cosanga-Pallatanga-Puna (CCPP) fault system, that plays a major role in accommodating plate tectonic processes in South America. We thus propose that static stress changes from the Pedernales megathrust earthquake triggered magma ascent below the Cayambe volcano, through reactivation of the CCPP fault system. Finally, volcanic hazards around Cayambe ice-capped volcano, previously dormant for the last 300 years, should be reassessed in light of this recent unrest

Effects of crustal assimilation on 238U-230Th disequilibria in continental arc settings

International audienc

Two new Mycale (Naviculina) Gray (Mycalidae, Poecilosclerida, Demospongiae) from the Paulista Biogeographic Province (Southwestern Atlantic)

Two new Mycale (Naviculiiia) Gray, 1867 are described, M. (N.) arcuiris sp. n. and M. (N.)purpurata sp. n., from the Paulista Biogeographic Province (Southwestern Atlantic). They both occur in the São Sebastião Channel area (São Paulo stale coast), the former extending its known distribution to "Arvoredo Marine Biological Reserve" (Santa Catarina state coast). The new species are sibling and differ from each other by a series of very small traits, the most notorious being live-color: yellow, orange, green, white, grey and beige, always light, in M. (N.) arcuiris sp. n. and bordeaux in M. (N.) purpúrala sp. n. Both species are compared with other known M. (Naviculiiia). Mycale (Aegogropila) henlscheli Sim & Lee, 2001 is transferred to subgenus Naviculiiia and given a new name, viz. M. (N.) chungue nom. n., as the specific name was preoccupied by M. (Curmia) henlscheli Bergquist & Fromont, 1988. An identification key for all the species hitherto assigned to the subgenus is provided