75 research outputs found
Belt-parallel mantle flow beneath a halted continental collision: The Western Alps
International audienceConstraining mantle deformation beneath plate boundaries where plates interact with each other, such as beneath active or halted mountain belts, is a particularly important objective of "mantle tectonics" that may bring a depth extent to the Earth's surface observation. Such mantle deformation can be mapped at scale lengths of several tens of kilometers through the analysis of seismological data and particularly by mapping seismic anisotropy from the splitting analysis of vertically-propagating SKS waves that largely reflect the strain-induced crystal preferred orientations of the rock-forming minerals within the upper mantle. In the present study, we analyse data from approximately 50 broadband seismic stations covering the Western Alps and we provide a coherent picture of upper mantle anisotropy beneath the belt. The large-scale anisotropy pattern is characterized by fast split directions that closely follow the trend of the belt. Moreover, the maximum anisotropy magnitude is not located beneath the internal zones of the belt but instead beneath external units. All suggests that the anisotropy is likely dominated by sublithospheric mantle deformation. We propose that the observed anisotropy pattern can be explained by recent or active mantle flow around the Eurasian slab presently plunging beneath the inner parts of the Alps
Burial and exhumation in a subduction wedge : mutual constraints from thermo-mechanical modelin and natural P-T-t data (Sch. Lustrés, W. Alps)
The dynamic processes leading to synconvergent exhumation of high-pressure low-temperature (HP-LT) rocks at oceanic accretionary margins, as well as the mechanisms maintaining nearly steady state regime in most accretion prisms, remain poorly understood. The present study aims at getting better constraints on the rheology, thermal conductivity, and chemical properties of the sediments in subduction zones. To reach that goal, oceanic subduction is modeled using a forward visco-elasto-plastic thermomechanical code (PARA(O)VOZ-FLAC algorithm), and synthetic pressure-temperature-time (P-T-t) paths, predicted from numerical experiments, are compared with natural P-T-t paths. The study is focused on the well constrained Schistes LustrĂ©s complex (SL: western Alps) which is thought to represent the fossil accretionary wedge of the Liguro-Piemontese Ocean. For convergence rates comparable to Alpine subduction rates (âŒ3 cm yrâ1), the best-fitting results are obtained for high-viscosity, low-density wedge sediments and/or a strong lower continental crust. After a transition period of 3-5 Ma the modeled accretionary wedges reach a steady state which lasts over 20 Ma. Over that time span a significant proportion (âŒ35%) of sediments entering the wedge undergoes P-T conditions typical of the SL complex (âŒ15-20 kbar; 350-450°C) with similar P-T loops. Computed exhumation rates (<6 mm yrâ1) are in agreement with observations (1-5 mm yrâ1). In presence of a serpentinite layer below the oceanic crust, exhumation of oceanic material takes place at rates approaching 3 mm yrâ1. In all experiments the total pressure in the accretionary wedge never deviated by more than ±10% from the lithostatic component
Crustal structure of the rifted volcanic margins and uplifted plateau of Western Yemen from receiver function analysis
International audienceWe analyse P-wave receiver functions across the western Gulf of Aden and southern Red Sea continental margins in Western Yemen to constrain crustal thickness, internal crustal structure and the bulk seismic velocity characteristics in order to address the role of magmatism, faulting and mechanical crustal thinning during continental breakup. We analyse teleseismic data from 21 stations forming the temporary Young Conjugate Margins Laboratory (YOCMAL) network together with GFZ and Yemeni permanent stations. Analysis of computed receiver functions shows that (1) the thickness of unextended crust on the Yemen plateau is âŒ35km; (2) this thins to âŒ22km in coastal areas and reaches less than 14km on the Red Sea coast, where presence of a high-velocity lower crust is evident. The average Vp/Vs ratio for the western Yemen Plateau is 1.79, increasing to âŒ1.92 near the Red Sea coast and decreasing to 1.68 for those stations located on or near the granitic rocks. Thinning of the crust, and by inference extension, occurs over a âŒ130-km-wide transition zone from the Red Sea and Gulf of Aden coasts to the edges of the Yemen plateau. Thinning of continental crust is particularly localized in a <30-km-wide zone near the coastline, spatially co-incident with addition of magmatic underplate to the lower crust, above which on the surface we observe the presence of seaward dipping reflectors (SDRs) and thickened Oligo-Miocene syn-rift basaltic flows. Our results strongly suggest the presence of high-velocity mafic intrusions in the lower crust, which are likely either synrift magmatic intrusion into continental lower crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the SDRs. Our results also point towards a regional breakup history in which the onset of rifting was synchronous along the western Gulf of Aden and southern Red Sea volcanic margins followed by a second phase of extension along the Red Sea margin
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We image the lithospheric and upper asthenospheric structure of western continental Yemen with 24 broadband stations to evaluate the role of the Afar plume on the evolution of the continental margin and its extent eastward along the Gulf of Aden. We use teleseismic tomography to compute relative P wave velocity variations in south-western Yemen down to 300 km depth. Published receiver function analysis suggest a dramatic and localized thinning of the crust in the vicinity of the Red Sea and the Gulf of Aden, consistent with the velocity structure that we retrieve in our model. The mantle part of the model is dominated by the presence of a low-velocity anomaly in which we infer partial melting just below thick Oligocene flood basalts and recent off-axis volcanic events (from 15 Ma to present). This low-velocity anomaly could correspond to an abnormally hot mantle and could be responsible for dynamic topography and recent magmatism in western Yemen. Our new P wave velocity model beneath western Yemen suggests the young rift flank volcanoes beneath margins and on the flanks of the Red Sea rift are caused by focused small-scale diapiric upwelling from a broad region of hot mantle beneath the area. Our work shows that relatively hot mantle, along with partial melting of the mantle, can persist beneath rifted margins after breakup has occurred
Installation de la station PR63 en février 2018 dans le cadre du projet HATARI (HAzard in TAnzanian RIft)
The Hatari project (HAzard in TAnzanian RIft) aims to study the impacts of geodynamic processes on the evolution of socio-ecosystems. For this project, an interdisciplinary team composed of geophysicists, petrogeochemists, geographers, didacticians and ecologists was formed to gather all the necessary information to address this interdisciplinary issue. This photo was taken during the February 2018 mission as part of the Hatari project. This mission made it possible to install eight seismological stations that recorded local and remote seismicity between Kilimanjaro and the Serengeti plains for one year continuously. These stations were installed in schools or "bombs". (Maasai villages) for security reasons.The stations installed as part of the project belong to the mobile instrument park (Sismob) of the French Seismological and Geodetic Network (RĂ©sif), a national research infrastructure dedicated to the observation and understanding of the structure and dynamics of the Inner Earth. RĂ©sif is based on high-tech observation networks, composed of seismological, geodetic and gravimetric instruments deployed in a dense manner throughout France. The data collected make it possible to study with high spatial and temporal resolution the deformation of the ground, surface and deep structures, seismicity on a local and global scale and natural hazards, and more particularly seismic events, on French territory. RĂ©sif is integrated into European (EPOS - European Plate Observing System) and global systems of instruments for imaging the Earth's interior as a whole and studying many natural phenomena.Le projet Hatari (HAzard in TAnzanian RIft) a pour objectif dâĂ©tudier quels sont les impacts des processus gĂ©odynamiques sur lâĂ©volution des socio-Ă©cosystĂšmes. Pour ce projet, une Ă©quipe interdisciplinaire composĂ©e de gĂ©ophysiciens, pĂ©tro-gĂ©ochimistes, de gĂ©ographes, de didacticiens et dâĂ©cologues a Ă©tĂ© constituĂ©e afin de rĂ©unir lâensemble des informations nĂ©cessaires pour rĂ©pondre Ă cette problĂ©matique inter-disciplinaire. Cette photo a Ă©tĂ© prise durant la mission de fĂ©vrier 2018 dans le cadre du projet Hatari. Cette mission a permis dâinstaller huit stations sismologiques qui ont enregistrĂ© un an en continu la sismicitĂ© locale et lointaine entre le Kilimanjaro et les plaines du Serengeti. Ces stations Ă©taient installĂ©es dans des Ă©coles ou dans des "bomas" (villages MasaĂŻ) pour des raisons de sĂ©curitĂ©.Les stations installĂ©es dans le cadre du projet appartiennent au parc d'instruments mobiles (Sismob) du RĂ©seau sismologique et gĂ©odĂ©sique français (RĂ©sif), une infrastructure de recherche nationale dĂ©diĂ©e Ă lâobservation et la comprĂ©hension de la structure et de la dynamique Terre interne. RĂ©sif se base sur des rĂ©seaux dâobservation de haut niveau technologique, composĂ©s dâinstruments sismologiques, gĂ©odĂ©siques et gravimĂ©triques dĂ©ployĂ©s de maniĂšre dense sur tout le territoire français. Les donnĂ©es recueillies permettent dâĂ©tudier avec une haute rĂ©solution spatio-temporelle la dĂ©formation du sol, les structures superficielles et profondes, la sismicitĂ© Ă lâĂ©chelle locale et globale et les alĂ©as naturels, et plus particuliĂšrement sismiques, sur le territoire français. RĂ©sif sâintĂšgre aux dispositifs europĂ©ens (EPOS - European Plate Observing System) et mondiaux dâinstruments permettant dâimager lâintĂ©rieur de la Terre dans sa globalitĂ© et dâĂ©tudier de nombreux phĂ©nomĂšnes naturels
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