The Poisson’s ratio of the Australian crust : geological and geophysical implications

The Poisson ratio, which depends on the VP/VS ratio, provides much tighter constraints on the crustal composition than either the compressional or the shear velocity alone. The crustal Poisson ratio can be determined from the joint analysis of the travel times of waves converted at the Moho and of crustal multiples reflected at the top of the Moho. We have analyzed the records of the permanent stations installed on the Australian continent, complemented by the data of the SKIPPY experiment. The results reveal substantial variations in the Poisson ratio in the different tectonic units. For the Proterozoic crust, an increase of the Poisson ratio with increasing crustal thickness is systematically observed while for the Phanerozoic crust, the Poisson ratio tends to decrease for increasing crustal thicknesses. These observations are in remarkable agreement with the results of the deep seismic soundings that were performed in the former Soviet Union. The variations observed in the Proterozoic provinces can perhaps be explained by underplating of mafic materials at the base of the crust

Seismic evidence for olivine phase changes at the 410- and 660-kilometer discontinuities

The view that the seismic discontinuities bounding the mantle transition zone at 410- and 660-kilometer depths are caused by isochemical phase transformations of the olivine structure is debated. Combining converted-wave measurements in East Asia and Australia with seismic velocities from regional tomography studies, we observe a correlation of the thickness of, and wavespeed variations within, the transition zone that is consistent with olivine structural transformations. Moreover, the seismologically inferred Clapeyron slopes are in agreement with the mineralogical Clapeyron slopes of the (Mg,Fe)2SiO4spinel and postspinel transformations

Seismic imaging of a mid-crustal low-velocity layer beneath the northern coast of the South China Sea and its tectonic implications

International audienceThis is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain

High-resolution imaging of the Pyrenees and Massif Central from the data of the PYROPE and IBERARRAY portable array deployments

International audienceThe lithospheric structures beneath the Pyrenees, which holds the key to settle long-standing controversies regarding the opening of the Bay of Biscay and the formation of the Pyrenees, are still poorly known. The temporary PYROPE and IBERARRAY experiments have recently filled a strong deficit of seismological stations in this part of western Europe, offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution. Here we report the results of the first tomographic study of the Pyrenees relying on this rich data set. The important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections. Beneath the Massif Central, the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere. Our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the Pyrenees. In contrast, they reveal the segmentation of lithospheric structures, mainly by two major lithospheric faults, the Toulouse fault in the central Pyrenees and the Pamplona fault in the western Pyrenees. These inherited Hercynian faults were reactivated during the Cretaceous rifting of the Aquitaine and Iberian margins and during the Cenozoic Alpine convergence. Therefore, the Pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the European plate

Subduction and volcanism in the Iberia-North Africa collision zone from tomographic images of the upper mantle

New tomographic images of the upper mantle beneath the westernmost Mediterranean suggest that the evolution of the region experienced two subduction-related episodes. First subduction of oceanic and/or extended continental lithosphere, now located mainly beneath the Betics at depths greater than 400 km, took place on a NW-SE oriented subduction zone. This was followed by a slab-tear process that initiated in the east and propagated to the west, leading to westward slab rollback and possibly lower crustal delamination. The current position of the slab tear is located approximately at 4°W, and to the west of this location the subducted lithosphere is still attached to the surface along the Gibraltar Arc. Our new P-wave velocity model is able to image the attached subducted lithosphere as a narrow high-velocity body extending to shallow depths, coinciding with the region of maximum curvature of the Gibraltar Arc, the occurrence of intermediate-depth earthquakes, and anomalously thick crust. This thick crust has a large influence in the measured teleseismic travel time residuals and therefore in the obtained P-wave tomographic model. We show that removing the effects of the thick crust significantly improves the shallow images of the slab and therefore the interpretations based on the seismic structureThis is a contribution of the Team Consolider-Ingenio 2010 TOPO-IBERIA (CSD2006-00041). Additional fundingwas provided by the SIBERIA (CGL2006-01171), RIFSIS (CGL2009-09727) and ALERTES-RIM (CGL2013-45724-C3-3-R) projects.Peer reviewe

Evolution of a low convergence collisional orogen: a review of Pyrenean orogenesis

The Pyrenees is a collisional orogen built by inversion of an immature rift system during convergence of the Iberian and European plates from Late Cretaceous to late Cenozoic. The full mountain belt consists of the pro-foreland southern Pyrenees and the retro-foreland northern Pyrenees, where the inverted lower Cretaceous rift system is mainly preserved. Due to low overall convergence and absence of oceanic subduction, this orogen preserves one of the best geological records of early orogenesis, the transition from early convergence to main collision and the transition from collision to post-convergence. During these transitional periods major changes in orogen behavior reflect evolving lithospheric processes and tectonic drivers. Contributions by the OROGEN project have shed new light on these critical periods, on the evolution of the orogen as a whole, and in particular on the early convergence stage. By integrating results of OROGEN with those of other recent collaborative projects in the Pyrenean domain (e.g., PYRAMID, PYROPE, RGF-Pyrénées), this paper offers a synthesis of current knowledge and debate on the evolution of this immature orogen as recorded in the synorogenic basins and fold and thrust belts of both the upper European and lower Iberian plates. Expanding insight on the role of salt tectonics at local to regional scales is summarised and discussed. Uncertainties involved in data compilation across a whole orogen using different datasets are discussed, for example for deriving shortening values and distribution

Pyrope : premiers retours d'expérience

Un an après sa fin officielle, c'est l'heure des premiers bilans de l'expérience Pyrope. Si nous sommes encore dans la phase préliminaire de l'exploitation des données récoltées, quelques résultats scientifiques importants ont déjà été obtenus

Pyrope : premiers retours d'expérience

Un an après sa fin officielle, c'est l'heure des premiers bilans de l'expérience Pyrope. Si nous sommes encore dans la phase préliminaire de l'exploitation des données récoltées, quelques résultats scientifiques importants ont déjà été obtenus