Seismic anisotropy as an indicator of mantle flow beneath the Himalayas and Tibet

SEVERAL models have been proposed for the geodynamical evolution of the Tibet-Himalayas collision zone1-6. It is now generally recognized that the high elevations of the region have been caused by mechanical thickening of the crust and flow in the mantle, but there is debate as to whether the thickening has occurred by the underthrusting of Indian crust under Tibet, or by distributed shortening and thickening of the Tibetan crust as India has pushed northwards into it. Here we address this question using seismic measurements of heterogeneity and anisotropy at depth, obtained with a temporary teleseismic array spanning 500km from the Lesser Himalayas to central Tibet (Fig. 1). We observe a significant change in seismic anisotropy across the Indus-Tsangpo suture (ITS), suggesting a change in mode or direction of deformation at depth. In the Himalayas, our results are consistent with the stacking of Indian and Tibetan lithospheres, whereas north of the ITS the data indicate ductile flow in the mantle and show no sign of the Indian lithosphere. © 1995 Nature Publishing Group.Peer Reviewe

Seismic structure and activity of the north-central Lesser Antilles subduction zone from an integrated approach: similarities with the Tohoku forearc

The 300 km long north-central segment of the Lesser Antilles subduction zone, including Martinique and Guadeloupe islands has been the target of a specific approach to the seismic structure and activity by a cluster of active and passive offshore-onshore seismic experiments coordinated within the ¿Thales was right¿ proposal to the European Union action (Laigle et al., Tectonophys., in rev.) The top of the subducting plate can be followed under the wide accretionary wedge by a dense grid of dip- and strike-lines of multichannel reflection seismics. This reveals the hidden updip limit of the contact of the upper plate crustal backstop thrust onto the slab. Two OBS refraction seismic profiles constrained a 26 km large crustal thickness from the volcanic arc throughout the forearc domain (Kopp et al., EPSL, 2011). These new observations imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the Caribbean plate with respect to a regular intra-oceanic subduction zone, in the common assumption that the upper plate Moho contact on the slab is a proxy of its downdip limit. Towards larger depth under the mantle corner, the top of the slab imaged from the conversions of teleseismic body-waves and the locations of earthquakes from the dense temporary array of 80 OBS and land seismometers appears with kinks which increase the dip from 10-20° under the forearc domain, to 60° on the segment from 70 km depth down to under the volcanic arc. There, at 140 km depth just north of Martinique the 2007 M 7.4 earthquake, largest for half a century, was accompanied by an increased seismic activity over the whole depth range, which provides a new focused image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is thus now resolved, as originally discovered in Tohoku ( NE Japan) and since in some other subduction zones. Other types of seismic activity uniquely observed in Tohoku, are resolved now here, such as ¿supraslab¿ earthquakes with normal-faulting focal mechanisms reliably located in the mantle corner and ¿deep flat-thrust¿ earthquakes at 45 km depth on the interplate fault under the Caribbean plate forearc mantle. None such types of seismicity should occur under the paradigm of a regular peridotitic mantle of the upper plate which is serpentinized by the fluids provided from the dehydrating slab beneath, and which is commonly considered as limiting the downward extent of the interplate seismic coupling. If the upper plate here comprised lithospheric segments related to the earlier formation of the Caribbean oceanic plateau by the material advection from a mantle plume, it could then be underlain by a correspondingly modified, heterogeneous mantle, which may impose regions of stick-slip behaviour on the interplate under the mantle corner among stable-gliding areas. The Tohoku 2011 M9 earthquake was unexpected not only in its slip reaching to the trench, but also in its slip reaching far under the mantle corner against the serpentinization decoupling paradigm, and its structural setting may be revisited for resolving corresponding structural heterogeneityPeer Reviewe

Complex images of Moho and variation of Vp/Vs across the Himalaya and South Tibet, from a joint receiver-function and wide-angle-reflection approach

Teleseismic receiver functions (RF) allow us to image the spatial variation of the crust-mantle boundary (Moho) along a tight array spanning from south of the Himalayas to the centre of the Tibetan Plateau. This approach is cross-tested with wide-angle reflection imaging (WARR). Highlighted by each of the two independent methods, a complex architecture of the Moho with dipping and overlapping segments indicating lithospheric imbrication, is confirmed. The joint use of the two methods reveals an increase of the average crustal P-to-S-wave-velocity ratio from south to the centre of the Lhasa block. This may be due to lowered S-wave velocity confined in specific layers, that may be interpreted as partial melt. This accounts for half of the relative increase in the delay of direct teleseismic S-wave arrivals with respect to P-wave arrivals from south to north, suggesting a similar anomaly in the shallower mantle.Peer Reviewe

Increase in melt fraction along a south-north traverse below the Tibetan Plateau: Evidence from seismology

A mid-crustal low-velocity zone and crustal melt fraction contrast the Lhasa block in the Maxiang Yangbajain zone with the Tethyan Himalayas in the south. Evidence from wide-angle reflections is extended by vertical reflection and differential S to P wave teleseismic delays. The temperature and rheology implied allow phase transitions which may change crustal buoyancy and elevation (Le Pichon et al., 1997), and may allow lateral mass flow. In the uppermost mantle, particular path geometries to a tight temporary teleseismic array allow detection of a decrease in velocity northward through the Lhasa block. A larger relative variation in late teleseismic S with respect to P indicates an increase in Poisson's ratio, hence of partial melt fraction. This may be seen as evidence of a higher position of the mantle asthenosphere. Level Tibet hence appears to be underlain by variation in the structure within the crust across the Indus-Tsangpo suture, and in the structure within the mantle, further northward of the middle Lhasa block. These variations are most readily attributable to spatial variation in partial melt fraction, and hence temperature, which also induces phase transformations. Mode and amount of deformation and control of buoyancy on elevation may vary accordingly. The dynamics of the system is evidenced by seismic anisotropy which we relate to ductile flow.Peer Reviewe

Mount Etna dense array local earthquake P and S tomography and implications for volcanic plumbing

Inversion for the three-dimensional velocity structure of Mount Etna is performed with a data set of arrival times of P and S waves of local earthquakes from temporary dense arrays of three-component seismographs. A high-Vp body revealed by the original tomography without nearby stations is confirmed, and its image is sharpened using new velocity constraints provided by refraction data. Synthetic tests of Vp and Vp/Vs and comparison with an independent artificial source tomography with a fundamentally different geometry consistently calibrate the significance threshold of the resolution indicators. The trustworthy part of the image shows a high-Vp body centered under the southern part of Valle del Bove above the 6 km below sea level deep basement, which extends towards sea level and may be rooted in or through the crust. It has a large contrast of over 1 km/s with the surrounding sediments and sharp lateral limits and can thus be regarded as made of intrusive material of magmatic origin. The massive high-Vp body is heterogeneous in Vp/Vs. The regions inside it where Vs is relatively low can then be suspected of containing a proportion of melt or be fractured and act as pressure links or transport zones. Such features may be structurally linked and appear to be activated in eruptive phenomena. By taking into account the heterogeneities in structure and physical state retrieved by seismic tomography a succession of seismic events, deformational episodes, and geochemical variation in lavas can be discussed with respect to the well-observed eruptions. Copyright 2000 by the American Geophysical Union.This research is part of the multimethod seismic approach of Etna funded partly by EC Environment Programme, Volcanic Hazard, under contract ETNASEIS EV5V-CJ92-0187 to A. Hirn, R. Nicolich, J. Gallart, and B. BrandsdottiPeer Reviewe

Spatially resolved electronic properties of MgO/GaAs(001) tunnel barrier studied by ballistic electron emission microscopy

International audienceThe spatially resolved electronic structure of the epitaxial Au/MgO/GaAs(001) tunnel junction has been studied by ballistic electron emission microscopy. The Schottky barrier height of Au on the MgO/GaAs heterostructure is determined to be 3.90 eV, in good agreement with spatially averaged x-ray photoelectron spectroscopy measurements. Locally, two well-defined conduction channels are observed for electrons energies of 2.5 and 3.8 eV, i.e., below the conduction band minimum of the oxide layer. These conduction channels are attributed to band of defect states in the band-gap of the tunnel barrier related to oxygen vacancies in the MgO layer. These defect states are responsible for the low barrier height measured on magnetic tunnel junctions with epitaxial MgO(001) tunnel barriers