104 research outputs found
Ambient-noise tomography of north Tibet limits geological terrane signature to upper-middle crust
No full textWe use ambient-noise tomography to map regional differences in crustal Rayleigh-wave group velocities with periods of 8–40?s across north Tibet using the International Deep Profiling of Tibet and the Himalaya phase IV arrays (132 stations, deployed for 10–24?months). For periods of 8–24?s (sensitive to midcrustal depths of ~5–30?km), we observe striking velocity changes across the Bangong-Nujiang and Jinsha suture zones as well as the Kunlun-Qaidam boundary. From south to north, we see higher velocities beneath the Lhasa terrane, lower velocities beneath the Qiangtang, higher velocities in the Songpan-Ganzi and Kunlun terranes, and the lowest velocities beneath the Qaidam Basin. Maps at periods of 34 and 40?s (sensitive to the middle and lower crust at depths of ~30–60?km) do not show evidence of changes across those boundaries. Any differences between the Tibetan terrane lower crusts that were present at accretion have been erased or displaced by Cenozoic processes and replaced almost ubiquitously by uniformly low velocities
Seismic Anisotropy from SKS Splitting beneath Northeastern Tibet
No full textThe northeastern boundary of the Tibetan high plateau is marked by a 2 km topographic drop and a coincident rapid change in crustal thickness. Surface tectonics are dominated by the Kunlun strike?slip fault system and adjacent Kunlun concealed thrust. The main objective of the current study is to map lateral variations of seismic anisotropy parameters in this region along the linear INDEPTH IV array in order to investigate the link between surface and internal deformation in the context of crust and mantle structure. To achieve this aim, we performed Minimum?Transverse?Energy based SKS splitting measurements using 23 stations of the INDEPTH IV array deployed across the northeastern margin of Tibet. Average fast polarization directions and splitting time delays are obtained by averaging stacked misfit surfaces of all analyzed events at each station. The agreement of fast directions with the strikes of major active strike?slip faults and strike?slip focal mechanisms, but not with fossil structures such as the Jinsha suture, implies that the anisotropy records lithospheric petrofabric formed by recent deformation within the lithosphere rather than representing frozen?in anisotropy or shear within the asthenosphere due to absolute plate motion. The distribution of large splitting delays throughout the northern plateau suggests that deformation is distributed rather than focused onto narrow shear zones associated with the Kunlun strike?slip faults. The drop in splitting delays toward the Qaidam is then a natural consequence of the much lower degree of deformation there
The seismological structure of the Tibetan Plateau crust and mantle down to 700 km depth
No full textStructure of the upper mantle in the north-western and central United States from USArray S-receiver functions
No full textWe used more than 40 000 S-receiver functions recorded by the USArray
project to study the structure of the upper mantle between the Moho and the
410 km discontinuity from the Phanerozoic western United States to the
cratonic central US. In the western United States we observed the
lithosphere–asthenosphere boundary (LAB), and in the cratonic United States we observed
both the mid-lithospheric discontinuity (MLD) and the LAB of the craton. In
the northern and southern United States the western LAB almost reaches the
mid-continental rift system. In between these two regions the cratonic MLD
is surprisingly plunging towards the west from the Rocky Mountain Front to
about 200 km depth near the Sevier thrust belt. We interpret these complex
structures of the seismic discontinuities in the mantle lithosphere as an
indication of interfingering of the colliding Farallon and Laurentia plates.
Unfiltered S-receiver function data reveal that the LAB and MLD are not
single discontinuities but consist of many small-scale laminated
discontinuities, which only appear as single discontinuities after longer
period filtering. We also observe the Lehmann discontinuity below the LAB
and a velocity reduction about 30 km above the 410 km discontinuity
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