Complex, multilayered azimuthal anisotropy beneath Tibet: evidence for co-existing channel flow and pure-shear crustal thickening

Abstract

SUMMARYOf the two debated, end-member models for the late-Cenozoic thickening of Tibetan crust, one invokes "channel flow" (rapid viscous flow of the mid-lower crust, driven by topography-induced pressure gradients and transporting crustal rocks eastward) and the other-"pure shear" (faulting and folding in the upper crust, with viscous shortening in the mid-lower crust). Deep-crustal deformation implied by each model is different and would produce different anisotropic rock fabric. Observations of seismic anisotropy can thus offer a discriminant. We use broadband phase-velocity curves-each a robust average of tens to hundreds of measurements-to determine azimuthal anisotropy in the entire lithosphere-asthenosphere depth range and constrain its amplitude. Inversions of the differential dispersion from path pairs, region-average inversions and phase-velocity tomography yield mutually consistent results, defining two highly anisotropic layers with different fast-propagation directions within each: the middle crust and the asthenosphere.In the asthenosphere beneath central and eastern Tibet, anisotropy is 2-4 per cent and has a NNE-SSW fast-propagation azimuth, indicating flow probably driven by the NNEward, shallow-angle subduction of India. The distribution and complexity of published shear-wave splitting measurements can be accounted for by the different anisotropy in the mid-lower crust and asthenosphere. The estimated splitting times that would be ac-2 M.R. Agius and S. Lebedev cumulated in the crust alone are 0.25-0.8 s; in the upper mantle-0.5-1.2 s, depending on location. In the middle crust (20-45 km depth) beneath southern and central Tibet, azimuthal anisotropy is 3-5 and 4-6 per cent, respectively, and its E-W fast-propagation directions are parallel to the current extension at the surface. The rate of the extension is relatively low, however, whereas the large radial anisotropy observed in the middle crust requires strong alignment of mica crystals, implying large finite strain and consistent with high-rate horizontal flow. Together, radial and azimuthal anisotropy suggest eastward mid-crustal channel flow in central Tibet, along the regional topography gradient.In NE high Tibet, mid-crustal azimuthal anisotropy is 4-8 per cent and has WNW-ESE and NW-SE fast-propagation directions, parallel to the net extension at the surface. These fast directions are inconsistent with channel flow following the SW-NE regional topography gradient. Instead, they suggest similar net deformation in the (decoupled) shallow and deep crust. In the brittle upper crust, it is accommodated by strike-slip faulting; in the ductile mid-lower crust-by shear oriented at ∼45 • to the faults. Although mid-crustal flow beneath NE Tibet may transport some material towards the plateau periphery at a low region-average rate, the dominant mid-crust deformation pattern is shear parallel to the plateau boundary. This implies that channel flow from central Tibet is not the main cause of the on-going crustal thickening farther northeast.

Mantle Flow and Deforming Continents: From India‐Asia Convergence to Pacific Subduction

“…In the case of East Asia, the vertical superposition of two perpendicular stretching/shearing directions, seen here at large scale, is confirmed by detailed studies of the stratification of anisotropy below Tibet (Agius & Lebedev, ; Huang et al, ) with different flow directions at lithospheric and asthenospheric depths. One can reasonably assume that the signal at 200 km for SL2013sv gives information on the asthenospheric flow as it samples the mantle well below the LAB in East Asia.…”

“…In the case of East Asia, the vertical superposition of two perpendicular stretching/shearing directions, seen here at large scale, is confirmed by detailed studies of the stratification of anisotropy below Tibet (Agius & Lebedev, 2017;Huang et al, 2017) with different flow directions at lithospheric and asthenospheric depths.…”

“…If one instead assumes that part of the deformation is driven from below by the viscous asthenosphere, the presence of a passively deforming mantle is no longer required. More detailed approaches of the stratification of anisotropy below Tibet show a different behavior at lithospheric and asthenospheric depths interpreted as different flow directions (Agius & Lebedev, ; Huang et al, ).…”

Reference Models for Lithospheric Geoneutrino Signal

“…The "CRUST" family of models predicts felsic-like densities in the entire crustal column in these regions (including the expected mafic lower crust; Figure 3). This prediction is in conflict with some regional studies (e.g., Bai et al, 2013;Monsalve et al, 2008) although in agreement with other global studies (e.g., Hacker et al, 2015) and regional seismic models (Agius & Lebedev, 2017;Gilligan & Priestley, 2018). Additionally, Pasyanos et al (2014) perturbed CRUST1.0 parameters up to 5% to fit observed surface waves to make LITHO1.0, which a comparison of CRUST1.0 and LITHO1.0 V P in the Himilayan region shows that this perturbation reached saturation (i.e., 5% change between LITHO1.0 and CRUST1.0).…”

Lateral variations in the crustal structure of the Indo–Eurasian collision zone

“…A recent study by Agius & Lebedev (2017), for example, finds that there is between 3% and 8% azimuthal anisotropy in the mid-crust beneath the Plateau. While the anisotropic component of velocity may trade-off with the heterogeneity in group velocity, synthetic tests (e.g.…”