Depth‐variant azimuthal anisotropy in Tibet revealed by surface wave tomography
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Abstract Azimuthal anisotropy derived from multimode Rayleigh wave tomography in China exhibits depth‐dependent variations in Tibet, which can be explained as induced by the Cenozoic India‐Eurasian collision. In west Tibet, the E‐W fast polarization direction at depths <100 km is consistent with the accumulated shear strain in the Tibetan lithosphere, whereas the N‐S fast direction at greater depths is aligned with Indian Plate motion. In northeast Tibet, depth‐consistent NW‐SE directions imply coupled deformation throughout the whole lithosphere, possibly also involving the underlying asthenosphere. Significant anisotropy at depths of 225 km in southeast Tibet reflects sublithospheric deformation induced by northward and eastward lithospheric subduction beneath the Himalaya and Burma, respectively. The multilayer anisotropic surface wave model can explain some features of SKS splitting measurements in Tibet, with differences probably attributable to the limited back azimuthal coverage of most SKS studies in Tibet and the limited horizontal resolution of the surface wave results.Keywords:
Asthenosphere
Seismic anisotropy
Rayleigh Wave
The article presents the data calculated from four different viscosity structures V1, V2 [1], SH08 [2], and GHW13 [3], as well as two tomography models S40RTS [4] and SAW642AN [5], using the joint modeling of lithosphere and mantle dynamics technique [3, 6-9]. Besides, the data contain the information on the viscosity variations of the lithosphere, asthenosphere, transition zone, and D″ layer based on the viscosity structure SH08.
Asthenosphere
Low-velocity zone
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As temperature increases with depth and the creep resistance of rock decreases exponentially, a high-viscosity sub-lithospheric layer, just beneath the 'elastic' lithosphere is expected to exist. Depending on the temperature profile, a low-viscosity asthenosphere may also exist if the temperature deeper down gets high enough. Since the temperature profile is expected to change laterally – especially from below the oceans to cratonic areas underneath continents, rock properties of the lithosphere, high-viscosity sub-lithosphere and low-viscosity asthenosphere are expected to change laterally. Our aim is to constrain sub-lithospheric properties (depth, thickness and viscosity), lateral lithospheric thickness variations and asthenospheric properties using observed GIA data. A Coupled Laplace-Finite Element Method is used to compute gravitationally self-consistent sea level with time-dependent coastline and rotational feedback in addition to changes in deformation, gravity and the state of stress. We start with the VM5a-ICE-6G_C model combination and then modify the lithospheric, sub-lithospheric and asthenospheric properties (including lateral thickness variation) while keeping the mantle viscosities the same as VM5a. Through this study, we confirm that the sub-lithospheric and asthenospheric properties can significantly affect the predicted global relative sea level (RSL), present-day gravity rate-of-change (g-dot) and uplift rate (u-dot) in Laurentia and Fennoscandia. In addition, incorporating the elastic lithosphere with lateral thickness variation, sub-lithosphere and asthenosphere can improve the fit to global RSL, but the predicted peak values of g-dot and u-dot in Laurentia may decrease slightly but not significant enough to affect the fit to the observed data. Our results prefer an elastic lithosphere that has maximum thickness of 140 km under continental cratons but reduces to 60 km underneath the oceans. The results preferred depth of the asthenospheric bottom is around 190–200 km with asthenospheric viscosity around 1020Pa s. Finally, we show that the best laterally heterogeneous mantle model we found in previous publication when combined with the lithosphere with lateral thickness variaion gives the best fit to global RSL and peak g-dot and u-dot in Laurentia simultaneously.
Asthenosphere
Lithospheric flexure
Laurentia
Post-glacial rebound
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Asthenosphere
Lithospheric flexure
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Asthenosphere
Base (topology)
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SUMMARY The lithosphere and asthenosphere are fundamental to plate tectonics and many other earth processes. Vertical motions can be measured quickly and accurately by the satellite Global Positioning System, GPS and other methods. This paper describes a new analytic method for inferring the elastic and viscous properties of the lithosphere and upper mantle from the uplift rate and history of an area that has been subjected to past surface load changes. The viscous response time of the centre of loading is determined from the dimensions of the load, a loading history comprised of linear segments, an estimate of the flexural rigidity of the lithosphere and a single constraint such as the current central uplift rate. The response time is then interpreted in terms of the elastic properties of the lithosphere, and the elastic and viscous depth profiles of the underlying mantle. The method is described mathematically and then illustrated through analysis of the isostatic adjustment observed in nine areas affected by Little Ice Age glaciation. The method replicates published conclusions, extracts insights from an extensive literature, provides new ways to separate the impacts of the lithosphere and asthenosphere on uplift rates and emergence, and indicates the widespread geographic distribution of a thin asthenosphere.
Asthenosphere
Lithospheric flexure
Post-glacial rebound
Hotspot (geology)
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Abstract Earth's cratonic mantle lithosphere is distinguished by high seismic wave velocities that extend to depths greater than 200 km, but recent studies disagree on the magnitude and depth extent of the velocity gradient at their lower boundary. Here we analyze and model the frequency dependence of S p waves to constrain the lithosphere‐asthenosphere velocity gradient at long‐lived stations on cratons in North America, Africa, Australia, and Eurasia. Beneath 33 of 44 stations, negative velocity gradients at depths greater than 150 km are less than a 2–3% velocity drop distributed over more than 80 km. In these regions the base of the typical cratonic lithosphere is gradual enough to be explained by a thermal transition. Vertically sharper lithosphere‐asthenosphere transitions are permitted beneath 11 stations, but these zones are spatially intermittent. These results demonstrate that lithosphere‐asthenosphere viscosity contrasts and coupling fundamentally differ between cratons and younger continents.
Asthenosphere
Low-velocity zone
Lithospheric flexure
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Asthenosphere
Collision zone
Lithospheric flexure
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Asthenosphere
Low-velocity zone
Lithospheric flexure
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Asthenosphere
Hotspot (geology)
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