We present a new upper-mantle tomographic model derived solely from hum seismic data. Phase correlograms between station pairs are computed to extract phase-coherent signals.Correlograms are then stacked using the time-frequency phase-weighted stack method to build-up empirical Green's functions. Group velocities and uncertainties are measured in the wide period band of 30-250 s, following a resampling approach. Less data are required to extract reliable group velocities at short periods than at long periods, and 2 yr of data are necessary to measure reliable group velocities for the entire period band. Group velocities are first regionalized and then inverted versus depth using a simulated annealing method in which the number and shape of splines that describes the S-wave velocity model are variable. The new S-wave velocity tomographic model is well correlated with models derived from earthquakes in most areas, although in India, the Dharwar craton is shallower than in other published models.
S U M M A R YThe Archean Dharwar craton in south India is known for long time to be different from most other cratons. Specifically, at station Hyderabad (HYB) the Ps converted phases from the 410-and 660-km mantle discontinuities arrive up to 2 s later than in other cratons of comparable age, which implies lower upper mantle velocities. To resolve the unique lithosphere-asthenosphere system of the Dharwar craton, we inverted jointly P and S receiver functions and teleseismic P and S traveltime residuals at 10 seismograph stations. This method operates in the same depth range as long-period surface waves but differs by much higher lateral and radial resolution. We observe striking differences in crustal structures between the eastern and western Dharwar craton (EDC and WDC, respectively): crustal thickness is of around 31 km, with predominantly felsic velocities, in the EDC and of around 55 km, with predominantly mafic velocities, in the WDC. In the mantle we observe significant variations in the P velocity with depth, practically without accompanying variations in the S velocity. In the mantle S velocity there are azimuthdependent indications of the Hales discontinuity at a depth of ∼100 km. The most conspicuous feature of our models is the lack of the high velocity mantle keel with the S velocity of ∼4.7 km s −1 , typical of other Archean cratons. The S velocity in our models is close to 4.5 km s −1 from the Moho to a depth of ∼250 km. There are indications of a similar upper mantle structure in the northeast of the Indian craton and of a partial recovery of the normal shield structure in the northwest. A division between the high S-velocity western Tibet and low S-velocity eastern Tibet may be related to a similar division between the northeastern and northwestern Indian craton.
S U M M A R YShear wave splitting in the seismic SKS phase provides a unique possibility to judge on deformations at depths inaccessible for direct observations. Fast S wave polarization direction in collisional belts is often parallel to the trend of the belt, although deformations of the mantle lithosphere in low-angle thrusts would lead to the fast polarization direction normal to the trend of the belt. These considerations suggested that the upper mantle in collisional belts is decoupled from the crust. However, SKS technique is notable by a poor depth resolution, and usually it assumes that the fast polarization direction is the same at any depth, which is hard to justify. Here, to investigate depth dependent azimuthal anisotropy in the mantle, we invert jointly P receiver functions and SKS particle motions at a number of seismograph stations. The technique involves azimuthal filtering of the receiver functions and provides a criterion to discriminate between the effects of azimuthal anisotropy and lateral heterogeneity of isotropic medium. A search for the optimum models is conducted with a technique similar to simulated annealing. Testing with synthetics demonstrates that this approach is robust. The results for 10 seismograph stations in the Tien Shan, the world's most active intracontinental collisional belt in Central Asia, reveal a pronounced change in the patterns of azimuthal anisotropy at a depth around 100 km. In the mantle lithosphere (at depths less than 100 km), anisotropy is relatively weak and fast wave polarization direction varies laterally in a broad range. This layer is not necessarily decoupled from the crust: its anisotropy can be a combined effect of present day thrusting and of deformations of the geologic past. In the lower layer (asthenosphere) the average azimuth of fast wave polarization is close to the trend of the belt, whereas magnitude of S wave anisotropy is stable and large (between 5 and 6 per cent). This anisotropy is a likely result of recent uniaxial shortening at right angle to the trend of the belt. At some stations the data require anisotropy in the crust. There is no evidence for anisotropy at depths exceeding 150-250 km.
We present a high‐resolution 3‐D lithospheric model of the Indian plate region down to 300 km depth, obtained by inverting a new massive database of surface wave observations, using classical tomographic methods. Data are collected from more than 550 seismic broadband stations spanning the Indian subcontinent and surrounding regions. The Rayleigh wave dispersion measurements along ~14,000 paths are made in a broad frequency range (16–250 s). Our regionalized surface wave (group and phase) dispersion data are inverted at depth in two steps: first an isotropic inversion and next an anisotropic inversion of the phase velocity including the SV wave velocity and azimuthal anisotropy, based on the perturbation theory. We are able to recover most of the known geological structures in the region, such as the slow velocities associated with the thick crust in the Himalaya and Tibetan plateau and the fast velocities associated with the Indian Precambrian shield. Our estimates of the depth to the Lithosphere‐Asthenosphere boundary (LAB) derived from seismic velocity Vsv reductions at depth reveal large variations (120–250 km) beneath the different cratonic blocks. The lithospheric thickness is ~120 km in the eastern Dharwar, ~160 km in the western Dharwar, ~140–200 km in Bastar, and ~160–200 km in the Singhbhum Craton. The thickest (200–250 km) cratonic roots are present beneath central India. A low velocity layer associated with the midlithospheric discontinuity is present when the root of the lithosphere is deep.
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