Tomographic Image of the Crust and Upper Mantle Beneath the Western Tien Shan from the MANAS Broadband Deployment: Possible Evid

Tomographic Image of the Crust and Upper Mantle Beneath the Western Tien Shan from the MANAS Broadband Deployment: Possible Evidence for Lithospheric Delamination Perturbations to P wavespeed (dVp/Vp) from an average 1D background model determined by inversion of the teleseismic travel time residuals. The locations of the slanted cross sections are shown at the top of the figure: the maximum resolution projection onto a plane extending from AA' to CC’ is shown on the upper left. A slightly displaced plane from BB’ to CC’ is shown on the upper right. The vertical axis in these sections is true depth in km (as opposed to downdip length). The lower two plots are NS sections taken at 74.5o (left) and 76.0 o (right) longitude. The 74.5 o section has better resolution at shallow depths in the north, while the 76.0 o section has better resolution at shallow depth in the south. Perturbations relative to the 1D average back- ground wavespeeds are in percent as indicated in the palettes below each section. Locations of the MANAS stations are shown as yellow triangles at the surface. Topography along each section is shown in the box above each section.
We combined teleseismic P arrival times from the MANAS deployment of broad band sensors with P and S arrival times from local events recorded by the GENGHIS deployment and analogue observations from the Kyrgyz Institute of Seismology to generate a high resolution (~20 km) image of elastic wavespeeds in the crust and upper mantle beneath the western Tien Shan. The total data set consists of 29,006 P and 21,491 S arrivals from 2176 local events recorded at 144 stations along with 5202 P arrivals from 263 teleseismic events recorded at 40 stations. The most significant feature in our image of the mantle beneath the Tien Shan is a pair of large, elongated high wavespeed regions dipping in opposite directions from the near surface to depths of at least 400 km. These regions appear to be continuous and extend upwards to bounding range fronts where the Tarim Basin is being overthrust by the Kokshal range on the south side, and the Kazach shield underthrusts the Kyrgyz range on the north side. While it is tempting to interpret these high wavespeed anomalies as evidence for contemporary subduction of continental lithosphere, such a scenario is difficult to reconcile with both 2the timing of the orogen and the size of the wavespeed anomaly. We suggest instead that they represent
downwelling side-limbs of a lithospheric delamination beneath the central part of the Tien Shan, possibly by siphoning of the bordering continental lithosphere as the central part descends.
</p><p>Acknowledgements: All of the broad band seismic equipment used in the MANAS and GHENGIS projects was provided by the IRIS PASSCAL program. This work was supported by the NSF Continental Dynamics Program (EAR-0309927).</p>


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