Seismic Crustal and Mantle Deformation Indicators Along the Himalayan Arc

Abstract:

We compare two seismological indicators of lithospheric deformation along the Himalayan arc from the foothills to the Tibetan plateau, from the eastern to the western syntaxis. The first method is shear wave splitting of core phases such as SKS, which has poor depth resolution but is typically presumed to be primarily sensitive to mantle anisotropy. The data come from a network in western Tibet as well as a worldwide compilation database of results from temporary and permanent networks.
The second method is an azimuthal harmonic analysis of receiver functions. Azimuthally varying arrivals in radial and transverse component receiver functions are generated by dipping isotropic contrasts as well as by contrasts in dipping foliation with anisotropy. Both show a two-lobed pattern with backazimuth that identifies the strike of the dipping isotropic contrast or of the foliation. If dipping isotropic contrasts and dipping foliation are formed in the same deformation regime, their strikes are expected to be parallel to one another. The method is different from splitting of P-to-S converted phases. We analyze receiver function data from temporary deployments and permanent stations extending from West Tibet to the eastern syntaxis.

SKS results show considerable variation across the arc, including variation in splitting times as well as fast axes on a short spatial scale of ~ 50 km. The scatter is larger within the Himalaya and towards the syntaxes than in the plateau interior. The foliation and dip signal in receiver functions is dominated by crustal arrivals. The mapped strikes are parallel to along-arc faults and to known strike-slip systems and show high regional coherence. We show maps of statistical correlations between splitting fast axes and receiver function strikes and show models of expected orientations seen by the two methods in crust and mantle for different deformation models and compositions. We compare the results to different orogens such as Taiwan, where SKS fast axes, receiver function strikes, and orogen axes are all parallel, and the Zagros, where receiver function strikes are orogen-parallel but SKS splits are null. We discuss the results in terms of tectonic models of the Himalaya and Tibetan Plateau, e.g. northward extent of Indian lithosphere and deformation mechanisms operating in the area.