Joint Analysis of GPS and Shear-Wave Splitting Data to Understand Large-Scale Continental Deformation in Central Asia

The recent increase in seismic-anisotropy and surface GPS data for Central Asia permits a comprehensive joint analysis of shear-wave-splitting and GPS observations to investigate the vertical coherence of lithospheric deformation in the region. In order to compare the surface deformation field inferred from GPS data and mantle deformation field inferred from shear-wave splitting data we calculate the surface kinematic vorticity from the velocity gradient tensor determined using GPS and Quaternary fault slip data. Kinematic voriticites range from -10 to 10. For ~50% of the region kinematic voritcities are greater than 1 indicating the region has undergone an external rotation in addition to the internal strain. We estimate the amount of external rotation using the line rotation method, using the observed shear-wave splitting orientations or Quaternary fault orientations as deformational invariant directions, i.e, orientations not rotated by the internal strain. The estimated external rotations generate a large-scale clockwise rotation of the eastern Himalayan syntaxis and a counter clockwise rotation of South China. The pattern of rotations reveal several regions of coherent rotation, producing patterns similar to published block models. We remove the external rotation from the velocity gradient tensor field and compare the updated surface deformation field with the mantle deformation field and find a strong correlation between the two. We find the observed spatial variations in anisotropy reflect the large-scale pattern of lithospheric deformation, as well as a change in deformational style from simple shear on the Tibetan Plateau that transitions to pure shear in surrounding regions. The close correspondence between the surface and mantle deformation fields argue for vertically coherent deformation, and given the large contribution of body forces to the surface field, additionally argues for strong crust-mantle mechanical coupling.