Crustal structure and deformation in the northeast India-Asia collision zone: constraints from receiver function analysis

Crustal structure across the IndiaAsia collision zone (Tidding Suture) in the northeast Himalaya bounded by the Eastern Himalayan Syntaxis (EHS) is investigated using the P-wave receiver function (RF) method. The analysed data included three-component waveforms of teleseismic earthquakes recorded by a linear array of 11 broad-band seismic stations. The RFs and inverted shear wave velocity models reveal azimuthally varying crustal structure. The RFs for earthquakes from the northeast back azimuths are conspicuous by absence of P-to-S converted phase at the Moho discontinuity. Inverted velocity model ascribe this to absence of the typical step velocity jump at the Moho in a narrow section of the EHS bordering the indenting Indian Plate and pierced by travelling waves. In contrast, teleseismic waves arriving from southeast back azimuth sample different litho-tectonic blocks of the Himalayan collision zone and inverted models show northeast dipping Moho across the Tidding Suture. Compared to an overall thickness of >70 km in the northwest and central Himalaya, the crust across the Tidding Suture is only about 55 km thick. This is attributed to a slower rate of convergence in this part of the collision zone. The Moho structure beneath Indian Plate and southeastern Tibetan Plateau reveal opposite dip directions with their colliding margin placed just east of the Walong Thrust. The inverted shear wave velocity models show evidence of intracrustal low-velocity layer whose strength varies across the Tidding Suture. The magnitude of velocity reduction beneath Lohit Plutonic Complex (Trans Himalaya) favour partial melt as a possible mechanism whereas south of the Tidding Suture, where velocity reduction is comparatively less, fluids generated by dehydration reactions appear to be the source for velocity reduction.