Deep electrical resistivity structure across the Gyaring Co Fault in Central Tibet revealed by magnetotelluric data and its implication

Abstract

As one of the most prominent deformation features on Tibetan plateau, a series of N-S systematic rifts and V-shaped conjugate strike-slip faults are well developed in central-southern Tibet. However the mechanism for the formation of the rifts and conjugate strike-slip faults is still controversial. An east-west trending magnetotelluric (MT) array has been operated across the Gyaring Co Fault (GCF) at the northern end of the Xainza-Dingjye Rift (XDR) in Lhasa block. Three-dimensional (3-D) inversions are employed to image lithospheric resistivity structure. Combined with the previous north-south MT 3-D inversion result, electrical resistivity models reveal obvious conductive layer in the mid-to-lower crust beneath the XDR and apparent resistivity change in the vicinity of the GCF. The calculated depth-integrated conductivity shows that the weak mid-to-lower crust (~30 km- ~ 60 km) characterized by high conductance (≥ ~10,000 S) and melt fraction of ~5–13%, is mainly distributed between the Indus-Yarlung suture (IYS) and the GCF, whereas the relatively rigid crust characterized by low conductance (≤ ~2000 S) and melt fraction of ~2–4% appears to the northeast of the GCF. The weak mid-to-lower crust in southern Tibet and the abrupt difference in the resistivity characteristics of the mid-to-lower crust in the vicinity of the GCF, suggest the formation of the rifts in Lhasa block is closely associated with the weak mid-to-lower crust. Taken together with the upper mantle seismic features, the weak mid-to-lower crust is believed to be attributed to the underplated Indian plate. An alternative geodynamic model is presented to respond to the eastward extrusion during the ongoing N-S convergence between the Indian and Eurasian plates, that the rigid crust in central Tibet is sandwiched by the weak mid-to-lower crust in southern and northern Tibet, and the rifts form above the weak mid-to-lower crust, whereas the conjugate strike-slip faults develop on the rigid crust in central Tibet.