Magnetotelluric data from a 150-km-long profile crossing the Banggong-Nujiang suture (BNS), central Tibet, acquired as part of the International Deep Profiling of Tibet and the Himalaya (INDEPTH) project, have been examined for crustal and upper mantle structure. Strike and dimensionality analyses demonstrate that regional-scale electrical structures are two-dimensional and oriented approximately parallel to surface geological strike. As seen elsewhere in Tibet, the double thickness crust is generally characterized by resistive upper crust (hundreds to thousands of ohm meters) overlying conductive middle and lower crust (tens to hundreds of ohm meters), but in detail, there are lateral variations at all levels. Regionally, a northward transition from thick (∼45 km) to thin (∼15 km) resistive upper crust coincides with (1) the surface trace of the BNS, (2) a prominent strand of the Karakorum-Jiali fault system, (3) northward decrease in upper mantle seismic velocities and increase in attenuation, and (4) pronounced northward onset of seismic polarization anisotropy. The latter two seismological features have been taken to mark the northern limit of Indian mantle lithosphere thrust beneath southern Tibet. On the basis of our electrical model, we speculate that (1) the resistive upper crustal root beneath the Neogene Lunpola and Duba basins was produced by crustal shortening localized along the northern edge of the Lhasa terrane; (2) the low midcrustal resistivity beneath the BNS reflects enhanced Neogene melting and/or metamorphic dewatering of relatively fertile subduction zone complex rocks; (3) observed steep upper crustal low-resistivity anomalies are produced by hydrothermal fluids within active faults localized within and adjacent to the BNS; and (4) these strike-slip and extensional fault arrays are surface manifestations of lithosphere-penetrating shear localized along the northern edge of the underthrust Indian plate.
- India-Asia collision
- magnetotelluric surveying