Three-dimensional magnetotelluric imaging of the SE Gonghe Basin: Implication for the orogenic uplift in the northeastern margin of the Tibetan plateau

Abstract

To better understand the upper crustal shortening and orogenic uplift in the northeastern margin of the Tibetan Plateau, we have utilized magnetotelluric (MT) data at 116 sites along two lines around the SE Gonghe Basin to determine a three-dimensional (3D) resistivity model by a 3D MT inversion package ModEM. Our resistivity model reveals a significant feature of step discontinuities between high and low resistivity blocks at different depths in the upper and middle crust. These discontinuities correspond well with the sharp topography variations, thus showing clear tendencies to be the thrust faults beneath the Gonghe Basin responsible for the formation of Gonghe Nan Shan (GHNS) thrust belt. Our 3D resistivity model also shows a large higher resistivity body at depths of 3–10 km located beneath the hot springs around the Basin. We interpreted this anomaly as resistive granite body, which acts as hot dry rocks (HDRs) to generate heat for the hot springs on the surface. Our results also reveal a pronounced conductive layer at depths of 10–25 km in the middle-upper crust, which is most likely caused by partial melting for heating the above HDRs. This high conductivity layer would behave in a ductile regime near the bottom of the high resistivity body and act as a decollement for the crustal shortening and deformation in the northeastern margin of the Tibetan Plateau.