Deep structure of the Longling-Ruili fault undemeath Ruili basin near the eastern Himalayan syntaxis: insights from magnetotelluric imaging

Magnetotelluric (MT) geophysical profiling has been applied to the determination of the deep structure of the Longling-Ruili fault (LRF), part of a convergent strike-slip fault system, underneath thick Caenozoic cover in Ruili basin in southwestern Yunnan, China. The recorded MT data have been inverted using a two-dimensional (2-D) nonlinear conjugate gradients scheme with a variety of smooth starting models, and the resulting models show common subsurface conductivity structures that are deemed geological significant. The models show the presence of a conductive (5-60 Omega m) cover sequence that is thickest (1-1.5 km) in the centre of the basin and rapidly pinches out towards the margins. A half-graben structure is interpreted for the Ruili basin. This is underlain by about 7-10 km thick upper crustal layer of high resistivity (>200-4000 Omega m) that is dissected by steep faults, which we interpret to flatten at depth and root into an underlying mid-crustal conductive layer at about 10 km depth. The mid-crustal layer does not appear to have been severely affected by faulting; we interpret it as a zone of partial melt or intracrustal detachment. The MT models suggest SE directed thrusting of basement rocks in the area. The Longling-Ruili fault is interpreted as a NW-dipping feature bounding one of the identified upper crustal fragments underneath Ruili city. We suggest that MT imaging is a potent tool for deep subsurface mapping in this terrain. (C) 2003 Elsevier Science B.V. All rights reserved.