Detection of Widespread Fluids in the Tibetan Crust by Magnetotelluric Studies

Abstract

Magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north-to-south width of the Tibetan plateau. The integrated conductivity (conductance) of the Tibetan crust ranges from 3000 to greater than 20,000 siemens. In contrast, stable continental regions typically exhibit conductances from 20 to 1000 siemens, averaging 100 siemens. Such pervasively high conductance suggests that partial melt and/or aqueous fluids are widespread within the Tibetan crust. In southern Tibet, the high-conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust. In northern Tibet, the conductive layer is at 30 to 40 kilometers and is due to partial melting. Zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow.

Finite frequency tomography in southeastern Tibet: Evidence for the causal relationship between mantle lithosphere delamination and the north–south trending rifts

“…Several recent Himalayan-Tibetan tectonic investigations have proposed a weak middle-lower crust, which allows crustal flow, to explain the building of the southeastern margin of the Tibetan Plateau [Clark and Royden, 2000;Beaumont et al, 2001;Schoenbohm et al, 2006]. This idea is supported by results from geophysical studies that show evidence for localized partial melt within the middle crust in Tibet [Kind et al, 1996;Wei et al, 2001] and a radial anisotropy that can be caused by channel flow within the mid-to-lower Tibetan crust [Shapiro et al, 2004]. However, contrary to the notion that the north -south rift zones in the Tibetan Plateau are shallow features, formed by the eastward motion of the shallow crust that are decoupled from the mantle lithosphere by a low-viscosity lower crust, our results suggest an upper mantle origin of the rift zones, at least in southeastern Tibet, where mantle lithosphere delamination plays a key role in the process of the rise of the plateau.…”

Crustal–lithospheric structure and continental extrusion of Tibet

“…Thus we can infer that from c. 10 Ma to the present time northern Tibet has had thick crust and hot mantle in the region north of the Bangong suture and south of the Kun Lun. It is possible that the Ulugh Muztagh peraluminous leucogranites are much more abundant at depth, and some of the 'bright spots' identified at high mid-crust levels in the INDEPTH III seismic profile (Tilmann et al 2003) and magnetotelluric studies (Wei et al 2001) may be pockets of Ulugh Muztagh type melts forming today.…”

“…Tomographic images show a subvertical high-velocity zone from c. 100 to 400 km depth located immediately south of the Bangong suture zone that has been interpreted as representing downwelling Indian mantle (Tilmann et al 2003). The INDEPTH III seismic profile extends north into the Qiangtang terrane, where the presence of crustal fluids in parts of eastern Tibet probably corresponds to low-viscosity ductile flow (Wei et al 2001;Fig. 6.…”

Analytical and numerical simulations of uplift processes at the Tibet-Sichuan boundary

“…Therefore, it should contain more radioactive elements, leading to high temperature in the lower crust. Magnetotelluric results reveal that the electrical resistivity of the lower crust in the Tibetan plateau is lower than ambient regions such as Sichuan basin and Tarim basin, suggesting the possible existence of partial melting (Nelson et al 1996;Wei et al 2001;Unsworth et al 2005). Second, according to the distribution of earthquake locations, the seismic activity is very weak in the lower crust (Priestley et al 2008), which Topography (m) Fig.…”

“…According to the results of seismic source mechanism, magnetotelluric imaging and anisotropic tomography results, the lower crust of the Tibetan plateau is probably a layer with high temperature and low viscosities (Nelson et al 1996;Wei et al 2001;Unsworth et al 2005;Jiménez-Munt and Platt 2006;Priestley et al 2008;Yang et al 2012;Liu et al 2014). First, the crustal thickness of the plateau is almost twice as much as the worldwide averaged value (Jiménez-Munt and Platt 2006).…”