Crustal rheology of the Himalaya and Southern Tibet inferred from magnetotelluric data

Abstract

The Cenozoic collision between the Indian and Asian continents formed the Tibetan plateau, beginning about 70 million years ago. Since this time, at least 1,400 km of convergence has been accommodated by a combination of underthrusting of Indian and Asian lithosphere, crustal shortening, horizontal extrusion and lithospheric delamination. Rocks exposed in the Himalaya show evidence of crustal melting and are thought to have been exhumed by rapid erosion and climatically forced crustal flow. Magnetotelluric data can be used to image subsurface electrical resistivity, a parameter sensitive to the presence of interconnected fluids in the host rock matrix, even at low volume fractions. Here we present magnetotelluric data from the Tibetan-Himalayan orogen from 77 degrees E to 92 degrees E, which show that low resistivity, interpreted as a partially molten layer, is present along at least 1,000 km of the southern margin of the Tibetan plateau. The inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in Southern Tibet.

Geochemical variations in Miocene adakitic rocks from the western and eastern Lhasa terrane: Implications for lower crustal flow beneath the Southern Tibetan Plateau

“…A model of channel flow in the middle crust has been proposed to explain these phenomena (Beaumont et al, 2001(Beaumont et al, , 2004Hodges, 2006;Hodges et al, 2001;Medvedev and Beaumont, 2006;Nelson et al, 1996;Unsworth et al, 2005). Subsequently, King et al (2007) proposed that the Kuday dykes are the result of partial melting of the Asian plate where it extends south of the IYS as a mid-crustal ductile channel structure beneath Southern Tibet.…”

“…Geophysical studies have shown that the middle crust below the Lhasa terrane and the High Himalaya range share some similar anomalies, including the bright spots in Southern Tibet among present-day plutons in the middle crust, which are analogous to the Miocene leucogranite plutons exposed farther south in the High Himalaya range (e.g., Chen et al, 1996;Gaillard et al, 2004;Nelson et al, 1996;Unsworth et al, 2005). A model of channel flow in the middle crust has been proposed to explain these phenomena (Beaumont et al, 2001(Beaumont et al, , 2004Hodges, 2006;Hodges et al, 2001;Medvedev and Beaumont, 2006;Nelson et al, 1996;Unsworth et al, 2005).…”

“…For example, geophysical studies have shown that the partially molten middle crust is being extruded southward below Southern Tibet on the south side of the IYS (Nelson et al, 1996;Unsworth et al, 2005), which demonstrates the existence of crustal channel flow beneath Southern Tibet. On the other hand, extensive crustal channel flow beneath the Himalaya area seems unlikely, because of the occurrence of the north-dipping rigid and cold Indian craton, as well as the Karakoram fault that interrupted possible channel flow processes in the western Himalaya (Leech, 2008).…”

“…Geophysical studies have revealed similar geophysical anomalies in the mid-crust of the Lhasa terrane (block) and the Himalaya range (Nelson et al, 1996;Unsworth et al, 2005), and a channel flow model has been proposed to explain this phenomenon (Beaumont et al, 2001(Beaumont et al, , 2004Clark and Royden, 2000;Hodges, 2006;Hodges et al, 2001;Klemperer, 2006;Medvedev and Beaumont, 2006;Nelson et al, 1996). Based on a study of the Kuday dykes emplaced on the south side of the Indus-Yalu suture (IYS), King et al (2007) proposed southward ductile flow of Asian crust beneath Southern Tibet.…”

Resistivity distribution from mid-crustal conductor to near-surface across the 1200 km long Liquiñe-Ofqui Fault System, southern Chile

“…The origin of these electric conductors has early been attributed to graphite (Frost et al 1989;Jödicke 1992). More recently, links to aqueous fluids and secondary minerals in fault zones (Korja et al 2008;Brasse et al 2009;Weckmann et al 2012), to partial melt (Unsworth et al 2005;Hill et al 2009), and to metamorphic processes (Wannamaker et al 2014;Zhang et al 2015) have been discussed. Worldwide, it is observed that these conductors have an upper depth limit near the brittle-ductile transition (Jones 1992;Jiracek 1995).…”