Crustal structure, restoration and evolution of the Greater Himalaya in Nepal-South Tibet: implications for channel flow and ductile extrusion of the middle crust

Abstract

Recent suggestions that the Greater Himalayan Sequence (GHS) represents a mid-crustal channel of low viscosity, partially molten Indian plate crust extruding southward between two major ductile shear zones, the Main Central thrust (MCT) below, and the South Tibetan detachment (STD) normal fault above, are examined, with particular reference to the Everest transect across Nepal-south Tibet. The catalyst for the early kyanite _ sillimanite metamorphism (650-680~ 7-8 kbar, 32-30 Ma) was crustal thickening and regional Barrovian metamorphism. Later sillimanite + cordierite metamorphism (600-680~ 4-5 kbar, 23-17 Ma) is attributed to increased heat input and partial melting of the crust. Crustal melting occurred at relatively shallow depths (15-19 km, 4-5 kbar) in the crust. The presence of highly radiogenic Proterozoic black shales (Haimanta-Cheka Groups) at this unique stratigraphic horizon promoted melting due to the high concentration of heat-producing elements, particularly U-bearing minerals. It is suggested that crustal melting triggered channel flow and ductile extrusion of the GHS, and that when the leucogranites cooled rapidly at 17-16 Ma the flow ended, as deformation propagated southward into the Lesser Himalaya. Kinematic indicators record a dominant southvergent simple shear component across the Greater Himalaya. An important component of pure shear is also recorded in flattening and boudinage fabrics within the STD zone, and compressed metamorphic isograds along both the STD and MCT shear zones. These kinematic factors suggest that the ductile GHS channel was subjected to subvertical thinning during southward extrusion. However, dating of the shear zones along the top and base of the channel shows that the deformation propagated outward with time over the period 20-16 Ma, expanding the extruding channel. The last brittle faulting episode occurred along the southern (structurally lower) limits of the MCT shear zone and the northem (structurally higher) limits of the STD normal fault zone. Late-stage breakback thrusting occurred along the MCT and at the back of the orogenic wedge in the Tethyan zone. Our model shows that the Himalayan-south Tibetan crust is rheologically layered, and has several major low-angle detachments that separate layers of crust and upper mantle, each deforming in different ways, at different times.The Tibetan plateau (Fig. 1), covering an area of >5 • 106 km, is an arid plateau with low relief and low erosion rates, and forms the largest area of high elevation (average elevation of 5023 m) and thick crust (65-80 km) on the planet (Fielding

Crustal–lithospheric structure and continental extrusion of Tibet

“…7) developed for the Greater Himalaya along the southern margin of the Tibetan Plateau was proposed initially from field geological mapping and strain data combined with P-T constraints and U-Pb dating of metamorphic rocks and leucogranites (Searle & Rex 1989;Grujic et al 2002;Searle et al 2003Searle et al , 2006Law et al 2004Law et al , 2006Searle & Szulc 2005;Godin et al 2006). The channel flow model infers that a partially molten middle crust layer was extruded south from beneath southern Tibet to the Greater Himalaya during the Early Miocene, at c. 23-15 Ma.…”

“…7. The Himalayan channel flow model after Searle et al (2006Searle et al ( , 2008Searle et al ( , 2010b showing the southward extrusion of a ductile partially molten layer of middle crust. Inset diagrams show the condensed right-way isograds beneath the South Tibetan Detachment and the condensed inverted isograds above the Main Central Thrust.…”

“…Photographs show the South Tibetan Detachment as exposed on Everest (top), and 70 km to the north of Everest at Dzakaa chu . Bottom diagram shows the restored South Tibetan Detachment profile for the Everest section, after Searle et al (2003Searle et al ( , 2006 during early Miocene leucogranite formation, and the flow pathways for its exhumation beneath the Lhotse detachment (LD) and the Qomolangma detachment (QD) branches of the South Tibetan Detachment. Chen & Yang (2004).…”

“…Clark & Royden 2000;Haines et al 2003) and (b) the Himalayan mid-crustal 'channel flow' model (e.g. Beaumont et al 2001Beaumont et al , 2004Grujic et al 2002;Searle et al 2003Searle et al , 2006Searle et al , 2010bLaw et al 2004Godin et al 2006).…”

Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction

“…Evidence for channel flow and/or ductile extrusion of mid-crustal rocks from the geologically recent Himalaya-Tibet orogen (Grujic et al 1996(Grujic et al , 2002Searle & Szulc 2005;Carosi et al 2006;Godin et al 2006;Hollister & Grujic 2006;Jessup et al 2006;Searle et al 2006) and related geodynamic models (Beaumont et al 2001Jamieson et al 2004Jamieson et al , 2006 …”