Himalayan inverted metamorphism and syn-convergence extension as a consequence of a general shear extrusion

Abstract

Two paradoxical geological features of the Himalaya are the syn-convergence extension and the inverted metamorphic isograds observed in the crystalline core zone of this orogen. This High Himalayan Crystalline Sequence corresponds to an up to 40 km thick sequence of amphibolite to granulite facies gneiss, bounded by the Main Central Thrust at the base, and by the extensional faults of the South Tibetan Detachment System at the top. Geochronological and structural data demonstrate that coeval movements along both the Main Central Thrust and South Tibetan Detachment System during Early to Middle Miocene times were related to a tectonically controlled exhumation of these high-grade metamorphic rocks. The High Himalayan Crystalline Sequence systematically shows an inverted metamorphic zonation, generally characterized by a gradual superposition of garnet, staurolite, kyanite, sillimanite + muscovite and sillimanite + K-feldspar isograds, from the base to the top of the unit. Recent kinematic flow analyses of these metamorphic rocks demonstrate the coexistence of both simple shear and pure shear during the ductile deformation. The simple shear component of such a general non-coaxial flow could explain a rotation of isograds, eventually resulting in an inversion. The pure shear component of the flow implies a thinning of the metamorphic sequence that must be balanced by a perpendicular stretching of the unit parallel to its boundaries. Inasmuch as seismic data show that both the Main Central Thrust and South Tibetan Detachment System converge at depth, a thinning of the wedge-shaped High Himalayan Crystalline Sequence should induce a ductile extrusion of these high-grade rocks toward the surface. Rapid extension at the top of the sequence could thus be the consequence of a general shear extrusion of this unit relative to its hanging wall. Moreover, this extensional movement should decrease with depth to become zero where the boundaries of the unit meet, accounting for the paradoxical convergence of the South Tibetan Detachment System toward the Main Central Thrust. Furthermore, a general flow combining simple shear and pure shear can reconcile inverted isograds with the lack of inverted pressure field gradient across the High Himalayan Crystalline Sequence, despite an intense non-coaxial deformation. In good agreement with the seismic, kinematic and P–T–t constraints on the Himalayan tectono-thermal evolution, general shear extrusion provides a consistent model accounting for both inverted isograds and rapid extension in a compressional orogenic setting.

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

“…(C) Ductile block deforming by pervasive simple shear. (D) Ductile block deforming by general shear with a pure shear component increasing towards the bottom of the wedge, as well as with time following a 'decelerating strain path' (Grasemann et al 1999;Vannay & Grasemann 2001).…”

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

“…Beaumont et al (2001,2004) presented a quantitative model in which this crustal channel was coupled to surface denudation at the High Himalayan front. In contrast, Grasemann et al (1999) and Vannay & Grasemann (2001) proposed a general shear model due to the requirements of strain compatibility, whereby the centre of the channel extruded by pure shear flow, rather than the heterogeneous simple shear model invoked by Grujic et al (1996).…”

Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

“…Alternative mechanisms include shear heating (England et al, 1992;Harrison et al, 1998), accretion of radiogenic crustal layers (Huerta et al, 1999;Guillot & Allemand, 2002), thermal conductivity contrast between the crystalline basement and its sedimentary cover (Pinet & Jaupart, 1987), large-scale fluid infiltration that lowers the melting point of overthrust terranes (Le Fort, 1975), slab break-off during collision (Kohn & Parkinson, 2002) and fast decompression of hot metamorphic rocks (Harris & Massey, 1994). Although differing in many respects, most recent models propose that both compressional and extensional faults exert a central role in the orographic development of the orogen (Harris & Massey, 1994;Grujic et al, 1996;Nelson et al, 1996;Beaumont et al, 2001;Vannay & Grasemann, 2001).…”