Empirical constraints on extrusion mechanisms from the upper margin of an exhumed high-grade orogenic core, Sutlej valley, NW India

Abstract

The Early-Middle Miocene exhumation of the crystalline core of the Himalaya is a relatively well-understood process compared to the preceding phase of burial and prograde metamorphism in the Eocene-Oligocene. Highly deformed rocks of the Greater Himalayan Sequence (GHS) dominate the crystalline core, and feature a strong metamorphic and structural overprint related to the younger exhumation. The Tethyan Sedimentary Series was tectonically separated from the underlying GHS during the Miocene by the South Tibetan Detachment, and records a protracted and complex history of Cenozoic deformation. Unfortunately these typically low-grade or unmetamorphosed rocks generally yield little quantitative pressure-temperature-time information to accompany this deformation history. In parts of the western Himalaya, however, the basal unit of the Tethyan Sedimentary Series (the Haimanta Group) includes pelites metamorphosed to amphibolite facies. This presents a unique opportunity to explore the tectono-thermal evolution of crystalline rocks which record the early history of the orogen. Pressure-temperature-time-deformation (P-T-t-d) paths modelled for two Haimanta Group pelitic rocks reveal three distinct stages of metamorphism: (1) prograde Barrovian metamorphism to 610-620 °C at c. 7-8 kbars, with garnet growing over an early tectonic fabric (S₁); (2) initial decompression during heating to 640-660 °C at c. 6-7 kbars, with development of a pervasive crenulation cleavage (S₂) and staurolite and kyanite porphyroblast growth; (3) further exhumation during cooling, with minor retrograde metamorphism and modification of the pervasive S₂ fabric. Monazite growth ages constrain the timing of initial garnet growth (> 34 Ma), the start of D₂ and maximum burial (c. 30 Ma), and the termination of garnet growth (c. 28 Ma). Muscovite Ar/Ar ages indicate cooling through c. 300 °C at c. 13 Ma, from which we derive an initial exhumation rate of c. 1.3 mm year^-1 for the Haimanta Group. The underlying GHS was exhumed at a rate of 2.2 to 3 mm year^-1 during this time. The difference in exhumation rate between these two units is considered to reflect Early Miocene displacement on the intervening South Tibetan Detachment. Slower exhumation (c. 0.6 mm year^-1) of both units after c. 13 Ma followed the cessation of major displacement on this structure, after which time the Haimanta Group and the GHS were exhumed as one relatively coherent tectonic block.