Metamorphic response to collision in the Central Himalayan Orogen

Abstract

Closure of the Neotethys Ocean and high-angle continent-continent collision between India and Asia after about 55 Ma resulted in low-angle subduction of the Indian plate below the Tibetan Plateau and by ~30 Ma established an arcuate 2300 km long, shallow north-dipping metamorphic fold-thrust belt in the foreland. This Himalayan Metamorphic Front quickly established an Upper-Plate/Lower-Plate paired metamorphic architecture centred on the median High Himal Thrust that largely controlled subsequent evolution. The Upper-Plate is a thick slab of high-T/moderate-P high-grade migmatized metamorphic rocks, whereas the Lower-Plate is an inverted series of moderate-T/high-P schists in two crustal wedges, the broad Main Central Thrust Zone and at lowest structural levels the Footwall, below the Basal Main Central Thrust. Spatial and temporal patterns of metamorphic response in the evolving Himalayan Metamorphic Front has been characterized in a large-scale integrated structural-metamorphic study based on 8 profiles across eastern Nepal. Metamorphic response at all structural levels is established using a large dataset (n ~ 160) of internally consistent quantitative PT determinations, petrology of metapelite samples, semi-quantitative P-T paths, metamorphic mapping and metamorphic field gradients. These results are integrated with previously published metamorphic studies, structural profiles and metamorphic chronology. From these datasets the architecture and evolution of the Himalayan Metamorphic Front is constrained by rock kinematics, metamorphic field gradients showing discontinuities, and diachronous metamorphism with contrasting P-T evolutions at different structural levels. Each of the three panels constituting the Himalayan Metamorphic Front: Upper-Plate, Main Central Thrust Zone and Footwall, experienced distinctly different tectono-metamorphic histories. Crustal processes operating during metamorphism and exhumation differ between the Upper- and Lower-Plates. The Upper-Plate experienced long-lived metamorphism starting from at least 28–38 Ma and tracking low ΔP/ΔT clockwise P-T paths that culminated at ~19–27 Ma in peak high-grade condition with 27–31 °C/km thermal regimes. Protracted high-grade conditions produced significant partial melt, which facilitated gravity driven southward extrusion involving internal ductile flow processes. Southward extrusion of the Upper-Plate was accommodated by coeval reverse movement on the High Himal Thrust and top down to the north, normal reactivation of the South Tibet Detachment System between ~22–10 Ma. Transport of this thick slab to the south resulted in further prograde burial of the Lower-Plate below, culminating in peak metamorphism at the highest pressures attained in the Lower-Plate rocks. The Lower-Plate consists of at least two deeply buried (7.5–9.4 kb) crustal wedges, both of which experienced steep ΔP/ΔT hairpin clockwise P-T paths with isothermal decompression. The Main Central Thrust Zone experienced 20–26 °C/km metamorphism at ~12–22 Ma and was exhumed along the Basal Main Central Thrust after ~11 Ma. Whereas, loading by the Main Central Thrust Zone gave rise to further prograde burial of the Footwall, which experienced 16–21 °C/km metamorphism at ~4–10 Ma and was exhumed along the Main Boundary Thrust at ~3–9 Ma. In contrast to the Upper-Plate, both crustal wedges were exhumed by a process of out-wedging, which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls. This three-stage foreland propagating lateral exhumation history resulted in telescoping of the Himalayan Metamorphic Front in concert with peak metamorphic events at different structural levels during main phase orogenesis, and is not the result of superimposed retrograde reactivation of the belt.