Early Miocene Exhumation of High-Pressure Rocks in the Himalaya: A Response to Reduced India-Asia Convergence Velocity

Giridas Maiti,Nibir Mandal

doi:10.3389/feart.2021.632806

Abstract

Low-viscosity channel flow, originating from a melt-weakened mid-crustal layer, is one of the most popular tectonic models to explain the exhumation of deep-seated rocks in the Greater Himalayan Sequence (GHS). The driving mechanism of such channel flow, generally attributed to focused erosion in the mountain front, is still debated, and yet to be resolved. Moreover, the channel flow model cannot explain eclogites in the GHS. In this study, we present a new two-dimensional thermo-mechanical numerical model, based on lubrication dynamics to demonstrate the exhumation process of deep crustal rocks in GHS. The model suggests that a dynamic-pressure drop in the Himalayan wedge, following a large reduction in the India-Asia convergence velocity (15 cm/yr at 50 Ma to nearly 5 cm/yr at ∼22 Ma) localized a fully developed extrusion zone, which controlled the pressure-temperature-time (P-T-t) path of GHS rocks. We show that the wedge extrusion, originated in the lower crust (∼60 km), was initially bounded by two oppositely directed ductile shear zones: the South Tibetan Detachment systems (STDS) at the top and the Higher Himalayan Discontinuity (HHD) at the bottom. With time the bottom shear boundary of the extrusion zone underwent a southward migration, forming the Main Central Thrust (MCT) at ∼17 Ma. Our model successfully reproduces two apparently major paradoxical observations in the Himalaya: syn-convergence extension and inverted metamorphic isograds. Model peak P (10–17 kb) and T (700–820°C) and the exhumation P-T-t path estimated from several Lagrangian points, traveling through the extrusion zone, are largely compatible with the petrological observations in the GHS. The model results account for the observed asymmetric P-T distribution between the MCT and STDS, showing peak P-T values close to the MCT. The lubrication dynamics proposed in this article sheds light on the fast exhumation event (&gt;1 cm/yr) in the most active phase of crustal extrusion (22-17 Ma), followed by a slowed-down event. Finally, our model explains why the extrusion zone became weak in the last 8-10 Ma in the history of India-Asia collision.

Highlights

Understanding the exhumation mechanism of high pressure (HP) and ultra-high pressure (UHP) rocks, such as eclogites is the key to modeling the crustal recycling processes in convergent plate boundaries (Ernst et al, 1997; Agard et al, 2009)
We show that the wedge extrusion, originated in the lower crust (∼60 km), was initially bounded by two oppositely directed ductile shear zones: the South Tibetan Detachment systems (STDS) at the top and the Higher Himalayan Discontinuity (HHD) at the bottom
Unlike the previous models that account for focused surface erosion at the mountain front to show the Greater (or Higher) Himalayan Sequence (GHS) extrusion in the form of a channel (Beaumont et al, 2001), the present model suggests that the extrusion process has been entirely controlled by the coupled tectonics of the Himalayan wedge and the Tibetan plateau, where the collapse of the plateau forced the deep-crustal materials in the Himalaya to extrude along a narrow zone, without any necessary involvement of localized rheological weakening or focused surface erosion

Summary

Introduction

Understanding the exhumation mechanism of high pressure (HP) and ultra-high pressure (UHP) rocks, such as eclogites is the key to modeling the crustal recycling processes in convergent plate boundaries (Ernst et al, 1997; Agard et al, 2009). The channel flow model, which explains the formation of GHS as a focused exhumation of low-viscosity mid-crustal materials through a channel-like passage between the Main Central Thrust (MCT) on the south and the South Tibetan Detachment System (STDS) on the north (Beaumont et al, 2001; Grujic et al, 2002) (Figure 1B), revolutionized the idea about the Himalayan tectonics. According to this model, the pressure gradient between the thick elevated Tibetan plateau and a relatively thin foreland resulted in a Poiseuille flow in the large hot orogens, coupled with active erosion at the mountain fronts to facilitate the exhumation of mid-crustal rocks. Several studies predicted the occurrence of similar narrow extrusion zones in subduction settings, which mediate the return flows and exhumation of ultrahigh pressure rocks (Warren et al, 2008; Li and Gerya, 2009; Malusà et al, 2015) in convergent plate margins

Results

Discussion

Conclusion