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

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 (>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.

Impact of Decelerating India‐Asia Convergence on the Crustal Flow Kinematics in Tibet: An Insight From Scaled Laboratory Modeling

“…The model estimate suggests an extensional strain of 0.8 to 2.2 × 10 −8 yr −1 in this region (Figure 8b‐ii), which fairly agrees with the geodetic estimates (1.6 to 2.2 × 10 −8 yr −1 , Ge et al., 2015). The time series analyses of our model crustal‐flow and the strain pattern support a two‐stage evolution of the Tibetan plateau: (a) a prolonged period of contraction since the beginning of collision to as late as 19 ± 3 Ma, which was coupled with (b) a phase of wide‐spread horizontal extension in the mid‐Miocene time (19 ± 3 Ma) preferentially in central Tibet (Maiti et al., 2020; Maiti & Mandal, 2021; Wang et al., 2014).…”

“…However, to investigate exclusively the effects of reducing convergence rate on the mode of Tibetan crustal deformations, we chose a kinematic model boundary condition by externally controlling the velocities at the model boundaries. This kinematic approach to modeling is similar to those used in many earlier India-Asia collision models (Bajolet et al, 2015;Bischoff & Flesch, 2018;Cook & Royden, 2008;Fournier et al, 2004;Maiti & Mandal, 2021;Schellart et al, 2019;Y. Yang & Liu, 2013).…”

“…The extensional tectonics is shown to have continued till the present‐day with a west to east younging trend in rift initiation (Bian, Gong, Zuza, et al., 2020; Cooper et al., 2015). During this period the contractional tectonics shifted its focus from the interior of the Tibetan plateau to its margins (S. Li, Currie, et al., 2015; Maiti & Mandal, 2021), including the regions of the Himalayan wedge (Main Central Thrust, 23‐13 Ma; Main Boundary Thrust, 12‐8 Ma; Main Frontal Thrust, 5‐0 Ma) (Huyghe et al., 2001; Meigs et al., 1995; Montemagni et al., 2018; Najman, 2006; Soucy La Roche et al., 2018). On the other side, the north‐eastern (Northern Qilian Shan thrust, 9‐0 Ma) and eastern (Longmen Shan thrust, 10‐0 Ma) margins of Tibet underwent fresh contractional deformation pulses at this late Cenozoic stage (Clark et al., 2005; E. Wang et al., 2012; Xu & Kamp, 2000; S. Yang et al., 2007).…”

India-Asia slowing convergence rate controls on the Cenozoic Himalaya-Tibetan tectonics

“…The timing of the rapid cooling phase varies but is typically <20 Ma (Searle, 2015). Shallow rock emplacement and cooling paths can only be reproduced by models with decreasing convergence, supporting the idea that exhumation is favoured by slowing convergence (Maiti and Mandal, 2021). The extrusion of deep crust resulted in the emplacement of the GHCS between the STD and the MCT (Godin et al, 2006), leading to inverted metamorphic isograds above the MCT, in a "hot-on-cold" sequence (Goscombe et al, 2018;Hunter et al, 2018;Searle, 2015) and right-way-up metamorphic isograds below the STD (Godin et al, 2006;Searle, 2015) (Figure 8c).…”