Age constraints on the deformation style of the South Tibetan Detachment System in Garhwal Himalaya

Montemagni, Chiara; Iaccarino, Salvatore; Montomoli, Chiara; Carosi, Rodolfo; Jain, Arvind K.; Villa, Igor M.

doi:10.3301/ijg.2018.07

Cited by 18 publications

(11 citation statements)

References 46 publications

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“…After 18 Ma, the southern shear boundary the wedge extrusion zone migrated southward, eventually resulting in initiation of the MCT after the most active phase of the GHS exhumation event (Figure 8B). This late initiation of MCT in the GHS evolution has been extensively reported in the recent publications (Montomoli et al, 2013;Carosi et al, 2016Wang et al, 2016;Montemagni et al, 2018) (Figure 8A).…”

Section: Frontiers Insupporting

confidence: 82%

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

Maiti

Mandal

2021

Front. Earth Sci.

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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.

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Section: Frontiers Insupporting

confidence: 82%

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

Maiti

Mandal

2021

Front. Earth Sci.

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“…Kinematic vorticity recorded at the base of the MCTz suggests a large pure shear component. Temporal bracket for the end of STDS shearing in the Alaknanda–Dhauli Ganga valleys from Iaccarino et al () and Montemagni et al (). The simple and pure shear components of the general flow are qualitatively represented in the grey boxes.…”

Section: Discussionmentioning

confidence: 99%

Three‐dimensional vorticity and time‐constrained evolution of the Main Central Thrust zone, Garhwal Himalaya (NW India)

et al. 2020

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Vorticity estimates based on porphyroclasts analysis are limited by the extrapolation to three dimensions of two‐dimensional data. We describe a 3D approach based on the use of X‐ray micro‐computed tomography that better reflects the real 3D geometry of the porphyroclasts population. This new approach for kinematic vorticity analysis in the Munsiari Thrust mylonites, the lower boundary of the Main Central Thrust zone (MCTz) in Indian Himalaya, indicates a large pure shear component during non‐coaxial shearing. 40Ar/39Ar ages of micas along the mylonitic foliation of the Munsiari and Vaikrita thrusts (the upper boundary of the MCTz) constrain thrust activity to 5–4 and 8–9 Ma, respectively. Available kinematic vorticity analyses of the Vaikrita mylonites suggest the dominance of a simple shear component. Combining these data, we suggest that the southward and structurally downward shift of deformation along the MCTz was accompanied by a progressive increase in the pure shear component in a general shear flow.

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“…It is widely acknowledged that dioctahedral micas are less susceptible to isotope resetting than coexisting trioctahedral micas, irrespective of the effective loss mechanism (whether volume diffusion, recrystallization or alteration -Dahl 1996). Several field-based studies have demonstrated a negligible re-equilibration of Ar isotopes only in white mica re-equilibrated under temperature conditions below 500-550 °C (e.g., Di Vincenzo et al 2004;Augier et al 2005;Warren et al 2012;Villa et al 2014;Montemagni et al 2018Montemagni et al , 2020. However, Ar isotopes diffusion can be efficient at medium-high temperature and/or for low pressure (e.g., for decompression) conditions (Warren et al 2012).…”

Section: Discussionmentioning

confidence: 99%

A thermal event in the Dolpo region (Nepal): a consequence of the shift from orogen perpendicular to orogen parallel extension in central Himalaya?

Nania

Montomoli

Iaccarino

et al. 2021

JGS

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In the Lower Dolpo Region (central Himalaya), structurally above the South Tibetan Detachment System (STDS), blastesis of static micas have been recognized. Nevertheless, until now, very little work has been done to constrain the tectonic meaning and the timing of this static mica growth. In this work we investigate samples from the STDS hanging wall, characterized by three populations of micas, defining (i) S1 and (ii) S2 foliations, and (iii) M3 static mineral growth cutting both foliations. New geochronological 40Ar/39Ar analyses on the microtexturally-different micas, complemented by microstructural and compositional data, allow to place temporal constraints on the static (re)crystallization at the STDS hanging wall. Results point out homogeneous chemical compositions and ages of micas within the investigated samples, irrespective of the structural positions. Phlogopite and muscovite on S1 and S2, and post-kinematic biotite yielded 40Ar/39Ar ages within 14-11 Ma with decreasing ages upward. We suggest that mica (re)crystallized under static conditions during a late thermal event at low structural levels (c. 15-18 km), after cessation of the ductile activity of the shear zone. We hypothesize that this later thermal event is kinematically linked to the switch from orogen perpendicular to orogen parallel extension in central Himalaya.Supplementary material: [Electron microprobe analyses of biotite and white mica] is available at https://doi.org/10.6084/m9.figshare.c.5509998Thematic collection: This article is part of the Isotopic Dating of Deformation collection available at: https://www.lyellcollection.org/cc/isotopic-dating-of-deformation

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Age constraints on the deformation style of the South Tibetan Detachment System in Garhwal Himalaya

Cited by 18 publications

References 46 publications

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

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

Three‐dimensional vorticity and time‐constrained evolution of the Main Central Thrust zone, Garhwal Himalaya (NW India)

A thermal event in the Dolpo region (Nepal): a consequence of the shift from orogen perpendicular to orogen parallel extension in central Himalaya?

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