Journal of Earth System Science ( IF 1.9 ) Pub Date : 2021-02-05 , DOI: 10.1007/s12040-020-01539-1 Lawrence Kanyan , Arvind K Jain , Sandeep Singh
Abstract
The Greater Himalayan Sequence (GHS), constituting the anatectic core of the Himalaya, is generally modelled as a mid-crustal southward extruding channel or wedge. Movements along the Main Central Thrust (MCT) in the south and the South Tibetan Detachment System (STDS) in the north and exhumation along the Himalayan front played an important role in the extrusion of the GHS from beneath the Tibetan plateau during the Miocene. To understand the kinematics of these orogen-scale shear zones, it is important to constrain the percentage of pure shear associated with them. In this paper, we present the kinematic vorticity data from the Main Central Thrust Zone (MCTZ), Alaknanda and Dhauli Ganga Valleys (Garhwal), Uttarakhand Himalaya. The mean kinematic vorticity number (Wm), which can be used to calculate the percentage of pure shear, has been estimated by analysing the rotational behaviour of rigid grains in a ductile matrix. The analysis reveals that pure shear provides significant contribution (30–52%) to the deformation associated with southward ductile shearing along the MCT, with the highest mean kinematic vorticity number (Wm) values close to the MCT. The results provide important quantitative constraints for the boundary conditions in the extrusion models. The Wm values from within the anatectic core have not been reported as most of the vorticity gauges fail due to increased deformation temperatures in this region.
Research highlights
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Orogen-scale mid-crustal southward extruding channel or wedge models deformation of the Great Himalayan Sequence (GHS) of the anatectic core, whose kinematics is to be understood by constraining the percentage of pure shear.
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Vorticity estimation near the Main Central Thrust Zone (MCTZ) is performed along the Alaknanda–Dhauli Ganga Valleys, Uttarakhand Himalaya along with critical analysis of published vorticity data from the other areas.
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Mean kinematic vorticity number (Wm), a quantitative estimator of pure shear percentage during non-coaxial deformation in a shear zone, varies between 0.675 and 0.875 within the MCTZ, corresponding to a pure shear percentage between 30% and 52%.
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A general trend of decreasing pure shear component towards the channel boundaries is explained by velocity profile within an extruding channel of hot and low-viscosity mid-crustal rocks and observed from the compiled vorticity data from other Himalayan traverses.
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Our results agree with the channel flow conceptual model and provide quantitative constraints on the percentage of pure shear associated with deformation within the GHS.
中文翻译：
北阿坎德邦喜马拉雅山Alaknanda–Dhauli Ganga谷（Garhwal）中央主推力带的涡度模式
摘要
大喜马拉雅层序（GHS）构成喜马拉雅山的南极挤压岩心，通常被建模为中地壳向南挤压通道或楔形。在中新世期间，南部的主要中央逆冲运动（MCT）和北部的南部藏传拆离系统（STDS）以及喜马拉雅山前线的掘尸活动在GHS从青藏高原的下方挤出中起了重要作用。要了解这些造山带尺度剪切带的运动学，重要的是限制与它们相关的纯剪切的百分比。在本文中，我们提供了来自北阿坎德邦喜马拉雅山的阿拉克南达和达卢里恒河谷（Garhwal）的主要中央逆冲带（MCTZ）的运动涡度数据。平均运动涡度数（W m可以用来计算纯剪切百分比的）是通过分析韧性矩阵中刚性晶粒的旋转行为来估算的。分析表明，纯剪切作用对沿MCT向南延性剪切的变形贡献显着（30％至52％），其最高平均运动涡度数（W m）接近MCT。结果为挤出模型中的边界条件提供了重要的定量约束。由于大多数涡度计由于该区域的变形温度升高而失效，因此尚未报道过来自于洋盐核心的W m值。
研究重点
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造山带规模的中地壳向南挤压通道或楔形模型模拟了南极核心喜马拉雅层序（GHS）的变形，应通过限制纯剪切力的百分比来理解其运动学。
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沿着北阿坎德邦喜马拉雅山的Alaknanda-Dhauli Ganga山谷进行了主要中央推力区（MCTZ）附近的涡度估算，并对来自其他地区的涡度数据进行了严格分析。
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平均运动涡度数（Wm）是剪切区域中非同轴变形过程中纯剪切百分比的定量估计值，在MCTZ范围内在0.675和0.875之间变化，对应于30％和52％之间的纯剪切百分比。
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沿通道边界减小纯剪切分量的总体趋势可以通过热和低粘度中地壳岩石挤压通道内的速度剖面来解释，并可以从其他喜马拉雅横断面的汇编涡度数据中观察到。
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我们的结果与通道流概念模型相符，并为与GHS内变形相关的纯剪力百分比提供了定量约束。
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