The Munsiari and Vaikrita thrusts are crucial to understanding the geodynamic evolution of the Himalayas. However, the structural architecture and kinematic evolution of these thrusts and associated deformation zones are still poorly understood. Furthermore, the existing criteria for field identification of the Vaikrita Thrust are varied and ambiguous. We address these issues through extensive large-scale mapping and structural analysis across the Alaknanda, Dhauliganga, and Bhagirathi valleys in the Garhwal Himalayas. Our results demonstrate that the Munsiari and Vaikrita thrust zones are the tectonic mosaics of high-strain cores and low- to moderate-strain damage zones. The large-scale mapping also brings out the tectonic interleaving of different group rocks in both thrust zones. New structural observations point to a shared history of progressive ductile shearing during which successive folds and mylonite foliation were developed and transposed in both thrust zones. The Munsiari Thrust consistently appears as a distinct litho-tectonic boundary across the valley sections. In contrast, the Vaikrita Thrust presents varied surface expressions. While manifesting as a ductile shear zone in the Bhagirathi Valley, it is exposed as a brittle-fault zone in the Dhauliganga Valley. The Vaikrita Thrust lacks distinct surface expression in the Alaknanda Valley. In such regions, it is routinely marked by the `kyanite-in isograd´ criterion, i.e., the first appearance of Barrovian kyanite in the Greater Himalayan Sequence. Our discovery of Barrovian kyanite occurrences in the Lesser Himalayan rocks challenges the tenability of the `kyanite-in isograd´ criterion for delineating the Vaikrita Thrust. Instead, we propose that identifying a high-strain anomaly within the strain profile from the footwall to the hanging wall represents the most reliable criterion for mapping the Vaikrita Thrust in such terrains. The shear zone framework approach is potentially useful for unraveling the geometry and kinematic evolution of an orogen.

中文翻译：

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印度加瓦尔喜马拉雅山 Munsiari 和 Vaikrita 冲断带的结构解剖


蒙西亚里和瓦克里塔逆冲断层对于了解喜马拉雅山的地球动力学演化至关重要。然而，人们对这些逆冲断层和相关变形区的结构体系和运动学演化仍然知之甚少。此外，现有的 Vaikrita Thrust 现场识别标准多种多样且不明确。我们通过对喜马拉雅山 Garhwal 的 Alaknanda、Dhauliganga 和 Bhagirathi 山谷进行广泛的大规模测绘和结构分析来解决这些问题。我们的结果表明，Munsiari 和 Vaikrita 逆冲带是高应变核心和低至中应变损伤区的构造镶嵌体。大比例尺测绘还揭示了两个冲断带不同群岩的构造交错。新的结构观察指出了渐进韧性剪切的共同历史，在此期间，连续的褶皱和糜棱岩叶理在两个冲断带中发育和转移。蒙西亚里逆冲断层始终表现为横跨山谷部分的独特岩石构造边界。相比之下，Vaikrita Thrust 呈现出不同的表面表情。虽然在 Bhagirathi 山谷中表现为韧性剪切带，但在 Dhauliganga 山谷中却表现为脆性断层带。外克里塔冲断层在阿拉卡南达山谷缺乏明显的地表表现。在这些地区，它通常以“等梯度蓝晶石”标准为标志，即巴罗夫蓝晶石首次出现在大喜马拉雅序列中。我们在小喜马拉雅岩石中发现的巴罗夫蓝晶石矿点对描述 Vaikrita 冲断层的“等梯度蓝晶石”标准的可行性提出了挑战。 相反，我们建议识别从下盘到上盘应变剖面内的高应变异常代表了在此类地形中绘制 Vaikrita 逆冲断层图的最可靠标准。剪切带框架方法对于阐明造山带的几何形状和运动学演化可能有用。