Abstract New tectono-metamorphic data along the Alaknanda-Dhauli Ganga valleys in the Garhwal Himalaya (NW India) are presented focusing on sheared rocks of the Main Central Thrust zone (MCTz), one of the first order regional structures in the Himalaya. In regard of large hot orogens (LHOs), the MCTz corresponds to the base of the low-viscosity infrastructure. Our new findings support that the MCTz is a c. 4 km thick mylonitic zone, affecting different lithologies and encompassing a possible (previously defined) protolith boundary. Four selected mylonitic samples, at different structural position along the MCTz, were studied with a multi-disciplinary approach in detail, supporting that an inverted metamorphic field gradient, with temperature (T) and pressure (P) ranging from c. 500 up to c. 700 °C and c. 0.50–0.60 GPa up to >1.0 GPa, was present. This inverted P-T array is also discernable by the distribution of Al-rich minerals (e.g. chloritoid, staurolite, kyanite) in the intercalation of sheared quartzite. In situ U-(Th)-Pb monazite geochronology indicates that the high-T shearing in the upper part of the MCTz mylonites developed from c. 20 Ma on after a stage of prograde metamorphism that lasted until c. 23 Ma. The integration of our data with available structural, metamorphic and geochronological information along the MCTz profile, in the study area, sheds light on the development of the associated inverted metamorphic sequence (IMS). These data carry also empirical finite patterns to be tested against theoretical models investigating the possible roles of viscous heating and “tectonic overpressure” in the development of the IMS in the Himalaya, and possibly in LHOs in general. Both phenomena, if have been active at all, are not obvious in regard of the current state of geothermobarometric methods. On the contrary, a model for the tectonic assembly of the IMS is favored. The anomalous apparent P and T inverted field gradients are interpreted to be the result of the differential and diachronous transport along a shear zone experiencing heterogeneous vertical thinning linked to the non-coaxial flow.

中文翻译：

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沿 Alaknanda 和 Dhauli Ganga 河谷（Garhwal Himalaya，印度西北部）的主要中央逆冲带：对倒置变质序列的洞察

摘要 介绍了 Garhwal 喜马拉雅（印度西北部）Alaknanda-Dhauli Ganga 河谷的新构造变质数据，重点是主要中央冲断带 (MCTz) 的剪切岩，这是喜马拉雅山的一级区域结构之一。对于大型热造山带 (LHO)，MCTz 对应于低粘度基础设施的基础。我们的新发现支持 MCTz 是 c。4 公里厚的糜棱带，影响不同的岩性并包含可能的（先前定义的）原岩边界。使用多学科方法详细研究了沿 MCTz 不同结构位置的四个选定糜棱岩样品，支持倒变质场梯度，温度 (T) 和压力 (P) 范围为 c。500 到 c。700 °C 和 c。0.50–0.60 GPa 至 >1.0 GPa，在场。这种倒置的 PT 阵列也可以通过剪切石英岩夹层中富含铝的矿物（例如绿泥石、星形石、蓝晶石）的分布来辨别。原位U-(Th)-Pb独居石年代学表明MCTz糜棱岩上部的高T剪切作用是从c. 经过一个持续到 c. 的顺行变质阶段之后的 20 Ma。23 马。我们的数据与研究区沿 MCTz 剖面的可用结构、变质和地质年代信息的整合，揭示了相关倒变质序列 (IMS) 的发展。这些数据还带有经验有限模式，将根据研究粘性加热和“构造超压”在喜马拉雅山脉 IMS 发展中的可能作用的理论模型进行测试，并且可能在一般的 LHO 中。就地温气压方法的当前状态而言，这两种现象如果一直活跃的话，都不是很明显。相反，IMS 的构造组合模型受到青睐。异常明显的 P 和 T 反转场梯度被解释为沿剪切带经历与非同轴流相关的非均质垂直变薄的差异和历时传输的结果。