We postulate that the inverted metamorphic sequence in the Lesser Himalayan Sequence of the Himalayan orogen is a finite product of its deformation and temperature history. To explain the formation of this inverted metamorphic sequence across the Lesser Himalayan Sequence with a focus on the Main Central Thrust (MCT) in eastern Bhutan, we determined the metamorphic peak temperatures by Raman spectroscopy of carbonaceous material and established the deformation temperatures by Ti‐in‐quartz thermobarometry and quartz c axis textures. These data were combined with thermochronology, including new and published ⁴⁰Ar/³⁹Ar ages of muscovite and published apatite fission track, and apatite and zircon (U‐Th)/He ages. To obtain accurate metamorphic, deformation, and closure temperatures of thermochronological systems, pressures and cooling rates for the period of interest were derived by inverse modeling of multiple thermochronological data sets, and temperatures were determined by iterative calculations. The Raman spectroscopy of carbonaceous material results indicate two temperature sequences separated by a thrust. In the external sequence, peak temperatures are constant across the structural strike, consistent with the observed hinterland‐dipping duplex system. In the internal temperature sequence associated with the MCT shear zone, each geothermometer yields an apparent inverted temperature gradient although with different temperature ranges, and all temperatures appear to be retrograde. These observations are consistent with the quartz microfabrics. Further, all thermochronometers indicate upward younging across the MCT. We interpret our data as a composite peak and deformation temperature sequence that formed successively and reflects the broadening and narrowing of the MCT shear zone in which the ductile deformation lasted until ~11 Ma.

中文翻译：

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不丹东部小喜马拉雅层序上的变形温度及其对主要中冲推力变形历史的启示

我们假设喜马拉雅造山带的小喜马拉雅序列中的倒置变质序列是其变形和温度历史的有限产物。为了解释横跨小喜马拉雅序列的该倒转变质序列的形成，重点是不丹东部的主要中央逆冲推力，我们通过碳质材料的拉曼光谱确定了变质峰温度，并通过Ti-in确定了变形温度石英热压法和石英c轴纹理。这些数据与热年代学相结合，包括新的和已发表的⁴⁰ Ar / ³⁹白云母的Ar年龄和已公布的磷灰石裂变径迹，以及磷灰石和锆石（U‐Th）/ He年龄。为了获得热年代学系统的准确变质，变形和闭合温度，通过对多个热年代学数据集进行逆向建模，得出感兴趣期间的压力和冷却速率，并通过迭代计算确定温度。碳质材料的拉曼光谱结果表明，两个温度序列被推力分开。在外部序列中，整个构造走向的峰值温度是恒定的，这与观测到的腹地-浸渍双工系统一致。在与MCT剪切带相关的内部温度序列中，每个地热仪都会产生一个明显的反向温度梯度，尽管温度范围不同，并且所有温度似乎都在逆行。这些观察结果与石英微织物一致。此外，所有温度计时器均显示MCT上方的年轻化现象。我们将数据解释为连续形成的复合峰和变形温度序列，反映了MCT剪切带的变宽和变窄，在MCT剪切带中塑性变形一直持续到〜11 Ma。