Deformation mechanisms and inverted thermal profile in the North Almora Thrust mylonite zone, Kumaon Lesser Himalaya, India
Deformation mechanisms and inverted thermal profile in the North Almora Thrust mylonite zone, Kumaon Lesser Himalaya, India
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印度小喜马拉雅库马翁北阿尔莫拉冲断糜棱岩带的变形机制和反演热剖面
DOI：
10.1016/0191-8141(95)00085-r
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发表时间：
1996
影响因子：
3.1
通讯作者：
G. Mitra
中科院分区：
文献类型：
--
作者：
P. Srivastava;G. Mitra
根据变形机制和微观结构解释了库马恩小喜马拉雅山（印度）2.2 公里厚的糜棱岩带的热结构。糜棱岩带位于北阿尔莫拉逆冲断层 (NAT) 沿线，这是一个 S 倾逆冲断层，标志着大型同形晶状飞峰 (阿尔莫拉飞峰) 的北侧，位于阿尔莫拉镇以北约 17 公里处。糜棱岩带是在始新世上层阿尔莫拉飞来峰（较大的蒙夏里冲断层的一部分）就位期间形成的，紧接着印度和欧亚大陆在中下始新世发生碰撞。该区域顶部附近的糜棱岩表现出长石和石英的动态再结晶，表明角闪岩级（500-600°C）的变形。在糜棱岩带的中心附近，长石发生断裂，石英发生动态再结晶，但程度低于该区域顶部附近的程度。这一证据，结合这些岩石中次生绿帘石+绿泥石、黑云母和方解石的出现，表明糜棱岩化发生在绿片岩级（500-400°C）。糜棱岩带底部附近存在黑云母，长石破碎并蚀变为绢云母，石英以波状消光和亚晶形成的形式表现出有限的塑性变形，表明变形过程中达到了 400°C 或更低的温度。因此，糜棱岩带岩石的变形机制和变质反应表明，变形温度从逆冲带顶部附近的 500-300°C 降低到其底部附近的约 400°C，这表明热剖面发生了倒转。我们将糜棱岩带中这种倒转的热剖面解释为由于逆冲时热量传递到冷下盘而导致该区域逐渐冷却的结果。
The thermal structure of a 2.2 km thick mylonite zone from the Kumaon Lesser Himalaya (India) is interpreted based on deformation mechanisms and microstructures. The mylonite zone lies along the North Almora Thrust (NAT), a S-dipping thrust that marks the northern flank of a large synformal crystalline klippe (Almora klippe), approximately 17 km north of the town of Almora. The mylonite zone evolved during the upper Eocene emplacement of the Almora klippe (part of a larger Munsiari thrust sheet), following the collision of India and Eurasia in lower-middle Eocene. Mylonitic rocks near the top of the zone show dynamicrecrystallization of both feldspar and quartz, indicating deformation at amphibolite grade (500–600°C). Near the center of themylonite zone, the feldspars are fractured and quartz is dynamically recrystallized though to a lesser degree than near the top of the zone. This evidence, combined with the occurrence of secondary epidote + chlorite, biotite and calcite in these rocks, suggests that mylonitization occurred at greenschist grade (500–400°C). Near the base of the mylonite zone, biotite is present, feldspars are fractured and altered to sericite, and quartz shows limited plastic deformation in the form of undulose extinction and subgrain formation, indicating that temperatures of 400°C or less were reached during deformation. Deformation mechanisms and metamorphic reactions in the mylonite zone rocks thus point to a decrease in the temperature of deformation from 500–300°C near the top of the thrust zone to about 400°C near its base, suggesting an inverted thermal profile. We interpret this inverted thermal profile in the mylonite zone to have resulted from gradual cooling of the zone by heat transfer to a cold footwall at the time of thrusting.