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Thermometry and Microstructural Analysis Imply Protracted Extensional Exhumation of the Tso Morari UHP Nappe, Northwestern Himalaya: Implications for Models of UHP Exhumation
Tectonics ( IF 3.3 ) Pub Date : 2020-11-25 , DOI: 10.1029/2020tc006482
Sean P. Long 1 , Matthew J. Kohn 2 , Buchanan C. Kerswell 2 , Jesslyn K. Starnes 1 , Kyle P. Larson 3 , Nolan R. Blackford 1 , Emmanuel Soignard 4
Affiliation  

Documenting the processes that facilitate exhumation of ultrahigh‐pressure (UHP) rocks at convergent margins is critical for understanding orogen dynamics. Here, we present structural and temperature data from the Himalayan UHP Tso Morari nappe (TMN) and overlying nappes, which we integrate with published pressure‐temperature‐time constraints to refine interpretations for their structural evolution and exhumation history. Our data indicate that the 5.5‐km‐thick TMN is the upper portion of a penetratively deformed ductile slab, which was extruded via distributed, pure shear‐dominated, top‐down‐to‐east shearing. Strain in the TMN is recorded by high‐strength quartz fabrics (density norms between 1.74 and 2.86) and finite strain data that define 63% transport‐parallel lengthening and 46% transport‐normal shortening. The TMN attained peak temperatures of ~500–600°C, which decrease in the overlying Tetraogal and Mata nappes to ~150–300°C, defining a field gradient as steep as 67°C/km. Within the overlying nappes, quartz fabric strength decreases (density norms between 1.14 and 1.21) and transport‐parallel lengthening and transport‐normal shortening decrease to 14% and 18%, respectively. When combined with published 40Ar/39Ar thermochronometry, quartz fabric deformation temperatures as low as ~330°C indicate that the top‐to‐east shearing that exhumed the TMN continued until ~30 Ma. Peak temperatures constrain the maximum depth of the overlying Mata nappe to 12.5–17.5 km; when combined with published fission‐track thermochronometry, this provides further support that the TMN was not underplated at upper crustal levels until ~30 Ma. The long‐duration, convergence‐subnormal shearing that exhumed the TMN outlasted rapid India‐Asia convergence by ~15 Myr and may be the consequence of strain partitioning during oblique convergence.

中文翻译:

测温和微结构分析暗示西北喜马拉雅山Tso Morari UHP推覆的旷日持久的延伸尸体化:对UHP尸体化模型的启示

记录有助于在收敛边缘挖掘超高压(UHP)岩石的过程对于理解造山带动力学至关重要。在这里,我们介绍了喜马拉雅超高压Tso Morari尿布(TMN)和上覆尿布的结构和温度数据,我们将这些数据与已发布的压力-温度-时间约束相结合,以完善对其结构演变和发掘历史的解释。我们的数据表明,厚度为5.5 km的TMN是渗透变形的延性板的上部,该板是通过分散的,纯剪切为主的,自上而下的向东剪切而挤出的。TMN中的应变由高强度石英织物(密度范数在1.74和2.86之间)和有限的应变数据记录,这些数据定义了63%的运输平行伸长和46%的运输正常伸长。TMN达到约500–600°C的峰值温度,在上覆的四足动物和马塔绒毛中下降到约150–300°C,从而定义了高达67°C / km的场梯度。在上覆的尿布中,石英织物的强度下降(密度标准在1.14和1.21之间),平行运输的伸长和正常运输的缩短分别降低到14%和18%。结合出版时40 Ar / 39 Ar热计时,石英织物的变形温度低至〜330°C,这表明发芽TMN的由顶向东的剪切一直持续到〜30 Ma。峰值温度将上覆马塔推覆的最大深度限制为12.5-17.5公里;当与已发表的裂变径迹测年法结合使用时,这进一步提供了TMN直到〜30 Ma才在地壳上层未进行底层镀覆的支持。TMN的长时间持续,辐合-次高剪切使印度-亚洲的快速辐合比〜15 Myr持久,这可能是斜向辐合过程中应变分配的结果。
更新日期:2020-12-15
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