Orogen‐parallel deformation of the Himalayan midcrust: Insights from structural and magnetic fabric analyses of the Greater Himalayan Sequence, Annapurna‐Dhaulagiri Himalaya, central Nepal
Orogen‐parallel deformation of the Himalayan midcrust: Insights from structural and magnetic fabric analyses of the Greater Himalayan Sequence, Annapurna‐Dhaulagiri Himalaya, central Nepal
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DOI：
10.1002/2016tc004244
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发表时间：
2016-11
期刊：
影响因子：
4.2
通讯作者：
A. Parsons;E. Ferré;R. Law;G. Lloyd;R. Phillips;M. Searle
中科院分区：
文献类型：
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作者：
A. Parsons;E. Ferré;R. Law;G. Lloyd;R. Phillips;M. Searle
位于尼泊尔中部安纳布尔纳峰-道拉吉里地区的喜马拉雅山（大喜马拉雅层序，GHS）的变质核心记录了中地壳演化过程中的造山带平行拉伸。磁化率各向异性和基于场的结构分析表明，GHS角闪岩相核心的中地壳变形发生在扁圆/亚扁圆应变状态下，并伴随着低角度北倾叶理的形成。位于 GHS 顶部的叶状结构内的磁性和矿物拉伸线状记录了右侧造山带平行拉伸。我们提出，中地壳流内的扁圆应变适应了印度和弓形造山锋之间的倾斜汇聚，而不需要在上地壳中进行应变分配。扁平化也可能促进了与造山带平行的熔体迁移，以及沿造山带走向以约 50-100 公里间隔的 km3 级淡色花岗岩顶峰之间熔体贫乏区域的发展。流动停止后，持续的倾斜会聚导致上地壳应变在逆冲断层上的造山带垂直会聚与正断层和走滑断层上的造山带平行伸展之间分配。在安纳布尔纳峰-道拉吉里喜马拉雅山，造山带平行拉伸线被解释为从中地壳造山带垂直挤压到上地壳造山带平行拉伸过渡的记录。我们的研究结果表明，中地壳流动和上地壳伸展不能同时维持，并支持喜马拉雅地区的其他研究，这些研究提出中中新世中期从中地壳造山带垂直挤压到上地壳造山带平行伸展的整个造山带过渡。扁圆应变和造山带平行拉伸的 3D 性质无法通过喜马拉雅造山带的 2D 数值模拟来复制。
The metamorphic core of the Himalaya (Greater Himalayan Sequence, GHS), in the Annapurna‐Dhaulagiri region, central Nepal, recorded orogen‐parallel stretching during midcrustal evolution. Anisotropy of magnetic susceptibility and field‐based structural analyses suggest that midcrustal deformation of the amphibolite facies core of the GHS occurred under an oblate/suboblate strain regime with associated formation of low‐angle northward dipping foliation. Magnetic and mineral stretching lineations lying within this foliation from the top of the GHS record right‐lateral orogen‐parallel stretching. We propose that oblate strain within a midcrustal flow accommodated oblique convergence between India and the arcuate orogenic front without the need for strain partitioning in the upper crust. Oblate flattening may have also promoted orogen‐parallel melt migration and development of melt‐depleted regions between km3 scale leucogranite culminations at ~50–100 km intervals along orogen strike. Following the cessation of flow, continued oblique convergence led to upper crustal strain partitioning between orogen‐perpendicular convergence on thrust faults and orogen‐parallel extension on normal and strike‐slip faults. In the Annapurna‐Dhaulagiri Himalaya, orogen‐parallel stretching lineations are interpreted as a record of transition from midcrustal orogen‐perpendicular extrusion to upper crustal orogen‐parallel stretching. Our findings suggest that midcrustal flow and upper crustal extension could not be maintained simultaneously and support other studies from across the Himalaya, which propose an orogen‐wide transition from midcrustal orogen‐perpendicular extrusion to upper crustal orogen‐parallel extension during the mid‐Miocene. The 3‐D nature of oblate strain and orogen‐parallel stretching cannot be replicated by 2‐D numerical simulations of the Himalayan orogen.