Extrusion vs. duplexing models of Himalayan mountain building 3: duplexing dominates from the Oligocene to Present

The Himalaya is a natural laboratory for studying mountain-building processes. Concepts of extrusion and duplexing have been proposed to dominate most phases of Himalayan evolution. Here, we examine the importance of these mechanisms for the evolution of the Himalayan crystalline core via an integrated investigation across the northern Kathmandu Nappe. Results reveal that a primarily top-to-the-north shear zone, the Galchi shear zone, occurs structurally above and intersects at depth with the Main Central thrust (MCT) along the northern flank of the synformal Kathmandu Nappe. Quartz <i>c</i>-axis fabrics confirm top-to-the-north shearing in the Galchi shear zone and yield a right-way-up deformation temperature field gradient. U-Pb zircon dating of pre-to-syn- and post-kinematic leucogranites demonstrates that the Galchi shear zone was active between 23.1 and 18.8 Ma and ceased activity before 18.8–13.8 Ma. The Galchi shear zone is correlated to the South Tibet detachment (STD) via consistent structural fabrics, lithologies, metamorphism, and timing for four transects across the northern margin of the Kathmandu Nappe. These findings are synthesized with literature results to demonstrate (1) the broad horizontality of the STD during motion and (2) the presence of the MCT-STD branch line along the Himalayan arc. The branch line indicates that the crystalline core was emplaced at depth via tectonic wedging and/or channel tunnelling-type deformation. We proceed to consider implications for the internal development of the crystalline core, particularly in the light of discovered tectonic discontinuities therein. We demonstrate the possibility that the entire crystalline core may have been developed via duplexing without significant channel tunnelling, thereby providing a new end-member model. This concept is represented in a reconstruction showing Himalayan mountain-building via duplexing from the Oligocene to Present.

喜马拉雅山脉（Himalaya）是研究造山作用过程（mountain-building processes）的天然实验室。挤出构造（extrusion）与双冲构造（duplexing）的相关概念被认为主导了喜马拉雅演化的多数阶段。本文通过跨加德满都推覆体（Kathmandu Nappe）北部的综合调查，探讨了这些构造机制对喜马拉雅结晶核演化的重要性。研究结果显示，一条以向北剪切为主的剪切带——加尔奇剪切带（Galchi shear zone）——在构造上位于向斜型加德满都推覆体北翼的主中央逆冲断层（Main Central thrust, MCT）之上，并沿该推覆体北翼在深部与其相交。石英c轴组构（quartz c-axis fabrics）证实了加尔奇剪切带内的向北剪切作用，并显示出正序变形温度场梯度。对前同构造-同构造期及后构造期淡色花岗岩（leucogranites）的锆石U-Pb定年（U-Pb zircon dating）结果表明，加尔奇剪切带的活动时限为23.1 Ma至18.8 Ma，并于18.8~13.8 Ma前停止活动。通过跨加德满都推覆体北缘四条剖面的一致构造组构、岩性、变质作用及定年结果，加尔奇剪切带可与藏南拆离系（South Tibet detachment, STD）对比。本研究将上述发现与已有文献成果综合分析后得出两点认识：其一，藏南拆离系活动期间整体近水平；其二，喜马拉雅弧沿线存在主中央逆冲断层-藏南拆离系分支线。该分支线表明，结晶核通过构造楔入及/或通道隧穿型变形在深部就位。我们进一步探讨了结晶核内部演化的启示，尤其是结合其中发现的构造不连续性。研究证实，整个结晶核可能仅通过双冲构造形成且未发生显著的通道隧穿变形，由此提出了一种全新的端元模型。该概念通过一则重建模型得以体现，该模型显示喜马拉雅造山作用自渐新世至今均以双冲构造为主导。