New constraints on the timing of partial melting and deformation along the Nyalam section (central Himalaya): implications for extrusion models

New structural, U–Th/Pb and Ar/Ar data along the Nyalam section constrain the timing of partial melting, crystallization and deformation in the Greater Himalayan Sequence. Prograde metamorphism was followed by the onset of partial melting at <em>c.</em> 30 Ma. In the central Greater Himalayan Sequence, <em>in situ</em> melts crystallized between 24 and 18 Ma. Subsequent cooling was very fast (<em>c.</em> 200 °C Ma⁻¹) and coeval with the emplacement of undeformed dykes that lasted until <em>c.</em> 15 Ma. In the upper Greater Himalayan Sequence, fast cooling continued until <em>c.</em> 13 Ma. Combined with published <em>P</em>–<em>T</em> and thermochronological data from the Langtang and Dudh Kosi valleys, these data imply that: (a) the partial melt zone thinned over time; (b) the end of melting preceded the end of motion on the Main Central Thrust and the South Tibetan Detachment by 6 and 2 Ma, respectively; (c) the South Tibetan Detachment possibly initiated at <em>c.</em> 25 Ma, probably reactivating a pre-existing thrust; and (d) the present-day topography has been established for &lt;6 Ma and focused erosion on the present-day southern slopes of the Himalaya was not active at the time of the exhumation of the Greater Himalayan Sequence. These observations suggest that the Main Central Thrust/South Tibetan Detachment systems are not passive structures induced by focused erosion, as has been suggested previously by some lower crustal channel flow models.

本研究获得的聂拉木剖面（Nyalam section）新构造数据、铀-钍/铅（U-Th/Pb）及氩-氩（Ar/Ar）定年数据，约束了大喜马拉雅序列（Greater Himalayan Sequence）内部分熔融、结晶及变形作用的发生时限。研究区在经历进变质作用（Prograde metamorphism）后，于约30 Ma时启动部分熔融过程。在大喜马拉雅序列中部，原位熔体的结晶作用发生于24~18 Ma之间。后续冷却速率极快（约200 ℃·Ma⁻¹），该阶段与持续至约15 Ma的未变形岩脉侵位事件同期。
在大喜马拉雅序列上部，快速冷却过程延续至约13 Ma。
结合已发表的朗塘（Langtang）与杜德科西（Dudh Kosi）河谷的温压（P-T）及热年代学数据，本研究得到以下认识：(a) 部分熔融带随时间推移逐渐变薄；(b) 熔融作用的终结分别早于主中央冲断层（Main Central Thrust）及藏南拆离系（South Tibetan Detachment）的活动终止时限约6 Ma和2 Ma；(c) 藏南拆离系可能于约25 Ma启动，其大概率活化了先期存在的逆冲断层；(d) 现今喜马拉雅地貌格局形成于距今不足6 Ma的时段，且在大喜马拉雅序列剥露作用（exhumation）时期，现今喜马拉雅南坡的集中侵蚀作用尚未活跃。
上述观测结果表明，主中央冲断层/藏南拆离系系统并非如部分下地壳通道流模型（lower crustal channel flow models）此前提出的那样，是由集中侵蚀作用诱发的被动构造。