| In the Nyainqentanglha Shan, S Tibet, a SE-dipping, low angle normal fault exposes a section of Tibetan middle crust. Field relations and structural analysis of the Nyainqentanglha shear zone (NSZ) are consistent with it originating at relatively low angle (<45°) and experiencing minor (10--15°) back-rotation. U-Th-Pb dating of footwall zircon and monazite reveals felsic intrusions of various ages, including Cretaceous-early Tertiary and Miocene granitoids. Miocene ages span >10 m.y., suggesting semi-continuous or episodic magmatism. Miocene granitoids appear to have been high-temperature melts (calculated saturation temperatures >800°C). Geochemical and isotopic analyses show Gangdese-arc affinity and indicate mantle heat and mass transfer in their formation. This calc-alkaline magmatism continued after the Tethys Ocean closed at the start of the Cenozoic, and throughout the Indo-Asian collision. The undeformed footwall granitoids suggests that Mesozoic and Cenozoic deformation of the Lhasa block was "thin-skinned", concentrated in supra-crustal assemblages (<15--17 km depth). This, and the lack of migmatites in the NSZ footwall, imply that at the granitoid emplacement level (∼17--30 km) the middle crust was not mobile, precluding an extensive mid-crustal partial melt zone and large scale channel flow. 40Ar/39Ar thermochronologic studies of footwall potassium feldspars reveal that samples collected within several kilometers below the detachment experienced cooling prior to emplacement of some of the young leucogranites, indicating that the background isothermal structure of the upper crust was little perturbed since ∼15 Ma. This, plus the granitoids' high melting temperatures and lack of penetrative deformation, requires that melts formed at lower crustal levels and were emplaced rapidly to the mid-crust. This could explain seismic reflection results showing highly conductive areas in the middle crust along the Nyainqentanglha rift, as magmas would be semi-continuously emplaced in the middle crust, and recent seismic studies may have imaged the youngest episode of magmatism. Alternatively, hydrothermal fluids could account for the "bright spot" anomalies seen by seismic imaging. Miocene magmatism could be due to either (1) steep subduction of Indian continental lithosphere below southern Tibet, or (2) lithosphere-scale rifting as exemplified by the Nyainqentanglha normal fault system and normal fault earthquakes in the lowermost crust or upper mantle. |
