The Arun mega\|antiform, a large N—S structure transversal to the tectonic trend of the E Nepal Himalaya, is a tectonic window offering a complete section of the Himalayan nappe pile, from the Lesser Himalayan zone t...The Arun mega\|antiform, a large N—S structure transversal to the tectonic trend of the E Nepal Himalaya, is a tectonic window offering a complete section of the Himalayan nappe pile, from the Lesser Himalayan zone to the Tethyan Himalaya. At the northern end of the Arun tectonic window (ATW), the Ama Drime—Nyonno Ri range of south Tibet exposes a section of that portion of the Main Central Thrust (MCT) zone and Lesser Himalayan Crystallines (LHC) which elsewhere in Nepal is concealed below the overlying Higher Himalayan Crystalline (HHC) nappe (Fig. 1). As throughout the Himalaya at the structural level of the MCT, the ATW is characterized by an inverted metamorphic field gradient characterized by a progression from chlorite to sillimanite grade from low to high structural levels of the nappe pile. Metamorphic peak temperatures rise from circa 400℃ in the pelitic and psammitic Precambrian metasediments of the Lesser Himalayan Tumlingtar Unit, to 550～620℃ in the overlying LHC, to over 700℃ in the muscovite\|free Barun Gneiss, the lowermost HHC unit in the Arun valley.展开更多
文摘The Arun mega\|antiform, a large N—S structure transversal to the tectonic trend of the E Nepal Himalaya, is a tectonic window offering a complete section of the Himalayan nappe pile, from the Lesser Himalayan zone to the Tethyan Himalaya. At the northern end of the Arun tectonic window (ATW), the Ama Drime—Nyonno Ri range of south Tibet exposes a section of that portion of the Main Central Thrust (MCT) zone and Lesser Himalayan Crystallines (LHC) which elsewhere in Nepal is concealed below the overlying Higher Himalayan Crystalline (HHC) nappe (Fig. 1). As throughout the Himalaya at the structural level of the MCT, the ATW is characterized by an inverted metamorphic field gradient characterized by a progression from chlorite to sillimanite grade from low to high structural levels of the nappe pile. Metamorphic peak temperatures rise from circa 400℃ in the pelitic and psammitic Precambrian metasediments of the Lesser Himalayan Tumlingtar Unit, to 550～620℃ in the overlying LHC, to over 700℃ in the muscovite\|free Barun Gneiss, the lowermost HHC unit in the Arun valley.
