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The Nyalam detachment is part of the east-west striking South Tibetan Detachment System exposed in the Nyalam area, southern Tibet. Seventeen muscovite and biotite 40Ar/39Ar age spectra and three K-feldspar multidiffusion domain modelling and cooling ages are presented for metamorphic rocks, leucogranite, granite and mylonite, collected from the Nyalam detachment and surrounding areas. The majority of the 40Ar/39Ar results are cooling ages related to exhumation, which therefore place important constraints on formation of the Nyalam detachment and exhumation history of the region. Muscovite 40Ar/39Ar ages from mylonite within the normal fault system and from the footwall of the fault are 16.1–15.2 Ma. Biotite 40Ar/39Ar ages from the same samples are 15.6–14.8 Ma, slightly younger than the muscovite cooling ages. K-feldspar multidiffusion domain modelling suggests that samples collected from both mylonite on the fault surface and from footwall rocks underwent rapid cooling between 16.1 Ma and 11.7 Ma.Ages and cooling histories in the Nyalam detachment and Greater Himalayan metamorphic sequence have similar characteristics and time constraints: the K-feldspar modelling indicates a sudden change in cooling rates for these regions during 15.5–14.0 Ma and c. 12 Ma, respectively. Taking the regional thermal history into account, cooling could be associated with significant northward surface movement triggered by detachment normal faulting in the Nyalam area. The Nyalam detachment and Greater Himalayan metamorphic sequence experienced similar cooling and exhumation histories during c. 17.0–11.7 Ma. Formation of the Nyalam detachment may have accompanied the southward extrusion of the Greater Himalaya zone along shear zones formed in response to underthrusting of the Indian plate beneath southern Tibet.

Abstract:Lying at the junction of the Dabashan, Longmenshan and Qinling mountains, the Micangshan Orogenic Belt coupled with a basin is a duplex structure and back‐thrust triangular belt with little horizontal displacement, small thrust faults and continuous sedimentary cover. On the basis of 3D seismic data, and through sedimentary and structural research, the Micangshan foreland can be divided into five subbelts, which from north to south are: basement thrust, frontal thrust, foreland depression‐back‐thrust triangle, foreland fold belt or anticline belt, and the Tongjiang Depression. Along the direction of strike from west to east, the arcuate structural belt of Micangshan can be divided into west, middle and east segments. During the collision between the Qinling and Yangtze plates, the Micangshan Orogenic Belt was subjected to the interaction of three rigid terranes: Bikou, Foping, and Fenghuangshan (a.k.a. Ziyang) terranes. The collision processes of rigid terranes controlled the structural development of the Micangshan foreland, which are: (a) the former collision between the Micangshan‐Hannan and Bikou terranes forming the earlier rudiments of the structure; and (b) the later collision forming the main body of the structural belt. The formation processes of the Micangshan Orogenic Belt can be divided into four stages: (1) in the early stage of the Indosinian movement, the Micangshan‐Hannan Rigid Terrane was jointed to the Qinling Plate by the clockwise subduction of the Yangtze Plate toward the Qinling Plate; (2) since the late Triassic, the earlier rudiments of the Tongnanba and Jiulongshan anticlines and corresponding syncline were formed by compression from different directions of the Bikou, Foping and Micangshan‐Hannan terranes; (3) in the early stage of the Himalayan movement, the Micangshan‐Hannan Terrane formed the Micangshan Nappe torwards the foreland basin and the compression stresses were mainly concentrated along both its flanks, whereas the Micangshan‐Hannan Terrane wedged into the Qinling Orogenic Belt with force; (4) in the late stage of the Himalayan movement, the main collision of the Qinling Plate made the old basement rocks of the terrane uplift quickly, to form the Micangshan Orogenic Belt. The Micangshan foreland arcuate structure was formed due to the non‐homogeneity of terrane movement.

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