Pelitic metasediments of Devonian to Triassic age of the Tethyan Himalaya (TH) in Western Nepal from Inner Dolpo to Marsyandi Valley, were sampled for tectono-metamorphic and paleomagnetic studies in several areas. In theses areas, the metamorphic conditions are still poorly understood. In order to constrain the age of remagnetization and the temperature reached during Himalayan orogeny, petrological and structural surveys, illite crystallinity, vitrinite reflectance and K/Ar dating on sub 2 μm illite fraction were performed. These constraints on the thermal evolution are necessary for the proper interpretation of paleomagnetic results. At least three ductile deformational (D1, D2 and D3) and one metamorphic (M1) event were recognized. The first deformation (D1) is shown by the well-developed bedding-parallel foliation (S1 = S0). Preferred orientation of the recrystallized minerals along S1 and also in pressure shadows of quartz clasts shows that the metamorphism (M1) was synchronous with D1. It means that south-vergent folding and shearing in the TH post-date the peak metamorphism. Therefore, the age of low-grade metamorphism in the TH should be older than the timing of MCT activity, granite emplacement and normal faulting along the South Tibetan Detachment System (STDS) around 20 Ma. The D2 is represented by mega-scale south-vergent folds as well as meso- and micro-scale drag folds, crenulation folds and crenulation and slaty cleavages. Strong top-to-the south shearing has occured along S2. No recrystallization has occurred along S2. Although the evidences of D3 were not seen in the thin sections, many north-vergent folds superposed on the south-vergent folds are seen in the outcrops. These folds represent the latest ductile deformation event in the area. Probably this event was related to the north-vergent collapse of the Tethyan sedimentary sequence along the STDS. In Dolpo area, according to illite crystallinity, the metamorphic conditions are very low (Kübler index: 0.45 < 1c < 0.46). Illite K/Ar from Shey Gompa give Late Cretaceous ages, indicating an incomplete resetting of the system as expected from the Ic data. In Hidden Valley, Ic is around 0.336 and the K/Ar age on illite is around 29-30 Ma. In Manang area, Ic = 0.26 ± 0.03 and the K/Ar ages range from 29 to 41 Ma and from 25 to 31 Ma for the 0.6-2μm and 0.2-0.6 μm fraction respectively. An incomplete resetting of the K/Ar ages cannot be excluded. Nevertheless those ages are consistent with previously published Ar/Ar biotite and phologopite ages from the Yellow Formation in Marsyandi valley in the hanging wall of the South Tibetan Detachment System, ranging from 29.9 to 27.1 Ma (Coleman and Hodges, 1998). Therefore we interpret our new ages as formation of new illites or resetting of detrital one during Eohimalayan metamorphism. In the studied area, the Eohimalayan metamorphism reached temperatures around 300-350°C implying a crustal thickening of the TH sedimentary cover. In the Kali Gandaki valley, sampling was carried out just west of Marpha. This area is clearly in the hanging wall of the STDS and close to the main faults limiting the Thakkhola graben. In Marpha area, K/Ar ages are surprisingly very young (12-15 Ma) in comparison to adjacent areas. Also Ic = 0.25 ± 0.3 indicating similar temperatures than in Manang area. But vitrinite reflectance as well as the occurrence of chloritoids indicates a temperature higher than in the other investigated areas. The dating results are in good agreement with the Thakkhola fault system initiation before 14 Ma based on hydrothermal muscovite (Coleman and Hodges, 1995) and with muscovite Ar/Ar cooling ages from Kali Gandaki just north of the Annapurna detachment (12-13 Ma, Godin et al., 1998). Therefore, Marpha area has undergone a second young thermal event probably related to large scale fluid circulation during the initial stage of the Thakkhola graben formation around 14-15 Ma, before the deposition of the Tetang Fm started. While the sedimentary sequence of the TH is very homogeneous all along the Himalayan belt, the results presented here demonstrate that it is not the case for its tectono-metamorphic evolution.
|Number of pages||2|
|Journal||Journal of Asian Earth Sciences|
|Publication status||Published - May 12 2001|
ASJC Scopus subject areas
- Earth-Surface Processes