Active tectonics exhumation of high-grade metamorphic rocks in the frontal part of the Himalayan orogen: 40Ar/39Ar and fission track geochronological evidences from the Sutlej Valley (NW India)

The Sutlej Valley is a natural cross-section across the entire Himalayan range in NW India. In the internal part of the orogen to the NE, the high-grade rocks of the High Himalayan Crystalline Sequence (HHCS) Crop out as a 10 km thick sequence of amphibolite facies and migmatitic paragneiss, bounded by the MCT at its base, and separated from the overlying, low-grade sediments of the Tethyan Himalaya by the extensional Sangla Detachment. Toward the foreland to the SW, the MCT becomes a shallow-dipping thrust, separating the HHCS from the low to medium-grade sediments of the Lesser Himalaya. In this external part of the orogen, large-scale 'folding of the MCT allows the HHCS to crop out in a large syncline (Luhri Syncline), which consists of basal mylonitic orthogneiss (MCT zone) superposed by greenschist facies paragneiss. Between the Luhri Syncline and the internal part of the orogen, the MCT is folded around the Larji-Kulu-Rampur (LKR) Window. Within the huge anticline formed by this window, greenschist facies, Early Proterozoic metasediments and granitoids of the Lesser Himalaya are overthrusted by the amphibolite facies Lesser Himalayan Crystalline Sequence (LHCS) along the Munsiari Thrust. The LHCS consists of mylonitic micaschist and Early Proterozoic orthogneiss (Wangtu Gneiss) of Lesser Himalayan affinity. The LHCS unit is separated from the overlying HHCS by the Karcham Normal Fault, a E-directed, brittle-ductile to brittle extensional structure cross-cutting the MCT mylonites in the lowermost HHCS (Janda et al., 2001). New zircon and apatite fission track ages (FT), as well as 40Ar/39Ar white micas ages, reveal a strongly discontinuous cooling trend along the Sutlej section, and significantly variable exhumation rates for the different units cropping out along this transect. Cooling of the basal part of the Tethyan Himalaya is recorded by Ar/Ar ages at 19.3-17.6 Ma. In the HHCS rocks from the internal part of the orogen. Ar/Ar white micas ages cluster around 16-15 Ma, whereas zircon and apatite FT ages vary between 16.3-13.7 and 4.9-2.7 Ma, respectively. Such a cooling is comparable to what is observed 70 km to the SE in the Bhagirathi Valley of Garhwal (e.g. Sorkhabi et al., 1996; Searle et al., 1999a, and it is consistent with an exhumation following an Early Miocene activation of the MCT in the NW Himalaya. In the frontal part of the orogen along the Sutlej section, zircon and apatite FT results from the HHCS mylonitic orthogneisses at the base of the Luhri Syncline indicate 12.2-10.7 and 4.1-3.0 Ma ages, respectively. These cooling ages are broadly comparable to those determined for the HHCS cropping out in a more internal position about 70-100 km to the NE, and they are in good agreement with the cooling trends from the frontal part of the HHCS along the Kulu Valley section, 50 km to the NW (La1 et al., 1999). The units cropping out in the LKR Window are characterized by a significantly younger cooling history. In the Lesser Himalaya, one sample of granite cropping out within the Proterozoic quartzites of the LKR Window yielded zircon and apatite FT ages at 4.8 and 1.7 Ma, respectively. The rocks from the LHCS record the youngest cooling evolution, characterized by Ar/Ar white micas ages between 6.6 and 4.3 Ma. The very young cooling of the LHCS is further confirmed by zircon and apatite FT ages at 2.7-1.7 and 0.9-0.7 Ma, respectively, in good agreement with the results of Jain et al. (2000). Together with structural, petrological and oxygen isotope geochemistry data (Vannay and Grasemann, 1998; Vannay et al., 1999; Janda et al., 2001), the geochronological results confirm that the metamorphic core of the Himalayan orogen in the Sutlej section consists of two distinct tectonometamorphic units. These results suggests the following tectonic scenario: (1) thrusting along the MCT during Early Miocene induced the exhumation...