New geochronological constraints on the thermal and exhumation history of the Lesser and Higher Himalayan Crystalline Units in the Kullu–Kinnaur area of Himachal Pradesh (India)

• Published in: Journal of Asian earth sciences Vol. 52; no. 3; pp. 98 - 116
• Main Authors: Thöni, M., Miller, C., Hager, C., Grasemann, B., Horschinegg, M.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 30.06.2012; Elsevier Science
• Subjects: Exhumation; Himachal Pradesh; Himalayan metamorphism; Indian Himalayas; Rb–Sr and Sm–Nd geochronology; Exhumation; Himalayan metamorphism; Indian Himalayas; Himachal Pradesh; Rb–Sr and Sm–Nd geochronology
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2012.02.015

► We report new Sm–Nd and Rb–Sr data from metamorphic wedges in the NW-Himalayas. ► Melting during the Eohimalayan event is constrained by Sm–Nd ages around 40Ma. ► Peak metamorphism and initial exhumation of the core is dated around 29Ma. ► Intrusion of plutons above the wedges around 19Ma clearly postdate the main fabric. ► 7Ma old pegmatite dykes constrain melting of the crust during ongoing collision. New geochronological, petrological and structural data from the Beas–Sutlej area of Himachal Pradesh (India) are used to reconstruct the tectonothermal and exhumation history of this part of the Himalayan orogen. Sm–Nd garnet ages at 40.5±1.3Ma obtained on a pegmatoid from the inverse metamorphic High Himalayan Crystalline (HHC) in the Malana–Parbati area probably mark local melting during initial decompression. Ongoing exhumation in ductilely deformed leuco-gneiss is constrained by Sm–Nd garnet ages at 29±1Ma and white mica Rb–Sr ages around 24–20Ma, while Bt Rb–Sr ages indicate a drop of regional metamorphic temperatures below 300°C between 15 and 12Ma. The deep Sutlej gorge exposes medium-grade paragneisses and Proterozoic orthogneisses of the Lesser Himalayan Crystalline (LHC), overthrust by the HHC along the Main Central Thrust (MCT). Mica cooling ages in the HHC are in the range of 14–11Ma. Above the extruded wedge of the HHC, the Leo Pargil leucogranite and associated dykes intrude the Haimanta Unit (HU) below the weakly metamorphic Palaeo-Mesozoic sediments of the Tethyan Himalayas (TH). The Leo Pargil leucogranite yielded a mean Sm–Nd garnet age of 19±1Ma and Rb–Sr muscovite and biotite cooling ages between 16.4 and 11.6Ma. Marked young extrusion of LHC units resulted in differentiated exhumation/cooling of more frontal parts of the orogen. Very young ductile deformation of LHC gneisses near Wangtu is constrained by late-kinematic pegmatite intrusions crosscutting the main mylonitic foliation. Sm–Nd garnet and Rb–Sr muscovite ages of these pegmatites range between 7.9±0.9 and 5.5±0.1Ma. Published apatite FT ages down to 0.6Ma also document accelerated diachronous sub-recent exhumation of different parts of the orogen. Together with geochronological data from the literature, the new results demonstrate that the HHC and the HU were deformed by shortening and crustal thickening during the Eohimalayan phase (Late Eocene–Oligocene), followed by a strong thermal overprint and intrusions of granitoids during the Neohimalayan Phase (Early to Middle Miocene). The LHC experienced amphibolite facies metamorphic conditions in the Late Miocene prior to extrusion between the HHC and the very low-grade Lesser Himalayan sediments. In conjunction with climate changes, young tectonic activity in this central part of the Himalayan orogen may have strongly influenced fluvial incision and erosion, and therefore, contributed to the accelerated uplift, as indicated by extensive accumulation of Late Miocene to Early Pleistocene fluviatile–lacustrine sediments in the Zanda basin, the Transhimalayan headwaters of the Sutlej, in Western Tibet.