Record of Crustal Thickening and Synconvergent Extension from the Dajiamang Tso Rift, Southern Tibet

North-trending rifts throughout south-central Tibet provide an opportunity to study the dynamics of synconvergent extension in contractional orogenic belts. In this study, we present new data from the Dajiamang Tso rift, including quantitative crustal thickness estimates calculated from trace/rare earth element zircon data, U-Pb geochronology, and zircon-He thermochronology. These data constrain the timing and rates of exhumation in the Dajiamang Tso rift and provide a basis for evaluating dynamic models of synconvergent extension. Our results also provide a semi-continuous record of Mid-Cretaceous to Miocene evolution of the Himalayan-Tibetan orogenic belt along the India-Asia suture zone. We report igneous zircon U-Pb ages of ~103 Ma and 70–42 Ma for samples collected from the Xigaze forearc basin and Gangdese Batholith/Linzizong Formation, respectively. Zircon-He cooling ages of forearc rocks in the hanging wall of the Great Counter thrust are ~28 Ma, while Gangdese arc samples in the footwalls of the Dajiamang Tso rift are 16–8 Ma. These data reveal the approximate timing of the switch from contraction to extension along the India-Asia suture zone (minimum 16 Ma). Crustal-thickness trends from zircon geochemistry reveal possible crustal thinning (to ~40 km) immediately prior to India-Eurasia collision onset (58 Ma). Following initial collision, crustal thickness increases to 50 km by 40 Ma with continued thickening until the early Miocene supported by regional data from the Tibetan Magmatism Database. Current crustal thickness estimates based on geophysical observations show no evidence for crustal thinning following the onset of E–W extension (~16 Ma), suggesting that modern crustal thickness is likely facilitated by an underthrusting Indian lithosphere balanced by upper plate extension.

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)

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 ± 1.3 Ma 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 ± 1 Ma and white mica Rb-Sr ages around 24-20 Ma, while Bt Rb-Sr ages indicate a drop of regional metamorphic temperatures below 300 °C between 15 and 12 Ma. 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 14-11 Ma. 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 ± 1 Ma and Rb-Sr muscovite and biotite cooling ages between 16.4 and 11.6 Ma. 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 ± 0.1 Ma. Published apatite FT ages down to 0.6 Ma 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.