Geochronological and metamorphic constraints on the exhumation of the Ama Drime Massif: Implications for the mid-Miocene evolution of Himalayan extensional structures

The east-west (E-W) trending South Tibetan Detachment System (STDS) and the north-south (N-S) trending normal faults and detachments have played significant roles in the Himalayan tectonic evolution since the Miocene. The Ama Drime Massif (ADM), which is bounded by the STDS in the north as well as the Dinggye and Kharta N-S trending detachments in the east and west, respectively, is a rare place where N-S and E-W extensions interacted and thus contains key information to detect the evolution of Himalayan extensional structures. Aiming at fully reconstructing the exhumation process of the ADM and figuring out the mid-Miocene evolution of Himalayan extensional structures, this contribution provides detailed field investigations and geochronological, petrological, as well as mineral compositional studies to constrain the activity of the detachments surrounding the ADM, the metamorphic process, and the thermal state of the ADM. The geochronological results suggest that both the STDS and the Dinggye detachment were active at 13 Ma. Unlike the Dinggye detachment, whose ductile deformation terminated at ~10 Ma, the Kharta detachment maintained its ductile deformation until at least 8.5 Ma. Geochronological, petrological, and mineral compositional analyses show that the ADM paragneiss underwent muscovite dehydration melting and garnet breakdown from at least 20 Ma to 12.5 Ma, indicating a long-term exhumation of the ADM. Importantly, biotite dehydration melting and garnet growth in the ADM orthogneiss suggest a heating event in the ADM at 15–13 Ma. Based on the temporal relationship of different extensional structures, the exhumation of the ADM was dominated by the STDS before 15 Ma, and subsequently by the E-W extension of the Dinggye and Kharta detachments since 15–13 Ma. The E-W extension led to significant uplift and melting of the ADM gneisses, which made the STDS reactivate as a passive stretching detachment after its ~15 Ma cessation. In the mid-Miocene E-W extensional regime, the ADM had exhumed asymmetrically with decreasing time and amount of the exhumation from its eastern side to the western side. Together with the heating event, the asymmetric exhumation indicates the lateral delamination of the subducted Indian lithosphere beneath the ADM in the middle Miocene. From a wider perspective, the mid-Miocene E-W extensions in the Himalayan Orogen were intrinsically controlled by the deep geodynamic process of the subducted Indian lithosphere and played a important role in the diachronous cessation of the STDS.