Investigating Timing of Slip along N-S Extensional Faults in the Modi Khola and Marsyandi Valleys, Annapurna Himalaya, Nepal

The South Tibetan Fault System (STFS), extending along the southern margin of the Tibetan Plateau, played a critical role in the evolution of the Himalaya. During the Early to Middle Miocene, fault slip along the STFS and concurrent movement along the Main Central Thrust system resulted in the extrusion of the Greater Himalayan Sequence. While current models of the evolution of the Himalaya are based on a premise that the STFS has been inactive since that time, multiple studies in recent years have found evidence of Pliocene-Pleistocene slip along strands linked to this fault system. If the isolated findings prove to be part of regional-scale extension along the entire range and not just localized slip confined to individual strands, these faults may have played a pivotal role in the evolution of the range more recently than has previously been postulated. Previous work by McDermott et al. (2015) revealed Pleistocene extension along fault strands in the hanging wall of the STFS in the Kali Gandaki and Myagdi Khola of the Annapurna-Dhaulagiri Himalaya. Our research builds on this existing body of work and aims to fill in geographic gaps by examining previously mapped fault strands in the Modi Khola and Marsyandi valleys to the west, which have been presumed to be correlative to those in the Kali Gandaki. Fieldwork conducted in late spring of 2016 yielded a suite of samples for low temperature thermochronology and structural analysis. Utilizing (U-Th)/He apatite and zircon thermochronology, we are examining cooling ages across both the Machhapuchare detachment in Modi Khola and its analogous structure, the Phu, in the Marsyandi. Discontinuities in cooling ages and metamorphic facies between hanging wall and footwall rocks may indicate recent movement along these strands. If there has been recent slip in the Modi Khola and Marsyandi, we would predict to find footwall cooling ages juxtaposed against significantly older cooling ages in the hanging walls, a finding consistent with other work along the range crest where Pliocene-Pleistocene slip has been documented. Confirmation of our hypothesis would indicate that models for the evolution of the Himalaya post-Miocene should be modified to accommodate this movement.