Polyphase (Miocene-Pleistocene?) slip on the South Tibetan Fault system in the Dhaulagiri Himalaya

The major detachments of the South Tibetan fault system (STFS) define a physiographic transition that can be traced along most of the length of the Himalayan orogen. The STFS has been assigned an Early-Middle Miocene initiation age, but recent geomorphic, structural and thermochronologic studies suggest that extensional deformation may have continued into the Pliocene and even Pleistocene epochs along some strands. Our recent work in the Kali Gandaki and Myagdi valleys of central Nepal (28°30'N-28°40'N; 83°20'E-83°45'E) adds to the mounting evidence for young STFS displacement. . The previously unmapped “Larjung detachment” is a low-angle (~15-20°), north-dipping structure that crops out within Cambrian(?) greenschist-facies calc-silicate rocks of Tibetan Sedimentary Sequence in the Dhaulagiri Himalaya. This detachment is roughly 2 kilometers structurally above and less steeply dipping than the previously mapped basal structure of the STFS in the region, the Annapurna detachment (AD). Our observations in the Kali Gandaki and Myagdi valleys reveal that the Larjung detachment expresses as a 1- to 10-m thick, brittle-ductile, shear zone. In the Myagdi valley, ductile fabrics indicate oblique slip on this structure in the direction N62°E, with normal and dextral components. On the basis of structural similarities, we link the Larjung detachment to the Machhapuchhare detachment in the Modi Khola drainage and the Phu detachment in the Marsyandi drainage farther east in the Annapurna Himalaya. We have collected a suite of samples in both the Kali Gandaki and Myagdi valleys for low-temperature thermochronometry aimed at constraining the ages of various strands of the STFS in the Dhaulagiri Himalaya. Preliminary (U-Th)/He dating of single crystals of zircon and apatite below the Larjung detachment exhibit consistently young cooling ages (<3.4 Ma). We found no statistically significant evidence of a distinctive age discontinuity across the AD, implying that the amount of Quaternary slip on that structure documented by Hurtado et al. (2001) was too small to be expressed as a thermochronologic discontinuity. On the other hand, one apatite (U-Th)/He date obtained thus far from the hanging wall of the Larjung detachment (12.26±0.49 Ma (2SE)) is significantly older than all sample dates from its footwall. Hurtado et al. (2001) GSA Bulletin, v. 113, p. 222