THE KUMHARSAIN SHEAR ZONE IN THE GARHWAL HIMALAYA: A SEGMENT OF THE FOLDED SOUTH TIBET DETACHMENT SOUTH OF THE GREATER HIMALAYAN CRYSTALLINES?

The South Tibet Detachment (STD), well defined in the central Himalaya, could not be traced west of longitude 77°30’E from the Garhwal to Zanskar Himalaya. Spatially associated with the apparent disappearance of the STD is the termination of the high-grade Greater Himalayan Crystallines (GHC); the latter forms an east-facing half window surrounded by the Late Proterozoic-Cambrian Haimanta Group of the basal Tethyan Himalayan Sequence and structurally above the Kullu-Larji-Rampur window exposing the Main Central Thrust (MCT). The Haimanta-GHC contact along the southern margin of the GHC half window was mapped as a depositional or N-dipping thrust contact. To clarify this relationship, we conducted mapping along a traverse from Narkanda where the MCT is located northward to the bottom of the Sutlej River valley where the GHC are exposed. We find that the Haimanta-GHC contact is a S-dipping fault at Kumharsain, juxtaposing highly folded Haimanta phyllite over a mylonitized gneiss unit of the GHC. Small drag folds in the 50-150 cm thick fault zone indicate top-south motion. Directly below the fault is the Kumharsain shear zone with complex kinematics. Its upper part (>200 m thick) contains top-south S-C fabric, the middle part (>50 m thick) exhibits top-north S-C fabric superposed by late south-verging folds, and the lower part (>100 m) shows top-south S-C fabric superposed by late top-north kink folds. Based on the cross-cutting relationships, we tentatively suggest the Kumharsain shear zone to have experienced three phases of deformation: (1) top-south motion, (2) top-north motion, and (3) top-south motion from older to younger, with the zone of deformation migrating upward with time. Because the Kumharsain fault is located at the same stratigraphic position as the STD mapped north of the GHC half window above the Kullu-Larji-Rampur window, it is possible that the two structures are segments of the same folded fault warped concordantly with the folded MCT below. Regional map relationships between the GHC and Haimanta strata require that the Kumharsain fault merges with the MCT in their updip directions, implying that the GHC was enclosed by the MCT below and the STD and Tethyan Himalayan strata above during its emplacement. This may explain why the GHC and STD reappear in the Zanskar region as a large tectonic window.