The East Jhomolari Fault System and the Timing of East-West Extension in NW Bhutan

Abstract

This investigation documents a little known, regionally significant, high angle, southeast dipping, system of normal faults in northwest Bhutan, the East Jhomolari Fault System (EJFS). The EJFS trends parallel to the Yadong Gulu rift (YG) and can be traced from Mt. Jhomolari to the northern border with Tibet. This fault zone has been demonstrably active since the last glacial maximum and there is geomorphic evidence of Holocene activity on some fault segments. The structurally lowest EJFS fault, the Lingshi fault appears on some published maps, but the full geographic extent, displacement, onset of faulting, and most recent activity have not been documented before. Northwest Bhutan is a geologically significant area due to the intersection of major extensional fault systems. The South Tibetan Detachment System (STDS) intersects the EJFS and YG at nearly right angles. These intersecting normal fault systems are evidence of a 90° change in active extension direction from N-S to E-W. This study used field mapping and observations combined with spectral analysis of ASTER imagery to document the geographic extent of the EJFS and the amount of displacement across it. This new mapping also resulted in a more detailed map of the upper South Tibetan Detachment (STDu) and the extent of Tethyan Sedimentary Sequence (TSS) outcrops in northwest Bhutan. To constrain the older slip history of the East Jhomolari Fault System, and concomitant exhumational history of the Jhomolari Himalaya in its footwall, a small suite of samples was collected from river sand transported by small (<90 km ^{2</super> drainage area) east-flowing catchments. Detrital apatite and zircon [U-Th]/He thermochronologic analyses produced apatite cooling ages ranging from 6.1 Ma to 10.4 Ma in the south, 2.4 Ma to 9.4 Ma in the central segments of the EJFS, and 6.5Ma to 11 Ma in the north. Zircon cooling ages range from 6.5-11 Ma in all samples. Modeling of these cooling ages implies exhumation rates ranging from 0.9 km/My to 0.2 km/My for the period of time from the oldest zircon to the youngest zircon in each sample; 2.5 km/My to 0.4 km/My for the period of time from the youngest zircon to the youngest apatite in each sample; and 0.8 km/My to 0.2 km/My for the period of time from the youngest apatite in each sample to the present. The spatially uniform exhumation suggested by the consistent range of zircon cooling age from all samples, followed by the divergent cooling histories recorded in the apatite data has been used to estimate the onset of EJFS faulting to 7 Ma. The onset of approximately east-west extension on the EJFS provides a minimum age for the initiation of east-west extension in NW Bhutan.}