Metamorphic constraints on the character and displacement of the South Tibetan fault system, central Bhutanese Himalaya

The South Tibetan fault system (STFS), a family of primarily extensional faults that separates the metamorphic core of the Himalaya (expressed as the Greater Himalayan sequence (GHS)) from overlying, predominately unmetamorphosed Tibetan sedimentary sequence (TSS) units, has been mapped for over 2,000 km coincident with the Himalayan range crest. In most areas, the immediate hanging wall of the STFS sole detachment consists of predominately carbonate rocks of lower Paleozoic age. However, in the Bhutan sector of the eastern Himalaya (ca. 89–92°E), the hanging wall of the sole structure is instead frequently mapped at the base of a metamorphosed, predominately siliciclastic succession (the Chekha Formation), and the base of the overlying predominately carbonate rocks (Pele La and Tang Chu Groups) is mapped as a less significant splay of the STFS. Unfortunately, poor exposures throughout central Bhutan make mapping and structural interpretation of these important contacts difficult, resulting in many disparities among geologic maps made by different research groups. The STFS in other parts of the Himalaya accommodates a significant metamorphic discontinuity that should also be apparent in Bhutan. Therefore, as an alternative approach, we have used the Raman spectroscopy on carbonaceous material (RSCM) thermometer to evaluate the evidence for a metamorphic discontinuity across both putative STFS structures.
RSCM thermometric data from 17 samples across three purported STFS klippen in central Bhutan (the Dang Chu, Ura, and Zhemgang klippen) suggest that the contact between the Chekha Formation and the underlying GHS is not a fault with large postmetamorphic displacement. We find no resolvable change in peak metamorphic temperature across the contact (ca. 560°C in both the Chekha and GHS), but we see a 130–140°C drop in paleotemperature across the higher contact between the Chekha Formation and overlying Pele La and Tang Chu groups. This change coincides with a major change in structural style, from high-strain, leucogranite-bearing rocks below to large-scale recumbently folded marbles above. Together, the change in deformational character and metamorphic grade suggest that the principal STFS detachment in Bhutan is the structural boundary of the Chekha Formation and the predominantly carbonate rocks above. The presence of an STFS detachment approximately 80 km south of the main STFS fault trace at the crest of the Himalaya, with no match between correlative footwall and hanging wall units along the direction of fault motion implies large displacements on the STFS in the eastern Himalaya.