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

The South Tibetan fault system, a family of primarily extensional faults that separates the metamorphic core of the Himalaya (expressed as the Greater Himalayan sequence) from overlying, predominantly unmetamorphosed Tibetan sedimentary sequence units, has been mapped for over 2000 km coincident with the Himalayan range crest. In most areas, the immediate hanging wall of the South Tibetan fault system sole detachment consists of predominantly carbonate rocks of Lower Paleozoic age. However, in the Bhutan sector of the eastern Himalaya (∼89°E-92°E), the hanging wall of the sole structure is instead frequently mapped at the base of a metamorphosed, predominantly siliciclastic succession (the Chekha Formation), and the base of the overlying predominantly carbonate rocks (Pele La and Tang Chu Groups) is mapped as a less significant splay of the South Tibetan fault system. 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 South Tibetan fault system 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 South Tibetan fault system structures. RSCM thermometric data from 17 samples across three purported South Tibetan fault system klippen in central Bhutan (the Dang Chu, Ura, and Zhemgang klippen) suggest that the contact between the Chekha Formation and the underlying Greater Himalayan sequence is not a fault with large postmetamorphic displacement. We find no resolvable change in peak metamorphic temperature across the contact (∼560°C in both the Chekha and Greater Himalayan sequence), 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 changes in deformational character and metamorphic grade suggest that the principal South Tibetan fault system detachment in Bhutan is the structural boundary between the Chekha Formation and the predominantly carbonate rocks abo