The South Tibetan detachment system facilitates ultra rapid cooling of granulite‐facies rocks in Sikkim Himalaya

Abstract

ABSTRACTThe eastern Himalaya is characterized by a region of granulites and local granulitized eclogites that have been exhumed via isothermal decompression from lower crustal depths during the India‐Asia collision. Spatially, most of these regions are proximal to the South Tibetan detachment system, an orogen‐parallel normal‐sense detachment system that operated during the Miocene, suggesting that it played a role in their exhumation. Here we use geo‐ and thermochronological methods to study the deformation and cooling history of footwall rocks of the South Tibetan detachment system in northern Sikkim, India. These data demonstrate that the South Tibetan detachment system was active in Sikkim between 23.6 and ~13 Ma, and that footwall rocks cooled rapidly from ~700 to ~120 °C between ~15‐13 Ma. While active, the South Tibetan detachment system exhumed rocks from mid‐crustal depths, but an additional heat source such as strain heating, advected melt and/or crustal thinning is required to explain the observed isothermal decompression. Cessation of movement on the South Tibetan detachment system produced rapid cooling of the footwall as isotherms relaxed. A regional comparison of temperature‐time data for the eastern South Tibetan detachment system indicates a lack of synchronicity between the Sa'er‐Sikkim‐Yadong section and the NW Bhutan section. To accommodate this requires either strike‐slip tear faulting or local out‐of‐sequence thrusting in the younger segment of the orogen.