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

Abstract

[1] The 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 outof-sequence thrusting in the younger segment of the orogen.

Geometry and kinematics of the Main Himalayan Thrust and Neogene crustal exhumation in the Bhutanese Himalaya derived from inversion of multithermochronologic data

“…Nonuniform, spatially variable early to middle Miocene exhumation of rock from midcrustal depths is documented across the GHS, including (1) in southeastern Bhutan (south of Tashigang, Figure ), where ~17–27 km of rapid exhumation in the MCT zone occurred between 18–13 Ma and present at rates of 3 to 9 mm/yr [ Daniel et al ., ], (2) further north in the footwall of the Kakhtang Thrust, rocks were exhumed from midcrustal depths by 21–17 Ma, possibly aided by synchronous slip on the O‐STD and MCT [ Chambers et al ., ; Kellett et al ., , ; Tobgay et al ., ; Warren et al ., ], and (3) in the hanging wall of the Kakhtang Thrust in NW Bhutan (Figure ), where granulitized eclogites experienced 20–44 km of exhumation in 1–2 Myr, implying exhumation rates of 10–44 mm/yr between 15 and 13 Ma, again possibly aided by coeval slip on the I‐STD [ Grujic et al ., ; Kellett et al ., ; Warren et al ., ]. Peak temperature metamorphic ages combined with muscovite 40 Ar/ 39 Ar ages across the GHS and upper LHS suggest very rapid cooling until 13–11 Ma (that may also be partly attributed to isotherm relaxation) and a common GHS‐LHS cooling history by 10 Ma [ Castelli and Lombardo , ; Gansser , ; Kellett et al ., , ; Long et al ., ; Maluski et al ., ; Stüwe and Foster , ]. Combined, these observations closely match predictions from numerical geodynamic models of channel flow tectonics in the Himalaya [ Beaumont et al ., ; Jamieson et al ., , ].…”

Mid-Miocene initiation of orogen-parallel extension, NW Nepal Himalaya

“…18 and 15 Ma. Studies along the South Tibetan detachment system in Bhutan and Sikkim documented a similar age progression, in which the growth of 16-15 Ma Y-rich monazite rims was interpreted to record retrograde Greater Himalayan Sequence metamorphism during final stages of Sdirected extrusion (Kellett et al, 2010(Kellett et al, , 2013.…”

A review on out-of-sequence deformation in the Himalaya

“…As geochronological approaches could not prove with certainty whether or not an out-of-sequence thrust exists inside the GHC, input from structural geology and metamorphic petrology might help to establish or refute the out-of-sequence thrust (Kellett et al 2013). For example, presuming a Himalayan D 2 deformation of top-to-the-south/SW ductile shear at c. 25 Ma Fig.…”

“…If this is proved, it could be an important input to understanding the seismicity within the GHC (Herman et al 2010). Kellett et al (2013) pointed out that if one of the strands of the South Tibetan Detachment acted as a passive roof structure, out-of-sequence thrusting is not required in the tectonics of the GHC. However, this argument does not negate the presence of an out-of-sequence thrust.…”