PRELIMINARY TIMING CONSTRAINTS on LESSER HIMALAYAN DUPLEX DEVELOPMENT FROM ZIRCON (U-TH)/HE THERMOCHRONOMETRY, EASTERN BHUTAN

We present new (U-Th)/He ages from zircon (ZHe) obtained from bedrock samples in Lesser Himalayan (LH) thrust sheets in eastern Bhutan, in order to constrain the timing of thrust-related cooling and exhumation. Samples are collected from multiple thrust sheets on an across-strike traverse, and are combined with kinematic and geometric data from a balanced cross-section in order to present preliminary timing constraints for LH duplexing, and estimate long-term shortening rates.

In eastern Bhutan, the LH section has been deformed into upper and lower duplex systems. For the lower LH duplex system the Main Central Thrust (MCT) was the roof thrust, while the Shumar Thrust (ST) was the floor thrust. As the system propagated, the ST became the roof thrust for the upper LH duplex. We obtained ZHe ages (~200ºC closure temperature) from 4 samples in the upper LH duplex (9.84±0.45 Ma, 9.69±0.34 Ma, 9.68±0.69 Ma 8.8±3.7 Ma; 2σ), and 6 samples in the lower LH duplex (11.9±1.1 Ma, 8.65±0.32 Ma, 7.5±1.7 Ma, 6.71±0.11 Ma, 6.6±1.4 Ma, 5.65±0.27 Ma; 2σ).

Previous work in Bhutan suggests that deformation of rocks within the lower LH duplex just beneath the MCT occurred at ~20-18 Ma. We obtain a 17.0±.05 Ma (2σ) age of zircon rim growth (LA-MC-ICP-MS) from a sample in the immediate footwall of the ST, which we interpret as the age of burial. We attribute this burial to emplacement of the ST hanging wall during construction of the lower LH duplex. We propose that the ca. 12-6 Ma ZHe ages obtained from the lower LH duplex are associated with rock uplift and erosion during younger deformation of upper LH and Subhimalayan rocks in the foreland, and post-date development of the lower LH duplex. We can broadly bracket the initiation of deformation and significant exhumation of rocks within the upper LH duplex, from a temperature range between ~280-450ºC to the ~200ºC isotherm, between the ~17 Ma burial age and the ~9-10 Ma ZHe ages.

Our data indicate a southward progression of LH duplex construction, bracketed between ~20 and 9 Ma. If all crustal shortening associated with the lower (102 km minimum) and upper (137 km minimum) LH duplexes was completed within this 11 myr time range, it would indicate a long-term minimum shortening rate of ~22 mm/yr. Future ³⁹Ar/⁴⁰Ar muscovite cooling ages from both duplexes will provide detailed timing on the change from burial to exhumation.