CALCULATING RATES OF DUCTILE THRUSTING USING MONAZITE GEOCHRONOLOGY
The Main Central Thrust (MCT) is a major Himalayan structure that has accommodated a large amount of movement during the India-Asia convergence. The Sikkim Himalaya presents some of the best exposures of the MCT across the Himalaya. Here, a late-stage duplex beneath the thrust has folded the MCT into a dome. This structural configuration allows a novel method for determining rates of thrusting along the MCT.
The folding has exposed two different structural levels within the fault at the surface. Assuming metamorphism is contemporaneous with thrusting, the age of the metamorphism can be used to calculate the rate of thrusting by comparing the age of samples from different levels within the thrust.
The timing of metamorphism has been established using monazite geochronology. The age of different chemically-distinguishable zones in monazite was determined using in-situ laser ablation HR-ICP-MS U-Th-Pb dating. The data show significant differences in the timing of attainment of equivalent metamorphic conditions across the MCT. Monazites in kyanite-bearing rocks in the southern Sikkim Himalaya yield peak metamorphic ages of ~16 Ma whereas monazite in equivalent-grade rocks in the northern Sikkim Himalaya at similar structural levels yield ages of ~11 Ma. These ages have been combined with P-T modeling of equivalent grade rocks to determine a rate of thrusting. Initial calculations yield rates of ~1cm/yr, which appear slower than rates reported in earlier studies. This either suggests differences in the rate of MCT movement along strike, or differences due to methods of calculation. Our approach is the first to exploit folding of a major fault to calculate rates of ductile thrusting at a mid-crustal level.