Abstract
Quantitative constraints on the rates of tectonic processes underpin our understanding of the mechanisms that form mountains. In the Sikkim Himalaya, late structural doming has revealed time-transgressive evidence of metamorphism and thrusting that permit calculation of the minimum rate of movement on a major ductile fault zone, the Main Central Thrust (MCT), by a novel methodology. U-Th-Pb monazite ages, compositions, and metamorphic pressure-temperature determinations from rocks directly beneath the MCT reveal that samples from ~50 km along the transport direction of the thrust experienced similar prograde, peak, and retrograde metamorphic conditions at different times. In the southern, frontal edge of the thrust zone, the rocks were buried to conditions of ~550°C and 0.8 GPa between ~21 and 18 Ma along the prograde path. Peak metamorphic conditions of ~650°C and 0.8–1.0 GPa were subsequently reached as this footwall material was underplated to the hanging wall at ~17–14 Ma. This same process occurred at analogous metamorphic conditions between ~18–16 Ma and 14.5–13 Ma in the midsection of the thrust zone and between ~13 Ma and 12 Ma in the northern, rear edge of the thrust zone. Northward younging muscovite 40Ar/39Ar ages are consistently ~4 Ma younger than the youngest monazite ages for equivalent samples. By combining the geochronological data with the >50 km minimum distance separating samples along the transport axis, a minimum average thrusting rate of 10 ± 3 mm yr−1 can be calculated. This provides a minimum constraint on the amount of Miocene India-Asia convergence that was accommodated along the MCT.
Highlights
Current rates of relative displacement within the crust in modern orogens can be precisely quantified using geodetic and geophysical data [Banerjee and Bürgmann, 2002; Bettinelli et al, 2006; Bilham et al, 1997; Larson et al, 1999]
Current convergence is taken up by a combination of lithospheric thickening of the Tibetan Plateau [Bettinelli et al, 2006; Bilham et al, 1997; Burgess et al, 2012; Larson et al, 1999; Lavé and Avouac, 2001] and tectonic escape along the eastern boundary [Clark and Bilham, 2008], in addition to that within the Himalaya. The majority of the latter is thought to be accommodated by displacement along the Main Himalayan Thrust (MHT) [Bilham et al, 1997; Larson et al, 1999]
All samples preserve evidence for multiple stages of synkinematic mineral growth, including spiral inclusion trails within both garnet and kyanite (Figures 2 and 3) and with crystals aligned along the dominant penetrative foliation caused by south directed thrusting
Summary
Current rates of relative displacement within the crust in modern orogens can be precisely quantified using geodetic and geophysical data [Banerjee and Bürgmann, 2002; Bettinelli et al, 2006; Bilham et al, 1997; Larson et al, 1999]. Current convergence is taken up by a combination of lithospheric thickening of the Tibetan Plateau [Bettinelli et al, 2006; Bilham et al, 1997; Burgess et al, 2012; Larson et al, 1999; Lavé and Avouac, 2001] and tectonic escape along the eastern boundary [Clark and Bilham, 2008], in addition to that within the Himalaya The majority of the latter is thought to be accommodated by displacement along the Main Himalayan Thrust (MHT) [Bilham et al, 1997; Larson et al, 1999]. The locus of the ~150–200 km of shortening in the MOTTRAM ET AL
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