Using U‐Th‐Pb petrochronology to determine rates of ductile thrusting: Time windows into the Main Central Thrust, Sikkim Himalaya

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 40 Ar/ 39 Ar 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 À1 can be calculated. This provides a minimum constraint on the amount of Miocene India-Asia convergence that was accommodated along the MCT.

Late Neogene tectonically driven crustal exhumation of the Sikkim Himalaya: Insights from inversion of multithermochronologic data

“…Other than the MHT, we do not simulate movements on hinterland structures including the MCT and the STDS, since there is no evidence for significant displacement on these structures after 12 Ma. We recognize that ductile shearing in the MCT zone may have occurred until 10 Ma [ Mottram et al ., , ]; however, since it does not disrupt the cooling age trend (Figure ), its Late Miocene activity must have had a limited impact on our thermochronological data set.…”

Petrogenesis of rare-metal pegmatites in high-grade metamorphic terranes: a case study from the Lewisian Gneiss Complex of north-west Scotland

“…A detailed description of the laboratory's monazite dating method is provided in Mottram et al (2015). Laser ablation and mass-spectrometer parameters are provided in Supplemental Table 3.…”

Geochronological challenges posed by continuously developing tectonometamorphic systems: insights from Rb–Sr mica ages from the Cycladic Blueschist Belt, Syros (Greece)

“…Progress has been made in tying monazite crystallization to deformation (Erickson et al, 2015) or reaction history using zoning in individual dated grains (e.g. Mottram et al, 2015). Garnet can be tied to metamorphic crystallization history in great detail through its chemical zoning and mineral inclusions and it can be dated by Lu-Hf and/or Sm-Nd, but results can be distorted by the presence of refractory inclusions (e.g.…”