METAMORPHIC HISTORY OF THE CENTRAL HIMALAYA, ANNAPURNA REGION, NEPAL AND IMPLICATIONS FOR MODELS OF TECTONIC EVOLUTION

Syntheses of Himalayan tectonics imply an average shortening rate of ~2 cm/yr across the Himalaya over the last 20-25 Myr. Fluctuations in the timing and rate of thrusting could have significant implications for strain partitioning across the Himalaya, and for partitioning during collisional orogenesis as a whole. We collected monazite ages and mineral chemical data to calculate P-T conditions and thrust rates from the Annapurna region of central Nepal. Peak P-T conditions increase from 650°C and 12.0 kbar at the base of the Greater Himalayan Sequence (GHS) thrust sheet, to 725-750°C and 12.5 kbar in the middle of GHS Formation I to 775°C and 12.5 kbar at the top of Formation I. Age probability diagrams demonstrate decreasing ages of metamorphism structurally downward, from peak-T metamorphism and anatexis at 25 ± 2 Ma at the top of Formation I to sub-solidus metamorphism at 21-16 Ma at the base of the GHS thrust sheet, consistent with progressive underthrusting of tectonic slices. Due to the observed chronology and thermobarometry of GHS rocks at Annapurna, we revise the original interpretation of the Bhanuwa Fault from a normal fault to a thrust, and identify a new thrust called the Sinuwa Fault structurally above the Bhanuwa thrust. The calculated slip rates for the Sinuwa, Bhanuwa, and Main Central Thrusts show P-T-t consistency with a 2 cm/yr convergence rate component across the Himalaya from ~25 to ~15 Ma. These results support models that presume constant rates since at least 25 Ma. Despite differences in absolute ages, the similarities among the chemical systematics of monazite, peak P-T conditions, and thrust rates calculated for Annapurna when compared to Langtang, ~200 km along strike to the east, imply that strain measurements in one part of an orogen can be realistically extrapolated to another within a few hundred kilometers, although the timing of movement on discrete thrust surfaces may differ. This lateral predictability may hold only for geologically similar regions of the orogenic belt. With distance may come a change in boundary conditions (like rotation at the syntaxes) that would prevent long-distance extrapolation of strain estimates. Additional measurements from other parts of the orogen, e.g., in India and Bhutan, would be needed to verify whether these results can be applied to the orogen as a whole.