Geochemical and Geochronological Constraints on the Tectonothermal History of the Central and Eastern Nepal Himalaya: Implications for Thermal-Mechanical Models

Syntheses of Himalayan tectonics imply an average shortening rate of ~2 cm/yr across the Himalaya over the last 20-25 Myr. However, despite decades of research, basic questions still remain about how strain was partitioned. 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. Due to the observed chronology and thermobarometry of Greater Himalayan Sequence (GHS) rock units in the Annapurna region of central Nepal, 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 (ST), Bhanuwa (BT), and Main Central (MCT) Thrusts show PT-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. The similarity in inferred peak ages for the ST, BT, and MCT could in principle indicate simultaneous initial cooling of all three sheets at ~22 Ma. A simple explanation would involve transport and cooling of each rock package along the same basal thrust (the MCT), and later juxtaposition of already-cooled rocks along the ST and BT. That is, this model implicitly requires as much as 100 km of out-of-sequence thrusting. Additional measurements from other parts of the orogen in India and Bhutan are needed to verify whether these estimates could be applied to the orogen as a whole. Lu-Hf dates from garnet separates in one relict eclogite from the Arun River valley in eastern Nepal indicate an age of 20.7±0.4 Ma, the first direct measurement of the timing of eclogitization in the central Himalaya. Four proximal garnet amphibolites from structurally lower horizons are 14-15 Ma, similar to post-eclogitization ages published for rocks along strike in southern Tibet. P-T calculations indicate three metamorphic episodes for the eclogite: eclogite-facies metamorphism at 23-16 Ma, a peak-T granulite metamorphism, and a late-stage amphibolite-facies metamorphism at ~14 Ma. Three models are considered to explain the observed P-T-t evolution. The first assumes that the Main Himalayan Thrust (basal thrust of the Himalayan thrust system) cuts deeper at the Arun River valley than elsewhere. While conceptually the most simple, this model has difficulty explaining both the granulite-facies overprint and the pulse of exhumation between ~25 and 14 Ma. A second model assumes that (aborted) subduction, slab breakoff, and ascent of India’s leading edge occurred diachronously: ~50 Ma in the western Himalaya, ~25 Ma in the central Himalaya of Nepal, and presumably later in the eastern Himalaya. This model explains the P-T-t path, particularly heating during initial exhumation, but implies significant along-strike diachroneity, which is generally lacking in other features of the Himalaya. A third model assumes repeated loss of mantle lithosphere, first by slab breakoff at ~50 Ma, and again by delamination at ~25 Ma; this model explains the P-T-t path, but requires geographically restricted tectonic behavior at Arun. The P-T-t history of the Arun eclogites may imply a change in the physical state of the Himalayan metamorphic wedge at 25-16 Ma, ultimately giving rise to the Main Central Thrust by 16-15 Ma