Geochronology and Himalayan Tectonics

Kyle Larson, University of British Columbia, Okanagan

Increasingly sophisticated geochronologic analytical techniques are providing the opportunity to collect data at spatial resolutions that were not achievable and in quantities not feasible previously. The advances in spatially constrained geochronologic techniques and the incorporation of simultaneously collected rare earth element data have made it possible to interpret detailed petrochronologic histories recorded in single grains. These techniques have been instrumental in providing new data and geochronologic constraints on the tectonic evolution of orogenic systems that can be tied to both the metamorphic and deformational histories through geochronometer mineral chemistry and textural setting. There are many recent examples of the unparalleled insight this type of study can provide. One such investigation, carried out in west-central Nepal, serves as particularly instructive case study on how petrochronologic data can help provide the information necessary to constrain complex tectonic histories and elucidate parts of that history that may not be readily apparent through other observational means or analyses.

New monazite U-Th/Pb geochronologic data from the Annapurna region of west-central Nepal outline a protracted growth history spanning ~ 31 myr from the Early Eocene (c. 49 Ma) to the Early Miocene (c. 18 Ma). Rare earth element data collected concomitant with the isotopic analyses is consistent with early prograde metamorphism between ~ 49 and 40 Ma, followed by continued high-grade metamorphism between ~ 38 and 30 Ma and finally large-scale anatexis between ~ 28 and 18 Ma. The timing of metamorphism recorded in these rocks is consistent with records of crustal shortening derived from ultra-high pressure terranes of the western Himalaya and related ranges and crosscutting dykes in deformed sedimentary rocks of southern Tibet. The ages extracted in this study are the first to extend the record of Early Eocene shortening, related to the initial collision of India with Asia, south into the main Himalaya. This has important implications for tectonic reconstructions and shortening rate estimates made for the Himalaya, which commonly ignore this early, poorly constrained history. Moreover, these new dates provide information on the timescale of initial continent-continent convergent margin evolution.