Initiation of crustal shortening in the Himalaya

New monazite U/Th-Pb petrochronological data from the Annapurna region of central Nepal outline a protracted thermal history spanning similar to 30Ma from the early Eocene (c. 48Ma) to the early Miocene (c. 18Ma). Rare earth element data collected concomitant with the isotopic analyses are consistent with prograde metamorphism and crustal thickening between similar to 48 and 30Ma and anatexis between similar to 28 and 18Ma. The timing of metamorphism recorded in these rocks is consistent with records of crustal shortening derived from ultrahigh-pressure rocks in the western Himalaya and exhumed metamorphic rocks in southern Tibet. Although previous records of early shortening/metamorphism related to the initial collision of India with Asia are spatially associated with the northern margin of the Indian plate, the ages presented in this study extend that early record south into the main Himalayan range. These new data provide important geological constraints on this early, poorly documented history.