Direct stratigraphic dating of India-Asia collision onset at the Selandian (middle Paleocene, 59+-1 Ma)

The collision of India with Asia had a profound influence on Cenozoic topography, ocean- ography, climate, and faunal turnover. However, estimates of the time of the initial collision, when Indian continental crust arrived at the Transhimalayan trench, remain highly contro- versial. Here we use radiolarian and nannofossil biostratigraphy coupled with detrital zircon geochronology to constrain firmly the time when Asian-derived detritus was first deposited onto India in the classical Sangdanlin section of the central Himalaya, which preserves the best Paleocene stratigraphic record of the distal edge of the Indian continental rise. Deep- sea turbidites of quartzarenite composition and Indian provenance are replaced upsection by turbidites of volcano-plutoniclastic composition and Asian provenance. This sharp transition occurs above abyssal cherts yielding radiolaria of Paleogene radiolarian zones (RP) 4–6 and below abyssal cherts containing radiolaria of zone RP6 and calcareous shales with nannofos- sils of the Paleocene calcareous nannofossil zone (CNP) 7, constraining the age of collision onset to within the middle Paleocene (Selandian). The youngest U-Pb ages yielded by detri- tal zircons in the oldest Asia-derived turbidites indicate a maximum depositional age of 58.1 ± 0.9 Ma. Collision onset is thus mutually constrained by biostratigraphy and detrital zircon chronostratigraphy as 59 ± 1 Ma. This age is both more accurate and more precise than those previously obtained from the stratigraphic record of the northwestern Himalaya, and sug- gests that, within the resolution power of current methods, the India-Asia initial collision took place quasi-synchronously in the western and central Himalaya.