Late Miocene Thermal Evolution of the Eastern Himalayan Syntaxis as Constrained by Biotite Ar-40/Ar-39 Thermochronology

We conducted biotite Ar-40/Ar-39 thermochronological research in the eastern Himalayan syntaxis and the neighboring Mesozoic and early Cenozoic Gangdese batholith belt with the aim of exploring the overall cooling pattern and thermal evolution of this remote region in the Himalayan orogen. A compilation of our newly acquired ages with existing data reveals a new temporal-spatial pattern. First, a temporal gap at 13-7 Ma exists in the cooling history of the study area, with ages ! 7 Ma within the syntaxis and ages 113 Ma in the Gangdese belt. The gap could be a manifestation of a renewed rapid cooling of the syntaxis since similar to 7 Ma within a general region of slow cooling. Second, the isochron contours of the cooling ages have an annulus shape, with a younging trend toward the Namche Barwa-Gyala Peri peaks at the core of the syntaxis at 7-4 and 4-2 Ma and along the margins of the syntaxis at 2-1 Ma. This pattern may be a result of a regionally progressive cooling from the margin to the core of the syntaxis that was punctuated by locally focused accelerated cooling and exhumation. Our new findings imply that the Cenozoic faults bounding the syntaxis controlled the development of the syntaxis. Additionally, new estimates of the exhumati rate suggest that the development of the syntaxis has been accelerating since 7 Ma.National Natural Science Foundation of China [41472182, 41002067, 40872149, 41472175]; Fundamental Research Funds for the Central Universities [2015QNA3013]SCI(E)ARTICLEgrsange@pku.edu.cn4369-38412