A HIMALAYAN SUSPECT TERRANE

The Himalayan upper and middle crust consists of two assemblages of mostly supracrustal and minor intrusive rocks. Most of Himalayan Assemblage A was deposited in the Paleo- to Mesoproterozoic, whereas Himalayan Assemblage B dominantly was deposited in the latest Neoproterozoic to earliest Cenozoic. Along most of its exposed length, Assemblage B meets the tripartite definition of a suspect terrane. It: (1) has a homogeneous internal history, (2) has a pre-Silurian history different than adjacent rock packages, and (3) is separated from adjacent rocks by major faults. Although recognition of a domain as suspect does not necessarily indicate long distance transport, regional tectonic relations suggest that a system of right-handed transcurrent faults might have emplaced Himalayan Assemblage B on the northern margin of India in the middle Paleozoic. Himalayan assemblages A and B may share lower to middle Paleozoic history in the eastern Himalaya, suggesting possible mid-Paleozoic transpressive collision there, unlike in the remainder of the Himalaya where transport may have been more purely transcurrent. If correct, this interpretation implies that Himalayan Assemblage B is a piece of Greater Arabian crust that formed in the late Neoproterozoic, adding to our estimates of the volume of new continental crust produced in this region in the late Neoproterozoic.