A review of Himalayan stratigraphy, magmatism, and structure

Abstract

"The Himalayan Orogen consists of two rock packages that parallel the topographic trend of the mountain belt between the eastern and western syntaxes. To avoid confusion with appellations previously used to identify elevation, Cenozoic metamorphic grade, or Cenozoic structural position, in this paper I introduce new names for these rock packages: Himalayan Assemblage A and Himalayan Assemblage B. Inclusion in an assemblage signifies that there was physical contiguity between adjacent members of the assemblage at the time of deposition or intrusion. Assemblage A and Assemblage B may not have shared depositional or intrusive relationships prior to Early Cretaceous time.

Himalayan Assemblage A mostly consists of sedimentary rocks deposited on the northern margin of India; the depositional substrate for these strata is not exposed anywhere in the orogen. Assemblage A comprises three main groups of rocks divided based on age of deposition or intrusion: Paleoproterozoic to Early Mesoproterozoic, Late Carboniferous to Permian, and terminal Cretaceous to Pleistocene. The oldest rocks exposed in the Himalaya, ca. 1900-1800 Ma clastic deposits and the ca. 1880-1830 Ma granite and gabbro that intruded them, may have formed in a continental rift setting. This rift system established depositional strike toward the northeast, at a high angle to the strike of Cenozoic thrusts in the western Himalaya, with obliquity decreasing eastward. The succeeding Upper Paleoproterozoic to Lower Mesoproterozoic strata were deposited in a passive margin setting or, alternatively, in an epi-cratonic basin. Upper Carboniferous to Permian strata are called the Gondwana Group; these deposits are present only in the eastern half of the orogen. This package is dominantly clastic and probably was deposited in extensional basins related to the breakup of Pangea. Depositional strike of the Gondwana Group was likely between 25 degrees and 50 degrees west of north. Upper Paleocene to Pleistocene, dominantly clastic, strata were deposited in the Himalayan foreland basin. Along the central two-thirds of the orogen, depositional age uncertainties extend the possible depositional ages of the lowermost of these strata into the latest Cretaceous Period."