Cenozoic basin evolution in the Indus-Yarlung suture zone and High Himalaya

Five genetic categories of sedimentary basins have been active within the Indus-Yarlung suture zone and in the neighboring High Himalaya since early Cenozoic time. These include: (1) the Xigaze forearc basin (Aptian–early Eocene), (2) the north Himalayan foreland basin (Paleocene–Eocene), (3) the Kailas extensional basin (Oligocene–Miocene), (4) the Liuqu wedge-top basin (early Miocene), and (5) a set of at least six rift and supradetachment basins that formed by arc-parallel extension (late Miocene–Pleistocene). The older basins (categories 1 and 2) were filled with predominantly deep-marine turbiditic deposits, which shoaled through time to subaerial (but very low) elevations. The other basins (categories 3–5) were filled with alluvial-fan, fluvial, and lacustrine sediments, and these formed at progressively higher elevations, culminating in category 5 basins at essentially modern (or slightly higher than modern) elevations (~4000–5000 m). Development of diverse basin types was a response to changing orientations and relative magnitudes of principal stresses in the upper crust of the suture zone and the northern Himalayan thrust belt. Through the Cenozoic, the orientation of maximum compressive principal stress (σ1) changed from approximately horizontal and north-south (Paleocene–Eocene) to approximately vertical with least compressive principal stress (σ3) oriented north-south (Oligocene–Miocene), to horizontal and north-south (early Miocene), to nearly vertical with σ3 oriented approximately east-west (late Miocene–present). Tectonic stresses associated with the degree of coupling between the converging plates were also potentially important, especially during the Oligocene–Miocene, when the subducting Indian slab was rolling backward relative to the upper Eurasian plate, and during middle to late Miocene time, when the Indian slab was subducting nearly flat beneath the High Himalaya and southern Tibet. Preservation of these extensive sedimentary basins in an orogenic system that is generally being eroded rapidly and deeply stems from original basin-forming mechanisms that produced very large-scale basins (the forearc and early foreland basins) and subsequent evolution of the Himalayan thrust belt in a manner that has isolated High Himalayan basins behind an orographic barrier that protects them from erosion. Recent incision by trans-Himalayan and orogen-parallel suture-zone rivers, however, threatens future preservation of these High Himalayan basins (particularly categories 4 and 5).