STRATIGRAPHIC RECORD OF THE INDIA-ASIA COLLISION: IMPLICATIONS FOR CONVERGENCE HISTORY AND COLLISION-RELATED MASS BALANCE

Exciting recently published detrital zircon dates from the pre-collisional Indian passive margin and initial collision-related foredeep basin along the central and western Himalayas fully confirm earlier detrital chromite-based interpretations as to the age of onset of India-Asia collision at circa 51 Ma. Pre-collisional detrital chromites and zircon age populations reflect derivation from Indian sources, whereas initial foredeep sediments reflect a combination of ophiolitic and Gandese arc sources with completely different age spectra. Preserved initial foredeep sequences in the Himalayas are remarkably thin (100’s m) given potential accommodation in excess of a km, and are typically unconformably overlain by red beds of fluvial origin, a relation that is not understood.

Initial collision at ~51 Ma requires an initial crustal volume of ~≥2.7*10⁸km³ in the collisional domain. Collision at ~51 Ma predates the abrupt slowdown in the India-Asia convergence rate at ~48 Ma, from velocities >120 km/my to ~60 km/my (for 28°N, 85°E). During this ~3 my interval ~400 km of convergence occurred. Slab-breakoff at ~48Ma is the likely cause of this abrupt slowdown. This accords with the present location of the Tethyan slab in the deep mantle. Slab break-off would have significantly changed the flexural characteristics, resulting in a significant decrease in flexural downwarping, potentially accounting for deposition of fluvial red beds above deep marine turbidites. Since ~48 Ma ~2650 km of convergence has been accommodated by crustal shortening, erosion, escape, and or lower crustal subduction.

The vast majority of erosional products from the India-Asia collision completely bypassed the Indian continent and currently reside in deep-sea fans (Bengal, Indus, Irrawaddy) or an approximately equal amount is in subduction accretion complexes in the Makran and Indoburman to offshore Sumatra as deformed Eocene and younger turbiditic sequences. In total, erosional products represent ~0.45*10⁸km³ of rock exported from the collision. Together with the excess crustal mass of the Tibetan Plateau (~1.3*10⁸km³) these represent ~63% of the required initial volume. Crustal subduction + escape are then required to accommodate the remaining ~37%, with each equivalent to the total erosional efflux, which may be too extreme.