Spasmodic rift reactivation and its role in the pre-orogenic evolution of the Himalayan region

Abstract

The evidence of several phases of pre-orogenic mafic magmatism of rift tectonic setting, associated with epeirogenic uplift, erosion and transgression, in the exposed Middle Proterozoic to Paleocene lithostratigraphic column of the Himalayas suggests that the region was the site of episodic rift reactivation for at least 1.5 Ga. The magmatism shows an overall progressive trend in the intensity, volume and spatial extent from the oldest (Middle Proterozoic) to the youngest (Early Cretaceous) phase. Also, the locus of magmatic activity shows a gradual south to north shift: the earliest eruptions were along the peripheral part of the basin, the middle ones moved towards the inner parts and the last phase occurred in the axial part of the basin. It is suggested that this peripheral to axial shift of magmatism indicates a gradual shift of the stress regime from an initial diffused pattern to progressive accentuation and final concentration along a narrow belt now represented by the Indus-Yarlung Zangbo Suture Zone. The long history of spasmodic tensional tectonics has certain critical regional and global implications. The regional implications include 1. (1)the rift-associated upper mantle volatile flux and high heat flow in the region inducing lithospheric buoyancy and epeirogenic uplift, erosion, transgression over the flank regions, and subcrustal anatexis; 2. (2)the control on lithological-geochemical associations (e.g., the deposition of carbonate rocks following each phase of rifting); and 3. (3)the evolution of the basin from an early wide depositional basin to finally a narrow linear trough by repeated rift reactivation. The global implication is examined in the light of possible mechanisms of rift initiation and its episodic reactivation. The convection current and mantle “plume” mechanisms within the plate tectonics framework are at variance with both the geological constraints of the region and the general considerations of these processes. Most importantly, the nature and age of the Indus-Yarlung Zangbo ophiolite and other geological data cannot be reconciled with the plate tectonics proposition that the ophiolite represents “ocean floor” rocks of the wide oceanic Tethys. Instead, the data indicate that the ophiolite represents mafic-ultramafic diapirism in an extremely attenuated lithosphere. The extraordinarily close synchroneity of the rifting spasms in the Himalayan region and the times of change in the Earth's rotation rate strongly point to some close relationship and the involvement of some basic aspect of Earth dynamics in the initiation and repeated reactivation of rifting in the Himalayan region.