Greater India'snorthernmarginprior toits collision with Asia

Greater India’s northern edge prior to collision with Asia is typically modelled as a rifted passive margin. We argue for a quite different geometry as a consequence of two tectonic episodes that happened sometime before the main impact. Whilst the western segment of India’s northern boundary had formed in the Late Triassic as a rifted margin, the central and eastern portions developed between 132 and 110 Ma when the sub-continent separated from Australia–Antarctica as the inner wall of a dextral ‘scything’ transform fault along the Wallaby–Zenith Fracture Zone off western Australia. Key features would have been (i) the very narrow (20–30 km wide) ocean–continent transition zone marking the sub-continent’s eastern northern boundary, and (ii) similar to the region offshore South Africa’s Garden Route coast, Greater India’s NE corner may have developed a series of ‘perched’ half grabens due to shearing related to its motion along the Wallaby–Zenith Fracture Zone, from initial break-up until it passed the Zenith Plateau (ca. 110 Ma). Differences in the development of NW Greater India may be reflected in restriction of ultra-high pressure metamorphic rocks to the western Himalaya where late Paleocene subduction of the rifted passive margin occurred at sub-equatorial latitudes beneath the intra-Tethyan arc. Further east, where the margin developed along the scything transform, the continent–ocean boundary would have been more abrupt and probably less strongly welded. Ophiolite emplacement appears to have been penecontemporaneous along the margin. A subsequent slab break-off episode then eliminated the original plate boundary. Thereafter, remaining oceanic lithosphere north of the arc sutured to the sub-continent, albeit rather weakly, was consumed beneath Eurasia, culminating in India–Asia collision.