| Summary: | 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.
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