THE APPLICATION OF SWEAT METHODOLOGY TO NEOPROTEROZOIC RIFTING IN NORTHERN INDIA

The SWEAT hypothesis offers a methodology for interpreting ancient rifting events and correlating crustal fragments. This methodology includes the recognition of rifted margins, establishing the timing of breakup and identifying conjugate fragments based on geologic similarities and piercing points. The possible Neoproterozoic rifting of a fragment from northern India and western Australia represents an event well suited for this methodology. Evidence for rifting in northern India includes 1. Neoproterozoic tholeiitic trap basalts having within plate continental basalt (WPCB) affinities; 2. A northward-thickening late Proterozoic to early Paleozoic miogeoclinal sequence; 3.Evaporites overlying crystalline basement in the basal sedimentary sequence; 4. Interstratified spilitic volcanics. In addition, lithotectonic elements along the margin of western Australia are truncated.

Fragments rifted off northern India in the Neoproterozoic drifted northward (present coordinates) across the Paleo-Tethys Ocean. These fragments and numerous island arcs were assembled into the pre-Tethyan Cimmeride accretionary collage. Despite the large number of terranes that make up the Cimmeride collage and their poorly understood provenance, possible candidates for the conjugate fragment of northern India are few. They are marked by four characteristics: 1. Large size (>2000 km); 2. Lower Paleozoic miogeoclinal sedimentation; 3. Pre-Carboniferous sutures; 4. Precambrian basement ages.

Of the possible candidates, Annamia (Indochina) is considered the best fit. It has a maximum dimension of ~2000 km if an extension under the South China Sea is included. Cambrian through Silurian shallow-water sediments are ~10 km in thickness. Annamia is joined to the South China (Yangtze) block by the Song Ma (Red River) suture and is bordered on the west by the Tamky-Phueson suture of pre-Devonian age. The ~2300 Ma basement ages of the Kontum Massif correspond with ages in the Indian Aravalli Supergroup. Similarly, 982 Ma ages from the Shan Highlands of Malaysia are contained within the Vindhyan Supergroup of India. Faunal assemblages are also consistent with an early separation of Annamia from Rodinia/Gondwana.