DISCOVERY OF A MAJOR INTRA-OCEANIC SUBDUCTION EVENT REVEALED BENEATH THE SOUTHERN INDIAN OCEAN

Global seismic tomography has unveiled numerous fast anomalies in the mantle that have been interpreted of subducted tectonic plates that have sunken deep into the Earth. Images of slabs of the ancient variety, such as the Farallon slab beneath North America, provide us with powerful information about ancient plate tectonic and dynamic processes. A new global image (Simmons et al. 2015, GRL) has recently revealed surprisingly positioned slab-like anomalies beneath the Southern Indian Ocean spanning depths from the upper mantle transition zone near the Southeast Indian Ridge (SEIR) to the deep mantle beneath the Indian Ocean and Australia. Portions of the slab anomalies bear a striking resemblance to the well-known Farallon slab anomalies and thus point to the existence of an ancient subduction event that was previous unrecognized. Based on the image and additional geoscientific observations, we postulate that the structure is an oceanic plate that sank into the mantle along a 7000-km intra-oceanic subduction zone that migrated southwestward across the ancient Tethys Ocean in the Mesozoic Era, perhaps beginning prior to 200 Ma. We refer to this new feature as the Southeast Indian Slab (or SEIS). Slab material still trapped in the transition zone is positioned near the former edge of East Gondwana ca. 140 Ma suggesting that subduction terminated near the margin of the ancient continent prior to breakup and subsequent dispersal of its subcontinents. The implications of this event are multifold including the need for re-evaluation of ancient global plate tectonic history and the likely existence of other intra-oceanic subduction events that have yet to be recognized or fully developed in global plate reconstructions.