Possible South-Dipping Mesozoic Subduction at Southern Tethys Ocean-Constrained from Global Tectonic Reconstructions and Seismic Tomography

The evolution of the Tethys Ocean involved several episodes of ocean opening (including the Paleo-Tethys and Neo-Tethys) along its southern margin as terranes rifting away from eastern Gondwana. These oceans were terminated by well observed north-dipping subduction as the same terranes accreted to southern Eurasia. However, the presence of south-dipping subduction, though geologically proposed by numerous studies, have generally been omitted in the reconstruction of Tethyan evolution. Here, we synthesize the Mesozoic south-dipping subduction evolutions in the global reconstruction and focus on two potential events located along the northern edges of Southwest Borneo Block and the Woyla Arc. We next evaluate their slab volumes after thermal diffusion in the current mantle. Fast velocity anomalies in the mantle beneath the same region are then converted to cold anomalies and their volumes are further estimated and compared to the volumes evaluated from these two Mesozoic south-dipping subduction. We further identify seismic fast velocity anomalies likely relevant to slab remnants of south-dipping Tethyan subduction in the present-day mantle beneath the Indian ocean and West Australia, and link them to arc systems in plate reconstructions. In addition, one more tectonic scenario relevant to the north-dipping subduction in our study region is also examined. We speculate the relationship and evolution between such south-dipping subduction and north-dipping subduction in the south of Tethys Oceans. The attempt to reconstruct intermittent south-dipping subduction systems in southern Tethys region represents an effort on assessing rifting mechanisms in the opening of the Tethys Ocean and break-up of eastern Gondwana.