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Subduction Initiation reconstructed from Neotethyan Kinematics (SINK): An iterative geological and numerical study of the driving forces behind plate tectonics

Final Report Summary - SINK (Subduction Initiation reconstructed from Neotethyan Kinematics (SINK): An iterative geological and numerical study of the driving forces behind plate tectonics)

Plate tectonics describes the outer shell of the Earth as a mozaic of rigid plates that move along discrete fault zones that form three types of plate boundaries. At mid-ocean ridges, plates move apart and new plate grows by cooling of upwelling mantle material. At transform faults, plates move along each other. And at subduction zones, one plate dives below another plate back into the mantle.

Although new subduction zones form geologically speaking frequently - about half of today's subduction zones formed in the last 65 million years - explaining why and how they form proves to be difficult. This is mainly because the process from the first fault to a full-blown subduction zone
takes millions of years, so we cannot witness the event in a life time.

The current physical hypothesis for the formation of subduction zones is that they form as a result of a change in the force balance on existing plates, after which a pre-existing weak zone localizes deformation and one plate is pushed below another plate. Once that plate is pushed deep enough, it
will undergo mineral reactions that make the plate denser than the mantle, and subduction will drive itself. For subduction zones that form within oceanic basins, with one oceanic plate subducting below another, those weakness zones were thought to be active or recent transform faults or mid-oceanic ridges.

The SINK project aimed to understand which processes create the force balance changes on plates leading to subduction initiation. If we understand those, we understand what keeps subduction - and hence plate tectonics - going.Therefore the project performed research on two fronts. During the first half of the project, we have reconstructed when and where subduction started within an oceanic domain that once existed between Africa, Arabia, and India to the South, and Eurasia to the North, and along what type of weakness zones. Relics of the overriding plate of that intra-oceanic subduction zone are found as rock massifs of oceanic crust that were uplifted in the Alpine-Himalayan mountain belt - so called 'ophiolites'. These ophiolites contain frequently crust that has already for long been thought to have formed during subduction initiation, but it remained unclear how.
The results of the SINK project have identified in which direction, when, and where subduction initiated along three subduction zones: We showed that ~170 million years ago between the Alps and southern Greece, subduction initiated along major fault zones - so-called oceanic detachment faults - along a mid-oceanic ridge. A second subduction zone that formed ~100 milion years ago from Turkey to Oman, formed along ancient fracture zones that existed along continental margins. A third subduction zone that formed ~150-130 million years ago along southern Tibet formed along a passive continental margin. Finally, a ~90 Ma old, east-dipping subduction zone that formed along western Sumatra and the Andaman islands formed within an old volcanic arc that formed above an originally west-dipping subduction zone - a so-called polarity flip. At all these subduction zones, so-called supra-subduction zone ophiolites formed, which were previously considered to form solely after subduction initiation close to mid-oceanic ridges.

The project then tested the physical plausibility of geodynamic scenarios that were built based on the reconstructions. First, we showed that the high temperatures that occur in young subduction zones, as deduced from so-called 'metamorphic sole' rocks below ophiolites and that were widely assumed to reflect subduction initiation, are also a logical feature in subduction initiation far away from ridges. Those settings were never tested before, and our work shows that the geological records are not as unique recorders of subduction initiation environments as previously thought. We then developed the hypothesis that the arrival of a mantle plume below southern India led to a small counterclockwise rotation of India relative to Africa and the Neotethys, generating the E-W convergence that initiated subduction. The modeling results are still underway, but preliminary results suggest that the rise of mantle plumes may indeed generate hemispheric subduction zones, thereby showing that subduction initiation may be externally triggered by plumes - one of the grand hypotheses at the start of SINK.