Rheological and kinematic conditions for forced subduction initiation at continental passive margins

Abstract

Subduction zones drive plate tectonics and are major sites of natural hazard generation such as volcanism, earthquakes and tsunamis. Therefore, understanding their formation and evolution is of paramount importance for both science and society. To date, it is still unclear which processes are involved in the nucleation of subduction zones, which parameters are controlling, and which tectonic settings are prone to the initiation of a new subduction zone. Uncertainties arise from the scarcity of observational constraints of modern and ancient subduction zones precluding the unambiguous identification of the initiation processes. In this PhD thesis a novel combination of state-of-the-art physical analogue and thermo-mechanical numerical modelling (chapter 2) is used for exploring geometric, kinematic and rheologic parameters that favour the initiation of new subduction zones. Our study focuses on the unresolved question of subduction zone formation at passive continental margins and aims at identifying and quantifying the deformational response of the ocean-continent transition to plate convergence to arrive at diagnostic features that can be tied to the subduction initiation process. The modelling results are validated by comparison with deformation structures from modern inverted passive margins and the geological record of past natural examples, which are now part of orogenic systems within the Tethys realm. The results of our modelling study reveal that subduction initiation at passive continental margins requires failure of the load-bearing crustal and mantle layers. As such, upon orthogonal convergence subduction initiation is controlled by the strength contrast at the ocean-continent transition, which is a function of the age dependant thickness of the oceanic and continental lithospheres, as well as the strength of the mid-oceanic ridge that varies through time by cooling.