Abstract
To address the question of the initiation and mechanisms involved in the process of subduction zone formation, we explored most of the available evidence for the subduction initiation (SI) during the Cenozoic. For this, we targeted a total of 70 candidate sites for subduction initiation cumulating ~70,000 km of trench, two thirds of which are still active and a majority still immature. Our strategy is to define four stages reached for each subduction initiation site (SIS) from the incipient-diffuse stage through incipient-localized stage and early arc magma production to self-sustained subduction. We have paid special attention to prematurely extinguished, i.e., aborted, subduction attempts in order to better understand the reasons for the termination of the process, and thus to clarify the conditions of success. The failure of SI results from a combination of hindering parameters (e.g., lithosphere cooling, frictional resistance, unfavorable age contrasts for intra-oceanic SISs) and insufficient external forcings (e.g., too low convergence velocity). From this comprehensive study, we find that new subduction zones regularly nucleate, at a mean rate of about once every Myr, and with a success rate of more than 70% to reach subduction maturity, generally in less than ~15 Myr, ~3-8 Myr for the shortest time between the very early stage and the self-sustained stage. A majority forms at the transition between an ocean and a continent, plateau or volcanic arc, demonstrating that large differences in composition, topography and/or lithospheric weaknesses favor the localization of the strain. Lithospheric forces are required to ensure the success of the process in the early (immature) stages, with the help of mantle forces in a third of the cases. Multiple triggers are common. Stress during the SI process is compressive in most, if not all, cases and oriented obliquely to the nascent plate boundary in more than half of the cases. The incipient plate boundary generally reactivates an old lithospheric fault, most often with a change in its kinematics, i.e., conversion of a transform plate boundary, a former normal or a detachment fault, or even a former spreading center. Sometimes, the new lithospheric fault reactivates a former subduction fault. There is no rule concerning the age of the subducting plate which varies from 0 to 140 Ma in the examples studied. In the same vein, the subducting plate is not necessarily older than the overriding plate when it is oceanic. Both situations are equally observed.