Forced subduction initiation near spreading centers: effects of brittle-ductile damage

Although positive buoyancy of young lithosphere near spreading centers does not favor spontaneous subduction, subduction initiation occurs easily near ridges due to their intrinsic rheological weakness when plate motion reverses from extension to compression. It has also been repeatedly proposed that inherited detachment faults may directly control the nucleation of new subduction zones near ridges subjected to forced compression. However, recent 3D numerical experiments suggested that direct inversion of a single detachment fault does not occur. Here we further investigate this controversy numerically by focusing on the influence of brittle-ductile damage on the dynamics of near-ridge subduction initiation. We self-consistently model the inversion of tectonic patterns formed during oceanic spreading using 3D high-resolution thermomechanical numerical models with strain weakening of faults and grain size evolution. Numerical results show that forced compression predominantly reactivates and rotates inherited extensional faults, shortening and thickening the weakest near-ridge region of the oceanic lithosphere, thereby producing ridge swellings. As a result, a new megathrust zone is developed, which accommodates further shortening and subduction initiation. Furthermore, brittle/plastic strain weakening has a key impact on the collapse of the thickened ridge and the onset of near-ridge subduction initiation. In contrast, grain size evolution of the mantle only slightly enhances the localization of shear zones at the brittle-ductile transition and thus plays a subordinate role. Compared to the geological record, our numerical results provide new helpful insights into possible physical controls and dynamics of natural near-ridge subduction initiation processes recorded by the Mirdita ophiolite of Albania.