Abstract

Although positive buoyancy of young lithosphere near spreading centers does not favor subduction, subduction initiation near ridges may occur upon forced compression due to their intrinsic rheological weakness. It has been repeatedly proposed that detachment faults may directly control the nucleation of new subduction zones. However, recent 3D numerical experiments suggested that direct inversion of a single detachment fault does not occur. Here, we further investigate numerically this controversy by focussing on the influence of brittle-ductile damage on the dynamics of near-ridge subduction initiation. We model self-consistently the inversion of inherited long-term spreading patterns using 3D high-resolution thermomechanical numerical models combining strain weakening of faults with grain size evolution in lithospheric mantle. Numerical results show that development and evolution of detachment faults are strongly affected by the brittle-ductile damage coupling. Forced compression predominantly thickens the weakest near-ridge region of oceanic lithosphere, and reactivates inherited extensional faults. This results in rotation of blocks along reactivated faults leading to their subsequent locking. As the result, the development of a new megathrust zone occurs, which accommodates further shortening and subduction initiation. Strain weakening has a key impact on the collapse of thickening mid-ocean ridge region and the occurrence of near-ridge subduction initiation. In contrast, grain size evolution of mantle plays a subordinate role in these processes by slightly modifying the localization of shear zones near brittle-ductile transition. Through comparing with the geological record, our numerical results provide new helpful insights into natural near-ridge subduction initiation processes recorded by the Mirdita ophiolite of Albani.