One of the most prominent plate tectonic processes is seafloor spreading. But its formation processes are poorly understood. In this study, we thoroughly address how the brittle-ductile weakening process affects the formation and development of tectonic patterns at spreading centers using 3D magmatic-thermomechanical numerical models. Grain size evolution and brittle/plastic strain weakening are fully coupled into the model. A spectrum of tectonic patterns, from asymmetric long-lived detachment faults in rolling-hinge mode, short-lived detachment faults in flip-flop mode, to symmetric conjugate faults in flip-flop mode are documented in our models. Systematic numerical results indicate that fault strength reduction and axial brittle layer thickness are two pivotal factors in controlling the faulting patterns and spreading modes. Strain weakening induced by localized hydrothermal alteration can lead to the variation of the fault strength reduction. Strong strain weakening with large fault strength reduction results in very asymmetric detachment faults developing in rolling-hinge mode, while weak strain weakening leads to small fault strength reduction, forming conjugate faults. Moreover, the thermal structure beneath the ridge is influenced by spreading rates, hydrothermal circulation, and mantle potential temperature, which in turn controls the thickness of the axial brittle layer and results in variation in tectonic patterns. Further, in order to test a damage mechanism with a physical basis, we investigate grain size reduction at the root of detachment faults. We found that its effect in the formation of detachment faults appears to play a subordinate role compared to brittle/plastic strain weakening of faults.

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.