Anna Gülcher , Stéphane J. Beaussier, Taras Gerya

Extensional detachment faults, which have been widely documented in slow-spreading and ultraslow-spreading ridges on Earth, can effectively localize deformation due to their weakness. After the onset of oceanic closure, these weak oceanic detachments may directly control the nucleation of a subduction zone parallel to the former mid-ocean ridge, as is suggested for the Neotethys in Middle Jurassic times. So far, this hypothesis has only been tested by 2D numerical models, whereas the geometry of detachment faults is intrinsically three-dimensional. Here, we conducted a series of 3D numerical thermomechanical experiments in order to investigate the formation of detachment faults in slow oceanic spreading systems and their subsequent response upon inversion from oceanic spreading to convergence.

Numerical results show that during the oceanic spreading stage, the formation of detachment faults strongly depends on the magnitude of the healing rate of faulted rocks in the oceanic lithosphere, that reflects the stability of hydrated minerals along fractured rocks. The detachment faults formed in our 3D numerical models deviate from the “rolling hinge model” of oceanic detachment faulting where fault footwalls are rotated and oceanic core complexes are thereby formed. Our results accentuate that the controlling physical parameters for the development of oceanic core complexes and detachment faults can differ, and that their coupled development in nature remains a key target for future research.

Upon modeled transition to compression, previously formed asymmetric spreading patterns are prone to asymmetric inversion, where one oceanic plate thrusts under the other. Our results suggest that detachment faults accommodate significant amounts of shortening during the initiation of oceanic closure, but, in contrast to the previously proposed simple conceptual model, no direct inversion of a single detachment fault into an incipient subduction zone is observed. Instead, a widespread interaction of multiple detachment faults occurs after the onset of convergence. Ultimately, the nascent subduction zone cuts through the base of several pre-existing detachment faults, thereby forming an initial accretionary wedge in the incipient fore-arc.