The Role of Microcontinent Strength and Basal Detachment in Accretionary Orogenesis: Insights From Numerical Models

During closure of an ocean through subduction and continental collision, bathymetric highs such as microcontinents can accrete, collide, or partially or completely subduct. Such interaction of future allochthonous terranes (FATs) with the overriding continent will modify the dynamics of the subduction zone, affecting its length and frictional resistance, and thus the force balance of the subduction system. Accreted microcontinents and microcontinental fragments are preserved in backarcs and collisional orogens, demonstrating that multiple terranes can accrete during a single Wilson-cycle, in what is termed accretionary orogenesis. In this study, we use thermo-mechanical numerical experiments of microcontinent-continent collision events to investigate parameters that influence whether microcontinents accrete, subduct, or collide. Our results indicate that multiple accretionary episodes are possible, but that a weak basal detachment layer within each FAT is paramount for such a scenario to occur. The introduction of a microcontinent, or FAT, in the subduction zone will affect the balance between slab-pull, far-field forces, and the subduction interface resistance. The strength (and rheological stratification) of the microcontinent determines the evolution of the subduction interface resistance throughout the collision event, exerting a first order control on the resulting geodynamic scenario. Collision with a strong microcontinent significantly increases the subduction interface resistance promoting terrane subduction and localization of deformation away from the subduction interface. In turn, collision with a weak microcontinent increases subduction interface resistance only mildly, allowing for multiple accretion events.