Three-dimensionality of slab detachment due to ridge-trench collision: Laterally simultaneous boudinage versus tear propagation

The detachment of subducted tectonic plates is a process that has been increasingly associated with collisional scenarios and the end of subduction in various locations worldwide. In particular, the propagation of slab detachment (tearing) of a subducting plate has been described in conceptual models as a cause for spatially and temporally progressing surface effects such as slab gap volcanism and uplift. However, there is little understanding of the causes and dynamics associated with three-dimensional (3-D) slab tearing, especially in the case of ridge-trench collision. Here we show using fully dynamic 3-D numerical models that the process of detachment due to ridge-trench collision depends on the geometry of the ridge segments approaching the trench. For a finite laterally symmetric slab, the 3-D detachment process occurs nearly simultaneously along strike by way of boudinage-type necking and opening of holes central to the slab. For a case involving the approach of two offset ridge segments to the trench, slab tearing occurs in the form of (1) a vertical propagating separation along the age offset boundary within the slab that was previously weakened by a transform weak zone and (2) horizontal propagating detachment controlled by lateral transfer of slab pull to adjacent surface plate segments. However, lateral decoupling between offset adjacent plate segments and the propagating nature of the vertical and horizontal tearing are dependent upon fracture zones remaining weak through the subduction zone. Whether detachment occurs simultaneously along strike or propagates laterally, the process is controlled by plastic yielding of the slab interior when young lithosphere entering the trench can no longer support slab pull.