Numerical models for slab tearing beneath southern Mexico and northern Central America

Abstract

An upper mantle vertical slab tear of the Cocos slab has been recently revealed by several seismological studies in southern Mexico. However, the origin of this slab tear is still not well understood, especially for southern Mexico where the tectonic evolution is still controversial since it is intimately linked with the tectonic evolution of the Chortis Block, a large continental fragment currently located in southern Mexico and the northern part of Central America. In this study we present 3D geodynamical time dependent kinematic models for two main proposed scenarios of tectonic reconstructions in southern Mexico, and analyze the origin and evolution of slab tear in the region. For each numerical model, top kinematic boundary conditions are imposed based on different tectonic evolution of the Chortis Block, and each simulation is integrated 45 Myr back in time. We also include the effect of Tehuantepec fracture zone (TFZ) which is treated as a weak zone with variable depth extents (from 0 to 100 km depth) and with a prescribed low viscosity zone two orders of magnitude lower than the upper mantle and located in the upper part of the oceanic lithosphere. Modeling results show that slab tearing cannot be achieved by purely slab contortions from flat slab to normal dipping slab. However, when a low viscosity TFZ is introduced in the simulations, we obtain a vertical slab tear consistent with seismological observations. The best fitting model includes a plate weakness in form of a mechanically weak linear feature equivalent to a serpentinized fracture zone that extends to 25–50 km depth into the mantle. This result is consistent with previous studies where the upper mantle where ocean floor faulting can gradually penetrate deep into the subducting plate before subduction and induce locally mantle serpentinization.