Numerical modelling of tectonic underplating in accretionary wedges

Accretionary wedge systems result from scraping clastic sedimentary sequences off a descending oceanic plate at subduction zones. Sediments covering the incoming, subducting oceanic plate may be accreted at the wedge front forming a typical imbricate fan. Buried parts of the stratigraphic sequence may underthrust the wedge body and get subsequently accreted at its base (underplating) or even descend further into the mantle. Tectonic underplating requires a step-down of the major décollement horizon representing in fact the subduction plate interface. Intense underplating at the rear of accretionary wedges ultimately leads to antiformal stacking and, in combination of surface erosion, to the uplift of sediments formerly accreted at the base of wedges. In the present study, I try to determine and quantify the main features leading to tectonic underplating and subsequent uplift of underthrust incoming sediments at subduction zones. To do so, a numerical model is set up defined by a downgoing rigid plate, an overlying sedimentary sequence and a rigid backstop resulting in sediment accretion. The implementation of two weak décollement layers allows for the potential development of tectonic underplating. Tested parameters for the tectonic evolution of such systems include décollement strength, surface erosion, elastic stiffness of the downgoing plate and geometry of the rigid backstop. The applied numerical code is based on the finite difference marker-in-cell technique with a visco-elasto-plastic rheology. Results indicate that underplating is intensified when i) the stratigraphically lower décollement is strengthened with respect to the upper one, ii) surface erosion is increased, or iii) the downgoing plate becomes stiffer. Modelling results are compared to natural cases of accrretionary wedges where sediments have been underplated (and eventually exhumed) as in the Makran, the Franciscan, or the Appalachians.