Geodynamic models of crustal-scale episodic tectonic accretion and underplating in subduction zones

We investigate the dynamics of crustal-scale episodic accretion at convergent margins using a finite element model of subduction. The numerical model incorporates effects from flexure, subduction loading, and large-scale variations in material properties. The model is used to assess the influence of these parameters on the discrete transfer of units from the lower to the upper plate during subduction. The primary controls investigated are spatial variations in density, material strength, and thickness of material layers. These controls correspond to natural examples where episodic accretion results from the thermal structure of subducting lithosphere, giving rise to areas of thermally activated ductile creep; attempted subduction of small crustal blocks (density, frictional and/or ductile strength variations); interaction between well-attached basement blocks and overlying, detachable sediments; and spatial changes in thickness of sediment and crust. In the experiments, accretion of small continental terranes within a model subduction zone can cause crustal-scale fold nappes and shear zones to develop, with accompanying tectonic underplating and/or frontal accretion. The model results may provide insight into real cases of episodic accretion, for example, the emplacement of the upper Penninic nappe stack of the Central Alps.