Rifting through a stack of inhomogeneous thrusts (the dipping pie concept)

Orogenic building leaves a complex heritage consisting of a stack of nappes that may have contrasting lithologic structures resulting in heterogeneous mechanical behavior of the system during the postorogenic stages. While the thermal state of the region is reequilibrating, strong lateral variations of the depth to the brittle-ductile transition develop as a consequence of these preexisting heterogeneities. We use a thermomechanical model to quantify how an inherited weak nappe influences the development of fault patterns resulting from postorogenic extension. The competence contrast between the nappe and the rest of the upper crust as well as the strength of the crust itself are the principal variable parameters of our experiments. The results suggest that a dipping weak nappe introduces a lateral velocity discontinuity and serves as a localization factor for deformation. The presence of a preexisting nappe with a low competence contrast is sufficient to localize strain along the nappe leading to the formation of a flexural rolling hinge. In this case, the migration of the basin is slow, continuous, and limited by gravity driven processes that lead to the rise of hot ( weak) material under the subsiding basin. In case of a high competence contrast, overall crustal strength is reduced by a dipping pie effect. Assuming overall high crustal strength, the presence of a contrasting nappe leads to a bimodal fault pattern governed by two types of faults: crustal-scale planar faults rooting on the brittle ductile transition of the crust and thin-skinned listric faults rooting on the nappe itself. This bimodality results in a jump-like migration of the basin downward along the dipping weak nappe. Applying this model to the case of the Gulf of Corinth ( Greece) allows us to explain, in the case of assumed high competence contrast, the observed microseismicity patterns, the asymmetry of the Gulf, and the kinematics of fault migration within the basin.