Influence of mobile shale on thrust faults: Insights from discrete element simulations

We study the effects of sedimentary loading of a mobile shale substrate, using two-dimensional discrete element modeling. We develop an updip extensional zone connected at depth to a downdip contractional zone, allowing us to study the formation of gravity-driven fold and thrust belts, and the evolution of decollements within this system. We compare our models to the Niger Delta type locale for shale tectonics. In general, most seismic interpretations of the Niger Delta include decollement(s) at the top of the mobile shale unit, but exact locations vary depending on area of investigation. Our models, particularly ones with mobile shale thicker than 2 km (1.2 mi), show more diffuse decollements, spanning the width of the mobile shale unit, which propagate out in front of the syntectonic sediment wedge and connect normal faults in the extensional zone with toe thrusts in the contractional zone. We also look at the distributions of stress and strain within our models, plotting the distributions of sigma(1) (maximum principal stress) and relating that to the vergence of thrust faults developed in our models. We quantify the amount of strain in different sections of the models, showing that extension is much greater than compression in the fold and thrust belt(s) alone, and that compression is distributed throughout the model, including in front of the fold and thrust belt.