Experiments on simultaneous faulting and folding above a basement wrench fault

Abstract

Wrench zones commonly show en-echelon patterns of folds in the stratified cover above a basement fault (examples are the Darien Basin in Panama and the Inglewood fault trend in California). Folds are often associated with synthetic and antithetic wrench faults, but the time relationships between folds and faults may not be obvious. We have done experiments where simultaneous faulting and folding occur above a basement fault. Four experiments were performed using a simple apparatus capable of imposing localised strike-slip motions. This apparatus consists of two plastic plates. One can be moved laterally past the other by a piston, thus modelling lateral motion on a single basement fault. Models were properly scaled to account for gravitational forces, brittle behaviour of sandstone and ductile behaviour of clay or shales in the upper crust. Each model is a vertical succession of silicone layers and sand layers. Each layer is 2 mm thick except for the basal silicone layer which is 2, 4, 6 or 8 mm thick in experiments 1, 2, 3 or 4, respectively. All models were deformed at the same slip rate on the basal fault (1 cm h −1). Experiments 1, 2 and 3, with 8 cm total horizontal sinistral offset, were designed to investigate the influence of basal silicone layer thickness on the deformation of a multilayered model. Experiment 4, with 20 cm of total offset, was designed to investigate the influence of an increasing left-lateral offset on folding and faulting. With a 2 mm thick basal silicone layer (experiment 1), synthetic strike-slip faults appeared, en-echelon above the basement fault. No folds were visible at the surface of the experiment, but the interfaces between sand and silicone layers showed millimetric folds en-echelon above the basement fault. With a 4 mm thick basal silicone layer (experiment 2), en-echelon synthetic strike-slip faults and folds appeared above the basement fault. Faults and folds rotated progressively anticlockwise, and synclines were activated as dextral antithetic shear zones. With a 6 mm thick basal silicone layer (experiment 3), synthetic faults and en-echelon folds appeared as before, but the fold amplitude was larger than in experiment 2. An increasing left-lateral offset produced anticlockwise rotation of faults and folds, as before, and synclines were activated as dextral antithetic shear zones. In experiment 4, after 16 cm of left-lateral offset, the anticlockwise rotation of the folds was such that antithetic dextral shear zones became almost parallel to the basement fault and were then reactivated as synthetic sinistral shear zones.