Experimental fracture, strain and subsidence patterns over en échelon strike-slip faults: implications for the structural evolution of pull-apart basins
Claybox experiments show that with progressive left-lateral displacement on basal left-stepping, en échelon strike-slip faults an extremely complicated zone of evolving fracture, strain and subsidence patterns develops in the overlying clay. The fracture pattern is more complex than predicted by theoretical models and consists of a progressively widening array of Riedel shears, conjugate Riedel shears, and tension gashes above basal master faults. Conjugate Riedel shears rotate anticlockwise and lengthen, forming S-shaped fractures. Riedel shears rotate both anticlockwise and clockwise. Those that rotate anticlockwise eventually open as tension gashes. Other tension gashes open where the ends of Riedel and conjugate Riedel shears link. Strain contours in the deformed clay show an asymmetrical Z-shaped pattern over the stepover area between basal en échelon faults. In the early stages of displacement, strain is distributed penetratively over a progressively broadening zone. In later stages of displacement strain is partitioned into a narrow middle zone. Maximum strain is sustained always in the middle of the sheared zone and increases exponentially with progressive displacement. Subsidence is accommodated in the stepover area by oblique-slip on many Riedel and conjugate Riedel shears distributed throughout the stepover area. Subsidence ( y) exhibits a linear relationship with master fault displacement ( x) described by y = 0.36 x - 1.4. The experimental fracture, strain and subsidence patterns in clay over en échelon faults can be used as interpretive and predictive tools with which to constrain interpretations of static field examples, especially in pull-apart basins on the crustal scale where many similarities with the models exist.