INVESTIGATION OF NEW FAULT DEVELOPMENT WITHIN CONTRACTION BENDS IN A CLAYBOX

We investigate the growth of new faults near a contractional bend within a strike-slip faults system using wet clay analog models. The models simulate conditions similar to that of the southern Big Bend of the San Andreas where new active fault traces have developed within the past 1 My. We explore a range of contractional bend fault geometries with a servo-controlled claybox. For each experiment, a contraction bend comprised of three kinked segments is cut into the clay. The kink angle and size of the step-over are systematically varied whereas fault dip is vertical in all models. Various basal plate geometries facilitate slip along the kinked fault geometries. Silicone putty is also introduced below the clay in some experiments to allow more distributed application of the basal displacements to the clay. During the experiment, the clay is periodically scanned with a 3D laser scanner to document slip, off-fault strain, new fault formation, and uplift. Contractional bends with large kink angle (e.g. 90˚) and large step-over length (e.g. 10 cm) do not exhibit significant strike slip. Instead, two new thrust faults develop to form a bivergent pop-up structure that promotes uplift within the contraction bend. Smaller step-over and smaller angle of fault kink promotes strike slip through the contractional bend via the development of new strike-slip faults that splay from the primary fault. The degree of new splay fault development decreases with decreasing step over size and kink angle. The mechanical efficiency of these evolving fault systems can be analyzed from the slip data collected. The results from this parametric study can be compared to the evolutionary history of the southern Big Bend of the San Andreas to provide insight into the processes controlling abandonment and initiation of active fault strands within this complex system.