Experiments on Faulting in Zones of Distributed Wrenching

Abstract

Analog model experiments were performed to investigate faulting in zones of distributed wrenching at basin and crustal scale. Three-dimensional fault shape and fault evolution were analyzed by x-ray-computed tomography. Stratified models were constructed with brittle analog materials (sand and glass powder) overlying a thin layer of viscous polydimethyl siloxane. Distributed wrenching is dominantly accommodated by synthetic strike-slip faults (R-shears) at low strain stages. They strike at about 20[degrees] to the shear direction. Antithetic strike-slip faults (R[prime]-shears) are rare and strike at 70-80[degrees]. Synthetic and antithetic Riede are subvertical and extend down to the base of the brittle layers. En echelon-arranged R-shears create push-up zones in the area of overlap. With increasing deformation, lower angle synthetic (R[sub L]) and lower angle antithetic faults (R[prime][sub L]) form. these secondary structures are confined mostly in between previously formed major R-shears. Newly formed R[prime][sub L]-shears strike at progressively lower angles to the shear direction with increasing wrenching and document local stress field modifications. In experiments with unconfined transversal borders, R[prime] and R[prime][sub L]-shears undergo considerable rotations about vertical axes. Space problems created by rotation of antithetic shears in between major R-shears are solved by upbulging of the free surface near their intersection. This resultsmore » in sigmoidal-shaped antithetic faults (plan view) with a dip-slip component and a dip direction that changes along strike. Possible structural traps are represented by push-up zones and areas of positive vertical relief near the intersection of antithetic and synthetic faults. There is good agreement between our models and natural examples of major wrench zones where block rotations have been documented by paleomagnetic,geodetic and seismic methods.« less