PHYSICAL MODELS OF INTRACONTINENTAL WRENCH ZONES

In a series of physical experiments on strike-slip zones, we investigated (1) the boundary conditions that promote the onset of wrenching motion within a continental plate subjected to regional shortening and (2) the influence of far-field stress conditions on the structural style of the wrench zone. Experiments demonstrate that the presence of preexisting faults or structural fabrics is not a prerequisite condition for wrenching. Wrench zones can spontaneously develop in an isotropic brittle layer overlying a weaker viscous layer if there are areas where the brittle layer is locally thinner, hence weaker (e.g., former extensional basins or thermal anomalies). When regional compression is applied, these weaker areas are preferentially shortened. A wrench zone forms, connecting the two weak areas and transmitting the displacements from one area to the other. Experiments also indicate that the structural style changes as far-field stress conditions are varied. In laterally confined models, the wrench zone is broad, trends slightly obliquely with respect to the direction of imposed movement, and comprises both transpressional and transtensional segments. In models having little or no lateral confinement, the imposed wrench movement is accommodated by a straight, linear, narrow fault zone trending exactly parallel to the direction of the imposed regional motion and along which pure strike slip takes place.