AAPG Annual Meeting 2013 INFLUENCE OF PRE-EXISTING STRIKE-SLIP FAULTS ON FAULT DEVELOPMENT DURING SUBSEQUENT PHASES OF DEFORMATION

We used scaled experimental (analog) models with wet clay to investigate how conjugate sets of steeply dipping strike-slip faults affect deformation patterns during a subsequent phase of extension. In all twophase experiments, an initial phase of strike-slip deformation produces a long, wide deformation zone consisting of subvertical Riedel shears; synthetic R-shears and antithetic R'-shears trend at ~15° and ~85°, respectively, relative to the long axis of the deformation zone. The second phase of deformation is extensional with extension directions trending 60° to 120° from the long axis of the deformation zone. Reactivation of the pre-existing R-shears only occurs in models in which the second-phase extension direction is subperpendicular to the R-shears. Reactivation includes changes in the sense of slip (strikeslip faults reactivated as oblique-slip faults) and length (propagation of pre-existing faults). In all models, new normal faults also form during the second phase of deformation. These new normal faults crosscut, terminate against, and/or initiate at pre-existing Riedel shears, creating zigzag geometries or "ladder" structures (normal faults bounded by subparallel strike-slip faults). In models where the extension direction is oblique to both sets of Riedel shears, the second-phase fault orientations resemble those that develop in models without a first phase of strike-slip deformation. In models where the extension direction is subperpendicular to the R-shears, the second-phase fault orientations resemble those of the Rshears. Although the new faults have the same strike as the R-shears, their moderate dip angles are more typical of normal faults. Layered models suggest that these faults develop when some steeply dipping Rshears, present only near the base of the model, are reactivated during the second phase of extension. As the faults propagate upward toward the surface, their dip angle decreases. In summary, reactivation of steeply dipping strike-slip faults can occur during extension, depending on the orientation of the extension direction relative to that of the strike-slip faults; additionally, the presence of pre-existing strike-slip faults can affect the geometry and spatial distribution of new normal faults.