ABSTRACT: Problems of Mechanics of Strike-Slip Faults

SYLVESTER, ARTHUR GIBBS, University of California, Santa Barbara, CA

Many of our perceptions and paradigms about strike-slip faults need to be revised, because they are inadequate to explain many faults encountered in the real world. Previous concepts involved parallel, horizontal slip of rigid fault blocks overlain by a massive, ductile cover such as clay or sand. These concepts led to the wrench model of strike-slip faulting with its overstepping synthetic and antithetic Riedel fractures; en echelon folds; pull-apart basins; "flower structures," both positive and negative; and a master fault extending vertically through basement to lithospheric depth. The model was extended to account for block rotation about a vertical axis. The wrench model fails in many cases, however, where strike-slip occurs on nonvertical faults; where folds are parallel to str ke-slip faults; where shallow crustal detachments exist beneath major faults such as the San Andreas; and where the crust behaves in a nonrigid manner over relatively short lengths of geologic time.

These shortcomings of the wrench model require us to reevaluate how the relative ductility of different kinds of basement rocks, such as massive granite or a heterogeneous ophiolitic basement, may influence the mode of deformation in the overlying cover rocks; how the thickness of cover rocks as well as their mechanical layering may dictate the style of deformation and basin formation; how horizontal detachments in the cover and basement rocks guide or modify strike-slip on vertical faults where they encounter one another; how a strike-slip fault is influenced where it is bent or intersected by another fault; how strike-slip faulting is affected and modified where a strike-slip fault occupies and reactivates a previous normal or reverse fault; and how and on what scale bulk strain is artitioned between strike-slip and dip-slip faults. Scaling factors also require closer scrutiny, because geometry and mechanics of faulting in laboratory models do not seem to self similar beyond a certain scale, no matter how attractive gross comparisons may be. The most elusive problem yet remaining for the explorationist, however, is confident, three-dimensional determination of fault displacement with limited subsurface data that rarely yield diagnostic piercing points. Without that information, it is usually academic to speculate on the answers to the other questions, be they directed to strike-slip faults or even to dip-slip faults.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)