Problems of wrench faults

Abstract

Some problems of wrench faults, mainly the geometry of their drag effects, and their determination and intersection are examined here. It is concluded that drag along faults, when not accompanied by shortening normal to the fault, cannot produce folds (the so-called “drag folds”). Drag folds indicate strike-slip movements, but not their magnitude or importance. The strike-slip movements of terminating wrench faults must be compensated by various structures — folds, thrusts, normal faults, etc. — which can be used to estimate the magnitude of the strike slip of faults crossing simply deformed terrains. The termination of wrench faults against other wrench faults or their intersection raise great geometrical difficulties. Wrench faults may be associated with many secondary structures resulting from the failure of the fault walls to match exactly, from movements normal to the fault trace, from variations in the magnitude of the strike slip along the fault, etc. These secondary structures can occur in several different combinations. A great tectonic role has lately been attributed to wrench faults. Nevertheless, an examination of the termination and intersection of these faults casts grave doubts on these interpretations and, furthermore, sets upper bounds to their possible horizontal offset. The great role of wrench faults may, therefore, be doubted, especially on platforms, unless much of the data are incomplete or our concepts about the rigidity of platforms have to be revised. Neither can wrench faults be considered as generally causing simple folding.