ABSTRACT: Geometry of Rollover: Origin of Complex Arrays of Antithetic and Synthetic Crestal Faults

R. E. Bischke, John Suppe

Rollovers form by the collapse or bending of a hanging wall as it slides over bends in a normal fault. Once we know the particle motion of the collapse, commonly antithetic Coulomb shear, we can develop a simple relationship between fault shape and rollover geometry that is a function of the relative rates of fault slip and sedimentation. The dip of the rollover is a function of the angle fault bend, which is verified in some structures. A fanning of dips in the rollover is predicted for continuously flattening faults. In contrast, numerous rollovers from the U.S. Gulf Coast, Brunei, and elsewhere do not show significant fanning of dips but show a nearly constant dip that is less than the magnitude predicted from Coulomb shear. These structures show complex arrays of smal synthetic and antithetic normal faults on the crest of the rollover, including keystone grabens. High-quality seismic images show that many of these crestal faults suddenly disappear downdip, apparently to run along bedding. These observations indicate that part of the rollover collapse is by bedding slip after a critical rollover angle is reached; the bedding slip extends from the master normal fault up to the crest of the rollover where it breaks upward to be the imaged synthetic and antithetic crestal faults. Modeling of this process shows that variation in the relative rates of fault slip and sedimentation predict the different observed styles of complex crestal faulting in offshore Texas, Brunei, and elsewhere.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990