Abstract: Growth Faults and Growth-Fault Mechanisms Observed in the Gulf of Mexico

Barry E. Bradshaw, Joel S. Watkins

Growth faults in the Gulf of Mexico typically occur as elements of extensive fault families. Isolated single faults are rare. Families in turn consist of a primary master fault and a variable number of submaster faults, all of which join a common detachment surface or group of related detachment surfaces at depth. Lateral changes in the incipient substrates control locations of master faults. Common triggers of master fault formation are bends or flexures, and changes from relatively high shear strength sediments to low shear strength sediments (salt or shale) across the edge of a basin. These triggers are preferred failure loci when sediments prograde over them and differentially load the substrate.

Mobilization and flow of the substrate (gravity spreading) creates a ridge (or salt "roller") down-dip of the master fault. Uplift of the ridge flexes overlying strata, and creates a new preferred failure location. This in turn triggers formation of a submaster fault. This process may repeat itself several times. A South Timbalier fault family, for example, indicates six repetitions. Upward and seaward migration of salt ridges may allow formation of counter-regional (landward-dipping) submaster faults.

Ridge and diapir formation accommodate most of the heave of the hangingwall. A few relatively small growth faults detach onto upper surfaces of salt sheets (gravity gliding). These faults are rare, and hangingwall heave is small in comparison to heave associated with gravity spreading. Gravity gliding heave is usually accommodated by toe-thrusting. Each master or submaster has an associated complex of synthetic and antithetic faults.

Both salt and shale serve as detachment media offshore Texas whereas salt is the medium for most detachments in offshore Louisiana and the MAFLA region.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas