Abstract: Using Antithetic Normal Faults to Map Growth Axial Surfaces

SPANG, JOHN H., and STEVEN L. DOROBEK
Department of Geology and Geophysics, Texas A&M University, College Station, TX 77843-3115

Changes in the curvature of growth axial surfaces can reflect changes in sedimentation rate, fault displacement rate, or amount of compaction of different sediment types that comprise

the hanging wall. An increase in the dip of the growth axial surface reflects an increase in sedimentation rate in the hanging wall depocenter, a decrease in the displacement rate along the growth fault or a change to sediment that undergoes less differential compaction. The shape of a rollover anticline in the hanging wall of a normal fault reflects the curvature of the bend in the fault where the fold develops. A gentle bend results in a rounded anticline while an angular bend forms an angular fold. Differential compaction of growth sediments over the anticline tends to dampen the amplitude and broaden the shape of the fold and increases closure area. Unless the fold is very angular, the actual location of the growth axial surface is difficult to determine using only the geometry of the folded layers. When antithetic faults form at the fault bend, they cut through the pre-growth sediments, and in the ideal case, they propagate all the way to the top of the growth sediments. The point where the antithetic fault intersects the top of the growth sediments is the location of the growth axial surface at that instant. With continued displacement and growth sedimentation, additional antithetic faults form as the older faults move away from the bend and are covered by younger unfaulted growth sediments. The upper termination of each antithetic fault, however, reflects the position of the growth axial surface when that antithetic fault formed. In combination with well constrained time lines, this method represents a powerful new tool to interpret the history of listric normal faults.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas