Three-dimensional geometry and evolution of a segmented detachment fold, Mississippi Fan foldbelt, Gulf of Mexico

Abstract

The frontal fold of the deep-water Mississippi Fan foldbelt is used to investigate the relationships between folding and faulting in detachment folds. Seismic coverage shows the entire three-dimensional geometry, from termination to termination, and the deformation history as recorded by asymmetric growth strata on fold limbs. The fold is a salt-cored detachment fold cut by reverse faults on both limbs. Its three-dimensional geometry is complex, consisting of four separate culminations, each associated with a distinct fault or fault segment. Consequently, profile geometries vary significantly, but unsystematically, along strike. Data analysis and structural restoration suggest a three-stage evolution during Mio-Pliocene shortening: (1) preexisting, en-echelon salt pillows served as buckling instabilities for the initiation of detachment folds that experienced relatively minor lateral propagation during growth and linkage; (2) an increase in shortening rate was accommodated by break-thrust folding; and (3) the faults became inactive upon a decrease in shortening rate, such that further fold amplification occurred by rotation and uplift of the backlimb. There is a direct correlation between fold and fault geometries, and abundant evidence indicates that the geometries of individual fold segments dictated fault geometries.