Synthetic Seismic-Reflection Profiles of Rift-Related Structures

Abstract

Published field data indicate that normal faults and associated secondary structures (i.e., minor normal faults, folds produced by drag on fault surfaces, and forced folds above faults) are common in continental rifts. The dip of the fault surfaces, the dip of the strata within the fault blocks, the displacement on the faults, and the position and size of the folds vary considerably. The synthetic seismic-reflection profiles presented in this paper show systematically how each of these variables, as well as rock velocity, influence the seismic expression of rift-related structures. The observed dip and curvature of any fault surface on an unmigrated seismic section depends, not only on the dip and curvature of the actual fault surface, but also on the velocity and dip of the overlying strata. The observed dip of a fault decreases as the velocity of the strata directly overlying the fault increases. Consequently, planar normal faults in rocks whose velocities increase with depth (e.g., most clastic sedimentary rocks) may appear to flatten with depth. The observed dip of a fault decreases as the angle between the fault surface and the strata directly overlying the fault decreases. Consequently, a normal fault dipping in the direction opposite to the strata may appear to have a greater dip than an identical normal fault dipping in the same direction as the strata, nd planar normal faults active during deposition (with beds on the downthrown sides having increasing dip toward the faults with depth) may appear to steepen with depth. The appearance of secondary structures associated with normal faulting on unmigrated seismic sections depends on the position and size of the secondary structures. An increased thickness of low-velocity rocks on the downthrown side of normal faults may disrupt and bend the reflections on the upthrown side. Depth, rock-velocity distribution, and fault displacement affect the severity of the distortion. This distortion may obscure secondary structures on the upthrown side of faults (i.e., minor faults, anticlines produced by drag) and can be interpreted erroneously as secondary faulting and folding. Synclines produced by drag on the downthrown side of normal faults generally have small radii of curvature relative to their burial depths. This relationship makes these synclines difficult o identify on unmigrated seismic sections. Many forced folds in rifts are gentle, shallow structures overlying normal faults. These folds are easier to identify because they are unaffected by the distortion beneath faults, and their synclines have large radii of curvature compared to their burial depths.