About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 571

Last Page: 572

Title: Seismic Reflection Models of Rift-Related Structures: ABSTRACT

Author(s): Martha Oliver Withjack, D. J. Drickman Pollock

Article Type: Meeting abstract


Published field data from several rifted basins indicate that normal faults and associated secondary structures (i.e., minor normal faults, folds produced by drag on fault surfaces, forced folds above faults) are common in rifts. The dip and curvature of the fault surfaces, the fault displacements, the dip of the strata within the fault blocks, and the position and size of the folds vary considerably. Our two-dimensional, seismic-reflection models systematically show how each of these variables, as well as rock velocity, influence the seismic expression of rift-related structures. These seismic models reveal several "pitfalls" of seismic interpretation common to rifts, many of which we have recognized on actual seismic data.

The observed dip and curvature of any fault surface on our unmigrated seismic models depend, not only on the dip and curvature of the actual fault surface, but also on the dip and velocity of the adjacent beds. The observed dip of a fault decreases as the angle between the actual fault surface and strata decreases. For example, normal faults dipping in the opposite direction as the strata appear to have greater dips than identical normal faults dipping in the same direction as the strata. Also, normal faults active during deposition (with beds on the downthrown side having increasing dip toward the faults with depth) appear to steepen


End_Page 571------------------------------

with depth more so than identical normal faults active after deposition. The observed dip of a fault decreases as the velocity of the adjacent rock increases. For example, normal faults in rocks whose velocities increase with depth (e.g., most clastic sedimentary rocks) appear to flatten with depth more so than identical normal faults in rocks with more uniform velocities.

The appearance of secondary structures associated with normal faulting on our unmigrated seismic models depends on the position and size of the secondary structures. The increased thickness of low-velocity rock on the downthrown side of normal faults disrupts and bends the reflections on the upthrown side. Depth, fault displacement rock velocity distribution, and the angle between the fault surface and adjacent beds affect the severity of the distortion. This distortion obscures any secondary structures present on the upthrown side of faults (i.e., minor faults, anticlines produced by drag) and can erroneously be interpreted as secondary faulting and folding. Synclines produced by drag on the downthrown side of normal faults have small radii of curvature relative to their burial depth . This relationship makes these synclines difficult to identify on unmigrated seismic sections. Many forced folds in rifts are gentle shallow structures overlying normal faults. These folds are the most easily identifiable because they are unaffected by the distortion beneath faults and the synclines have large radii of curvature compared to their burial depths.

End_of_Article - Last_Page 572------------

Copyright 1997 American Association of Petroleum Geologists