SCHOLLNBERGER, E., and A.W. BALLY, Rice University, Houston

Abstract: Passive Margins as Precursors of Foreland Foldbelts

Reconstructions of foreland foldbelts often allow us to infer outlines of passive margins. For exploration, it is important to differentiate pre-orogenic structures and unconformities from orogenic features. In this context, we attempt to model forward deformation of present-day passive margins into foldbelts. This modeling is performed both 'in sequence' and 'out of sequence'.

Regional seismic profiles of selected passive margins around the Atlantic Ocean are interpreted and depth converted. Decoupling levels and detachment planes include major transgressive systems tracts, evaporites, the sediment-basement interface, the bounding faults of half graben systems, and the brittle/ductile transition layer within the crust.

The deformed passive margin examples provide the following insights relevant to exploration in foldbelts. Deep sea fans would be in a younger-over-older configuration if transported onto slope unconformities form ties. If a slope unconformity is used as a flattening datum in a reconstruction, a false basement uplift is simulated. If transgressive shale (possibly hydrocarbon source beds) is involved in the decoupling process it may lose its continuity due to shale flowage. The evaporites would be decoupled primarily at their base, thus bodily transporting existing salt structures. Facies changes on passive margins occur both along and across the strike to the margin, however the strike of the future foldbelt may be oblique to the changes. These facies changes lead to lateral changes in tectonic style along the strike of a foldbelt due to the shift in preferred decoupling levels. Extensional half graben systems would be inverted in the process of deformation if they strike more or less parallel to the foldbelt, while they would not be inverted, but preserved if they are perpendicular to the foldbelt.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England