--> ABSTRACT: Extensional Forced Folds: Experimental Models and Geologic Examples, by Martha O. Withjack, Jon E. Olson, Eric T. Peterson; #91003 (1990).
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ABSTRACT: Extensional Forced Folds: Experimental Models and Geologic Examples

Previous HitMarthaTop O. Withjack, Jon E. Olson, Eric T. Peterson

We have simulated the development of forced folds above buried normal faults using clay models. In single-layer models, moderately dipping master normal faults produce wide folds with gentle limb dips. Upward-steepening secondary normal faults form. Steeply dipping master faults produce narrower folds with steeper limb dips. Upward-steepening secondary normal faults become high-angle reverse faults at shallow depths. In models with layer-parallel detachments, folds are wider and limb dips are gentler. Slip on the lowermost detachment transfers extension from the master normal fault toward the footwall block where secondary normal faults form. The opposite sense of slip occurs on overlying detachments.

The folds in our clay models resemble extensional forced folds observed in the field and on seismic data. Dipping beds, normal faults, and reverse faults preserved in the hanging walls of through-going normal faults in the Gulf of Suez area resemble the structures in our single-layer models. Normal faults near the anticlinal axial surfaces of extensional forced folds from offshore Norway resemble the secondary normal faults in our multilayer models.

Insight gained from our models can improve the structural interpretation of field and seismic data. A significant amount of forced folding can precede the upward propagation of buried normal faults. Interpreters using geometric models to relate the shape of a fault-bend fold to the shape of a normal fault must consider whether extensional forced folding contributed to the shape of the hanging-wall fold. Our models show that both normal and reverse faults can develop during extensional forced folding. Therefore, the presence of reverse faults does not necessarily indicate an episode of compressional tectonics. Finally, our models show that interpreters can use surface observations of extensional forced folding to estimate the location and dip of master normal faults and to infer the pr sence of layer-parallel detachments at depth.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990