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AAPG Bulletin

Abstract


Volume: 74 (1990)

Issue: 7. (July)

First Page: 1038

Last Page: 1054

Title: Experimental Models of Extensional Forced Folds (1)

Author(s): MARTHA OLIVER WITHJACK, JON OLSON, and ERIC PETERSON (2)

Abstract:

We have used single-layer and multilayer clay models to study the development of forced folds above normal faults. Our modeling results show that the deformation patterns associated with extensional forced folding depend on the dip of the underlying normal fault and the presence of layer-parallel detachments.

In single-layer clay models, extensional forced folds are upward-widening monoclines. Anticlinal axial surfaces dip in the same direction as underlying master normal faults, and synclinal axial surfaces dip in the opposite direction of master normal faults. Most secondary faults are upward-steepening normal faults. If master normal faults are steeply dipping, however, many secondary normal faults become high-angle reverse faults at shallow depths. The propagation and linkage of secondary faults into through-going normal faults terminates the development of extensional forced folds. More folding occurs prior to fault linkage if the master normal fault is steeply dipping rather than gently dipping. Most dipping beds and secondary faults are preserved in the hanging walls of the through- oing normal faults.

In multilayer clay models with layer-parallel detachments, extensional forced folds are also upward-widening monoclines. Slip on the lowest detachment laterally transfers extension induced by normal faulting and forced folding from the master normal fault to the detachment edge. Slip on overlying detachments accommodates minor thickness changes associated with upward-widening of the fold. Secondary faults include low-angle normal faults near the anticlinal axial surface, minor thrust faults near the synclinal axial surface, and high-angle normal faults above the detachment edge.

The model-predicted deformation patterns are similar to those of extensional forced folds from the Gulf of Suez and offshore Norway. This similarity suggests that our modeling results apply to extensional forced folds and can provide guidelines for interpreting field, well, and seismic data.

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