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Abstract: Effect of Initial Fault Geometry on the Development of Fault-Propagation Folds with Fault-Cored Anticlines and Footwall Synclines

SPANG, JOHN H., Department of Geology and Geophysics and Center for Tectonophysics, Texas A&M University, College St., TX 77843-3115, U.S.A.

Summary

Asymmetric synclines have often been observed in the footwall of thrust faults. These structures are typically found in well layered sedimentary strata and develop on a wide range of scales. Fault-propagation folds which develop a wedge-shaped syncline in the footwall of the propagating fault (Spang, 1995) have a forelimb anticlinesyncline pair with dips on their axial surfaces which are less than the dip of the fault. The wedge-shaped syncline in the footwall of the fault is due to distributed layerparallel simple shear below the stratigraphic level of the tip line. In the model the trailing edge is vertical above the stratigraphic level of the tip line but is inclined below it due to the distributed layer-parallel shear. With increasing displacement the tip line propagates upward in the stratigraphy and the wedged shaped trailing edge and footwall syncline increase in size. The models are balanced using a combination of line length and area balancing and are checked using the excess-area method (Epard and Groshong, 1993). The geometry of the self-similar models can be varied to yield constant bed thickness or thickening/thinning on the common fold limb between the forelimb anticline and the syncline. In this case, the relative amount of layer-parallel shear varies with stratigraphic level, which simulates a stratigraphy with varying relative ease of bedding plane slip. The fault-propagation syncline can develop at the tip line of a propagating layer layer-parallel detachment, of a propagating ramp, or beneath a block uplift, and the effect of these initial geometries is discussed below.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah