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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


AAPG Bulletin, V. 87, No. 5 (May 2003),

P. 757-780.

Copyright 2003. The American Association of Petroleum Geologists. All rights reserved.

Origin of drag folds bordering salt diapirs

D. D. Schultz-Ela1

1Bureau of Economic Geology, University of Texas at Austin, Austin, Texas, 78713; email: [email protected]


Dan Schultz-Ela specializes in numerical modeling and analysis of salt structures. Since 1989, he has been a research scientist in the Applied Geodynamics Group of the Bureau of Economic Geology. He taught for two years at Colorado College after receiving his Ph.D. from the University of Minnesota for strain analysis of an Archean greenstone belt. Prior work included an M.S. degree from Brown University and a B.A. degree from Carleton College.


Many thanks to Martin Jackson for discussions, thorough reviews of this paper, and for pointing out key references. T. Lawton and M. Rowan provided detailed and helpful reviews; their expert comments are greatly appreciated. I gratefully acknowledge ongoing interaction with Mark Rowan, who with Kate Giles and Tim Lawton has developed and orally presented similar ideas through analysis and reconstructions of a variety of salt structures in parallel to the modeling and interpretation work by our group. Our work was supported by the members of the Applied Geodynamics Laboratory Consortium, namely, Anadarko Petroleum, BHP Petroleum (Americas), BP International, Chevron Petroleum Technology, ENI-Agip, Enterprise, ExxonMobil, Marathon Oil, PanCanadian Petroleum, Petroleo Brasileiro, Phillips Petroleum, Shell Oil, Statoil, Texaco Exploration and Production, TotalFinaElf, Unocal-Spirit, and Woodside. This paper is published by permission of the Director, Bureau of Economic Geology, University of Texas at Austin.


Drag folds bordering salt diapirs are commonly attributed to shear from rising salt. Finite-element models demonstrate that shear deformation is only significant for extremely weak overburdens, such as those having high overpressure and ductile interlayers. Protrusions of overburden into and onto the diapir are most susceptible to folding. So-called drag folds are much more likely to initiate as drape folds of strata onlapping a downbuilding diapir, here termed flap folds for clarity. Potential for flap folding is greatest where the salt/overburden contact dips moderately and for episodic or variable timing and thickness of deposition. Moderate dip allows strata to onlap far across the diapir crest and be carried upward into a high-amplitude fold with the rising salt. Depositional hiatuses allow time for such rise without additional layers that would strengthen the onlapping wedge. Previously folded strata may block onlap of later layers. Stretching and rotation of the layers adjacent to steep parts of the diapir may cause disaggregation, slumping, and debris deposits that can be overridden as the diapir crest spreads laterally onto the overburden surface. Large changes in the relative rates of salt rise and sediment deposition can create cycles of onlap, flap folding, and spreading.

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