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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes

Abstract

K. R. McClay, 2004, Thrust tectonics and hydrocarbon systems: AAPG Memoir 82, p. 303-323.

Copyright copy2004. The American Association of Petroleum Geologists. All rights reserved.

Shear Fault-bend Folding

John Suppe,1 Christopher D. Connors,2 Yikun Zhang3

1Department of Geosciences, Princeton University, Princeton, New Jersey, U.S.A.
2Department of Geology, Washington and Lee University, Lexington, Virginia, U.S.A.
3Department of Geosciences, Princeton University, Princeton, New Jersey, U.S.A.; Present address: Department of Earth Sciences, Nanjing University, Nanjing, China.

ACKNOWLEDGMENTS

We are especially grateful to Roy D. Hyndman of the Pacific Geoscience Centre for providing us with copies of the Cascadia seismic lines, and to Greg F. Moore of the University of Hawaii for providing the Nankai Trough line. We are also extremely grateful for the very careful and thoughtful reviews of the manuscript by Josep Poblet of the University of Oviedo and Dave Waltham of Royal Holloway. The final manuscript was substantially improved because of their stimulating reviews. Finally, we take pleasure in thanking Ken R. McClay for having the vision and energy to host three stimulating Thrust Tectonics conferences and to see the volumes through to publication.

ABSTRACT

Shear fault-bend folding produces ramp anticlines with very distinctive shapes. They are characterized by long, gentle backlimbs that dip less than the fault ramp, in contrast with classical fault-bend folding. Backlimb dips and limb lengths increase progressively with fault slip, by a combination of limb rotation and kink-band migration. We summarize and apply two simple end-member theories of shear fault-bend folding involving a weak deacutecollement layer of finite thickness at the base of a ramp: (1) simple-shear fault-bend folding, in which the layer undergoes an externally imposed bedding-parallel simple shear with no basal fault, and (2) pure-shear fault-bend folding, in which this basal layer slides above a basal fault and shortens and thickens above the ramp, with no externally applied bed-parallel simple shear. In the limit of large displacement, the fold geometry in pregrowth strata approaches the geometry of classical fault-bend folding, with a backlimb dip that approaches the ramp dip. However, even in these cases, growth strata may record the history of limb rotation that is characteristic of a shear fault-bend fold heritage. We demonstrate that these theories are in agreement with well-imaged seismic examples from the Nankai Trough and Cascadia accretionary wedges, which show substantial shears (40–65deg) over stratigraphic intervals of a few hundred meters.

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