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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 8. (August)

First Page: 1353

Last Page: 1354

Title: Foreland Deformation--A Critique of Cause: ABSTRACT

Author(s): John K. Sales

Article Type: Meeting abstract


The cause of foreland deformation has been argued for nearly 100 years, and despite definitive stratigraphy, superb exposure, and extensive seismic and well data, the mechanism and geometry remain elusive. It is necessary to separate arguments for cause from arguments that only support a particular geometric interpretation.

The following statements are presented for discussion and are not advanced as "the answer."

(1) Foreland deformation is caused by end-load buckling in a plate tectonic setting driven by abnormally shallow subduction. (2) The crust is the active (competent) unit, and the plastic subcrust is a passive cushion allowing the great vertical component, though a slight slope forced on it by the subducting slab may have aided telescoping of the crust above. (3) No strictly vertical cause can allow the basins to go down as much as they did, and active (causal) intrusion under uplifts only is not likely. (4) While reverse faults and modest shortening predominate, fault attitudes can

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have greater diversity than in the thin-skinned regime because the plastic substratum allows easy vertical motion; nearly vertical faults are allowable. (5) The basement does fold, but with difficulty and in broad wavelength. (6) Because of the broad arching, a "neutral surface" exists well within basement over uplifts, allowing high-level features, such as Rattlesnake Mountain, to be bounded by high-angle normal faults. (7) This same neutral surface forces out-of-the-basin crowding, causing the steep flanks of most basin folds to face toward the adjacent uplift.

(8) The "Hafner approach" illustrates the diversity of curved faults that can be generated in a vertically sinusoidally loaded beam, and which can be generated equally well in an end-loaded sinusoidally buckled beam, as long as it sits on a passive plastic substratum. (9) The Sanford model is excellent for depicting the fault configuration generated in sediments above a high-angle fault. (10) Faults such as Dinosaur Monument can be seen to steepen downward, but my models suggest that they go listric at their lower transition with the plastic substratum. (11) The COCORP trace of the Wind River fault indicating a nearly planar 35° dipping fault most of the way through the crust is probably real; arguments that you cannot see a fault of "granite" against "granite" do not apply. (12) Gravity highs over uplifts, models, and later collapsed uplifts speak for a flexed and jostled slab configuration and against a buoyant root configuration.

(13) Thrusting from several different directions appears not to be a problem when viewed in the context of "jostled slabs." (14) Blocky corners do place limits on amount of thrust and strike-slip translation. (15) The argument for pure verticality to solve a presume "space problem" in the Piney Creek structure loses validity as soon as the bounding tears are allowed to stray from a perfectly vertical dip. (16) The argument at Elk Mountain that the dip of the bounding structure must be at least as low-angle as the degree of overturning of sedimentary panels is wrong (proven by Five Springs).

(17) Sales' eastward crowding of the Colorado Plateau and development of a "Wyoming couple" north of it still seems cogent. (18) The Chapin and Cather, and Gries subdivision into movement phase also appears to be correct. (19) If horizontal compression is a reality, Stone must be correct in principle; there have to be logical connecting structures. (20) The crust can transmit stress over great distances because it is weak enough; southeast Asia tectonics require greater distances of stress transmittal than Laramide foreland tectonics.

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