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Regional Structural and Geophysical Studies of Interaction and Overlap—Primary and secondary basement controls on thrust sheet geometries

By
Nicholas B. Woodward
Nicholas B. Woodward
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Published:
January 01, 1988

Active buttressing of thin-skinned structures by simultaneous thick-skinned structures is a primary basement control for thrust belt geometries. Stratigraphic variations in the sedimentary cover are a form of secondary basement control that can explain many changes in thrust geometry better than primary controls. Thin-skinned structural style changes related to secondary basement control lack the precise temporal and geographic proximities that provide the critical evidence for primary control. Secondary controls, however, are ubiquitous throughout orogenic belts, not just near present basement uplifts, and they provide a powerful mechanism to understand regional and local changes in thrust geometries. Secondary controls are derived from the downwarp behavior of the basement surface when loaded by sediments. Variable subsidence produces different stacking sequences of lithic units (and structural lithic units for later deformation). When deformed by thrusting, lateral ramps and changes in duplex and imbricate geometries result from the regionally variable stratigraphic package being folded and faulted.

An unexposed Snake River fault zone has been proposed previously to explain apparent offsets of stratigraphic and structural trends across the Snake River Plain. This feature is often considered to involve foreland basement rocks as well, and to be part of a foreland structure that affected the thrust belt’s evolution. On a palinspastic map, isopachs and many facies boundaries south of the plain trend northeast-southwest. Once these stratigraphic variations within thrust sheets of the Idaho-Wyoming-Utah thrust belt are understood, the need for any such fault zone is removed because correlative structures across the plain carry different stratigraphic sections.

Basement asperities as the cause of thrust ramps are examined in two field examples in which basement and sedimentary strata are involved in the same thrust sheet at the surface. In both the Ogden, Utah, and the Mountain City, Tennessee, areas, the presence of basement seems to have no effect on the structural geometries developed above them, although stratigraphic layering does. As a result, basement faulting in general seems a less likely direct cause for thrust ramp locations than does stratigraphic control. The inference is that, because different stratigraphic packages vary at different rates and in different directions, they are the most likely control on the areal and vertical locations of thrust ramp positions.

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GSA Memoirs

Interaction of the Rocky Mountain Foreland and the Cordilleran Thrust Belt

Christopher J. Schmidt
Christopher J. Schmidt
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William J. Perry, Jr.
William J. Perry, Jr.
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Geological Society of America
Volume
171
ISBN print:
9780813711713
Publication date:
January 01, 1988

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