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The AAPG/Datapages Combined Publications Database

Rocky Mountain Section (SEPM)


Mesozoic Systems of the Rocky Mountain Region, USA, 1994
Pages 27-64

The Sevier Orogenic Belt of the Western United States: Recent Advances in Understanding its Structural and Sedimentologic Framework

David R. Lageson, James G. Schmitt


The Sevier orogenic belt of the western United States has been a prolific laboratory for the generation of new ideas in structural geology and sedimentology during its 150+ years of exploration and mapping. As a result of the mid- to late 1970s boom in petroleum exploration, the Sevier orogen is arguably one of the best understood back-arc fold and thrust belts in the world, and yet, there are still major problems left to be solved. Although many geometric, kinematic, and dynamic concepts of fold-thrust deformation were established prior to the mid-1970s (e.g. duplex fault zones, balanced cross sections), their widespread application to the Sevier orogen has led to a greater depth of understanding and to the emergence of new paradigms. In broad terms, the “rules of behavior” and “family of structures” for fold and thrust belts have become more complex and diverse. For example, out-of-sequence thrust faults are now widely recognized at all scales throughout the Sevier orogen, and are an expected result of wedge thickening in the critical-taper wedge model of fold-thrust belts.

Balanced cross sections have become a standard tool for deciphering the structural geometry of the Sevier orogen, and for restoring it to a pre-contractional state. Although extremely useful as a first-approximation, all balanced cross sections inherently underestimate the amount of regional shortening due to the assumptions of constant volume and area (plane-strain assumptions). Additional problems are introduced in balanced cross sections when lithotectonic zones (i.e. structural lithic units) based on observable field relations are not incorporated, and when kink fold models are applied a priori to entire thrust sheets, regardless of stratigraphic anisotropy. Despite these problems, balanced cross sections have significantly contributed to an enhanced understanding of regional tectonic relations in the Sevier orogenic belt, particularly in the Idaho-Wyoming-Utah salient. Also, through detailed finite strain and fabric studies, it is now possible to estimate the amount of internal deformation that has contributed to regional shortening and incorporate these values into balanced cross sections, improving their accuracy.

Cross-strike structural discontinuities have served an important role in the structural evolution and sediment dispersal history of the Sevier orogen. Usually recognized as transverse lateral ramps, these structures have longitudinally compartmentalized the Sevier orogen into segments of contrasting structural style and geomorphic expression. Major cross-strike structural discontinuities are invariably controlled by long-lived, deep-seated fault zones (e.g. southwest Montana transverse zone) and are not simple tear faults in thrust sheets.

The Sevier orogenic belt and associated foreland basin have served as stimuli for development of ideas concerning relations between sedimentation and tectonism in foreland basins. Recognition of the complex structural and geomorphic controls on sediment dispersal within the thrust belt and adjacent foreland basin has tempered the utilization of conglomerate provenance in reconstruction of the timing and sequence of thrust emplacement. At the same time, new techniques such as provenance modeling have been developed to help address some of the uncertainties inherent in provenance determinations. The Sevier orogen and foreland basin have also been a primary stimulant for new ideas concerning the tectonic significance of conglomerates. Although there is likely no simple answer, merely addressing the question of whether conglomerates are indicative of tectonic uplift or quiescence has profoundly influenced how sedimentary geologists think about all tectonically active sedimentary basins.

Consideration of the structural and geomorphic controls in the Sevier orogen on foreland basin sedimentation has led to application of such concepts as interiorramp-supported uplifts and structural culminations, cross-strike structural discontinuities and transverse drainage systems, and a topographically elevated hinterland as fundamental keys to understanding the delivery of synorogenic sediment to the foreland basin. Detailed, field-based studies involving structural mapping in deformed synorogenic strata have shown the usefulness of syntectonic progressive unconformities in deciphering the kinematic history of thrust systems. Recognition of allochthonous piggyback basins as important loci of sedimentation which are structurally and spatially separate from the adjacent subsiding foreland basin has profound implications for analysis of basin subsidence histories. Major conceptual advances concerning the interaction between the structural evolution of contractional orogens utilizing the critical-taper wedge model, erosion, climate, and sedimentation, have provided a paradigm for thought concerning fold-thrust orogens. Thus, future investigations will incorporate the disciplines of structural geology, stratigraphy, sedimentology, and geomorphology in a synergistic manner to improve the understanding of tectono-sedimentary relations in fold-thrust orogens.

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