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AAPG Bulletin

Abstract


Volume: 65 (1981)

Issue: 3. (March)

First Page: 509

Last Page: 525

Title: Structural Geometry of Meade Thrust Plate in Northern Blackfoot Mountains, Southeastern Idaho

Author(s): Richard W. Allmendinger (2)

Abstract:

One of the principal generalizations concerning structural geology in foreland thrust belts is that thrust faults climb to the surface by running parallel with incompetent units and cutting upsection (ramping) across competent members, thus forming folds in the hanging wall but not in the footwall. Detailed geologic mapping at the front of the Meade plate in the northern Blackfoot Mountains, southeastern Idaho, shows that this rule can be applied only to the initial phase of deformation. The initial forms of folds created over ramps in the basal decollement have strongly influenced the geometry of subsequent imbricate thrusts within the Meade plate so that later faults were locally required to cut downsection in the direction of translation.

The allochthon has been divided into three subsidiary thrust plates by two major imbricate thrusts. Folds within thick-bedded upper Paleozoic strata of the Meade plate have sharp hinges and planar limbs, with curved axial surfaces that progressively steepen to the west. These folds are of a modified box or kink form. Folds in less competent Mesozoic rocks below the Meade thrust (in the parautochthon) are more open and concentric. Fold geometry was apparently determined by thickness of overburden during folding, proximity to ramps in the thrust surface, and mechanical properties of the strata involved.

Major thrust imbrications dip gently westward, locally cutting downsection toward the east (direction of transport) where they cross more steeply west-dipping upright fold limbs. This geometry implies that folding preceded imbricate faulting, and that anisotropy of the sequence no longer controlled stratigraphic levels of thrusting after bedding became moderately inclined to the west (generally greater than 35°, depending on competence).

The inferred ramp-associated genesis of folds within the Meade allochthon in the Blackfoot Mountains implies that these folds are rootless. Although rootless hanging-wall folds are productive farther east in the Idaho-Wyoming thrust belt (e.g., above the Absaroka thrust in southwestern Wyoming), those described here in the Blackfoot Mountains, as a result of erosional level and shallow dip of the Meade thrust, do not provide significant structural traps. Folds of the underlying parautochthon (upper plate Absaroka/St. Johns thrust) are more promising.

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