Geometry,mechanisms, and significance of extensional folds from examples in the RockyMountain Basin and Range province

Document Type

Article

Journal/Book Title/Conference

Journal of Structural Geology

Volume

20

Publisher

Elsevier

Publication Date

1998

First Page

841

Last Page

856

Abstract

Geologic mapping and structural analysis in Paleogene half graben of Idaho and Montana have revealed over 70 extensional folds that form orthogonal sets with mostly NE- and SE-trending axes. On a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults. Transverse folds, at a high angle to the fault, and folds oblique to the fault, comprise more than half of the folds and reflect the highly three-dimensional nature of the bulk strain. Detailed geometric analysis of 15 folds in the Salmon, Horse Prairie and Medicine Lodge basins shows that they typically have an upright to steeply inclined bisecting surface, an interlimb angle of 141°, and a plunge of 19°.

Based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics, we were able to determine how some of the investigated folds formed. Fault-bend folding above both fault-parallel and fault-perpendicular bends in the underlying normal fault produced most of the folds in the study area, but displacement gradients along normal faults, fault-drag, isostatic adjustments, and transtension were also factors in the deformation. Most of the compound folds, which result from more than one mechanism of folding, are oblique to all adjacent normal faults. Recognition of such extensional folds is critical, because they might be misinterpreted as contractional structures, they influence sediment thickness patterns and dispersal in rift basins, and may control the migration and trapping of petroleum and groundwater resources.

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