C. J. VanDenburg

GEOMETRY, MECHANISMS AND SIGNIFICANCE OF EXTENSIONAL FOLDS FROM EXAMPLES IN THE ROCKY MOUNTAIN BASIN AND RANGE PROVINCE, U.S.A.

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.

Long-distance longitudinal transport of gravel across the Cordilleran thrust belt of Montana and Idaho

Two newly identified middle Eocene paleovalleys (≥ 100 km long) preserved on top of the southwest Montana reentrant of the Cordilleran fold-and-thrust belt indicate long-lived longitudinal flow across the thrust belt and resolve a long-standing debate about the source of the voluminous quartzite debris in the Upper Cretaceous to lower Tertiary Divide, Harebell, and Pinyon conglomerates of Montana, Idaho, and Wyoming. Geologic mapping, stratigraphic, provenance, and geochronologic studies revealed that Eocene volcanic and sedimentary rocks in the paleovalleys are as thick as 2 km, onlap preexisting bedrock, and interfinger with well-rounded conglomerate derived from formations exposed only to the west. The middle Eocene paleovalleys are the youngest expression of a major paleoriver system that transported sediment toward the foreland during the Sevier orogeny. An Eocene subcrop map shows that the headwaters of the Eocene paleovalleys coincided with structural culminations in the thrust belt that supplied sediment to the Divide conglomerate of the Upper Cretaceous to lower Tertiary Beaverhead Group. Ultimately, the Lemhi Pass and Hawley Creek paleovalleys provided several thousand cubic kilometers of quartzite debris to the Pinyon and Harebell conglomerates of northwest Wyoming 200–350 km away, and formed the northwest half of a giant longitudinal drainage system. Sevier contraction, not the rising Idaho batholith, first uplifted vast culminations beneath the headwaters of this river system.

Three-dimensional strain produced by >50 My of episodic extension, Horse Prairie basin area, SW Montana, U.S.A.

Abstract The Horse Prairie basin of southwestern Montana is a complex, east-dipping half-graben that contains three angular unconformity-bounded sequences of Tertiary sedimentary rocks overlying middle Eocene volcanic rocks. New mapping of the basin and its hanging wall indicate that five temporally and geometrically distinct phases of normal faulting and at least three generations of fault-related extensional folding affected the area during the late Mesozoic (?) to Cenozoic. All of these phases of extension are evident over regional or cordilleran-scale domains. The extension direction has rotated ∼90° four times in the Horse Prairie area resulting in a complex three-dimensional strain field with ≫60% east–west and >25% north–south bulk extension. Extensional folds with axes at high angles to the associated normal fault record most of the three-dimensional strain during individual phases of extension (phases 3a, 3b, and 4). Cross-cutting relationships between normal faults and Tertiary volcanic and sedimentary rocks constrain the ages of each distinct phase of deformation and show that extension continued episodically for more than 50 My. Gravitational collapse of the Sevier fold and thrust belt was the ultimate cause of most of the extension.