Paper No. 5
Presentation Time: 2:10 PM


ERSLEV, Eric A., Department of Geology and Geophysics, University of Wyoming, 1000 E. University Ave, Dept. 3006, Laramie, WY 82071 and KENNEDY, Laura E., Weatherford Laboratories, Golden, CO 80403,

New structural geology research is generating a comprehensive framework for understanding the Laramide and post-Laramide deformation of Rocky Mountain basins. Deep seismic data from the NSF/EarthScope Bighorn Project have confirmed that the Moho in this area was not involved in Laramide thrusting, with the Bighorn Arch master thrust rooted in a lower crustal detachment at ~30 km depth. The resulting detachment model for the Laramide Orogeny requires marginal and internal transpressional zones, which have been documented as both discrete oblique-slip faults, commonly reactivating Precambrian weaknesses, and as broad zones of distributed strike-slip minor faulting.

Bighorn Project fracture studies encircling the Bighorn Arch have revealed a radial component to shortening directions in the steep, arcuate forelimb. This indicates dual mechanisms for Laramide shortening: (1) regional ENE-WSW shortening and (2) localized radial shortening away from arch highs. The addition of a radial, probably gravitational component to late-stage shortening explains evidence for the diverse directions of late-Laramide shortening (e.g., N-S shortening next to E-W arches, and E-W shortening next to N-S arches) without invoking regional temporal changes in the Laramide compression direction. A gravitational component is also consistent with diversely-oriented syn-Laramide gravity slides and the common sequence of out-of-the-basin thrusting followed by into-the-basin thrusting.

Post-Laramide fracturing and basin formation occurred during at least two separate episodes. Post-Laramide magmatism locally caused fracturing, with these fractures probably partially due to hydrocarbon maturation in organic-rich strata like the Niobrara Formation. Miocene to Holocene extension reactivated Laramide thrust faults along the current-day Rio Grande Rift and in several E-W arches, causing crestal collapse of arches and basin formation. Extensive regional E-W to NW-SE fracturing in the center of basins is continuing to occur while at basin margins above several Laramide master thrusts, post-Laramide extensional fractures strike parallel to the back-slipping thrust systems.