FORMATION AND FRACTURING OF LARAMIDE BASINS

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.