Thrust Faults as a Mechanism for Attenuation in the Steep Limbs of Laramide Folds in Colorado

Abstract

Key exposures of several Laramide, basement-cored folds in Colorado provide views of an ordered architecture of faults in their forelimbs. The faults provide an important mechanism for thinning and extending the strata in the steep limbs as displacement on the master, basement fault increases. One location demonstrates thinning and extension of a shale unit entirely by brittle faulting, rather than ductile flow.

A conjugate set of thrust faults may form when the strata have been uplifted and rotated about 60-65 degrees from horizontal—perhaps when the steep limb evolves from compression into extension. One set of thrust faults dip toward the uplifted, basement block and displace older strata over younger strata. The other set of thrust faults dip away from the uplifted, basement block and displace younger strata over older strata. A thrust fault that displaces younger strata over older strata may appear counter-intuitive—as might a thrust fault in an extensional environment.

The sets of conjugate faults in these forelimbs evolve in an orderly way. As the displacement on the basement fault increases, the dip of the strata in the forelimb steepens (even to overturned) and the conjugate set of faults is rotated within the limb. The thrust faults (<45°) become reverse faults (>45°) as the dip of the limb increases and consequently the dip of the younger-over-older set of faults steepen. With rotation, the younger-over-older set becomes more favorably oriented to slip and experiences greater displacement. This system appears to be scale independent as it is observed operating on the centimeter scale and the hundreds-of-meters scale. Indeed, the steeply dipping limbs of Colorado’s range-front monoclines are significantly displaced by these conjugate faults at Golden and Garden of the Gods. Other locales in Colorado contain similar phenomena.

AAPG Datapages/Search and Discovery Article #90374 © 2020 AAPG Rocky Mountain Section Meeting, 2020 Vision: Turn Hindsight to Foresight, Grand Junction, Colorado, September 13-15, 2020 (CANCELLED)