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

Dr. Vince Mathews
Retired - State Geologist and Director of the Colorado Geological Survey
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About the Speaker:

Dr. Vince Matthews retired as State Geologist and Director of the Colorado Geological Survey at the beginning of 2013. He then served as Interim Executive Director of the National Mining Hall of Fame and Museum and still serves on the Board of Directors.

Vince received Bachelor’s and Master’s degrees in Geology from the University of Georgia and a Ph. D. from the University of California, Santa Cruz and holds Outstanding Alumnus Awards from both institutions. He taught at six institutions of higher education and served as an executive in four natural-resource companies.

Dr. Matthews is a Senior Fellow in the Geological Society of America where he served as General Chair of the 125th Anniversary Meeting in 2013. He is the 2014 recipient of the Pioneer Award from the American Association of Petroleum Geologists. Vince is the author of the multiple, award-winning book Messages in Stone: Colorado’s Colorful Geology and is currently finishing up a book titled Land of Ice: Jaunts into Colorado’s Glacial Landscape.


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.

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