HOW EXTENSIVE IS NEOGENE FAULTING IN COLORADO?

The Colorado Geological Survey's “Late Cenozoic Fault and Fold Database and Map Server” contains 350+ faults that cut Miocene or younger rocks. Ninety-two of these faults cut Quaternary deposits. The catalog is incomplete because it does not include faults that cut Oligocene rocks. However, the catalog is probably far more incomplete because many areas of the state are devoid of post-Laramide strata, thus making recognition of Neogene faults more difficult.

Colorado has the largest expanse of Precambrian crystalline rocks in the western United States. It is a challenge to determine whether any of the thousands of faults that cut these crystalline rocks have moved during the Neogene. The character of Neogene faulting is such that the faults are not prone to easy recognition, other than by geomorphology. Significant Neogene fault offset in the subsurface can be represented at the surface only by a geomorphic saddle.

Laramide faulting in Colorado occurred under relatively high confining pressure and a compressional regime that was accompanied by significant calc-alkaline igneous activity. Neogene faulting occurred under lower confining pressure and an extensional regime that is accompanied by basalt/rhyolite igneous activity. Because of this difference in boundary conditions, exposures of Neogene fault zones contrast with exposures of Laramide fault zones. Laramide faults are highly brecciated, fault zones are commonly well preserved (cemented), and associated strata are commonly folded. Neogene fault zones contain gouge rather than cemented breccia, are not well preserved at the surface, and associated strata are commonly faulted rather than folded.

Offset of geomorphic erosion surfaces, offset of post-Laramide igneous bodies, apatite fission track data, and high elevation are useful in detecting potential Neogene tectonism in the Precambrian core of Colorado's ranges. In the Front Range there appears to be a tectonic front, west of which virtually all faults become suspect as undergoing post-Laramide movement.