POST-LARAMIDE DEFORMATION IN CENTRAL COLORADO

Understanding the post-Laramide tectonic evolution of Central Colorado is essential to understanding the region's uplift history and linkage to processes on the plate margin. This study analyzed faults in the abundant post-Laramide igneous rocks to determine the deformation history of the region.

Previous hypotheses for post-Laramide deformation include transmission of dextral shear from the Cordillera causing north-south strike-slip faulting, counterclockwise rotation of the Colorado Plateau relative to the southern Rocky Mountains causing east-west to southwest-northeast extension, and gravity-driven collapse of flanking thrust belts and/or volcanic piles.

In the San Juan volcanic center, mapped fault strikes are distinctly bimodal, with northwest-striking normal faults and north-south faults that parallel the Rio Grande rift to the south. Minor faults from the 28 Ma Fish Canyon tuff in South Fork, and the 28 Ma Bachelor Mountain tuff in Creede, are dominated by north-south left-lateral and northwest right-lateral strike-slip faults as well as NNW normal faults. These faults indicate ENE-WSW extension, with the abundance of strike-slip faults possibly indicating a transtensile stress regime.

In the northwest Front Range near the Wyoming border, the 29 Ma Mt. Richthofen batholith deforms Cretaceous sedimentary strata. A large north-south fault zone extending over 60 kms cuts the batholith. Minor fault data from this zone indicates north-south strike-slip faulting, locally overprinted by normal slip, suggesting northeast-southwest transtension followed by east-west extension.

In conclusion, post-Laramide deformation in Central Colorado appears to include east-west extension, north-south strike slip faulting and northeast-southwest extension, indicating a complex structural evolution.