INSIGHTS ON THE KINEMATIC EVOLUTION OF THE UTE PASS FAULT ZONE FROM INVESTIGATION OF MESOSCOPIC BRITTLE FAULTS

The Ute Pass fault (UPF), Colorado Front Range, forms the eastern margin of the Pikes Peak massif, where Proterozoic crystalline rocks are upthrown with respect to Phanerozoic strata, consistent with west-side-up reverse faulting during the Laramide orogeny. Units as young as Late Cretaceous are vertical or overturned in the footwall of the fault. However evidence also exists for strike-slip and normal sense movement in Cenozoic time, suggesting a complex movement history for the UPF. To gain kinematic information about each deformation stage, a new study of mesoscopic brittle faults within and bordering the UPF is underway in the area around Manitou Springs, CO.

In this area, the ~N-S-striking, moderately west-dipping UPF that bounds the southern Front Range changes in orientation to ~ 320º, subvertical, along a very linear trace suggestive of a strike-slip fault. The NW fault segment trends obliquely across the Front Range, and steep NW-striking brittle shears in the zone do exhibit shallow, NW-plunging striae indicative of sinistral (linked to west side up) oblique strike slip. Deformation is focused where the change in fault geometry occurs: the UPF divides in to three or more fault strands; there is a wide zone of brittle cataclasis and gouge; and footwall strata undergo an abrupt change in orientation. Abundant “sandstone dikes” in the UPF hanging wall are penetratively brittlely sheared. Work to date distinguishes three distinct fracture arrays associated with the Ute Pass Fault. These are 1) subvertical NW-striking shears with shallowly plunging fault striae; 2) steeply S-dipping ENE-WSW shears with shallowly plunging striae; and 3) a conjugate array oriented N65W, exhibiting down-dip, normal-sense striae. Preliminary results from kinematic analysis (P, T axis maxima; e.g. Marrett & Allmendinger 1990) indicate that the NW and ENE sets likely formed in response to generally E-W shortening during the Laramide Orogeny.The third array, identified with Pikes Peak granite bedrock, may be a record of NNE-directed stretching, and will be the focus of further study to determine whether the normal faults are a result of the Neogene extension that has been documented (Steven & others, 1997) to the northwest in the Woodland Park graben.

Marrett, R. & Allmendinger, R., 1990, J. Structural Geology, v. 12, p. 973-996.

Steven et al., 1997, in Bolyard & Sonnenberg, eds., RMAG, p. 114-124.