TRANSITIONAL TECTONICS: EARLY LARAMIDE STRIKE-SLIP DEFORMATION OF THE NORTHEASTERN FRONT RANGE, COLORADO

The role of strike-slip faulting in Laramide uplift of the eastern flank of the northern Front Range was analyzed through kinematic analysis of 97 minor (<100m trace) faults. The dominant fault population was oriented approximately perpendicular to bedding, with lineations sub-parallel to bedding. Rotating bedding to horizontal showed these faults to have the pattern of a strike-slip conjugate set.

Prior to Laramide folding, left-lateral faults had an average orientation of (287, 87N) with lineations to (287, 01); right-lateral faults had an orientation of (065, 88S) with lineations to (245, 00). The timing of motion on these faults is constrained by the age of the youngest units in which they are observed (~98 Ma), and the age of the folds that rotated them (70-64 Ma from the age of synorogenic conglomerates). The conclusion is that strike-slip motion was active during the earliest Laramide.

Plots of the principal strain axes from these faults, after rotation, give an average shortening axis orientation of (276, 03) and an average extension direction of (006, 02). The calculated shortening axis orientation is consistent with that of later Laramide deformation, supporting the hypothesis that strike-slip deformation occurred in the northeast Front Range during the earliest Laramide.

Reconnaissance of the Thompson Canyon and other basement faults was undertaken in search of evidence that one or more of these faults may have controlled early Laramide strike-slip minor faulting. Due to poor exposure, no direct evidence was found, but sparse minor left-lateral faults along Big Thompson Canyon, apparent left-lateral offsets mapped by Braddock et al. (1970) across the Thompson Canyon Fault, and geometric similarity to measured minor faults suggest that it may have experienced left-lateral motion, and may have been a controlling basement fault during the early Laramide.

Analysis of minor faults on the eastern flank of the northern Front Range shows that strike-slip faulting was a locally important deformation mechanism at the onset of the Laramide orogeny. The work suggests that the regional tectonic regime was one of east-west shortening and north-south extension prior to the onset of the main phase of Laramide deformation, at which time the regional strain field rotated to one of east-west shortening and vertical extension.