PALEOMAGNETIC AND GEOCHRONOLOGIC DATA BEARING ON THE DEFORMATION AND TIMING OF THE CRIPPLE CREEK DIATREME COMPLEX AND RELATED ROCKS, FRONT RANGE, COLORADO

Paleomagnetic data, combined with high-precision ⁴⁰Ar/³⁹Ar age determinations, show that parts of the Front Range of Colorado, in this case the Cripple Creek Diatreme complex and surrounding rocks, experienced modest amounts of deformation since mid-Tertiary time. Paleomagnetic data from the Cripple Creek diatreme complex identify localized block rotations and a more regional, district-wide tilting. The overall paleomagnetic dataset consistently yields magnetizations with inclinations that are steeper than the expected field direction (normal (D=3.7°, I=65.6°, a95=4.0, k=68.02, N=31 sites) and reverse (D=168.2°-, I=-65.9°-, a95=3.7, k=60.07, N=43) polarity). There is no correlation between degree of north-side down tilting of the diatreme complex and age relations, (i.e. older rocks do not have steeper inclinations than younger rocks, suggesting that tilting followed diatreme formation). Block rotations are identified in the Cresson and Altman pits and are consistent with major sinistral and dextral faults that bisect the district. These rotations distinguished by the paleomagnetic data are relatively local in extent and do not represent the entire diatreme. Initial igneous events are represented by relatively felsic rocks, at 31.59±0.36 Ma phonolite, followed by mafic to ultramafic intrusions of 31.11±0.1 Ma tephriphonolite, 31.15±0.12 Ma phonotephrite, and 30.41±0.13 Ma lamprophyre. Igneous activity ceased for about 2 m.y. and a final episode of ultramafic activity is represented by the 28.39±0.12 Ma lamprophyre Cresson Pipe. The high precision geochronologic combined with paleomagnetic data from rocks sampled for this study indicate that magmatism took place between polarity chrons 13R and 10R. We interpret the total available collection of paleomagnetic data to indicate that the diatreme and host rocks in the immediate vicinity were deformed by a slight yet distinguishable amount of north-side down tilting since the early Oligocene. Such deformation could have been accommodated by motion along faults active in part of a northwest-directed transtensional setting during mid-Tertiary and younger extension.