Petrologic, paleomagnetic, and structural evidence of a Paleozoic rift system in Oklahoma, New Mexico, Colorado, and Utah
A west-northwest–trending dike swarm consisting of at least 50 mafic dikes cuts Precambrian basement rocks in the area in and south of the Black Canyon of the Gunnison, Colorado, and in Unaweap Canyon near the Colorado-Utah border. The dikes have been studied petrographically, petrochemically, and paleomagnetically, and they have been radiometrically dated. Lithologically, the dike rocks are very similar, consisting of tholeiitic diabase composed primarily of labradorite, augite, and granophyric intergrowths of quartz and alkali feldspar. A Rb-Sr isochron based on mineral separates from one dike yields an age of 495 ±15 m.y. which is statistically identical with a previously reported Rb-Sr date. Pooling the age data for these two dikes gives an age of 497 ± 16 m.y. Whole-rock K-Ar dates on four of the dikes are scattered around the Rb-Sr date and indicate that the dikes have been relatively unaffected by reheating or hydrothermal activity since emplacement. Paleomagnetic directional data from nine of the dikes are well grouped and consistent after both alternating-field (AF) and thermal demagnetization. Several lines of evidence indicate that the remanence reflects the dipolar geomagnetic field direction at the time the dikes were emplaced. Paleopoles corresponding to the mean directions are located at 37.0°N, 101.0°E, δp = 8.6°, δm = 16.4° (AF demagnetization) and 37.0°N, 102.3°E, δp = 4.9°, δm = 9.4° (thermal demagnetization). The consistency of the paleomagnetic data corroborates the idea that the dike swarm is the result of one short pulse of mantle-derived magmatic activity. The dike swarm and other Cambro-Ordovician igneous rocks in Colorado (Powderhorn alkalic complex, Wet Mountains alkalic complexes and dikes) are aligned along a linear west-northwest trend. Cambrian diabase dikes in northeastern New Mexico and a bimodal suite (basalt–gabbro-rhyolite–granite) in southern Oklahoma (Wichita and Arbuckle Mountains, Anadarko Basin) are on line with this trend. Petrologically, all of these Cambrian and Cambro-Ordovician igneous rocks represent types that are typically associated with extensional tectonic regimes. From these data and consideration of documented Paleozoic tectonic activity along the trend of the plutons, it is concluded that throughout the Paleozoic, a west-northwest–trending tectonic zone extended from southeastern Oklahoma, through northeastern New Mexico and Colorado, and into southeastern Utah. Bimodal (basalt-rhyolite) and alkalic igneous activity was restricted to the Cambrian and Cambro-Ordovician, but tectonic activity occurred intermittently along the zone throughout the Paleozoic.