THE PALEOPROTEROZOIC MYLONITE OF KINGS CANYON, COLORADO: A CA. 1.6 GA GREENSCHIST FACIES MYLONITE FAR TO THE NORTH OF THE CONCURRENT ACTIVE TECTONIC MARGIN

We present the Kings Canyon mylonite as a case study of ca. 1.6 Ga reactivation and deformation far to the north of the active tectonic margin of Laurentia. The Kings Canyon mylonite is a narrow band (<25m) of intensely deformed Proterozoic rocks that crops out 16.7 miles north of Walden, Colorado, on CO Highway 127. This field area was targeted for three reasons: 1) its proximity to other well documented greenschist grade reactivations at ~1.6 Ga, 2) presence of greenschist grade assemblages, and 3) the orientation of the mylonite is subparallel to a 1.6 Ga strike-slip (thrust-tear) fault ~25km away. Our study includes detailed field mapping, microstructural and petrographic analysis, and direct dates on deformation.

Our study substantially revises paleogeographic reconstructions, establishes that newly accreted lithosphere was strong enough to transmit stress 1500 km, and fills in the apparent North American tectonic gap. Filling the North American Tectonic gap also calls into question the use of ~1.6 Ga detrital zircon ages as uniquely non-Laurentian. We directly dated extensive greenschist grade remineralization to specific pulses between ~1.65 and 1.55 Ga, based on U-Pb dates of syn-deformational titanite and epidote growth at Kings Canyon. The dates are consistent with the youngest stages of deformation in the Soda Creek-Fish Creek shear zone (SCFCSZ), which crops out ~50km to the SW of the Kings Canyon mylonite, as recorded by monazite, titanite, and zircon. These results fit into a broader model of foreland thrusting from 1.68-1.55 Ga within the lithosphere west of a proposed N-S transform fault that runs from 5 Points shear zone in the Wet Mountains, to the Battle Lake Fault zone in the Sierra Madre. In this model, breakout thrusts progress to the north starting from the active tectonic margin of Laurentia. Our data indicate older, high-temperature shear zone reactivations in the south with younger, low-temperature shear zone reactivations in the north. The results presented here are just several pieces of evidence for widespread deformation, magmatism, and metamorphism at ~1.6 Ga.