- UW Department of Geology and Geophysics
Detailed mapping and stratigraphic studies at the intersection of the Proterozoic Homestake shear zone with the early Paleozoic section exposed on the margins of the Laramide Sawatch anticline in central Colorado provide limits on the timing and magnitude of brittle reactivation during Phanerozoic time. An episode of displacement rooted within two distinct ductile branches of the Homestake shear zone in the northeastern Sawatch Range generated an 8-km-wide, Late Cambrian fault block that had 4–20 m of paleotopographic relief prior to depositional onlap by sandstones of the Sawatch Formation. An episode of up-to-south displacement (<30 m) subsequently occurred along the southern part of the shear zone in the northeastern Sawatch Range and in the western Sawatch Range during deposition of the Lower Ordovician Manitou Formation. A third episode of reactivation during Early to Middle Ordovician time (post-Manitou Formation, pre-Harding Sandstone) resulted in renewed, decameter-scale uplift of the Late Cambrian fault block in the northeastern Sawatch Range. A fourth episode of reactivation during Late Cretaceous Laramide deformation produced localized strike-slip displacement along brittle faults on the northeastern flank of the Sawatch Range. The three early Paleozoic episodes of reactivation occurred on a cratonic platform and are genetically linked to extension and thermal uplift during intrustion of a suite of bimodal, rift-related plutonic rocks (Iron Hill and Wet Mountains intrusive rocks). They were emplaced ∼135 km south of the Homestake shear zone along the Cimarron-Red Rocks fault and Apishapa fault system. The reactivation history model proposed herein differs from some previous interpretations that cumulatively suggest at least eleven episodes of Phanerozoic reactivation, including relatively large-magnitude late Paleozoic displacement.