Proterozoic history of the Idaho Springs-Ralston shear zone: evidence for a widespread ca. 1.4 Ga orogenic event in central Colorado, The

Abstract

The Idaho Springs-Ralston shear zone (IRSZ) is one of several Proterozoic NE-trending zones within the central-eastern Colorado Front Range. The zone is composed of multiple mylonitic strands of a few meters to several kilometers in width and is mapped from ~16 km NNW of Denver, CO to ~26 km to the SW (near Idaho Springs, CO). The IRSZ and other NE-trending zones have previously been interpreted as ~1.45 Ga reactivated shear zones, following a pre-existing crustal-scale weakness, such as a suture zone that formed at ~1.7 Ga. The interpreted suture zone was based on the presence of tectonic mélange at the St. Louis Lake shear zone, ~40 km NNW of the IRSZ, suggesting the presence of a subduction zone. New field data suggest that the IRSZ is not as extensive as previously interpreted. Additionally, a lack of pinch outs and offset of major units, as well as similar deformation histories and metamorphic conditions on either side suggest that the IRSZ did not form as a continental suture zone. Isoclinal F1 folds are overprinted by F2 folds. F2 folds NW of the IRSZ have subvertical NE-trending axial planes and shallowly NE-plunging fold hinge lines. SE of the IRSZ they also plunge shallowly NE, but axial planes dip shallowly ENE. We suggest that the isoclinal F1 folds are folded first by asymmetric NW-side-up meter-scale F2 folds, followed by a several km-scale NE-plunging, NW-dipping F3 folds, that are responsible for the current fold pattern. NW-side-down movement along the IRSZ may have been a result of flexural slip on the NW steeply dipping limb of the NE-plunging, NW-dipping F3 fold. U-Pb laser ablation inductively coupled mass spectrometry monazite dates revealed ~1.68 Ga and ~1.43 Ga events, both within and adjacent to the IRSZ. Relationships between microstructures and monazite grains suggest F1 folds formed at ~1.68 Ga and F2 and F3 folding and associated shearing along the IRSZ occurred at ~1.43 Ga. The relationship between shearing and widespread folding at ~1.43 Ga suggests that Mesoproterozoic deformation was much more extensive than movement along shear zones, as previously interpreted. Folding at ~1.43 Ga is interpreted to be associated with the Picuris orogeny recognized in New Mexico, and the IRSZ is interpreted as a result of folding, rather than as a major reactivated suture zone.