FOLD-RELATED STRUCTURES OF THE PALEOPROTEROZOIC IDAHO SPRINGS-RALSTON SHEAR ZONE AND IMPLICATIONS FOR ITS EVOLUTION AS A CRUSTAL-SCALE FEATURE

The Idaho Springs-Ralston shear zone (IRSZ) is one of several 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 wide and is mapped from the Great Plains ~16 km NNW of Denver, CO to ~26 km to the SW (near Idaho Spring, CO). The shear zone is located entirely in early to mid-Proterozoic metamorphic rocks consisting mainly of schist and gneiss with some plutonic rocks such as granodiorite in the NE-most extent. The IRSZ has steeply NW-dipping foliations and steep mineral lineations along its entire length, it is easily recognizable as a typical mylonite with visible S-C fabrics, shear bands, and sigma and delta clasts within its strands. The IRSZ along with other NE-trending shear zones within the Colorado Front Range have long been thought of as having originated along a crustal-scale weakness, such as a continental suture zone, during the ~1.8-1.7 Ga amalgamation of the Yavapai province onto the Wyoming craton. It was hypothesized that this suture zone localized younger deformations, including those that controlled the Colorado Mineral Belt, which is generally located along some of the NE-trending shear zones found in the Front Range. New field data suggest that the SW termination of the IRSZ is ~1 km NE of Idaho Springs. There, the average width is ~60-80 meters and the maximum width is ~2 km, and thus not as extensive as previously mapped. Rock types, structures and metamorphic grade on either side of the shear zone are similar and no major displacements have been recognized. Folds NW of the IRSZ have subvertical NE-trending axial planes and shallowly NE-plunging fold hinge lines. SE of the IRSZ folds also plunge shallowly NE, but axial planes dip shallowly ENE. Tens of meter-scale NE-plunging open to close folds overprint the isoclinal folds. We suggest that the isoclinal folds are folded by a several km-scale NE-plunging, NW-dipping fold. The IRSZ may have formed on the NW limb of that fold and be a result of the folding. Based on the limited length of the IRSZ, its potential relationship with regional folds, similarities between rock types, minor structures and metamorphic grade on either side of the shear zone and lack of evidence for displacement, there is no evidence that the shear zone was a continental suture zone.