PROTEROZOIC TECTONICS IN COLORADO: INVESTIGATING THE STRUCTURAL EVOLUTION OF THE IDAHO SPRINGS-RALSTON SHEAR ZONE

The Idaho Springs-Ralston Shear Zone (IRSZ) has been interpreted as one of several persistent zones of weakness in the Proterozoic crust of Colorado. The structure has been associated with the assembly of North America and with later development of the Colorado Mineral Belt. In view of the proposed roles of the IRSZ as a crustal-scale weakness and a control on the localization of Late Cretaceous-Early Tertiary mineral deposits, we have reinvestigated the structural history of the zone and adjacent rocks. Two fundamental questions are addressed: 1) is the description of the IRSZ as a major through-going crustal-scale shear zone accurate?, and 2) is there evidence for reactivation within the shear zone supporting interpretations that it is a long-lasting zone of weakness? Kinematic analyses of rare meter-scale, discontinuous mylonites show no evidence of ductile strike-slip movement along the shear zone, but show both north-side and south-side-up sense of shear. In order to unravel the complex geometry and history of structures in the basement rocks, structural domain analyses were used to identify large-scale fabric patterns and to test how regional-scale structural fabrics are influenced by deformation along and adjacent to the IRSZ. A 40 km long by 1-2 km wide domain of parallel foliation is defined by the analyses and supports the existence of the shear zone. There is, however, no truncation of Proterozoic rock units or offset of mapped contacts across the IRSZ, which is inconsistent with a major through-going crustal structure. The eastern half of the exposed IRSZ does, however, run adjacent to a km-scale and upright synform that flanks its south limb. A change in regional foliation and lineation orientations across the fold-shear zone pair shows that the zone marks a boundary between structural fabric domains. This boundary indicates that either pre-IRSZ fabrics had different orientations or that the folding, shearing, or both caused regional realignment of structures. Realignment of structures is investigated by retrodeformation of foliations and lineations across the IRSZ. The results indicate that deformation along the zone involved a component of dip-slip shear along fold limbs at both local and regional scales.