STRUCTURAL ANALYSIS OF THE IDAHO SPRINGS-RALSTON SHEAR ZONE: A NEW LOOK AT AN ANCIENT STRUCTURE

The Idaho Springs-Ralston Shear Zone (IRSZ) has been described and mapped as a regionally important long-lived and reactivated crustal structure in the Front Range and Central Rocky Mountain regions of Colorado. It has been viewed as one of several deformational structures and long-standing zones of weakness in the continental crust associated with the Proterozoic assembly of North America and later development of the Colorado Mineral Belt. We are reinvestigating the nature of the structure in order to answer three fundamental questions: 1) is the description and interpretation of the IRSZ as a major shear zone accurate, 2) is the IRSZ a major through going crustal structure, and 3) what role, if any, did the IRSZ play in the geologic evolution of Laurentia? A structural domain analysis of foliation and lineation data was used to determine if any large-scale fabric patterns exist that indicate the presence of a major shear zone. Petrographic analyses were used to decipher variations in lithologies and metamorphic grade associated with movement along the IRSZ. Kinematic analyses were used to determine overall versus localized movement sense throughout the IRSZ. Structural domain analysis shows the presence of a large scale lineament supporting the existence of the shear zone, however, mapped units do not show any significant variation in rock types adjacent to and across the shear zone as would be expected at a major through going crustal structure. Petrographic analyses indicate an increase in metamorphic grade, from east to west along the ~40 km extent of the IRSZ, suggesting exposure of different crustal levels or variations in deformation intensity. There is however no step in metamorphic grade from south to north across the shear zone. The presence of discrete mylonitized zones along the eastern portion of the IRSZ and lack thereof along the western portion further supports variations in deformation intensity. Localized zones of high strain may have distributed displacements allowing for the formation of a large scale zone of intense strain made up of smaller anastomosing shear zones. This pattern may explain the observation of differing movement sense, north-side-up and south-side-up, within the whole IRSZ. Although a through-going lineament is present, no features of the zone are indicative of major crustal movement.