MAPPING AND 3D MODEL RESTORATION OF FAULTS BORDERING THE SILVERTON AND SAN JUAN CALDERAS, SAN JUAN MOUNTAINS, COLORADO

The western San Juan Mountains are one of the major mineralized areas in the Rocky Mountain region (Lipman et al., 1973), with calderas serving primarily as the structural control for late intrusions and associated hydrothermal systems (Lipman, 2000). The town of Silverton, Colorado sits inside two principal calderas; the younger Silverton caldera (~27.5 Ma) is nested inside the older San Juan caldera (~28 Ma). Several intersecting north-south (N-S) and east-west (E-W) trending faults in Deadwood Gulch, located just south of the approximate caldera boundaries, were mapped and modeled using 3D structure modeling software, Move^TM. The orientations of the N-S and E-W trending faults in Deadwood Gulch appear to be related to the boundary orientations of the Silverton and San Juan calderas respectively. Three-dimensional model analysis made it possible to visualize offset between Paleozoic sedimentary layers in the study area, which were otherwise concealed in the field; in one area within the model, bed shapes implied by the outcrop traces suggested the existence of a fault that was not recognized in the field. This hypothesized fault was later confirmed in the field and added to the 3D model. Restoration of the 3D model constrained fault displacement values ranging from 16 to 100 m. The measured displacement values exposed a fault pattern showing increased displacement towards the north, where the boundaries of the Silverton and San Juan calderas reside. This pattern found using the model suggested that subsidence of the Paleozoic rocks at the edge of each caldera was likely the dominant cause for faulting in Deadwood Gulch. The combination of vegetation, road blast, and other erosional remnants concealing outcrop can all contribute to standard field mapping error. Comparing the 3D fault model with the field map proved to be a useful tool for minimizing this type of field mapping error.