STRUCTURAL SETTING AND KINEMATICS OF THE OLIGOCENE CRIPPLE CREEK DIATREME COMPLEX, COLORADO

The Cripple Creek diatreme was emplaced on the SW flank of Pikes Peak Colorado around 30Ma. Detailed mapping within active open pit mines indicates that the district was emplaced during a structurally dynamic time. Several lines of insight suggest that the setting for the diatreme is quite complex and most likely roots in the existing Precambrian structural geometry (Tweto and Sims, 1969). Regionally important features include a NE trending fault zone that intersects with NW and N-S trending features at the site of diatreme emplacement. Fabrics developed in the 1.8 to 1.0 Ga wall rocks suggest that this area may have played an active role during proposed terrain accretion as described by Selverstone, (1999) and may help to explain the location of the diatreme more than 1 b.y. later. Three-dimensional mapping of pit geology indicates pervasive faulting throughout the diatreme along primary fault zones that appear to be along trend of regional structures found within the Precambrian wall rocks. Offsets across faults are typically oblique and therefore the use of kinematic analysis to describe the orientation of the strain field is required. Kinematic indicators demonstrate that the maximum shortening direction during emplacement of the diatreme varied from NNE to NW. Relative relations between volcanic activity and the faulting within the diatreme indicates a complex pattern of deformation that may have first provided for diatreme emplacement (transtension) that was followed by a moderate phase of north/south directed thrusting within the diatreme. It is entirely possible that all of this strain is related to long-lived transtension during and following diatreme emplacement. Such kinematics appear to be internal to the volcanic center and ultimately had a profound effect on the pattern of mineralization within the district. We interpret that strain accommodation on reactivated basement fault segments began prior to and following diatreme emplacement. Strain characteristics of the diatreme, including secondary fault development, are variable but remain fairly consistent with the overall pattern of regional dextral transtension during the Oligocene and highlight the interactions of fault development and kinematics between neoform structures and reactivated structures found outside the diatreme.