CORRELATING PROTEROZOIC DUCTILE DEFORMATION IN VALLECITO CONGLOMERATE AND UNCOMPAGHRE GROUP, EASTERN NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO

An important question regarding tectonic evolution of Proterozoic metasedimentary units in the Needle Mountains concerns the correlation and comparison of deformational histories between the Vallecito Conglomerate and the Uncompaghre Group. The units are of similar lithology, but are nowhere in contact with each other and their deposition is constrained only between the age of the underlying Yavapai age “basement” gneiss suite and the intrusive 1.43 Ga Eolus Granite. Understanding the deformation histories of these two units is of primary importance for establishing a stratigraphic framework for Needle Mountains “cover” sequences and increasing our understanding of the extent and character of deformation during the ~1.65 Ga Mazatzal orogeny and the ~1.45 to 1.35 Ga magmatic and thermal event. Structural analysis of the Vallecito Conglomerate indicates that it has undergone three major phases of ductile deformation. The first is cryptic but may have involved layer parallel shear. The second resulted in macroscopic folding of sedimentary layering (and S1) into close to tight, north-northeast trending folds and development of a dominant tectonic foliation. A later event is associated with west side up shearing of the Irving formation against the Vallecito. Macroscopic F2 folds in the Uncompaghre trend east-west to northwest-southeast, forming a wide arc across the Needle Mountains. In the eastern Needle Mountains, the two units are in close proximity to each other and deformation sequence is similar, but structural trends differ markedly. We are actively investigating these relationships to determine whether second generation macroscopic folds in the Uncompaghre trend directly into major structures in the Vallecito, which will suggest the two units are of similar age and share a similar deformation history, or alternatively, that macroscopic folds in the Vallecito refold second generation structures in the Uncompaghre, which will suggest that the Vallecito is younger than the Uncompaghre. Cover fabrics within the two units show a similar high angle relationship with basement fabrics, hypothesized to represent decoupling of basement from cover during cover deformation and rotation of both S2 in the cover and basement fabrics into a regional domal pattern during 1.43 Ga emplacement of the Eolus pluton.