CONSTRAINING THE DEFORMATION GEOMETRY OF THE YAVAPAI-MAZATAL OROGENY USING THREE-DIMENSIONAL STRAIN ANALYSIS IN THE PROTEROZOIC VALLECITO CONGLOMERATE, NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO

The Needle Mountains in southwestern Colorado consist of Proterozoic igneous and metasedimentary rocks that have experienced multiple deformation events associated with the Yavapai-Mazatal accretionary boundary at ~1.65 Ga. In the SW Needle Mountains, the Vallecito Formation unconformably overlies 1.8 Ga metavolcanic rocks of the Irving Formation. We conducted three-dimensional strain analysis of conglomerates in the Vallecito Formation to quantify the cumulative deformation and better understand the kinematic history of the Yavapi-Mazatal orogeny. Clasts in matrix-supported Vallecito conglomerates are composed of Irving metavolcanic, chert, banded iron formation, and vein quartz. Near Table Mountain the Vallecito Conglomerate is exposed in a 15 km2 inlier. Bedding in the conglomerates is folded into an open, asymmetric syncline that plunges moderately to the SSW. Foliation in the Irving and Vallecito formations is similarly folded. Penetrative lineations in Vallecito plunge to the SSW. Asymmetric structures in the conglomerate consistently indicate top-to-the-N, reverse sense of shear. The metamorphic mineral assemblage and microstructures are consistent with deformation under lower to middle greenschist facies conditions. Finite strain analysis was performed using the Rf/phi method on detrital clasts from 10 hand samples and 12 outcrop faces of the Vallecito conglomerates. Three sections were analyzed per sample and combined to convert two-dimensional strain ratios into three-dimensional strain ellipses. K-values range from 2.1-4.2, with a majority of samples plotting in the field of apparent constriction. Constrictional strain and the SSW plunging lineations indicate extension parallel to the fold axes. Deformation values range from 1.2-1.3 and, for a volume constant deformation, require 180 to 250% extension parallel to fold axes and 30 to 53% shortening normal to foliation.