STRAIN, VORTICITY, & KINEMATIC ANALYSIS OF THE PROTEROZOIC VALLECITO CONGLOMERATE, NEEDLE MOUNTAINS, SOUTHWESTERN COLORADO: IMPLICATIONS FOR THE YAVAPAI-MAZATAL OROGENY

Proterozoic igneous and metasedimentary rocks in the Needle Mountains, southwest Colorado, experienced multiple deformation events associated with accretion along the Yavapai-Mazatal boundary at ~1.65 Ga. In the southeastern Needle Mountains, the Vallecito Formation unconformably overlies 1.8 Ga metavolcanic rocks of the Irving Formation. We conducted three-dimensional strain and vorticity analysis of conglomerates in the Vallecito Formation to quantify the cumulative deformation and better understand the kinematic history of the Yavapi-Mazatal boundary zone. Vallecito conglomerates are primarily matrix supported with clasts of vein quartz, chert, mafic to intermediate metavolcanic, and banded iron formation. Microstructures and the metamorphic mineral assemblage (qtz + mus ± bt ± chl) are consistent with lower to middle greenschist facies conditions during deformation. In the Table Mountain area, the Vallecito conglomerate crops out in a ~2 km² inlier, and is folded into open to tight, asymmetric folds that plunge moderately to the SSW. Foliation in both the Vallecito and underlying Irving formations is similarly folded. Penetrative lineations in Vallecito conglomerate plunge to the SSW. Finite strain analysis was performed using the Rf/phi method on detrital clasts from hand samples (n =12) and outcrop faces (n=11) of the Vallecito conglomerates. Three sections were analyzed per sample and combined to convert two-dimensional strain ratios into three-dimensional strain ellipsoids. Deformation values range from 1.2- 2.5 and K-values range from 2.1- 5, strain estimates are similar at both the hand sample and outcrop face scale. X-axes are parallel to the penetrative lineation in the conglomerate as well as local fold axes. Vorticity analysis is consistent with general shear to pure shear dominated flow. The observed constrictional strains are compatible with 150 to 250% extension parallel to fold axes and could be the product to two superposed deformation events or a single transtensional deformation.