DEVELOPMENT OF RETROGRADE SHEAR ZONES ALONG QUARTZITE-GNEISS CONTACTS WITHIN THE NEEDLE MOUNTAINS, SOUTHWEST COLORADO

The northwestern Needle Mountains of southwest Colorado expose Proterozoic quartzite that rests in unconformity on amphibolite and orthogneiss. Shear zones within the Needle Mountains are localized along the gneiss-quartzite contact suggesting the unconformity was a significant rheological boundary. Prior work has established that the quartzites were extruded vertically during approximately north-south shortening resulting in the formation of a kilometer scale gently plunging synclinal structure cored by quartzite. These east-west trending folds were then warped by a series of retrograde shear zones. At Snowden Peak, the southern quartzite-gneiss contact strikes east-west and dips moderately to steeply north with the quartzite structural above the gneisses. To the west, at Lime Creek, the contact takes on a northeast strike, and the gneiss sits structurally above the quartzite with steeply southeast dips along the contact. Kinematic indictors developed parallel to a sub-horizontal lineation record consistent left-lateral sense of motion, with the gneisses up relative to the quartzite. Deformation appears to have progressed from penetrative ductile strain as recorded by the foliation, to localized shearing on C’-type shears, to brittle faults. Taken together, these structures are consistent with development during cooling from the peak of metamorphism within the quartzite. Regional cooling ages and the proximity and geometry of these retrograde shear zones to the 1.44 Ga Eolus granite, suggests they formed at 1.4 Ga. The overall geometry of the shear zones is consistent with regional northwest-southeast horizontal shortening.