TO BE PRE- OR NOT PRE-1.7 GA, THAT IS THE QUESTION: TESTING COMPETING HYPOTHESES ON THE DEFORMATIONAL HISTORY OF THE TENMILE GRANITE, SOUTHWESTERN COLORADO

Previous work on the Proterozoic Tenmile Granite in southwestern Colorado generated several different hypotheses on the state of deformation in this unit, which in turn led to competing ideas about the timing of deformation and its significance. Early studies recognized deformational fabrics in the pluton but did not attribute them to a tectonic event. Later work established the deformational record in Tenmile Granite, and concluded that the deformation was either synorogenic at ~1.7 Ga or linked to deformation between 1.7 and 1.4 Ga.

Recent field studies on the ~1.7 Ga Tenmile Granite document that the pluton is composed of several generations of sill- and dike-shaped bodies of granite and diorite. Variation in the deformation states of intrusive phases related to the pluton vary from concordant to discordant, and record a continuum of deformation from highly mylonitized to undeformed. Deformation in the Tenmile Granite developed from a subhorizontal N-S compressive-strain field with related E-W extension. Early intrusive phases preserve isoclinal to tight F₁ folds, transposed dikes and sills, prominent S_1-2 subvertical E-W foliation, and stretching lineation with shallow to moderate east plunges developed on foliation planes. Late-intrusive phases in the pluton are not deformed and cross cut all earlier deformational fabrics. This evidence clearly links emplacement of the Tenmile Granite to deformation and metamorphism at ~1.7 Ga during the regional Yavapai orogenic event.