STRUCTURAL ANALYSIS OF THE MOORE GULCH SHEAR ZONE: TESTING ALTERNATIVE MODELS OF THE PROTEROZOIC EVOLUTION OF CENTRAL ARIZONA

The Moore Gulch shear zone is a lithologic, structural, and metamorphic discontinuity in the Proterozoic basement rocks of central Arizona. Two hypotheses have been proposed to explain the juxtaposition of distinct lithotectonic terranes across the Moore Gulch shear zone, each with important implications for the Proterozoic tectonic evolution of central Arizona. In the first hypothesis, lower grade rocks of the southeastern province were unconformably deposited atop structurally deeper rocks of the northwestern province and juxtaposed by normal-sense motion across the Moore Gulch shear zone. A second hypothesis posits that rocks of the southeastern province evolved separately from those of the northwestern province and were juxtaposed by thrust-sense motion across the Moore Gulch shear zone. Despite its central role in both hypotheses, no detailed structural analysis of the Moore Gulch shear zone has yet been conducted. Here, we present the results of preliminary field characterization of the Moore Gulch shear zone. The Moore Gulch shear zone is a >1 km wide zone of localized ductile and brittle deformation. The main ductile fabric present in the Moore Gulch shear zone is a northeast-striking foliation which dips steeply to the northwest. The character of this foliation varies markedly with lithology; in a diorite exposed to the northwest of Moore Gulch the fabric ranges from proto-mylonite to mylonite. In contrast, a parallel fabric is developed as a schistosity to slaty cleavage in the chlorite-schist exposed within Moore Gulch itself. Deformation within the chlorite-schist exhibits both brittle and ductile styles, with brittle faulting and jointing becoming more intense towards the Moore Gulch fault. Stretching lineations measured in the ductile foliation plunge to the northwest. Rare kinematic indicators indicate northwest-side-up motion across the shear zone. These preliminary results suggest that the Moore Gulch shear zone is a reverse sense shear zone that uplifted the northwestern block against the southeastern block. Reverse sense, northwest side up shearing along the Moore Gulch shear zone is inconsistent with the hypothesis that Proterozoic rocks of the southeastern terrane were thrust up and over those of the northwestern terrane, and instead favor the first hypothesis.