STRUCTURE AND METAMORPHISM IN THE NORTHERN HUALAPAI MOUNTAINS, ARIZONA: IMPLICATIONS FOR THE PALEOPROTEROZOIC TECTONICS OF THE SOUTHWESTERN UNITED STATES

The boundary between the Mojave and Yavapai Proterozoic crustal provinces in Arizona has been the subject of much debate in recent years. A transition zone between the provinces with a well-defined eastern boundary has been proposed based on isotopic evidence. The western boundary of this zone is not as clear, but the Gneiss Canyon shear zone in the Lower Granite Gorge of the Grand Canyon has been proposed as a structural, metamorphic, and isotopic boundary. This zone has previously been projected 100 km southwest, through the northern Hualapai Mountains. New mapping in the northern Hualapai Mountains indicates that structures there are not compatible with those found in the Lower Granite Gorge. Rocks within the Gneiss Canyon shear zone in the Lower Granite Gorge are migmatitic and contain mylonitic LS fabrics with strong northwest-side-up kinematic indicators. Very few areas in the northern Hualapai Mountains are migmatitic and virtually no kinematic indicators are present, although some rocks are highly strained. Rocks in the northern Hualapai Mountains are dominantly S tectonites and commonly contain the regional, steeply dipping, northeast-striking D2 fabric, as well as some zones of the regional, moderately-to-steeply dipping, northwest-striking D1 fabric. The rocks in the northern Hualapai Mountains also contain evidence for a local(?) deformational fabric temporally between D1 and D2. This fabric is exposed as crenulation folding with fold axes dissimilar in orientation to both D1 and D2. Granulite-facies assemblages are present across the projected trace of the Gneiss Canyon shear zone, indicating the rocks in the northern Hualapai Mountains have a metamorphic history more similar to the Mojave province than the Yavapai province. This observation is compatible with recent metamorphic work in the Peacock Mountains and Pb isotopic studies that place the western margin of the transition zone significantly east of the Hualapai Mountains. Therefore, we propose that the Gneiss Canyon shear zone may project south from its exposure in the Grand Canyon rather than southwest.