FABRIC REACTIVATION IN THE HUALAPAI MOUNTAINS, NW ARIZONA: FURTHER CONSTRAINTS ON 1.4 GA TECTONICS

One of the important questions for the Proterozoic of SW USA is the tectonic setting of Mesoproterozoic "anorogenic" magmatism. Recent structural studies of plutons and their aureoles over a wide geographic area indicate kinematics involving SE-NW shortening. Detailed structural study in the contact aureole of the 1.4 Ga Boriana granite in Boriana Canyon, Hualapai Mtns provides additional information on 1.4 Ga tectonism. This area also provides a unique locality to document the thermal evolution of Proterozoic crust where the deformational and thermal effects of Paleo- and Mesoproterozoic events can be clearly delineated. The main fabric element in Proterozoic sedimentary rocks in Boriana Canyon is a steeply dipping, NE-striking foliation, S2. Andalusite and garnet porphyroblasts outside of the contact aureole of the Boriana granite are synkinematic with S2. Kinematics of this event is west side up with a sinstral component. The age of the S2 is bracketed to be syn- or post-1.68 Ga by the observation that both the Antler and Hualapai granites are strongly foliated with fabrics parallel to S2. Within the contact aureole of the Boriana granite, S2 is deformed by S3 crenulation cleavage. Within 100 m of the granite contact, S3 crenulation cleavage is the dominant fabric element. Andalusite and garnet porphyroblasts in the contact aureole overgrow S3 and are truncated by reactivation of S2 fabrics to form a new NE-striking foliation, S4. The absolute age of S3 and S4 is not known; however based on the spatial association with the Boriana granite, these fabrics appear to be related to 1.4 Ga tectonism. Conjugate pegmatite veins and fracture orientations associated with 1.4 Ga plutonism indicate a SE-NW sub-horizontal shortening direction. These observations emphasis the importance of recognizing the potential for reactivation of Paleoproterozoic fabrics during later tectonism, especially within contact aureoles of Mesoproterozoic plutons.