EMPLACEMENT OF THE MIOCENE IRON AXIS LACCOLITHS, SW UTAH: RESULTS FROM MAGNETIC FABRIC, PALEOMAGNETIC, AND FIELD MAPPING STUDIES

Iron Axis magmatism in SW Utah consisted of intrusion of early Miocene (22-20 Ma) calc-alkaline hypabyssal laccoliths and associated volcanism immediately west of the present Colorado Plateau. Laccolith groups on the Colorado Plateau (e.g., Henry and La Sal Mountains) consist of distinct central intrusions from which numerous smaller intrusions radiate, whereas laccoliths of the Iron Axis are single intrusions distributed along a NE-SW linear trend, most of which are spatially associated with a NE-SW striking, NW dipping Sevier thrust fault. The distribution of Iron Axis laccoliths thus appears to be influenced, in part, by crustal discontinuities. One possibility we are considering is that the fault zone served as a planar conduit along which magma migrated from a deeper source region to the northwest (i.e., the Escalante Desert). Another possibility is that the laccoliths may have been fed vertically by NE-striking subvertical dikes and emplaced preferentially along the Sevier fault. Anisotropy of Magnetic Susceptibility (AMS) results, combined with field mapping data, can provide valuable magma flow fabric information important for understanding laccolith emplacement. Results from the Stoddard Mountain pluton suggest internal flow geometry of the laccolith resulted from lateral magma flow from a western source. Preliminary results from the Granite Mountain-Three Peaks pluton reveal moderately dipping magnetic foliations that generally follow country rock contacts and magnetic lineations plunging moderately but lacking an obvious trend. Two laccoliths not spatially associated with thrust faults appear to be fed by sub-laccolith dikes. The Iron Peak pluton has an exposed central feeder dike system beneath its floor and preliminary AMS results reveal subvertical flow in and near the dikes and more shallowly plunging lineations that reflect variably directed subhorizontal flow away from the dikes. The Pine Valley mega-laccolith has a NE-SW elongate shape that mimics the trend of nearby steep pre-laccolith faults. For these two laccoliths, we suggest that pre-laccolith fault(s) may have tapped a magma source and created avenues for vertical magma ascent.