THE EASTERN BOUNDARY OF THE CALIENTE - ENTERPRISE ZONE, SW UTAH: PALEOMAGNETIC AND GEOCHRONOLOGY DATA

The Miocene Caliente-Enterprise Zone (CEZ) is a 20 km - 50 km wide east-northeast trending left-lateral transfer zone that borders the western margin of the stable Colorado Plateau. Previous paleomagnetic results from the central and western CEZ show significant counterclockwise vertical axis rotations of strike-slip bounded fault blocks that vary in magnitude both across and along the strike of the zone. Results of detailed geologic mapping and new geochronologic data to the east allow us to extend paleomagnetic studies into the eastern CEZ. Two new ⁴⁰Ar/³⁹Ar age determinations from the Harmony Hills tuff and the Rencher Formation yield plateau age estimates of 22.03 +/- 0.15 Ma and 21.46 +/- 0.40 Ma, respectively. Collectively, these age determination data and previously published ages tightly constrain the age of key units sampled for this study. New paleomagnetic data from four regionally extensive Miocene (24-22 Ma) ignimbrite sheets and from three 22-20 Ma Iron Axis laccoliths from the eastern CEZ reveal significant amounts and similar spatially variable components of counterclockwise rotations. Rotations are assessed relative to a non-to minimally-rotated section just north of the Colorado Plateau (Grass Valley). Accepted tilt-corrected paleomagnetic data from sites away from the reference area are discordant in declination from the individual ash flow tuff expected directions with statistically significant rotation (R) and flattening (F) estimates that range from R = -2° to -84° and F = +15° to -14°. Relative to the expected Miocene direction, results from the Three Peaks laccolith yield R = -22.2° and F = 8.8°, the Granite Peak laccolith yield R = -8.0° and F = 21.4°, and the Iron Mountain laccolith yield R = -40.0° and F = 3.0°, respectively suggesting that deformation effects a considerable thickness of the upper crust. We argue that the boundary of the eastern CEZ extends east to within a few kilometers of the breakaway with the Colorado Plateau. The transitional zone between the eastern CEZ and Colorado Plateau, therefore, is very abrupt occurring over a narrow zone northwest of Cedar City, UT.