Paleomagnetic results from the eastern Caliente-Enterprise zone, southwestern Utah: Implications for initiation of a major Miocene transfer zone

Abstract

The Miocene Caliente-Enterprise zone (CEZ) in southwestern Utah (USA) is a 20–50-km-wide east-northeast–trending left-lateral transfer zone that displaces north-south–trending crustal blocks of the eastern Basin and Range Province to the west. Previous paleomagnetic results from the central and western CEZ show significant counterclockwise vertical axis rotations of strike-slip–bounded fault blocks, and these rotation estimates vary in magnitude both across and along the strike of the zone. Results of recent detailed geologic mapping and new geochronologic data in the area east of previous studies allow us to extend paleomagnetic studies into the easternmost CEZ. New paleomagnetic results include data from 4 regionally extensive latest Oligocene to early Miocene (ca. 24–22 Ma) ignimbrite sheets and from 3 ca. 22–20 Ma Iron Axis laccoliths. These data reveal significant magnitudes and similar spatially variable components of counterclockwise vertical axis rotation. Rotation estimates from the ignimbrites are assessed relative to what we interpret to be a nonrotated to only minimally rotated reference section just north of the Colorado Plateau (Grass Valley) and from several low-extension areas in southeast Nevada. Accepted tilt-corrected paleomagnetic data from sites away from the reference areas are discordant in declination from the expected individual ash-flow tuff directions, with rotation (R) and flattening (F) estimates that range from R = −2° to −84° and F = +15° to −14°. Many of the rotation estimates are large and statistically significant. For example, site P-18 from the Bauers Tuff yields an R = −61.1° ± 5.3° and F = −0.6° ± 5.0°. Relative to the expected Miocene direction, in situ paleomagnetic data from the Iron Axis laccoliths, specifically the Three Peaks laccolith, yield a mean that is discordant in declination, with estimated R = −22.2° and F = −8.8° values. These rotation and flattening estimates, although consistent with the overall data set from volcanic rocks, must be considered of lesser quality, as we are unable to accurately correct these data for possible effects of local tilting. If the rotation estimate is viable, then we suggest that this component of deformation involves much of the upper crust, and we furthermore propose that the boundary of the eastern CEZ extends farther east than previously envisioned, to within a few kilometers of the breakaway with the Colorado Plateau. The transitional zone between the eastern CEZ and Colorado Plateau is therefore abrupt and occurs within a narrow zone near Cedar City, Utah.