Paleomagnetism of Tertiary intrusive and volcaniclastic rocks of the Cerrillos Hills and surrounding region, Española Basin, New Mexico, U.S.A.: Assessment and implications of vertical-axis rotations associated with extension of the Rio Grande rift

Paleomagnetic data from parts of the northern Rio Grande rift provide evidence for clockwise (CW) and counterclockwise (CCW) vertical-axis block rotations associated with strike-slip deformation along basin-bounding faults during rift evolution. Despite the spatial consistency of the results, the quality and statistical significance of data sets are difficult to evaluate because of small sample size and potential failure to average secular variation. To understand the extent, importance, and origin of such rotations, we report paleomagnetic data from Tertiary intrusive and volcaniclastic rocks in the Cerrillos Hills and surrounding areas in the Espanola Basin. Paleomagnetic data from in situ Tertiary intrusions and tilt-corrected volcaniclastic strata of the Oligocene Espinaso Formation sampled at four localities yield a grand mean of declination = 342.9° and inclination = 58.3° (α 95 = 3.5°; N = 32 sites of normal polarity/21 reverse). Correction for minor postemplacement tilt of the Cerrillos Hills and La Cienega data sets yields a grand mean (declination = 349.5°, inclination = 55.3°, α 95 = 3.4°) that is indistinguishable from the 30 Ma reference direction for the study area, and there is no evidence of rotation (R = 1.8° ± 6.4°). However, if an alternative reference direction is used, minor CCW rotation (−6.6° ± 5.8°) is possible. Our data suggest that the magnitude of rotation in the Espanola Basin is significantly less than previously estimated and may be negligible. Regardless, paleomagnetic data from elsewhere in the basin suggest that CCW rotations may be an important component of recent rift extension and deformation.