Paleomagnetic and 40Ar/39Ar Geochronology of Tertiary Intrusive and Volcanic Rocks in the Espanola Basin, New Mexico: Further Evidence for Counterclockwise Vertical-Axis Block Rotations in the Rio Grande Rift

Published paleomagnetic data from the northern Rio Grande rift provide evidence for the presence of counterclockwise vertical axis block rotations, perhaps associated with left-slip on faults during rift extension. Despite the apparent consistency of results, the significance of some of these results is difficult to evaluate because of problems associated with small sample size, potential failure to average secular variation, etc. In order to better document and understand the extent, significance and origin of such rotations, we have obtained new paleomagnetic data from intrusive and volcanic rocks in the Cerrillos Hills and surrounding areas in the Espanola basin, south of Santa Fe, New Mexico. These rocks include both alkaline and calc-alkaline rock suites that are associated with magmatism in the Ortiz Porphyry Belt. New 40Ar/39Ar and published K-Ar dates indicate that most of these igneous rocks were emplaced between about 36 and 28 Ma. Preliminary paleomagnetic results from 40 sites from in situ intrusive and associated tilt-corrected volcaniclastic rocks of the Oligocene Espinaso Formation yield well-defined site-mean directions, with a group-mean direction of Dec.=343.6°, Inc.=57.4° (k=35, α95=4.0°; 26 sites of normal polarity and 14 of reverse polarity). Given the distribution of site-mean directions, the time-span of activity indicated by the isotopic dates in this area, the diversity of rock types sampled, and the presence of both normal and reverse polarity site-mean directions, we interpret our data to have adequately averaged secular variation; thus, we consider the results to represent an accurate recording of the Oligocene geomagnetic field. Comparison of our group-mean direction with the expected direction for this area (e.g., Dec.=355.3°, Inc.=53.9°, Irving and Irving, 1979) indicates that it is discordant ( R=-11.7°±7.4°) and that the result is significant at the 95% confidence level. Thus, our data confirm the results of earlier studies and demonstrate that significant counterclockwise rotation is an important component of recent rift extension and deformation, but the overall magnitude of rotation indicated by our data is somewhat less than suggested by previous studies. The sense and magnitude of the rotation determined from this study is consistent with recently reported results from Pliocene-Pleistocene flows of the Cerros del Rio volcanic field ( R=-9.0°±6.3°; Hudson et al., 2004). If these results are correct, then most of the rotation experienced within the Espanola basin must have occurred during the last 2 m.y. The exact cause of the observed rotation is problematic. Our data are consistent with the hypothesis idea that left slip occurred during development of the Rio Grande rift resulting in significant, although minor counterclockwise vertical axis rotation during rift development. Alternatively, other mechanisms such as right slip along rift-bounding faults during transtension could also account for domains of counterclockwise rotation within the rift.