PALEOMAGNETISM OF TERTIARY INTRUSIVE AND VOLCANIC ROCKS FROM THE CERRILLOS HILLS, NEW MEXICO: EVIDENCE FOR CRUSTAL BLOCK ROTATIONS IN THE RIO GRANDE RIFT

Published paleomagnetic results from the northern Rio Grande rift provide evidence for counterclockwise vertical axis rotations, perhaps associated with left-slip on faults during rift extension. In order to better document and understand the extent, significance and origin of such rotations, we have collected new paleomagnetic data from intrusive and volcanic rocks in the Cerrillos Hills and surrounding areas, 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; isotopic dates indicate that most of these igneous rocks were emplaced between about 36 and 28 Ma. Preliminary paleomagnetic results from 31 sites in intrusive and volcaniclastic rocks yield well-defined site-mean directions, with a group-mean direction of Decl.=345.6°, Inc.=57.6° (k=28, α₉₅=4.4°; 21 sites of normal polarity and 10 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 and consider our 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., Decl.=355.3°, Inc.=53.9°, Irving and Irving, 1979) indicates that it is discordant (R=-9.6°±7.4°) and that the result is significant at the 95% confidence level. The apparent rotation is consistent with the results of previous paleomagnetic studies documenting the presence of counterclockwise rotation in this part of the rift, but the overall magnitude of rotation indicated by our data is substantially less than previously reported. Our data are not inconsistent with the hypothesis idea that left slip occurred during development of the Rio Grande rift resulting in significant, although very small magnitude counterclockwise vertical axis rotation during rift development. Alternatively, such a small magnitude of counterclockwise rotation can be readily explained in an extensional setting that does not involve left slip parallel or sub-parallel to the rift.