Paleomagnetism of Lower Permian Abo and Yeso Formation, Carizzo Arroyo, Lucero Uplift, New Mexico

We report paleomagnetic data from Lower Permian hematite-cemented sandstones and siltstones from Carrizo Arroyo, on the eastern edge of the Lucero uplift along the west-side of the middle Rio Grande rift, to test the hypothesis that the rift margins have accommodated extensional strain via vertical axis rotation. In addition, we present a revised interpretation of the structural setting and deformation history of the area, were late-Tertiary transtensional stresses have produced the majority of the structures in the area. The paleomagnetic data are discussed in the context of this hypothesis. In the Rio Grande rift area, a mid-Cenozoic and younger extensional feature defining the eastern margin of the Colorado Plateau, relatively little work has been done to assess the magnitude and sense of vertical axis rotations of fault-bounded crustal blocks within and at the margins of the plateau. A growing body of evidence shows that the Colorado Plateau has experienced some degree of vertical axis rotation and some magnitude of northward translation, although the magnitudes of the rotation and translation have been subject to considerable debate. Eight to ten oriented samples from 50 sites have been fully demagnetized with all sites yielding interpretable results: 41 sites from three sections in the Lower Permian Abo Formation, and 9 sites in the Meseta Blanca Member of the overlying Yeso Formation. In most cases, progressive thermal demagnetization resulted in a nearly univectorial decay of the magnetization to the origin that is well grouped at the site level. After correcting for modest dip of strata, the 50 sites in Carrizo Arroyo yield an estimate group mean (D = 162.1°, I = -4.1°, α₉₅ = 6.8°, k = 10.18). Overall, the data from this part of the west side of the rift are discordant, in a clockwise since, with Early Permian (about D = 140°, I = -2.0°) and mid-Permian (about D = 145°, I = -4.0°) expected directions. We interpret the paleomagnetic data from Carrizo Arroyo to be consistent with modest clockwise vertical axis rotation of these sections since the latest Paleozoic, arguably during Laramide deformation and subsequent Rio Grande extension. We hypothesize that a component of tectonic rotation has accommodated extensional strain along the margins of the Rio Grande rift since inception. The observed rotation could be the result of oblique-slip displacement of the hanging wall block; in this case, the observed rotation is apparent. Alternatively, the rotation is a true block rotation where the block is decoupled from the underlying strata along a detachment surface. Additional field mapping will help to distinguish between these two possible models.