PALEOMAGNETISM AND FAULT KINEMATIC ANALYSIS OF THE EASTERN LUCERO UPLIFT, NEW MEXICO

We report paleomagnetic data from Lower Permian hematite-cemented sand- and siltstones and fault kinematic analysis of small-scale, brittle faults from the eastern edge of the Lucero uplift along the west side of the middle Rio Grande rift (RGR). These data provide a field test to evaluate the hypothesis that parts of the RGR margin exhibit components of extension linked to vertical axis rotation. At this latitude, little work has been done to assess the magnitude and sense of vertical axis rotation within the eastern margin of the Colorado Plateau. Elsewhere within the plateau and further north along the eastern plateau margin, some data suggest that the plateau has experienced vertical axis rotation and northward translation. Eight to ten oriented samples from 32 sites in the Lower Permian Yeso Formation have been fully demagnetized with 23 sites yielding interpretable results. Progressive demagnetization resulted in a near univectorial decay of the magnetization to the origin that is well grouped at the site level, after removal of a randomly oriented secondary overprint. The 23 sites yield a stratigraphically corrected group mean of D=174.3°, I=-8.3°, a95=6.3°, k=24.2. Overall, the data from this part of the RGR are discordant clockwise with an inferred rotation of +31.8° +/- 11.9° and a minimum flattening of -5.3° +/- 5.4° compared to an Early Permian (140°, -2.0°) and mid-Permian (145°, -4.0°) expected direction. Previous paleomagnetic data from 50 sites yield similar results. Fault kinematic analysis of small-scale, brittle faults from normal faults within the Yeso Fm yield a bimodal distribution of extensional directions with modeled mean extension direction (n=56) that trends 265°. The shortening directions are scattered about a great circle normal to the intermediate strain axes. This suggests two phases of faulting accommodated both extension and rotation. Additional analysis of polished fault surfaces will provide further insight. Paleomagnetic data reveal that a component of vertical axis rotation accommodated part of the transtensional strain along the margins of the RGR since inception. The absence of overprinted small-scale structures consistent with earlier Laramide deformation leads us to contend that most of the deformation, including the modest vertical axis rotation, is associated with RGR.