Paleomagnetic and geochemical data from the late Miocene Lobato Formation adjacent to the Santa Clara fault system, Chili quadrangle, Rio Arriba County, New Mexico

Abstract

New paleomagnetic and petrologic data from late Miocene volcanic rocks in the northern Espanola Basin of north-central New Mexico help constrain the late Tertiary tectonic history and landscape development of the area. We studied a 100-m-thick (328-ft-thick) section of the ~ 10 Ma Lobato Formation in Arroyo de la Plaza Larga, an east-trending drainage in the northeastern Jemez Mountains. The Lobato Formation represents some of the earliest precaldera mafic volcanism associated with the Jemez Mountain volcanic field and coincides with an episode of crustal extension in the Espanola Basin. At Arroyo de la Plaza Larga, Lobato Formation flows are subhorizontal for nearly 2 km (1.25 mi) southeast from their eruptive source in the Cerro Roman volcanic center. These flows take on a monoclinal geometry with an apparent northeast-trending fold axis where they flow over an erosional escarpment adjacent to the Santa Clara fault, a prominent northeast-striking structure along the western part of the Espanola Basin. Here we show that the apparent monocline is not of a structural origin but formed due to lava flow emplacement down an escarpment formed by displacement along the Santa Clara fault. One hundred thirty-four oriented samples were collected for paleomagnetic analysis from 16 sites from the hinge zone and east limb of the apparent monocline. Paleomagnetic data reveal a single-component magnetization that decays to the origin with less than 10% of the natural remanent magnetization remaining after treatment in 120 mT fields. In situ results from sites located in the hinge zone and those from the east fold limb yield statistically indistinguishable remanence directions. Following structural correction, based on the strike and dip of the individual flows, the dispersion between the two data sets increased, indicating failure of the fold test at the 95% confidence level. We argue that the Lobato Formation basalts from Cerro Roman were emplaced into a paleovalley of considerable relief adjacent to the Santa Clara fault and that during the late Miocene, it was an active structure that influenced the topography and drainage systems of the western margin of the Rio Grande rift.