New Mexico Geological Society Annual Spring Meeting — Abstracts

The Late Miocene Lobato Formation comprises predominantly fine-grained and vesicular olivine- and plagioclase-phyric alkaline basalts. Lobato volcanism represents some of the precaldera mafic volcanism in the Jemez Mountain Volcanic Field and coincided with an episode of crustal extension in the Espanola Basin. We examined a 100 meter thick sequence of the Lobato Formation on the north side of Arroyo de la Plaza Larga Canyon. Here, the Lobato flows are subhorizontal to gently dipping for nearly 2 kilometers southeast from the Cerro Roman volcanic center, then dip steeply about a roughly northwest trending axis, then abut and dip modestly against the Santa Clara fault – a major structure on the western margin of the Espanola Basin. We studied the disposition of these lava flows and attempted to distinguish between (1) lava flow emplacement into a paleovalley which existed in the late Miocene along the Santa Clara fault system or (2) post-emplacement drag folding against the Santa Clara fault. Paleomagnetic data were collected from sixteen sites along a transect representing the arcuate structure in order to conduct a paleomagnetic fold test. Remanent magnetizations were measured using a AGICO JR6-A Dual-Speed magnetometer at the New Mexico Highlands University paleomagnetic-rock magnetic laboratory. Specimens were progressively AF demagnetized in 10 to 15 steps to a maximum field of 120 mT to isolate the geological important characteristic remanent magnetization. Paleomagnetic data reveal a single component magnetization that decays to the origin with less than 10 percent of the natural remanent magnetization remaining after treatment in 120 mT fields. In situ results from sites located in the subhorizontal 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 increases, indicating a negative paleomagnetic fold test. We argue that the lava flows were emplaced into a paleovalley of considerable relief adjacent to the Santa Clara Fault during the late Miocene. These data indicate that the Santa Clara Fault was a prominent structure that influenced the paleotopography of the western margin of the Rio Grande Rift in the late Miocene.