UNDERSTANDING THE COMPLEXITY OF CINDER CONE VOLCANOES USING PALEOMAGNETIC, ROCK MAGNETIC AND STRUCTURAL ANALYSIS; A CASE STUDY FROM THE CIENEGA VOLCANO, CERROS DEL RIO VOLCANIC FIELD, NEW MEXICO

The Cerros del Rio volcanic field, west of Santa Fe, NM, is situated on the La Bajada constriction, between the Espanola and the Santo Domingo basins of the Rio Grande Rift. Pliocene to Pleistocene volcanism resulted in about 60 volcanoes with the majority of them cinder cones; including the La Cienega volcano. Cinder cone volcanoes are commonly described as a loosely consolidated pile of pyroclastic materials formed by ascension of magma through simple feeder conduit geometry. Recent field and laboratory studies, however, reveal that the magma follows numerous pathways during the emplacement process resulting in more complex and variable feeder geometries. Here, we investigate the La Cienega Volcano, an excavated cinder cone that exposes in near three-dimensions the magma plumbing system We inspected the magmatic plumbing system using different field (mapping) and laboratory methods (thin section petrology, paleomagnetic and anisotropy of magnetic susceptibility (AMS) data). To evaluate magma emplacement and subvolcanic magma feeder system evolution, we collected samples across each preeminent outcrop of the feeder system. The in situ results for six paleomagnetic sites provide a group mean of D=350.9º, I=19.2º, α95= 4.6º, 5/6, that is statistically distinct from the Pliocene expected field direction. The virtual geomagnetic pole dispersion of the group mean yields a value of 4.9º, which is significantly lower than the predicted VGP dispersion estimate of 15º for the paleolatitude of the site (35.7°N). The low VGP dispersion and the group mean result likely indicates that the sampled dikes were emplaced quickly relative to secular variation of the geomagnetic field. When comparing the remanence direction between individual dikes, we tested the hypothesis that one dike shared a common true mean direction with another dike. Several dikes are statistically distinct at the 95% confidence level to the other dikes, while a few dikes are statistically indistinguishable at the 95% confidence level. The AMS data show magma flow patterns that support the field observation of macroscopic structures. Preliminary paleomagnetic and AMS results indicate that the cinder cone plumbing systems are more complicated than common models predict and that the magma uses different pathways as it flows through the volcanic edifice.