VOLCANISM AND EXTENSIONAL TECTONISM OF THE SOUTHWESTERN ESPAÑOLA BASIN, NORTHERN RIO GRANDE RIFT, NEW MEXICO

The Plio-Pleistocene Cerros del Rio volcanic field (CdRVF) in northern N. Mex. is one of the Rio Grande rift's largest (>700 km²) basaltic volcanic fields: it records the late-stage, volcano-tectonic evolution of the SW part of the Española basin. The CdRVF reflects both regional proclivity toward Pliocene basaltic volcanism following protracted Neogene extensional tectonism and localized eruptive response to migration of basin- bounding faults. Approximately 180 km³ of flat-lying basalt, andesite, and minor dacite lava flows and pyroclastic deposits of the CdRVF were erupted from > 50 exposed vents between 2.8Ma and 1.14 Ma. In the subsurface, volcanic deposits are interbedded with Santa Fe Group basin-fill sediment deposited in actively subsiding sub-basins of the southernmost Española Basin. Major basin-bounding faults in this area are characterized by predominantly N- and NW-trending, inward dipping faults having mostly dip-slip and subordinate strike-slip displacement. Although major basin-bounding structures were established prior to the onset of volcanism, protracted extension resulted in basinward migration of graben-bounding faults. In the area of the CdRVF, this resulted in a westward migration of active faulting. Phases of coeval volcanism at 2.8-2.6 Ma, 2.5-2.2 Ma, and 1.5-1.1Ma, were of decreasing eruptive volume and are delineated on the basis of mapped stratigraphy, argon geochronology, paleomagnetic and aeromagnetic properties, and records a simultaneous westward migration of eruptive centers. The alignment of vent areas with mapped or inferred faults strongly suggests deep magmatic conduits are utilizing local structures (faults and fractures, developed in response to regional stress). However, near-surface feeder dikes associated with eroded cinder cones record orientations that are not typically correlative with regional fault patterns suggesting near-surface conduits are independent of regional strain.