INTEGRATION OF GEOLOGIC MAPPING, GEOCHRONOLOGY, PALEOMAGNETISM AND AEROMAGNETIC MAPPING IN THE CERROS DEL RIO VOLCANIC FIELD, RIO GRANDE RIFT, NEW MEXICO

Three principal phases of volcanism are preserved in the >700 km² Cerros del Rio volcanic field (CdRVF) within the Rio Grande rift of northern New Mexico: (1) 2.7-2.6 Ma; (2) 2.5-2.2 Ma; and (3) 1.5-1.1 Ma. The predominantly mafic CdRVF comprises mostly flat-lying lava flows and pyroclastic deposits erupted from multiple basaltic to dacitic volcanic centers that formed marginal to the large-volume intermediate to silicic volcanoes of the Jemez Mountains. The >120 km³ CdRVF erupted between the actively subsiding, fault-bounded, Española and Santa Domingo sub-basins through basin-fill sediments of the rift. We have mapped the geology of the CdRVF at 1:24,000 scale, sampled more than 50 widely distributed sites for paleomagnetic analysis, analyzed more than 45 units for ⁴⁰Ar/³⁹Ar age determinations, and integrated these data sets with high-resolution aeromagnetic data. We interpret the magnetic anomalies observed in the aeromagnetic data to mostly reflect remanent magnetization captured when the volcanic rocks erupted and cooled during normal or reversed polarities of the Gauss or Matuyama geomagnetic chrons. Compared to paleomagnetic polarities for mapped units, prominent aeromagnetic highs or lows for topographic features underlain by single geologic units reflect remanent magnetic polarity. Where aeromagnetic signatures are unclear, paleomagnetic determinations enable interpretation of the aeromagnetic polarities. In some areas, where polarity of surface lava flows is opposite that of aeromagnetic anomalies, we believe that the flows are not thick enough to mask the opposite aeromagnetic signature of underlying deposits. Combined with geologic mapping and ⁴⁰Ar/³⁹Ar dates, the integrated magnetic studies yield significantly better map definition of younger, reversed-polarity volcanic strata erupted during the Matuyama chron (<2.58 Ma) where they overly or are inset into older normal-polarity units of the Gauss chron (> 2.58 Ma) and identification of normal-polarity units erupted during Reunion events (~2.2-2.1 Ma) of the Matuyama chron.