A FAILED PALEOMAGNETIC REVERSAL TEST INDICATES AN EPISODIC ERUPTIVE HISTORY FOR THE CERROS DEL RIO VOLCANIC FIELD, NEW MEXICO

Mafic volcanic fields within the Rio Grande rift contain lava flows, cinder deposits and/or dikes, commonly erupted from multiple vent locations, but the duration and rate at which such fields were emplaced are often poorly known. The Pliocene to Quaternary Cerros del Rio volcanic field of northern New Mexico forms a dissected basaltic plateau sourced by more than 50 recognized vents. Its preserved stratigraphic sequence reflects three principal phases of volcanism: (1) 2.7-2.6 Ma, (2) 2.5-2.2 Ma, and (3) 1.5-1.1 Ma. Paleomagnetic data collected from 85 sites spanning the volcanic field mainly sample either phase-1 deposits that record the normal-polarity Guass chron or phase-2 deposits that record the reversed-polarity Matuyama chron. Paleomagnetic site directions from individual eruptive centers have low scatter compared to the full data set, indicating that such centers formed quickly (probably within a few 100 yrs) compared to rates of geomagnetic paleosecular variation. To avoid a time-sampling bias, we grouped sites from the individual centers to calculate eruptive group means (EGM). Although a grand mean of 40 EGM is D = 350.6°, I = 52.0°, k= 38, a95 = 5.6°, the means of normal- and reversed-polarity EGM are not statistically antipodal. This failed reversal test suggests that limited time was sampled within one or both of the polarity groups. Modal analysis demonstrates multiple directional clusters among the normal-polarity EGM and the mean normal-polarity inclination is significantly shallower (38°) than an expected (55°) dipole inclination. Both of these directional characteristics suggest that voluminous phase-1 deposits of the Cerros del Rio volcanic field were erupted episodically during short time intervals; the similarity of some EGM directions imply that several individual volcanic centers erupted synchronously (e.g, the andesite of Cerrita Portrilla and the basalt of La Bajada). As a similar rapid eruption was indicated by paleomagnetism of the basaltic Pleistocene Albuquerque Volcanoes (Geissman et al., 1990), such episodic development may well be the norm for mafic volcanism within the rift.