Geophys. J . Int. (1990) 102, 73-88 Brunhes chron excursion/polarity episode recorded during the late Pleistocene, Albuquerque Volcanoes, New Mexico, USA J . W. Geissman,' L. Brown,2 B. D. T~rrin,~ . D. McFadden' and S. S. Harlan' 'Department of Geology, University of New Mexico, Albuquerque, NM 87131, U SA 'Department of Geology and Geography, University o f Massachusetts, Amhe rst, M A 01003, USA ' U S Geological Survey, Mail Stop 941, 345 Middlefield Road, Menlo Park, €A 94305, U S A Accepted 1989 December 29. Received 1989 December 11; in original form 1989 April 11 SUMMARY All basaltic lava flo ws o f t he Al buqu erq ue Volcanoes (lat.: 35.2"N, long.: 253.2"E), Albuque rque-Belen Basin, New Mexico, record a short excursion/polarity episode. K-Ar isotopic age determinations (weighted average: 1 5 5 f 7 ka) and evaluation of soil profiles on flow surfaces suggest the late Pleistocene (circa between 250 and 80 ka) as the time of extrusion. Result s from 63 sit es in a minimum o f eight flo ws yield a mean direction of D = 101. lo, Z = -36. lo, ag 5 = 1.2" (a;, 0.7, a & = 1.2), k = 2219 (N = 8 flows) and a corresponding virtual geomagnetic pole position (VGP) of 354.1"E, 20.2"S, dp = 0.8", dm = 1.4" ( A& = 0.5, = 1.4), and VGP angular standard deviation (ASD) = 1.8". The unusual magnetization in flows o f the Albuquerque Volcanoes is carried by fine-grained [single-domain (SD) and/or pseudo-single-domain (PSD)] low-Ti magnetite and is apparently not a function of complex sub-solidus altera tion of magnetic phases. Underlying baked soils contain a relatively low coercivity magnetization with directions comprising two groups. One is roughly antipodal to the lava remanence; the other intermediate between the lava remanence and present field direction. Because all flows yield statistically indistin- guishable directions, a field directional path clearly cannot be defined. At the precision level of our age determinations, the Albuquerque feature may correlate with other s hort polarity episode s or excursions o f late Pleistocene age (e.g., Blake or Jamaica). Though characteristically limited in directional morphology, well-dated polarity episodes (i.e. sub-chrons) and excursions recorded in volcanic rocks may provide information on the frequency of significant dynamo instabilities. K ey words: dynamo, excursion, Pleistocene, polarity episode. INTRODUCTION Geomagnetic excursions represent major deviations in field behaviour over relatively short periods o f geologic time (Denham & Cox 1971; Cox 1975; Verosub & Banerjee 1977; Merrill & McElhinny 1983). Virtual geomagnetic pole positions (VGPs) differ by more than 45" from the time-averaged position for that chron (Watkins 1976); these are often in part located in the opposite hemisphere. Excursions are not necessarily associated with field transitions; the full reversal o f magnetic field direction has been discussed as a polarity event, sub-chron, or 'episode', and regarded as a potential stratigraphic tool (Denham 1976; Harland et 01. 1982; Tucholka et a l. 1987). On the basis o f palaeomagnetic records o f variable quality, excursions and/or sub-chrons have been suggested for several distinct times during the Brunhes Chron (Tarling 1983; Jacobs 1984; Champion, Lanphere & Kuntz 1988). Most features have been reported from continental and marine sedimentary sequences, leaving the fidelity, or even the validity o f each record often in question (Verosub 1982; Verosub & Banerjee 1977; Tarling 1983). Unusual field features based on thermoremanent magnetization (TRM) data from lava flows, baked sediments, or artificially baked clays typical ly are characterized by only a few instantaneous parts of each feature. Nonetheless, additional documenta- tion o f well-dated complete or even partial excursions in sequences o f lava flows would be o f considerable use. Furtherm ore, identification o f records of identical, anoma- lous field features at different locations may provide considerable insight into the nature o f short-term field phenomena and the generation and frequency o f dynamo 73 atU n iv ersity o fCalifo rn ia, San D ieg o o n Ju n e19, 2013http ://g ji. o x fo rdjo u rn als. o rg /D o w lo adedfro m
