Brunhes chron excursion/polarity episode recorded during the late pleistocene, Albuquerque Volcanoes, New Mexico, USA

Abstract

All basaltic lava flows of the Albuquerque Volcanoes (lat.: 35.2°N, long.: 253.2°E), Albuquerque‐Belen Basin, New Mexico, record a short excursion/polarity episode. K‐Ar isotopic age determinations (weighted average: 155 ± 47 ka) and evaluation of soil profiles on flow surfaces suggest the late Pleistocene (circa between 250 and 80 ka) as the time of extrusion. Results from 63 sites in a minimum of eight flows yield a mean direction of D= 101.1°, I=−36.1°, α95= 1.2° (α195= 0.7, α295= 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° (A195= 0.295, = 1.4), and VGP angular standard deviation (ASD) = 1.8°. the unusual magnetization in flows of 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 alteration 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 indistinguishable directions, a field directional path clearly cannot be defined. At the precision level of our age determinations, the Albuquerque feature may correlate with other short polarity episodes or excursions of 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. Copyright © 1990, Wiley Blackwell. All rights reserved