HR: 15:45h
AN: GP43A-08    [Abstracts]
TI: Did Episodes of Very Rapid Field Change Occur During the Steens Mountain Reversal?
AU: * Coe, R S
EM: rcoe@pmc.ucsc.edu
AF: University of California, Earth and Planetary Sciences Department, Santa Cruz, CA 95064, United States
AU: Jarboe, N A
EM: njarboe@pmc.ucsc.edu
AF: University of California, Earth and Planetary Sciences Department, Santa Cruz, CA 95064, United States
AU: Glen, J J
EM: jglen@usgs.gov
AF: United States Geological Survey, 345 Middlefield Road, MS989, Menlo Park, CA 94025, United States
AU: Renne, P R
EM: prenne@bgc.org
AF: Berkeley Geochronology Lab, 2455 Ridge Road, Berkeley, CA 94079, United States
AB: We have discovered new evidence relating to the hypothesis of very rapid jumps in direction during the 16.6 Ma reversal recorded by Steens Basalt flows. The benchmark record of the polarity transition is punctuated by three large directional gaps (Mankinen et al., 1985; Prevot et al., 1985). An important question is whether any of the gaps mark impulsive changes in the field, as suggested earlier, or just signify longer than usual hiatuses between successive flows. Within each of the first and second directional gaps a lava flow was found with directions prominently streaked in such a way that it might have resulted from the field changing at an astonishing rate (several degrees/day) as the flow cooled. An alternate explanation could involve some kind of subtle overprinting mechanism that remagnetized samples in the interior of the flow more than nearer the top. An important additional observation is that the directions in the flow within the second gap are streaked along the great circle containing the directions of the underlying and overlying flows, but in the flow within the first gap they are not. Our new results from other contemporaneous sections of Steens Basalt (see abstract by Jarboe et al.) shed new light on this problem. Lava flows at the position of the second directional gap display a whole new swing in field direction that is missing in the Steens benchmark record. Thus, the second gap marks a significant hiatus, not field change during cooling of a single flow, and the streaked within-flow directions probably signify varying degrees of partial remagnetization induced by the overlying flow by a mechanism not yet identified. At the position of the first gap there are also flows with directions new to the Steens benchmark record, but these directions lie near the end of the directional streak, i.e., near the direction recorded by the samples in the flow interior that were the last to cool through their principal blocking temperatures. This finding is most consistent with rapid field change as the flow cooled at the first directional gap.
DE: 1530 Rapid time variations
DE: 1533 Remagnetization
DE: 1535 Reversals: process, timescale, magnetostratigraphy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly