REVISITING THE INITIAL SITES OF GEOMAGNETIC-FIELD IMPULSES DURING THESTEENS-MOUNTAIN POLARITY REVERSAL

We present a new palaeomagnetic investigation of the two sites from th e Steens Mountain (Oregon) volcanic record of a Miocene polarity rever sal which were supposed to record very fast changes of the geomagnetic held or impulses (Mankinen et al. 1985; Prevot et al. 1985a,b). Appro ximately 130 cores were first drilled from the two initial sites, belo nging to sections A and B, in order to obtain at least one detailed an d complete vertical sampling of each lava flow. Thermal analyses of na tural remanent magnetization, complemented by some alternating field t reatments, low-held thermomagnetic curves, microscopic observations an d electron probe analyses of key magnetic phases, lead us to somewhat different conclusions for the first and the second impulses. At site B (first impulse), we find that the dependence of the remanence directi on on the sample vertical position in flow B51 does not imply a direct ional field change during flow cooling, but is better explained by a t hermochemical overprinting due to the overlying B50 flow. However, thi s conclusion does not challenge the existence of the first impulse bec ause this field change seems to be recorded some 25 m away in flow B51 (Coe & Prevot 1989), at a place where it is thick enough for this rec ord not to have been erased by the baking due to B50. Regarding the se cond impulse, restudied at site A, our new findings are more comprehen sively explained by a change in the field direction during cooling of flow A41-2 than by some overprinting. Using a simple model of flow coo ling, the angular rate of change of the held is estimated to have been of the order of 2 degrees-3 degrees or 250-350 nT per day during the impulse. This figure is similar to that previously obtained from site D, some 250 m away. However, the directional paths describing the held change are somewhat different at the two sites. New investigations ar e planned to try to understand the origin of this discrepancy.