Evidence from lava flows for complex polarity transitions: the new composite Steens Mountain reversal record

Abstract

S U M M A R YGeomagnetic polarity transitions may be significantly more complex than are currently depicted in many sedimentary and lava-flow records. By splicing together paleomagnetic results from earlier studies at Steens Mountain with those from three newly studied sections of Oregon Plateau flood basalts at Catlow Peak and Poker Jim Ridge 70-90 km to the southeast and west, respectively, we provide support for this interpretation with the most detailed account of a magnetic field reversal yet observed in volcanic rocks. Forty-five new distinguishable transitional (T) directions together with 30 earlier ones reveal a much more complex and detailed record of the 16.7 Ma reversed (R)-to-normal (N) polarity transition that marks the end of Chron C5Cr. Compared to the earlier R-T-N-T-N reversal record, the new record can be described as R-T-N-T-N-T-R-T-N. The composite record confirms earlier features, adds new west and up directions and an entire large N-T-R-T segment to the path, and fills in directions on the path between earlier directional jumps. Persistent virtual geomagnetic pole (VGP) clusters and separate VGPs have a preference for previously described longitudinal bands from transition study compilations, which suggests the presence of features at the core-mantle boundary that influence the flow of core fluid and distribution of magnetic flux. Overall the record is consistent with the generalization that VGP paths vary greatly from reversal to reversal and depend on the location of the observer. Rates of secular variation confirm that the flows comprising these sections were erupted rapidly, with maximum rates estimated to be 85-120 m ka −1 at Catlow and 130-195 m ka −1 at Poker Jim South. Paleomagnetic poles from other studies are combined with 32 non-transitional poles found here to give a clockwise rotation of the Oregon Plateau of 11.4 • ± 5.6 • with respect to the younger Columbia River Basalt Group flows to the north and 14.5 • ± 4.6 • with respect to cratonic North America (95 per cent confidence interval).

Dynamical similarity of geomagnetic field reversals

“…A threshold of eight north VGPs with latitudes less than 60u was the second selection criterion. Seven detailed studies of R-N transitions 8,[23][24][25][26][27] and three records of N-R transitions 23,28,29 meet these requirements. One record 32 could not be retained owing to lack of normal polarity directions.…”

Demise of the rapid-field-change hypothesis at Steens Mountain: The crucial role of continuous thermal demagnetization

“…And even though the rock-magnetic mechanism for the streaked directions at the second directional gap is not yet known, the many new transitional directions at this point in the Steens reversal from two other sections (Fig. 3) show that the gap in the record at the original section must be explained instead by an unusually long temporal gap between superposed flows (Jarboe et al, 2011).…”

“…(Adapted from Fig. 15 of Jarboe et al, 2011. ) (Mankinen et al, 1985) and studied in detail nearby (Coe et al, 1995).…”

“…The simplest interpretation of these new directions weighed against the hypothesis of rapid field change, but, because an equally large gap still remained in the composite record with A and B results spliced together, the hypothesis could not be ruled out. The recent composite record of Jarboe et al (2011), however, splices ten new directions into the gap in the Steens A path that were recorded by 17 flows from two much more distant sections, Poker Jim Ridge 80 km to the west of Steens Mountain and Catlow Peak 70 km to the south-southeast (Fig. 2).…”

Evidence of Unusual Geomagnetic Regimes Recorded in Plio‐Pleistocene Volcanic Sequences from the Lesser Caucasus (Southern Georgia)

“…Specifically, lava flow sequences can be particularly helpful in revealing the behavior of the EMF during an extended time. This type of records can be found in sequences characterized by sequential image lava flow eruptions (e.g., Caccavari et al, ; Camps et al, ; Chauvin et al, ; Herrero‐Bervera & Valet, ; Jarboe et al, ; Kissel et al, ; Leonhardt et al, ; Mankinen et al, ; Moulin et al, ; Prévot et al, ).…”