The Steens Mountain (Oregon) Geomagnetic Polarity Transition: 3. Its regional significance

Abstract

Study of the variations of direction and intensity of the geomagnetic field as recorded by the Miocene lava flows on Steens Mountain, southeastern Oregon, has resulted in a detailed description of total field behavior during a reversal in polarity. In addition to information about the polarity reversal itself, the detailed paleomagnetic record includes several thousand years of geomagnetic history preceding and following the polarity transition at 15.5 Ma. In order to test the feasibility of using this record as a means of correlation in this part of the western United States, comparisons are made of reconnaissance and previously published paleomagnetic records obtained from what has been thought to be the Steens Basalt or rocks of equivalent age. Despite the fact that many of these earlier studies were not done in detail and were not intended for correlation purposes, convincing similarities among some of the records are evident. The Steens Basalt paleomagnetic record does, indeed, have potential as a correlation tool during this time of widespread basaltic volcanism. Additionally, paleomagnetic data from flows that were sampled in detail yield a middle Miocene paleomagnetic pole at 88.3°N, 209.0° (α95 = 6.3°) for the High Lava Plains of Oregon. This pole position is statistically indistinguishable from the earth's rotational axis and implies that no tectonic rotation of this region has occurred since these lava flows were erupted. Data from selected sites within the coeval part of the Columbia River Basalt Group yield a paleomagnetic pole at 88.7°N, 171.6°E (α95 = 4.0°). The Columbia River Basalt Group pole is statistically indistinguishable from either the rotational axis or from the High Lava Plains pole. These findings indicate no post‐20 Ma differential rotation between south‐eastern Washington and south‐central Oregon, in contrast to previous interpretations.

Regional Surficial Geochemistry of the Northern Great Basin

“…The Miocene basalts associated with the northern Nevada rift do not have high factor 1 scores; they presumably lack the chalcophile element enrichment shown by the Steens-like rocks. The correlations suggested here are in agreement with those proposed by Mankinen et al (1987) but extend the occurrence of Steens-like rocks farther to the south, into central Nevada.…”

“…Widely exposed throughout southeastern Oregon, this distinctive 16.6-Ma basalt represents the remnants of a Miocene shield volcano that may have been 100 km in diameter (Mankinen et al, 1987;Johnson et al, 1998). These basalts are the same age and generally the same composition as those of the Columbia River basalt province.…”

Assessing vertical axis rotations in large‐magnitude extensional settings: A transect across the Death Valley extended terrane, California

“…This mean is similar to and within statistical error of the mid-Miocene expected direction (358°, +55°, a 95 = 6.0° [Mankinen et al, 1987]) and is consistent with no statistically significant vertical axis rotation (rotation (R) = 1.2°± 14.1°) of the northern Greenwater Range since 7 Ma. The slightly shallower inclination (flattening (F) = 8.3°± 10.1°) is also statistically insignificant and may result from inability to adequately remove a younger normal polarity overprint from the reversed polarity sites.…”

“…This value, which we consider a maximum, also yields groupmean direction close to the Miocene direction, D = 194°, I = À50°, with R = +16°and F = +5°. The late Miocene deformational history of the central Panamint Mountains is consistent Mankinen et al [1987]). See Table 1 for site bedding corrections used.…”

“…It is possible that the moderately discordant paleomagnetic directions likely reflect either high-amplitude paleosecular variation or a transitional field during a reversal. Mankinen et al [1987]). See Table 1 for site bedding corrections used.…”