We carried out an extensive paleointensity study of the 15.5±0.3 m.y. Miocene reversed-to-normal polarity transition recorded in lava flows from Steens Mountain (south central Oregon). One hundred eighty-five samples from the collection whose paleodirectional study is reported by Mankinen et al. (this issue) were chosen for paleointensity investigations because of their low viscosity index, high Curie point and reversibility, or near reversibility, of the strong field magnetization curve versus temperature. Application of the Thellier stepwise double heating method was very successful, yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows. After grouping successive lava flows that did not differ significantly in direction and intensity, we obtained 51 distinguishable, complete field vectors of which 10 are reversed, 28 are transitional, and 13 are normal. The record is complex, quite unlike that predicted by simple flooding or standing nondipole field models. It begins with an estimated several thousand years of reversed polarity with an average intensity of 31.5±8.5 μT, about one third lower than the expected Miocene intensity. This difference is interpreted as a long-term reduction of the dipole moment prior to the reversal. When site directions and intensities are considered, truly transitional directions and intensities appear almost at the same time at the beginning of the transition, and they disappear simultaneously at the end of the reversal. Large deviations in declination occur during this approximately 4500±1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site. The transitional intensity is generally low, with an average of 10.9±4.9 μT for directions more than 45° away from the dipole field and a minimum of about 5 μT. The root-mean-square of the three field components X, Y, and Z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude. However, a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions, which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field. Changes in the field vector are progressive but jerky, with at least two, and possibly three, large swings at astonishingly high rates. Each of those transitional geomagnetic impulses occurs when the field intensity is low (less than 10 μT) and is followed by an interval of directional stasis during which the magnitude of the field increases greatly. For the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700±2700 nT yr−1, which is about 15–50 times larger than the maximum rate of change of the nondipole field observed during the last centuries. The occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions. The record closes with an estimated several thousand years of normal polarity with an average intensity of 46.7±20.1 μT, agreeing with the expected Miocene value. However, the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable.

The Steens Mountain (Oregon) geomagnetic polarity transition: 2. Field intensity variations and discussion of reversal models

Burke et al. (1980) have considered geologic and seismic data from Jamaica, while Mann et al. (1984) have examined data from Hispaniola. The present investigation is concerned with the neotectonics of the entire Caribbean region, taking into account the recent results in the northeastern Caribbean and a more comprehensive compilation of seismicity and neotectonic structures from other areas than has been available to most previous authors. It is hoped that this study will complement seismic network results in helping to pinpoint areas of seismic hazard. It is also intended to provide a framework for high-precision geodetic surveys of Caribbean plate motion using measurements from satellites and astronomical observations.

Neotectonics of the Caribbean

High-resolution record of the Upper Olduvai transition from Po Valley (Italy) sediments: support for dipolar transition geometry?

We present new paleomagnetic data from three Middle Neoproterozoic carbonate units of East Svalbard, Norway. The paleomagnetic record is gleaned from 50 to 650 m of continuous, platformal carbonate sediment, is reproduced at three locations distributed over >100 km on a single craton, and scores a 5‐6 (out of 7) on the Van der Voo (1990) reliability scale. Two >50° shifts in paleomagnetic direction are coincident with equally abrupt shifts in ! 13 C and transient changes in relative sea level. We explore four possible explanations for these coincidental changes: rapid plate tectonic rotation during depositional hiatus, magnetic excursions, nongeocentric axial-dipole fi elds, and true polar wander. We conclude that the observations are explained most readily by rapid shifts in paleogeography associated with a pair of true polar wander events. Future work in sediments of equivalent age from other basins can test directly the true polar wander hypothesis because this type of event would affect every continent in a predictable manner, depending on the continent’s changing position relative to Earth’s spin axis.

Combined paleomagnetic, isotopic, and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard, Norway

Brunhes-Matuyama polarity transition

Equatorial paleopoles and behavior of the dipole field during polarity transitions

A small Lower Cretaceous igneous province on Jamaica has been assigned an Albian or Aptian age on the basis of the fauna of associated sediments. Despite pronounced tropical weathering, it was possible to find three reasonably fresh lava exposures, paleomagnetic investigations of which suggest that during the Lower Cretaceous period the southern polarity geomagnetic pole, as viewed from Jamaica, was situated in the present day northeast Pacific. An interpretation of this result in terms of plate tectonics yields a movement of the Caribbean tectonic plate over a distance of 1000 km to the northeast since Lower Cretaceous. This result is in good agreement with similar deductions made from previously investigated Upper Cretaceous rocks on Jamaica. It suggests also that no appreciable movement of the Caribbean plate or polar wonder had occurred during the Cretaceous age. One of the three lava units reveals an equatorial direction of magnetization also found previously in Upper Cretaceous rocks of the island. This direction could have been caused by a non axial dipole configuration during a reversal of the geomagnetic field. This interpretation is supported by other evidence of a field reversal at the close of Aptian and beginning of Albian.

Stanislaw A. Vincenz

Papers

Equatorial paleopoles and behavior of the dipole field during polarity transitions

Paleomagnetism of lower cretaceous Lavas on Jamaica

Paleomagnetism of a paleocene pluton on jamaica

Origin of the magnetization of Permo-Carboniferous sediments of Spitsbergen, Svalbard Archipelago

Interpretation of Gravity Data For Evaluation of Tectonic Structure In the Northern Mississippi Embayment