Paleomagnetism of the Late Cretaceous‐Early Tertiary north‐central Montana Alkalic Province

Abstract

Paleomagnetic results have been obtained from the Late Cretaceous‐early Tertiary igneous complexes of the north‐central Montana alkalic province. Data from 94 sites in Eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82.0°N, 170.2°E (A95 = 3.5°; k = 18.6), while 36 sites in Paleocene intrusions yield a paleomagnetic pole at 81.8°N, 181.4°E (A95 = 5.4°; k = 20.2). These poles differ by only 1.6° and are not significantly different statistically. The 130‐site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander (APW) during the period of magnetization of the igneous centers. Postmagnetization structural complications in these rocks are minimal. The presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration. The northcentral Montana data together with other early Tertiary, Cretaceous, and mid‐Tertiary paleomagnetic results require modification of our earlier APW chronology [Diehl et al., 1980]. It now appears that APW relative to North America since the Early Cretaceous consists of a polar still‐stand during much of the Cretaceous (120–75 m.y. B.P.), a period of rapid movement in latest Cretaceous time (75–65 m.y. B.P.), and a period of slow polar movement thereafter. The onset of this period of rapid APW correlates well with a major change in plate motions at ∼80 m.y. and the beginning of the Laramide Orogeny.

Tectonic implications of magnetic fabrics and remanence in the Cooper Mountain pluton, North Cascade Mountains, Washington

“…The mean direction of the magnetite component from 16 specimens is D = 342.7°, I = 72.8°, α 95 = 9.2°, and k = 17. The means of these two components are unlike the present day field direction (D = 20.9°, I = 71.7°) but similar to each other and the direction expected for the Eocene (D = 349.5°, I = 67.4°; calculated from North American pole of Diehl et al 1983) (Fig. 8c).…”

“…Fabrics are more coherent in some zones than others. For instance, zone F has Also plotted are the present-day field direction (star) and the North American expected Eocene direction (gray diamond, calculated for the location from pole of Diehl et al (1983); α 95 circle of confidence for the expected Eocene direction is as small as the symbol drawn). fairly uniform southwest dips and northwest plunges, and zone B has steep northeast dips and steep southeast plunges.…”

“…Using the Diehl et al (1983) reference pole, the margin of North America in this region was oriented essentially north-south. Based on analysis of regional patterns of magmatism, geochemical variations of the magmas, and incorporation of existing plate motion models and other geological data, Breitsprecher et al (2003) concluded that the Kula-Farallon ridge was being subducted beneath North America near the latitude of the CMP at 50 Ma.…”

Magnetostratigraphy and Tectonic Rotation of the Eocene-Oligocene Makah and Hoko River Formations, Northwest Washington, USA

“…These results include the middle Eocene Humptulips Formation in the southwestern Olympics [57,58] [59]) also yielded a counterclockwise rotation. These rocks yielded both normal and reverse polarity magnetizations, which passed a reversal test and resulted in a formation mean direction of D = 308.4, I = 51.0, k = 6.7 and α 95 = 9.5.…”

Paleomagnetism of the Flores volcanics, Vancouver Island, in place by Eocene time

“…Diehl et al (1983); (6)Shive and Pruss (1977). Craton mean is average of the three previous entries.FIG.…”