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.