Paleomagnetic poles and polarity zonation from the Middle Proterozoic Belt Supergroup, Montana and Idaho

Abstract

Twelve paleomagnetic poles and a preliminary polarity zonation are reported from geologically mapped, stratigraphically controlled sections of the middle Proterozoic Belt Supergroup in western Montana and northern Idaho. Although gaps exist in the stratigraphic coverage, the lower Belt, Ravalli Group, and middle Belt carbonate appear to be largely if not entirely of normal polarity. A switch to reversed polarity takes place near the base of the overlying Missoula Group (base of Snowslip Formation), and two comparatively narrow intervals of mixed polarity containing at least 18 reversals are found in the lower and middle Snowslip. Seven reversals, mostly widely spaced in stratigraphic position, have been found in overlying strata of the Missoula Group. Poles from strata of the Ravalli Group through Missoula Group are well defined and tightly clustered. They plot in the south‐central Pacific Ocean and display only a small (∼20°) southeast to northwest shift in pole path. Poles from Belt strata in the eastern basin plot systematically east of poles from the more westerly collection sites. A counterclockwise shift in declination of 7°–10° is seen in strata from the easterly sites, which has given rise to a curving pole path that closely parallels the path for the westerly sites. This easterly path appears explainable as a consequence of a counterclockwise structural rotation of eastern Belt strata relative to western and central Belt strata produced during west to east thrust faulting of late Mesozoic and early Cenozoic age. An additional counterclockwise rotation of the same magnitude exists between the northern and southern parts of the Montana disturbed belt of the eastern Belt basin. Moreover, anomalously steep inclinations, not yet explained, have been reported from some of the strata sampled at the north end of the disturbed belt. The steep inclinations result in poles that plot distinctly east of poles reported from this study. The Belt pole path lies in a part of the North American apparent polar wander path that has been assigned an age of about 1500–1200 m.y. This is in broad agreement with isotopic ages reported for strata of the lower Belt to the middle Ravalli Group. The Belt path trends toward, and its northwest end overlaps, the beginning of the pole path for the Grand Canyon Supergroup, which formed in the interval from ≲1300 to ∼830 m.y. ago. Because polarity zonations differ between strata of the upper Belt and lower Grand Canyon Supergroups, Belt strata would appear, on paleomagnetic grounds, to be largely or entirely older than strata of the Grand Canyon Supergroup. However, isotopic ages for the Missoula Group (1100 m.y. and younger) presently require an alternative interpretation, i.e., that the Belt terrane did not participate in inferred plate motion which gave rise to a large north‐trending loop in the interval ∼1200 to ∼1100 m.y. ago and to later motion seen in the Grand Canyon path. Additional dating is needed for the Purcell Lava of the Missoula Group to support or modify the 1100 m.y. age determined by the K‐Ar method.