Paleomagnetism and geochronology of St. Matthew Island, Bering Sea

Abstract

Oriented samples from a series of flow units, pyroclastics, tuffs, tuff breccias, and some intrusive rocks from St. Matthew Island were studied paleomagnetically in an attempt to constrain possible paleolatitudes of the Bering Sea shelf. The volcanic sequences have apparently stable magnetic directions and contain a reversed–normal–reversed–normal polarity succession.The Late Cretaceous paleolatitude deduced from the paleomagnetic data was 67°N, and the island has rotated in a clockwise sense by a nominal 10°. This implies that the rocks analyzed were formed about 12 °south of their present location with respect to cratonic North America. These data argue against tectonic models for the Bering Sea shelf region that require large-scale Late Cretaceous and Tertiary latitude changes.New K–Ar age dates combined with previously reported K–Ar ages give a best estimate of the age of the St. Matthew volcanic units of about 78.7 ± 0.4 Ma. The reversed–normal–reversed polarity changes are thought to represent part of the sequence seen between marine anomalies 32 and 33 that are usually assigned an age of about 74 Ma, which appears to be about 5 Ma too young.

Paleomagnetism and Microplate Tectonics

“…In the Aleutian Islands, new paleomagnetic data from early Oligocene rocks show significant clockwise rotation but little latitudinal displacement when compared to North America [ Harbert , 1987]. This result is in agreement with other new paleomagnetic data from Upper Cretaceous units of St. Matthew Island in the Bering Sea [ Wittbrodt et al , 1989]. Deformation and clockwise block rotation along the Aleutian arc massif also have been observed using reflection seismic methods [ Geist et al , 1988; Ryan and Scholl , 1989] and are consistent with the paleomagnetic results.…”