Paleomagnetism of the Methow region, north-central Washington: structural application of paleomagnetic data in a complexly deformed, variably remagnetized terrane

Abstract

Jurassic through Paleocene rocks of the Methow–Pasayten belt were studied in order to use paleomagnetic directions to resolve the question of Cretaceous northward transport. In the end, circumstances prevented us from doing so. However, three independent studies of these rocks, summarized here, indicate that several units retain strong and stable magnetizations that are different from the present axial dipole field direction. When partly or completely corrected to paleohorizontal, these magnetizations become less dispersed, suggesting that they were acquired before at least the last stage of Late Cretaceous deformation. A pervasive northeast–southwest streak of magnetizations at various stages of structural correction indicates that at least some of the rocks were partly to completely remagnetized at different times during folding. This complex structural–remagnetization history and consequent loss of paleohorizontal prevent a simple analysis of paleolatitude during remagnetization. However, analysis of the youngest layered rocks of the Goat Peak syncline indicates that much of the remagnetization occurred when the structure was more open but still well developed. Subsequent tighter folding followed intrusion of the Fawn Peak stock. Our results are a useful case study of some of the problems that arise in studying the paleomagnetism of a complicated orogenic terrane.