PRELIMINARY GRAVITY AND MAGNETIC STUDY OF THE METHOW BASIN, WASHINGTON

The Methow Basin (MB), located on the eastern slope of the North Cascades in Washington and British Columbia, contains Cretaceous and Jurassic strata that overlies allochthonousTriassic ocean crust of the Methow Terrane. Rece The basin contains more than 6 kilometers of Cretaceous fluvial and alluvial clastic and volcanic sediments and an unknown thickness of Jurassic sediments. The Cretaceous strata are exposed in elongated structural block that is bounded by Pasayten fault on the east and the Ross Lake and Fraser-Straight Creek fault systems on the west. The evolution of the basin is complex and there have been various theories of its origin which have included an extensional origin, a wrench-fault system, a pull apart basin with significant dextral motion along the bounding faults to the most recent theory, compressional tectonics forming a broad syncline. Despite the numerous geological investigations on the MB, there have been no geophysical studies within the basin. The lack of geophysical studies and drill holes penetrating the basement hinder the determination of the three-dimensional geometry of the basin and hence the tectonic origin of the basin. The present study will collect and analyze gravity and magnetic data within the basin. The gravity and magnetic data will be analyzed via a series of anomaly maps using a variety of techniques including regional-residual anomaly separation methods, and horizontal and vertical derivatives. The data (1 km spacing), collected along profiles perpendicular to the strike of the basin, will be modeled via 2.5-D forward modeling techniques constrained by surface mapping and densities/magnetic susceptibilities of surface samples. The 2.5-D models will then be interpreted to determine the basin’s geometry and thickness and to estimate the origin of the basin.