GRAVITY AND MAGNETIC ANALYSIS OF THE METHOW BASIN, NORTH-CENTRAL WASHINGTON

The Methow Basin (MB), located on the eastern slope of the North Cascades in Washington, contains Cretaceous and Jurassic strata that overlie allochthonousTriassic ocean crust within the Methow Terrane. 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 an 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. There have been a number of different theories on the evolution which include extension, wrench-faulting, a pull apart basin with significant dextral motion along the bounding faults to the most recent theory involving compressional tectonics which formed a broad syncline. Despite the numerous geological investigations on the MB, there have been no geophysical studies on 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 involved the collect and analysis of gravity and magnetic data within the basin. The preliminary analysis of the gravity and magnetic data included the constructed a series of anomaly maps using a variety of techniques including regional-residual anomaly separation methods, and horizontal and vertical derivatives. The preliminary map analysis of the data indicates that gravity anomaly due to the basin is small in amplitude and lateral extent. The southern end of the basin is dominated by a gravity maximum that is probably caused by mafic material within the Methow Terrane. The data collected every 1 km along available roads 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.