THE GEOMETRY OF THE METHOW BASIN IN NORTH-CENTRAL WASHINGTON VIA A GRAVITY AND MAGNETIC ANALYSIS

The Methow Basin (MB) is on of several basins on the eastern slope of the Cascades in Washington. It is the northernmost basin and extends partially into British Columbia. The basin 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 that overlies an unknown thickness of Jurassic forearc 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 of basin including extension, wrench-faulting, strike-slip faulting and compression that 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 is part of a long term effort to collect and analyze of gravity and magnetic data within and surrounding the basin. The collection of gravity data is hindered by the lack of roads due to wilderness areas in the basin, so data have been slowly collected along trails in the wilderness. The preliminary analysis of the gravity and magnetic data included the constructed a series of anomaly maps using a variety of techniques including wavelength filtering, and horizontal and vertical derivatives. The map analysis of the data indicates that gravity anomaly due to the basin is small in amplitude and lateral extent. A residual gravity anomaly map clearly indicates a small amplitude gravity minimum in the central portion of the basin which implies that the underlying metamorphic and igneous lithologies have a relatively low density. The southern end of the basin is dominated by a gravity and magnetic maximum that is probably caused by mafic material within the Methow Terrane. The geometry of the basin will be estimated using Euler deconvolution and 2.5 dimensional forward modeling of both the gravity and magnetic data.