STRUCTURAL ANALYSIS OF THE METHOW BASIN, WASHINGTON USING GRAVITY AND MAGNETIC DATA

The Methow Basin (MB) is the northernmost basin along the eastern slope of the Cascades Mountain range in Washington. The basin contains Cretaceous and Jurassic strata formed in fluvial and forearc basin environments, respectively. These strata overlie allochthonous Triassic ocean crust located within the Methow Terrane. There are approximately 6 kilometers of stratigraphically determined Cretaceous fluvial and alluvial clastic and volcanic sediments that overlie an unknown thickness of Jurassic forearc sediments. The known Cretaceous edges of the northwestern-trending basin are the Pasayten fault on the east and the Ross Lake and Fraser-Straight Creek fault systems on the west. The origin of the basin is controversial and hence, there have been numerous evolutional theories 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 within the basin and this lack of geophysical studies and drill holes penetrating the basement have hindered the determination of the three-dimensional geometry of the basin and hence the tectonic origin of the basin. To remedy the lack of geophysical data, a long term effort has been initiated to collect and analyze 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 within the wilderness areas. The analysis of the gravity and magnetic data have included the constructed a series of anomaly maps using a variety of techniques including isostatic residuals, wavelength filtering, and horizontal and vertical derivatives. The map analysis of the data indicates that the 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.