Gravity and Structure of Continental Margin: Oregon to Southeastern Alaska: ABSTRACT

Abstract

Reconnaissance surface-ship gravity measurements over the continental margin of western North America extend from northern California to southern Alaska. The gravity measurements, spaced approximately 2.5 km apart along tracklines approximately 40 km apart, have an estimated RMS uncertainty of approximately 5 mgl. A negative free-air anomaly along the base of the continental slope is attributed to the dip of the Mohorovic discontinuity, lateral density variations in the upper mantle, and in some locations a sediment-filled trough. Off the north end of Vancouver Island a free-air anomaly greater than -150 mgl occurs over the Scott Islands fracture zone, suggesting a sediment thickness in the fracture zone of 4-6 km. Hypothetical crustal cross sections of the continental ma gin constrained by the free-air anomalies and the available seismic refraction data suggest crustal thicknesses are approximately 20 km in western Oregon and Washington, and 25-30 km in the Insular Belt of British Columbia and the Alexander Archipelago. The relatively thin crust in the region between the continental shelf and Coast Mountains of British Columbia and the Cascade Range in Oregon and Washington is characteristic of the transition from oceanic to continental structure in western North America. End_of_Article - Last_Page 432------------