TECTONIC SETTING OF THE PORTLAND-VANCOUVER AREA, OREGON AND WASHINGTON - CONSTRAINTS FROM LOW-ALTITUDE AEROMAGNETIC DATA

Seismic activity in the Portland-Vancouver metropolitan area may be as sociated with various mapped faults that locally offset volcanic basem ent of Eocene age and younger. This volcanic basement is concealed in most places by young deposits, vegetation, and urban development. The U.S. Geological Survey conducted an aeromagnetic survey in September 1 992 to investigate the extent of these mapped faults and possibly to h elp identify other seismic and volcanic hazards in the area. The surve y was flown approximately 240 m above terrain, along flight lines spac ed 460 m apart, and over an area about 50 x 50 km. These magnetic data indicate a pronounced northwest-striking magnetic lineation east of t he Willamette River in downtown Portland associated with a fault conce aled beneath Quaternary sedimentary deposits and previously inferred f rom shallow well data. The magnetic lineation confirms the existence o f the fault and suggests that it has had a prolonged history: (1) Alth ough well data indicate <200 m of vertical offset of underlying volcan ic basement, models based on the aeromagnetic data from down-town Port land suggest reverse faulting with up to 1 km of offset deeper in the section. (2) The magnetic lineation associated with this fault extends southeast to the Clackamas River drainage, a distance of 50 km and co nsiderably beyond the mapped extent of the fault. A northwest-striking magnetic anomaly located southwest of the Tualatin Mountains correspo nds closely with another mapped fault and with mixed reverse and strik e-slip faulting during a seismic swarm (M less than or equal to 3) in 1991. We believe these and other anomalies in the aeromagnetic data re flect the Portland Hills fault zone, believed to be the southwestern b oundary of a structural basin now occupied by Portland and Vancouver. The postulated northeastern boundary of the basin, the Frontal fault z one, is also evident, although less well represented in the aeromagnet ic data. Aeromagnetic anomalies, geologic mapping, and earthquake foca l-plane solutions demonstrate a complex deformational history in the P ortland-Vancouver area since middle Miocene time that includes element s of compression, extension, and dextral slip. These complexities refl ect Portland-Van couver's unique position within a north-south transit ion in tectonic styles along the Cascadia margin, from compressional i n the north to extensional in the south.