THE CANBY-MOLALLA FAULT, OREGON

We have conducted a variety of geologic and geophysical investigations to determine the offset on an inferred fault marked by a remarkably linear aeromagnetic anomaly near the towns of Canby and Molalla, Oregon. The aeromagnetic lineament strikes north-northwest, is 60 km long, and is on strike at its northern end (about 25 km south of Portland) with faults mapped in Columbia River basalt (CRB). We have measured four detailed ground-magnetic profiles, collected 1000 additional ground-magnetic stations, conducted a seismic-reflection profile, and compiled logs from more than 70 water wells to further define the cause of the aeromagnetic lineament.

A geologic cross-section, based on a detailed ground-magnetic profile east of Canby, suggests that the aeromagnetic lineament is caused by at least 150 m of near vertical, down-to-the-west offset of CRB, the only significant magnetic lithology near the surface in this area. This model is consistent with water wells from the area: Two wells immediately east of the fault penetrated CRB at 50 m depth, whereas CRB was not encountered in any wells (150 m maximum depth) west of the fault. In addition to vertical displacement, lateral offset of a small magnetic anomaly at the northern end of the aeromagnetic lineament indicates approximately 4 km of right-lateral displacement.

We also conducted a high-resolution seismic-reflection transect across the aeromagnetic anomaly east of Canby. The transect was 600 m in length, was acquired with 2 m shot and receiver spacings, and provided 30-fold stacked coverage. The seismic cross section shows near-vertical truncation of CRB, consistent with the ground magnetic model. Youngest deformed sediments in the footwall are probably Missoula flood deposits, raising the possibility of Holocene deformation. A small, laterally restricted berm lies within 100 m of our estimated location of the fault and also suggests young deformation.

The linearity of the aeromagnetic anomaly, its association at its northern end with mapped faults in CRB, and the abrupt offset demanded by ground-magnetic, seismic, and well data indicate that the anomaly is a 60-km-long oblique strike-slip fault. Given its proximity to Portland and Salem, additional studies are warranted to assess possible Holocene deformation.