COMPLEX RIGHT-LATERAL FAULTING AT THE NORTHERN END OF THE PORTLAND BASIN: GEOLOGIC, AEROMAGNETIC, AND PALEOMAGNETIC EVIDENCE

New geologic mapping in and adjacent to the northern Portland basin, in concert with aeromagnetic and paleomagnetic studies and ⁴⁰Ar/³⁹Ar dating, is revealing new details of the complex geologic structure.

Late Eocene (c. 37-35 Ma) volcanogenic strata of the Cascade Range in southwestern Washington are regionally deformed into large-amplitude SE-plunging folds, and bedding generally dips E and S at about 20°. A few km E of the Columbia River near Kalama, WA, however, bedding swings abruptly to the NNW. Across the river in Oregon, sparse measurements indicate that strikes return to a roughly E-W orientation. This pattern suggests the Paleogene rocks are deformed into a large drag fold by right-lateral movement on a fault beneath the NNW-trending Columbia River valley. This structure is also reflected in a complex, NW-striking aeromagnetic anomaly that crosses the Columbia River about midway between Kalama and Saint Helens, OR.

South of Kalama, exposures of middle Miocene Grande Ronde Basalt are plastered against valley walls on both sides of the Columbia River. Near Saint Helens, basalt outcrops exhibiting scabland morphology poke through alluvium of the valley floor. Chemical data indicate that many of these outcrops are erosional remnants of a single Grande Ronde flow that once filled the ancestral valley. Paleomagnetic measurements from 8 sites in this flow indicate that the flat-lying rocks on the valley floor are rotated 20-30° clockwise relative to outcrops on the valley walls. We interpret the valley-floor rocks as microblocks caught up in a broad right-lateral shear couple.

The geologic and paleomagnetic results reflect deformation within a complex system of N- to NW-striking faults at the northern end of the Portland basin inferred from linear aeromagnetic anomalies. Motion on these structures is predominantly right-lateral strike-slip, although high-angle reverse offset is inferred for some of them. Collectively, these dextral oblique faults may have accommodated a significant part of the northward translation of the Oregon Coast Range crustal block relative to interior North America that is recorded by transpressive structures in the Washington Coast Range. Sparse geodetic data indicate that this motion is continuing. Thus some or all these faults should be considered as potentially active.