DEVELOPMENT OF PALEOMAGNETIC AND GEOCHEMICAL REFERENCE SECTIONS IN MIOCENE COLUMBIA RIVER BASALT GROUP FLOWS ON THE COLUMBIA PLATEAU FOR STRATIGRAPHIC, STRUCTURAL, AND TECTONIC STUDIES IN THE PORTLAND METRO AREA AND COAST RANGES OF OREGON AND WASHINGTON

Some two-dozen Columbia River Basalt Group (CRBG) flows potentially serve as important stratigraphic markers in the greater Portland Metro area (PMA) and Coast Ranges (CR) of western OR and WA. Exposures are poor and only a few flows are usually present at any one locality. Identification of flow units within the CRBG, however, is essential to determining fundamental stratigraphic, structural, and tectonic information. The most reliable flow identifications are made using both paleomagnetic (PM) and geochemical (GC) data. Additional CRBG flows from the Columbia River Gorge, PMA, and CR have been found with no known correlatives in Columbia River Plateau sections. Furthermore, accurate estimates of vertical-axis rotations can only be made using well-defined PM reference directions from the same units on the stable Plateau. Selected stratigraphic sections on the Plateau (Patrick grade-25 flows, Armstrong Canyon-15 flows, Rock Creek grade-10 flows, and Buttermilk Canyon-11 flows) have been (re)sampled for both GC and PM analysis. The focus of GC characterization (XRF, ICP-MS) is on (1) development of flow unit discriminators using immobile trace and minor element ratios, and (2) application of these discriminators to moderately weathered flows in areas where variable depletion of mono- and divalent cations commonly renders conventional TiO₂-MgO data unreliable. Initial results from the CR indicate that immobile element ratios remain unaffected across a spectrum of moderate surficial weathering. Combined GC and PM data, along with known stratigraphic positions from Plateau sections, allows identification of specific CRBG flows. CRBG flows within the CR block are uniformly rotated ≈30° (102 sites) clockwise relative to their counterparts on the Plateau. East of the NW-trending en échelon Mt. Angel–Gales Creek, Portland Hills, and unnamed faults to the north, and north of the CR block's northern boundary (≈Columbia River), CW rotations abruptly drop to ≈15° (39 sites) where offsets correspond to bounding fault zones of the Portland and Willamette pull-apart basins. Agreement of vertical-axis rotation rates estimated from CRBG magnetizations with those determined from GPS velocities indicates a relatively steady rate of deformation over the last 10 to 15 Myr due to oblique subduction of the Juan de Fuca Plate.