Backarc-forearc links across the Oregon-Washington Cascade Range interpreted from high-resolution magnetic anomalies

Abstract:

A two-decade program of airborne data acquisition by the U.S. Geological Survey has yielded >150,000 km² of high-resolution aeromagnetic data over the Cascadia forearc and backarc of Oregon (OR) and Washington (WA). These data provide a rich tapestry of information about subsurface geology and tectonic history when interpreted in combination with geologic mapping, lidar topography, and other geophysical methods. Previous work used these data to map structural connections across the WA Cascade Range, between active faults in the Puget Lowland in the forearc to faults of the Yakima fold belt (YFB) in the backarc. Forearc-backarc linkage implies that documented dextral shear on, for example, the Southern Whidbey Island fault may transfer to the backarc, as supported by recent paleoseismic evidence for late Quaternary right-lateral slip in the YFB. Here we investigate possible links between YFB structures in the backarc and the NW-striking seismic zones and active NW-striking dextral faults in the southern WA Cascade Range and the greater Portland area, OR.

Miocene basalts of the Columbia River Basalt Group (CRBG) produce pronounced WSW-striking magnetic anomalies where deformed within the Horse Heaven Hills and Columbia Hills anticlines of the YFB, which are well mapped to the eastern margin of the Cascade Range. Boundary analysis and other interpretive techniques applied to the aeromagnetic data allow us to extend these YFB structures through the Cascade Range and into the Portland and Tualatin forearc basins, where they abut active, NW-striking dextral faults, including the Gales Creek, Oatfield, and East Bank faults. These faults, seismic zones, and related anomalies form a suite of left-stepping linears that link YFB anticlines to shorter, E-W oriented, YFB-style structures in the forearc (e.g., Grays River, Doty, and Tacoma fault zones) to form a series of restraining bends consistent with coast-parallel dextral slip. Dextral slip may be pervasive in the forearc and arc, given ~9 km of right-lateral offset of Miocene CRBG recently documented on the Gales Creek fault. The pattern is consistent with the northward motion of Oregon that increases westward. Work is in progress in the Columbia River Gorge to evaluate similar kinematic linkages within the Cascade Range.