I. Direction of maximum horizontal compression in western Oregon determined by borehole breakouts. II. Structure and tectonics of the northern Willamette Valley, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/z603r267c

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  • Elliptical borehole enlargements or "breakouts" caused by systematic spalling of a borehole wall due to regional maximum horizontal stresses were identified in 18 wells drilled in the Coast Range and Willamette Valley of western Oregon. The breakouts generally indicate a NNW to NNE orientation of maximum horizontal compression (oH[subscript max]) that agrees with the predominant direction of Gllmax determined from earthquake focal mechanisms, from post-middle Miocene structural features, and from alignments of Holocene volcanic centers in the Pacific Northwest. However, this orientation is inconsistent with the N50°E convergence between the Juan de Fuca and North American plates determined by Riddihough [1984] from Juan de Fuca plate magnetic lineations as young as 730 ka (the Brunhes-Matuyama boundary). The predominant NNW to NNE orientation of Gllmax may be due to the complex interaction of a northwestward-moving Pacific plate driving into the Gorda and Juan de Fuca plates and indirectly transmitting N-S compression across the strongly coupled Cascadia subduction zone into the overriding North American plate [Spence, 1989]. Alternatively, the predominant NNW to NNE orientation of cillmax may be due to a landward counterclockwise rotation of the direction of oHmax from N50°E compression offshore to N-S compression in the Coast Range. The northern Willamette Valley lies on the eastern flank of the broad northnortheast- trending Oregon Coast Range structural arch. Eocene to Oligocene marine sedimentary rocks crop out along the western side of the northern Willamette Valley and form a gently eastward dipping homocline. However, beneath the center of the Willamette Valley, Eocene to Oligocene strata are structurally warped up. During the Eocene several major volcanic centers subdivided the Coast Range forearc region into shallow to deep marine basins. Several such volcanic centers occur adjacent to the northern Willamette Valley and are associated with residual gravity anomaly highs and lineations. The top of basalt in the northern Willamette Valley (middle Miocene Columbia River basalt except near the valley margins) is contoured based on petroleum exploration wells, water wells, and seismic-reflection data. It is structurally downwarped to an altitude of less than -500 m just north of Woodburn. The downwarp is bounded to the south by the NE-trending Waldo Hills range-front fault and in part to the north by the NE-trending Yamhill River-Sherwood fault zone. The NW-trending Mt. Angel fault extends across the northern Willamette Valley between Mt. Angel and Woodburn and deforms middle Miocene Columbia River basalt and overlying Pliocene and Miocene fluvial and lacustrine deposits. The top of Columbia River basalt is vertically separated, NE side up, roughly 100 m based on seismic-reflection data near Woodburn, and 250+ m based on water-well data near Mt. Angel. The Mt. Angel fault is part of a NW-trending structural zone that includes the Gales Creek fault west of the Tualatin basin; however, a connection between the Gales Creek and Mt. Angel faults does not occur through Willamette River alluvial deposits. A series of small earthquakes (6 events with me = 2.0, 2.5, 2.4, 2.2, 2.4, 1.4) occurred on August 14, 22, and 23, 1990 with epicenters near the northwest end of the Mt. Angel fault. Routine locations indicate a depth of about 30 km. The preferred composite focal mechanism is a right-lateral strike-slip fault with a small normal component on a plane striking north and dipping steeply to the west. Both recent mapping of the Mt. Angel fault and the recent seismicity suggest that the Gales Creek-Mt. Angel lineament is similar to the Portland Hills-Clackamas River lineament found to the north. Together, these two lineaments may take up right-lateral strike-slip motions imposed on the upper plate by oblique subduction. Boring Lava appears to occur extensively in the subsurface of the northeastern portion of the northern Willamette Valley based on seismic data. Many of the faults in the area are interpreted to be largely caused by doming from influx of Boring magma or subsidence associated with evacuation of Boring magma. Such faults occur at Petes Mountain, at Parrett Mountain, along the Molalla River, and possibly near Curtis. The fault along the Molalla River appears to offset the Pleistocene (?) Rowland Formation 1 m (Glenn, 1965).
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