Connecting the Yakima fold and thrust belt to active faults in the Puget Lowland, Washington

Abstract

[1] High-resolution aeromagnetic surveys of the Cascade Range and Yakima fold and thrust belt (YFTB), Washington, provide insights on tectonic connections between forearc and back-arc regions of the Cascadia convergent margin. Magnetic surveys were measured at a nominal altitude of 250 m above terrain and along flight lines spaced 400 m apart. Upper crustal rocks in this region have diverse magnetic properties, ranging from highly magnetic rocks of the Miocene Columbia River Basalt Group to weakly magnetic sedimentary rocks of various ages. These distinctive magnetic properties permit mapping of important faults and folds from exposures to covered areas. Magnetic lineaments correspond with mapped Quaternary faults and with scarps identified in lidar (light detection and ranging) topographic data and aerial photography. A two-dimensional model of the northwest striking Umtanum Ridge fault zone, based on magnetic and gravity data and constrained by geologic mapping and three deep wells, suggests that thrust faults extend through the Tertiary section and into underlying pre-Tertiary basement. Excavation of two trenches across a prominent scarp at the base of Umtanum Ridge uncovered evidence for bending moment faulting possibly caused by a blind thrust. Using aeromagnetic, gravity, and paleoseismic evidence, we postulate possible tectonic connections between the YFTB in eastern Washington and active faults of the Puget Lowland. We suggest that faults and folds of Umtanum Ridge extend northwestward through the Cascade Range and merge with the Southern Whidbey Island and Seattle faults near Snoqualmie Pass 35 km east of Seattle. Recent earthquakes (M W ≤ 5.3) suggest that this confluence of faults may be seismically active today.

Contemporary Seismicity in and around the Yakima Fold-and-Thrust Belt in Eastern Washington

“…Miocene volcaniclastic and continental sedimentary interbeds are variably found within the flows, and Miocene and more recent fluvial, glacial, and Missoula flood deposits (12-14 ka ago) overlay the CRB (Reidel et al, 2002;Blakely et al, 2011). Regional deformation was probably active prior to and throughout CRB emplacement (Blakely et al, 2011). Numerous faults are found in the YFTB, with reverse faults typically found low on the flanks and bending-moment normal faults on the crests of anticlines (Yeats, 2009).…”

“…Regional lore suggests swarms occur preferentially in the synclines. Recently both Blakely et al (2011) and Wicks et al (2011) suggest that modern seismicity is a response to active regional deformation and speculate that the Wooded Island earthquake swarm manifests ongoing folding in the YFTB, involving buckling of competent layers and slip along less competent bedding planes of the Columbia River basalts that blanket the region.…”

“…These questions must be answered for big picture studies that use contemporary seismicity patterns to delineate major block boundaries (e.g., McCaffrey et al, 2007). The occurrence of the vigorous 2009 Wooded Island earthquake swarm beneath the Hanford nuclear reservation (Wicks et al, 2011), the completion of a multidisciplinary geophysical and geological study of the active tectonics of the Yakima foldand-thrust belt (YFTB) (Blakely et al, 2011), and a heightened interest in the seismic hazard within the Columbia basin (Wong et al, 2008;Yeats, 2009) also motivated this study (Fig. 1).…”

“…A basement of Miocene, Oligocene, and Eocene sedimentary rocks (Ohanapecosh, Fifes Peak, and Wenatchee formations) and pre-Tertiary rocks consisting of sedimentary, metamorphic, and granitic rocks underlies the CRB Campbell and Reidel, 1994;Burns et al, 2011). Miocene volcaniclastic and continental sedimentary interbeds are variably found within the flows, and Miocene and more recent fluvial, glacial, and Missoula flood deposits (12-14 ka ago) overlay the CRB (Reidel et al, 2002;Blakely et al, 2011). Regional deformation was probably active prior to and throughout CRB emplacement (Blakely et al, 2011).…”

“…east-west and northwest-southeast trending anticlinal ridges and wide synclinal valleys deforming part of the Columbia River basalt group (CRB) (Reidel et al, 1989;Blakely et al, 2011) (Fig. 1).…”