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  • 2010-2014  (4)
  • 1
    Publication Date: 2014-04-03
    Description: The Tualatin basin, west of Portland (Oregon, USA), coincides with a 110 mGal gravity low along the Puget-Willamette lowland. New gravity measurements (n = 3000) reveal a three-dimensional (3-D) subsurface geometry suggesting early development as a fault-bounded pull-apart basin. A strong northwest-trending gravity gradient coincides with the Gales Creek fault, which forms the southwestern boundary of the Tualatin basin. Faults along the northeastern margin in the Portland Hills and the northeast-trending Sherwood fault along the southeastern basin margin are also associated with gravity gradients, but of smaller magnitude. The gravity low reflects the large density contrast between basin fill and the mafic crust of the Siletz terrane composing basement. Inversions of gravity data indicate that the Tualatin basin is ~6 km deep, therefore 6 times deeper than the 1 km maximum depth of the Miocene Columba River Basalt Group (CRBG) in the basin, implying that the basin contains several kilometers of low-density pre-CRBG sediments and so formed primarily before the 15 Ma emplacement of the CRBG. The shape of the basin and the location of parallel, linear basin-bounding faults along the southwest and northeast margins suggest that the Tualatin basin originated as a pull-apart rhombochasm. Pre-CRBG extension in the Tualatin basin is consistent with an episode of late Eocene extension documented elsewhere in the Coast Ranges. The present fold and thrust geometry of the Tualatin basin, the result of Neogene compression, is superimposed on the ancestral pull-apart basin. The present 3-D basin geometry may imply stronger ground shaking along basin edges, particularly along the concealed northeast edge of the Tualatin basin beneath the greater Portland area.
    Electronic ISSN: 1553-040X
    Topics: Geosciences
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  • 2
    Publication Date: 2012-08-01
    Description: Magnetic anomalies provide insights into the tectonic implications of a swarm of ~1500 shallow (~1 km deep) earthquakes that occurred in 2009 on the Hanford site, Washington. Epicenters were concentrated in a 2 km 2 area near Wooded Island in the Columbia River. The largest earthquake ( M  3.0) had first motions consistent with slip on a northwest-striking reverse fault. The swarm was accompanied by 35 mm of vertical surface deformation, seen in satellite interferometry (InSAR), interpreted to be caused by ~50 mm of slip on a northwest-striking reverse fault and associated bedding-plane fault in the underlying Columbia River Basalt Group (CRBG). A magnetic anomaly over exposed CRBG at Yakima Ridge 40 km northwest of Wooded Island extends southeastward beyond the ridge to the Columbia River, suggesting that the Yakima Ridge anticline and its associated thrust fault extend southeastward in the subsurface. In map view, the concealed anticline passes through the earthquake swarm and lies parallel to reverse faults determined from first motions and InSAR data. A forward model of the magnetic anomaly near Wooded Island is consistent with uplift of concealed CRBG, with the top surface 〈200 m below the surface. The earthquake swarm and the thrust and bedding-plane faults modeled from interferometry all fall within the northeastern limb of the faulted anticline. Although fluids may be responsible for triggering the Wooded Island earthquake swarm, the seismic and aseismic deformation are consistent with regional-scale tectonic compression across the concealed Yakima Ridge anticline.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 3
    Publication Date: 2012-08-01
    Description: We present evidence that the Seattle fault zone of Washington State extends to the west edge of the Puget Lowland and is kinematically linked to active faults that border the Olympic Massif, including the Saddle Mountain deformation zone. Newly acquired high-resolution seismic reflection and marine magnetic data suggest that the Seattle fault zone extends west beyond the Seattle Basin to form a 〉100-km-long active fault zone. We provide evidence for a strain transfer zone, expressed as a broad set of faults and folds connecting the Seattle and Saddle Mountain deformation zones near Hood Canal. This connection provides an explanation for the apparent synchroneity of M7 earthquakes on the two fault systems ~1100 yr ago. We redefine the boundary of the Tacoma Basin to include the previously termed Dewatto basin and show that the Tacoma fault, the southern part of which is a backthrust of the Seattle fault zone, links with a previously unidentified fault along the western margin of the Seattle uplift. We model this north-south fault, termed the Dewatto fault, along the western margin of the Seattle uplift as a low-angle thrust that initiated with exhumation of the Olympic Massif and today accommodates north-directed motion. The Tacoma and Dewatto faults likely control both the southern and western boundaries of the Seattle uplift. The inferred strain transfer zone linking the Seattle fault zone and Saddle Mountain deformation zone defines the northern margin of the Tacoma Basin, and the Saddle Mountain deformation zone forms the northwestern boundary of the Tacoma Basin. Our observations and model suggest that the western portions of the Seattle fault zone and Tacoma fault are complex, require temporal variations in principal strain directions, and cannot be modeled as a simple thrust and/or backthrust system.
    Electronic ISSN: 1553-040X
    Topics: Geosciences
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  • 4
    Publication Date: 2012-08-01
    Description: Magnetic anomalies provide insights into the tectonic implications of a swarm of approximately 1500 shallow ( approximately 1 km deep) earthquakes that occurred in 2009 on the Hanford site, Washington. Epicenters were concentrated in a 2 km (super 2) area near Wooded Island in the Columbia River. The largest earthquake (M 3.0) had first motions consistent with slip on a northwest-striking reverse fault. The swarm was accompanied by 35 mm of vertical surface deformation, seen in satellite interferometry (InSAR), interpreted to be caused by approximately 50 mm of slip on a northwest-striking reverse fault and associated bedding-plane fault in the underlying Columbia River Basalt Group (CRBG). A magnetic anomaly over exposed CRBG at Yakima Ridge 40 km northwest of Wooded Island extends southeastward beyond the ridge to the Columbia River, suggesting that the Yakima Ridge anticline and its associated thrust fault extend southeastward in the subsurface. In map view, the concealed anticline passes through the earthquake swarm and lies parallel to reverse faults determined from first motions and InSAR data. A forward model of the magnetic anomaly near Wooded Island is consistent with uplift of concealed CRBG, with the top surface 〈200 m below the surface. The earthquake swarm and the thrust and bedding-plane faults modeled from interferometry all fall within the northeastern limb of the faulted anticline. Although fluids may be responsible for triggering the Wooded Island earthquake swarm, the seismic and aseismic deformation are consistent with regional-scale tectonic compression across the concealed Yakima Ridge anticline.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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