Tectonic setting of the Portland-Vancouver area, Oregon and Washington: Constraints from low-altitude aeromagnetic data

Blakely, Richard J.; Wells, Ray E.; Yelin, Thomas S.; Madin, Ian P.; Beeson, Marvin H.

doi:10.1130/0016-7606(1995)107<1051:tsotpv>2.3.co;2

Cited by 34 publications

(46 citation statements)

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“…The late Miocene and younger Portland sedimentary basin is believed to be a ϳ30 km by ϳ80 km pull-apart basin between two of these northwest-trending fault zones ( Fig. 1) (Beeson et al, 1985;Yelin and Patton, 1991;Blakely et al, 1995). On the basin's southwest edge lie the nearly parallel Portland Hills and East Bank faults (Fig.…”

Section: Geologic Settingmentioning

confidence: 96%

“…1). The former is inferred from geologic and topographic evidence to be a southwest-dipping, transpressional fault along the linear edge of the Portland Hills (Balsillie and Benson, 1971;Beeson et al, 1991;Swanson et al, 1993;Blakely et al, 1995). The East Bank fault lies beneath Portland basin sediments 2 to 4 km northeast of the Portland Hills fault and is interpreted to be a northeast-dipping reverse fault (Beeson et al, 1991, Blakely et al, 1995.…”

Section: Geologic Settingmentioning

confidence: 96%

“…The former is inferred from geologic and topographic evidence to be a southwest-dipping, transpressional fault along the linear edge of the Portland Hills (Balsillie and Benson, 1971;Beeson et al, 1991;Swanson et al, 1993;Blakely et al, 1995). The East Bank fault lies beneath Portland basin sediments 2 to 4 km northeast of the Portland Hills fault and is interpreted to be a northeast-dipping reverse fault (Beeson et al, 1991, Blakely et al, 1995. The East Bank fault was originally interpreted from a ϳ200-m step in the top of Miocene volcanic rocks, but modeling of aeromagnetic data suggests there is a vertical displacement of 1 km or more on the base of the volcanic rocks (Blakely et al, 1995).…”

Section: Geologic Settingmentioning

confidence: 97%

“…1) (Nelson and Personius, 1996), local crustal earthquakes may cause stronger shaking in the Portland-Vancouver urban area (e.g., Wong et al, 1993). Shallow faults (Ͻ1 km depth) are mapped or are inferred from geophysical data to lie directly beneath the Portland-Vancouver urban area (Beeson et al, 1989(Beeson et al, , 1991Blakely et al, 1995). The seismic potential of these faults is uncertain, although the Portland Hills fault is interpreted to cut shallow strata of late Miocene to Pliocene age (Beeson et al, 1991).…”

Section: Introductionmentioning

confidence: 98%

“…Shallow strata can amplify seismic waves because of low impedance, resonance effects caused by strong reflectors Figure 1. Aeromagnetic map of Portland area (Blakely et al, 1995) showing inferred faults (heavy dashed lines), faults interpreted from seismic profiles (short-dash lines), and locations of our seismic profiles (heavy black lines for land profiles, thin lines for river profiles). The heavy red lines show the approximate location of the ancestral Columbia River paleochannel delineated by the seismic reflection profiles and drillholes.…”

Section: Introductionmentioning

confidence: 99%

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Late Pleistocene and Holocene Tectonics of the Portland Basin, Oregon and Washington, from High-Resolution Seismic Profiling

Pratt

Odum

Stephenson

et al. 2001

Bulletin of the Seismological Society of America

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High-resolution seismic reflection profiles were acquired to aid earthquake hazard assessment in the Portland-Vancouver urban area of Oregon and Washington, western North America. The profiles show (1) a strong reflector at the base of unconsolidated deposits; (2) the ancestral Columbia River channel where it has eroded into the unconformity at the base of the unconsolidated deposits; and (3) evidence consistent with late Pleistocene or Holocene faulting. The seismic data consist of marine profiles along 40 km segments of the Columbia and Willamette Rivers and two 1.5-km-long land profiles across the East Bank and Portland Hills fault zones. The marine profiles show a strong reflector as deep as 85 m that correlates with the unconformity at the base of unconsolidated, late Pleistocene and Holocene sediments penetrated in nearby drillholes. A ϳ1.5-km-wide, up to 85-m-deep paleochannel filled by unconsolidated sediments marks the course of the ancestral Columbia River. Apparent vertical displacements of late Pleistocene or Holocene reflectors at the East Bank fault are consistent with recent faulting. The Portland Hills fault zone also shows what could be late Pleistocene to Holocene deformation, but other interpretations of these features are possible. No obvious faulting of the late Pleistocene unconformity is observed on our profiles across the inferred location of the Frontal Fault zone. The strong reflection from the unconformity and a large contrast in measured S-wave velocities between the unconsolidated sediments (ϳ250 m/sec) and the underlying strata (477 to 817 m/sec) indicates the shallow layer could amplify and trap seismic energy during an earthquake. These results indicate the East Bank and Portland Hills faults may represent significant seismic hazards to the PortlandVancouver urban area and emphasize that further characterization of the shallow strata is crucial to estimating the shaking potential at sites above the Portland basin.

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Section: Geologic Settingmentioning

confidence: 96%

Section: Geologic Settingmentioning

confidence: 96%

Section: Geologic Settingmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

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Late Pleistocene and Holocene Tectonics of the Portland Basin, Oregon and Washington, from High-Resolution Seismic Profiling

Pratt

Odum

Stephenson

et al. 2001

Bulletin of the Seismological Society of America

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Evidence for distributed clockwise rotation of the crust in the northwestern United States from fault geometries and focal mechanisms

et al. 2017

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Paleomagnetic and GPS data indicate that Washington and Oregon have rotated clockwise for the past 16 Myr. Late Cenozoic and Quaternary fault geometries, seismicity lineaments, and focal mechanisms provide evidence that this rotation is accommodated by north directed thrusting and right‐lateral strike‐slip faulting in Washington, and SW to W directed normal faulting and right‐lateral strike‐slip faulting to the east. Several curvilinear NW to NNW trending high‐angle strike‐slip faults and seismicity lineaments in Washington and NW Oregon define a geologic pole (117.7°W, 47.9°N) of rotation relative to North America. Many faults and focal mechanisms throughout northwestern U.S. and southwestern British Columbia have orientations consistent with this geologic pole as do GPS surface velocities corrected for elastic Cascadia subduction zone coupling. Large Quaternary normal faults radial to the geologic pole, which appear to accommodate crustal rotation via crustal extension, are widespread and can be found along the Lewis and Clark zone in Montana, within the Centennial fault system north of the Snake River Plain in Idaho and Montana, to the west of the Wasatch Front in Utah, and within the northern Basin and Range in Oregon and Nevada. Distributed strike‐slip faults are most prominent in western Washington and Oregon and may serve to transfer slip between faults throughout the northwestern U.S.

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Crustal Magnetism

Purucker

2015

Treatise on Geophysics

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Tectonic setting of the Portland-Vancouver area, Oregon and Washington: Constraints from low-altitude aeromagnetic data

Cited by 34 publications

References 0 publications

Late Pleistocene and Holocene Tectonics of the Portland Basin, Oregon and Washington, from High-Resolution Seismic Profiling

Late Pleistocene and Holocene Tectonics of the Portland Basin, Oregon and Washington, from High-Resolution Seismic Profiling

Evidence for distributed clockwise rotation of the crust in the northwestern United States from fault geometries and focal mechanisms

Crustal Magnetism

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