Subducting Juan De Fuca Plate Research Articles

Two‐dimensional magnetotelluric inversion of the EMSLAB Lincoln Line

Two‐dimensional, Backus‐Gilbert inversion of the EMSLAB land magnetotelluric (MT) data along the 200‐km‐long Lincoln Line has yielded optimally smooth geoelectric sections. Inversions were performed on the apparent resistivity and impedance phase data approximating the transverse magnetic (TM) mode. The land portion of the Lincoln Line traverses the edge of the North American plate that is being underthrust by the Juan de Fuca plate system. The inversion reveals three centralized conductive zones in the depth range of 20–40 km. A slightly conducting (<100 S) zone is centered at 30–35 km depth under the Oregon Coast Range; this feature may be the top of the subducting Juan de Fuca plate since there is complementary evidence here from Consortium for Continental Reflection Profiling seismic data. A prominent conduction zone of several hundred Siemens (S) is also detected at 30–35 km depth under the very resistive (>1000 ohm m) Western Cascades. Here the depth is too shallow for the zone to be the subducting plate. There is also evidence for a highly conducting (>1000 S) lower crust east of the High Cascades on the east end of the Lincoln Line. Two vertical conductive regions are also exposed in the inversion model. One occurs at 70–80 km from the coast under the Willamette Valley where a postulated Eocene trench may have left a suture zone. The second region is coincident with surface hydrothermal activity along the Western‐High Cascades boundary. There are ample sources of water in the crust, e.g., in subducted sediments, from dehydration reactions along the upper plate boundary, and in volcanic arc magmas, to lead us to believe that hot, saline water is the major source of the conductive occurrences along the Lincoln Line. However, the various zones appear to be distinct, and the water may be trapped by different mechanisms.

Cenozoic active margin and shallow Cascades structure: COCORP results from western Oregon

Two deep seismic reflection profiles totaling 98 km were recorded by the Consortium for Continental Reflection Profiling (COCORP) in western Oregon in 1984, providing some of the first onshore deep seismic reflection data from the forearc and arc of an active convergent margin. These data are of lower quality than most COCORP data, but when combined with other geological and geophysical data, they provide some useful insights into the subduction zone beneath Oregon and into extensional structures in the Cascades. Line 1 crossed the Eocene sediments and underlying pillow basalts of the Coast Range and western Willamette Valley. Reflections from the Willamette Valley clearly define the lower and western boundary of flat-lying, rhythmically bedded Eocene sediments. These sediments appear to be underlain by as much as 8 km of seismically transparent Eocene pillow basalts. Layered reflections at depths of 8-16 km may indicate a remnant crust upon which the Eocene pillow basalts were erupted. East-dipping reflections at depths of 35-40 km may represent the decollement above the subducting Juan de Fuca plate. Oregon line 2 crosses the Cenozoic volcanic arc terrains of the Western and High Cascades. Interpretation of COCORP seismic data suggests modification of existing models for normal faulting that postulate a symmetric graben or grabens in the Cascades of Oregon. In the High Cascades along COCORP Oregon line 2, reflections suggest a large, gently west-dipping half-graben with major offset only on the west side of the range. The seismic reflection data along with other geological and geophysical data suggest that the High Cascades were built on a series of blocks, rather than a single graben. The blocks were differentially faulted during the Pliocene. Seismic data from the Western Cascades indicate several normal faults with down-to-the-east offset, including a major, previously unidentified fault of probable Miocene age.

Subduction of the Juan de Fuca Plate: Thermal consequences

Terrestrial heat flux was measured in fjords, in boreholes, and in offshore wells at sites across the convergent margin of southwestern British Columbia from the continental shelf landward to the Garibaldi Volcanic Belt. Temperatures in the offshore wells were corrected for drilling disturbances, and formation thermal conductivities were modeled using measurements on cuttings and downhole geophysical logs. Marine measurements in the fjords were corrected for the large effects of refraction as well as aperiodic temperature variations in the bottom waters. There was excellent agreement between marine measurements and those from nearby onshore boreholes. The heat flux above the subducting Juan de Fuca plate steadily decreases landward from over 50 mW m−2 on the shelf to 25 mW m−2 seaward of the Garibaldi Volcanic Belt. An abrupt increase to 80 mW m−2 over a distance of 20 km is centered 30 km seaward of the volcanic zone. Very large variations in heat flux occur locally within the Pleistocene volcanic area, the result of advective cooling of intrusive magmas. The measured heat generation of crustal samples along the entire profile is low, 0.6–0.8 μW m−3. A landward dipping, seismically reflective zone above the subducting oceanic plate beneath Vancouver Island appears to be nearly profile is low, 0.6–0.8 μW m−3. A landward dipping, seismically reflective zone above the subducting oceanic plate beneath Vancouver Island appears to be nearly isothermal. It is postulated that dehydration of the subducting oceanic crust at and above approximately 450°C absorbs heat and produces water which flows updip along this zone in the overlying subduction complex, effectively redistributing the heat seaward to where the water is reabsorbed in hydration processes. A relatively cool crustal wedge lies above the deeper subducting oceanic crust, and at its thick, landward side the abrupt increase in surficial heat flux must be caused by a shallow (10 km depth) heat source produced by ascending magma.

Tomographic image of the Juan de Fuca Plate beneath Washington and western Oregon using teleseismic P‐wave travel times

Backprojection tomography is applied to teleseismic P‐wave travel‐time residuals to produce a detailed image of the upper mantle beneath Washington and western Oregon. Beneath the Cascades volcanic arc from central Oregon to central Washington the subducted Juan de Fuca plate is imaged as a high‐velocity quasi‐planar feature dipping ∼65°E. Resolution analysis shows that beneath southern Washington, where resolution is best, the steeply dipping slab extends to a depth of only about 300 km. The slab may end near this depth, or low‐velocity structure in the vicinity of the slab or a shallowing of the slab dip may produce this result. Beneath Oregon, where resolution is poor, the slab extends steeply into the mantle to at least 150 km depth. The uppermost mantle beneath northeastern Washington is high in velocity, preventing the resolution of the slab geometry there.

Automated editing and true-amplitude stacking of seismic data

Lateral changes in recording conditions often require that trace amplitudes be balanced, with consequent loss of information on the lateral amplitude variation of reflected energy. We present a quality-control and automated editing algorithm that recognizes source and geophone coupling problems and different noise levels along the survey line. Problem traces are discarded, and true-amplitude stacking of the remaining ones is possible with constant scaling factors. Application of the algorithm to one of the Vancouver Island LITHOPROBE profiles gives a nominal signal-to-noise improvement of 15 dB and a better understanding of recording problems in the field. Our analysis shows that the varying strength of a reflection from near the top of the subducting Juan de Fuca plate cannot be explained by changing recording conditions alone. Results suggest that the extra effort involved in the automated optimization of common midpoint stacks of low signal-to-noise deep seismic data is warranted only if lateral amplitude information is to be preserved.

Distribution of Late Cenozoic volcanic vents in the Cascade range: Volcanic arc segmentation and regional tectonic considerations

Spatial, temporal, and compositional distributions of approximately 4000 volcanic vents formed since 16 Ma in Washington, Oregon, northern California, and northwestern Nevada illustrate the evolution of volcanism related to subduction of the Juan de Fuca plate system and extension of the Basin and Range province. Vent data were obtained from published map compilations and include monogenetic and small polygenetic volcanoes in addition to major composite centers. On the basis of the distribution of 2821 vents formed since 5 Ma, the Cascade Range is divided into five segments, with vents of the High Lava Plains along the northern margin of the Basin and Range province in Oregon forming a sixth segment. Some aspects of the Cascade Range segmentation can be related to gross structural features of the subducting Juan de Fuca plate. The orientation of the volcanic front of segments one and two changes from NW in northern Washington to NE in southern Washington, paralleling the strike of the subducting Juan de Fuca plate. Segments one and two are separated by a 90‐km volcanic gap between Mount Rainier and Glacier Peak that is landward of the portion of the subducting plate having the least average dip to a depth of 60 km. A narrow, N‐S trending belt of predominantly andesitic vents in Oregon constitutes a third segment, which is landward of the seismically quiet portion of the subduction zone. The narrowness of this segment may indicate steep dip of the subducting plate beneath the Cascade arc in Oregon. Vents are sparse between segment four (containing the Mount Shasta and Medicine Lake centers) and segment five (containing Lassen Peak), where the Juan de Fuca and Gorda North plates are characterized by differing age, amounts of subcrustal seismicity, and probably geometry. From the relation between seismicity at depth of 60 km and the position of the volcanic front of vents formed since 5 Ma, transitions between subducting‐plate segments of varying geometry likely occur near boundaries between independently defined volcanic segments in northern Oregon and northern California. In the Basin and Range province east of the Cascade arc, volcanism migrated into the region adjacent to the Cascade Range during the interval 5–10 Ma. Since 5 Ma, the impingement of the two provinces is characterized by cessation of basin‐range volcanism in southern Oregon, continuation of basaltic volcanism in northeastern California where the impingement process may not yet be complete, and contraction of the area of mafic volcanism around Mount Shasta, Medicine Lake, and Lassen Peak. In central Oregon where the northern margin of basin‐range volcanism (the High Lava Plains) intersects the Cascade arc, impingement of basin‐range extensional volcanism approximately coincides in time and space with the development of the High Cascade graben between Three Sisters and Mount Jefferson.

Geometry of the Juan de Fuca plate beneath Washington and northern Oregon from seismicity

Abstract Earthquake hypocenters within the subducting Juan de Fuca plate beneath Washington and northern Oregon are interpreted as showing that the direction of plate dip changes from northeast beneath the Puget Sound region to east-southeast beneath southwestern Washington. The shallowest hypocenters within the Juan de Fuca plate are between 30- to 40-km depth, and the distribution of these events strikes north-northeast from near the mouth of the Columbia River to the northern Olympic Mountains. The distribution of hypocenters between 40 to 50 km generally strikes parallel with the shallowest events, but shows a significant broadening beneath the eastern Olympic Mountains and Puget Sound. Events with depths greater than 50 km south of the 1965 Seattle earthquake (mb = 6.5) strike north-northeast, approximately parallel with the shallower distributions; however, north of this event, the distribution of these deeper hypocenters strikes northwest. This change in the distribution of earthquake hypocenters reflects an upward arching of the Juan de Fuca plate plate beneath Puget Sound compared with the depth of the plate beneath southwestern Washington. The T axis calculated for the 1949 South Puget Sound earthquake (MS = 7.1) is oriented to the southeast, and the 20° plunge of the T axis is in good agreement with the plate dip angle determined from the earthquake hypocenters. We conclude that the 1949 earthquake resulted at least in part from down-dip tensional forces within the subducting Juan de Fuca plate. One consequence of the change in the direction of plate dip is that volcanic front in Washington is everywhere perpendicular to the dip of the Juan de Fuca plate.

Results of a magnetotelluric traverse across western Oregon: crustal resistivity structure and the subduction of the Juan de Fuca plate

As part of project EMSLAB, we have collected and analysed wideband magnetotelluric data along an east-west transect in western Oregon. Preliminary modelling of the data using one-dimensional inversions based upon rotationally-invariant earth response functions was followed by finite-element two-dimensional modelling. The models produced indicate the presence of an electrical conductor beneath the Oregon Coast Range dipping eastward at 12–18° from a depth of 23–32 km. We believe that this conductor includes the thrust surface of the subducting Juan de Fuca plate and/or adjacent water-saturated rocks. Its high conductance (about 200 S) is thought to be due to one or more of the following mechanisms: (1) sediments subducted atop and with the Juan de Fuca plate, (2) saline fluids produced by dehydration of the former, or (3) seawater contained within subducted oceanic basalts. There is a distinct possibility that the high conductivity is due primarily to the presence of subducted sediments, in contrast with the notion that the subduction of young, buoyant lithosphere retards sediment subduction at this convergent margin. The conductive layer is overlain by relatively resistive rocks presumed to be accreted oceanic lithosphere. Model-determined resistivities for the upper part of the Coast Range section are in good agreement with deep well-log data. A strong electrical contrast appears in the determinant phase pseudosection between the Coast Range and the Willamette Valley suggesting a structural boundary between the two provinces. A surficial conductor is present in the valley to depths of 1–2 km and is due to alluvial fill. Induction arrow data show the geomagnetic coast effect and a smaller effect by the Willamette Valley alluvial fill.

LITHOPROBE—southern Vancouver Island: Cenozoic subduction complex imaged by deep seismic reflections

The LITHOPROBE seismic reflection project on Vancouver Island was designed to study the large-scale structure of several accreted terranes exposed on the island and to determine the geometry and structural characteristics of the subducting Juan de Fuca plate. In this paper, we interpret two LITHOPROBE profiles from southernmost Vancouver Island that were shot across three important terrane-bounding faults—Leech River, San Juan, and Survey Mountain—to determine their subsurface geometry and relationship to deeper structures associated with modem subduction.The structure beneath the island can be divided into an upper crustal region, consisting of several accreted terranes, and a deeper region that represents a landward extension of the modern offshore subduction complex. In the upper region, the Survey Mountain and Leech River faults are imaged as northeast-dipping thrusts that separate Wrangellia, a large Mesozoic–Paleozoic terrane, from two smaller accreted terranes: the Leech River schist, Mesozoic rocks that were metamorphosed in the Late Eocene; and the Metchosin Formation, a Lower Eocene basalt and gabbro unit. The Leech River fault, which was clearly imaged on both profiles, dips 35–45 °northeast and extends to about 10 km depth. The Survey Mountain fault lies parallel to and above the Leech River fault and extends to similar depths. The San Juan fault, the western continuation of the Survey Mountain fault, was not imaged, although indirect evidence suggests that it also is a thrust fault. These faults accommodated the Late Eocene amalgamation of the Leech River and Metchosin terranes along the southern perimeter of Wrangellia. Thereafter, these terranes acted as a relatively coherent lid for a younger subduction complex that has formed during the modem (40 Ma to present) convergent regime.Within this subduction complex, the LITHOPROBE profiles show three prominent bands of differing reflectivity that dip gently northeast. These bands represent regionally extensive layers lying beneath the lid of older accreted terranes. We interpret them as having formed by underplating of oceanic materials beneath the leading edge of an overriding continental place. The upper reflective layer can be projected updip to the south, where it is exposed in the Olympic Mountains as the Core rocks, an uplifted Cenozoic subduction complex composed dominantly of accreted marine sedimentary rocks. A middle zone of low reflectivity is not exposed at the surface, but results from an adjacent refraction survey indicate it is probably composed of relatively high velocity materials (~ 7.7 km/s). We consider two possibilities for the origin of this zone: (1) a detached slab of oceanic lithosphere accreted during an episodic tectonic event or (2) an imbricated package of mafic rocks derived by continuous accretion from the top of the subducting oceanic crust. The lower reflective layer is similar in reflection character to the upper layer and, therefore, is also interpreted as consisting dominantly of accreted marine sedimentary rocks. It represents the active zone of decoupling between the overriding and underthrusting plates and, thus, delimits present accretionary processes occurring directly above the descending Juan de Fuca plate. These results provide the first direct evidence for the process of subduction underplating or subcretion and illustrate a process that is probably important in the evolution and growth of continents.

A magnetotelluric sounding across Vancouver Island detects the subducting Juan de Fuca plate

Magnetotelluric1 (MT) and geomagnetic2 depth-sounding measurements have been made at 18 sites (7 with remote reference3) across Vancouver Island (Fig. 1), beneath which the Juan de Fuca plate is underthrusting4,5. Vancouver Island is part of the old accretionary Wrangellia terrane6 and is located in the central portion of a 350-km forearc region that extends from the sediment-filled trench7 to the Garibaldi volcanic belt8. Gravity9 and seismic refraction studies10 suggest that the descending plate has a shallow northeasterly dip of 8°–16° with perhaps higher dips beneath northeastern Vancouver Island. A high-resolution seismic reflection survey11 along lines 1 and 3 (Fig. 1) located a zone of strong acoustic reflections (the E-horizon11) near the top of the subducting Juan de Fuca plate at depths of 23–34 km. Our model of the broad-band MT data defines a sloping, highly conducting zone also near the top of the subducting plate at the same depths as the seismic E-horizon. The high electrical conductivity in this zone (the E-conductor) is the result of saline fluid within the pore spaces of sedimentary and mafic materials of the upper oceanic crust that have been subducted to those depths beneath Vancouver Island. These are the first data which clearly define a dipping conductive layer associated with the boundary region between converging plates; they have significant implications for thrust earthquakes and metamorphic reactions that occur in subduction zones.

Upper mantle structure from teleseismic P wave arrivals in Washington and northern Oregon

Teleseismic P wave travel time residuals are used to detect lateral velocity heterogeneities in the upper mantle beneath Washington and northern Oregon. The results of an inversion for three‐dimensional velocity variations resolves an east dipping high‐velocity zone that we interpret as the subducting Juan de Fuca plate. The plate is characterized by 3–8% higher velocities than those in the surrounding upper mantle. Inversion of the travel time data and ray trace modeling indicate that the plate extends to a depth of 200–300 km. The plate dips at a moderate angle of 45° to the east‐northeast beneath the central Washington Cascade Range north of Mount Rainier, with 5% faster velocities than the surrounding upper mantle. Beneath the North Cascade Range of Washington, the plate strikes to the northwest and has 6–8% faster velocities than the upper mantle to the west. South of 47°N, beneath the Cascade Range in southern Washington and northern Oregon, the plate dips steeply to the east and has 3–4% faster velocities than the surrounding upper mantle. Based on changes in the geometry and velocity structure of the subducted Juan de Fuca plate east of about 123°W, we propose that the subducted slab is segmented into three sections beneath Washington and northern Oregon.

Changing concepts of geologic structure and the problem of siting nuclear reactors: Examples from Washington State

The conflict between regulation and healthy evolution of geological science has contributed to the difficulties of siting nuclear reactors. On the Columbia Plateau in Washington, but for conservative design of the Hanford reactor facility, the recognition of the little-understood Olympic-Wallowa lineament as a major, possibly still active structural alignment might have jeopardized the acceptability of the site for nuclear reactors. On the Olympic Peninsula, evolving concepts of compressive structures and their possible recent activity and the current recognition of a subducting Juan de Fuca plate and its potential for generating great earthquakes - both concepts little-considered during initial site selection - may delay final acceptance of the Satsop site. Conflicts of this sort are inevitable but can be accommodated if they are anticipated in the reactor-licensing process. More important, society should be increasing its store of geologic knowledge now, during the current recess in nuclear reactor siting.

Ridge subduction, eduction, and the neogene tectonics of southwestern north america

Subduction of a spreading ridge beneath a continent, the ridge half-spreading rate being greater than the continent-ridge convergence rate, leads to the possibility of lateral flow of mantle material beneath the continent and its emergence at the continental margin. The emergence, here termed eduction, provides a mechanism for exhumation of formerly subducted blueschists. An example is provided by the overriding of the East Pacific Rise south of the Mendocino transform fault by North America (~30 Ma ago). The relative motion between the Pacific and North American plates being in the direction of the future San Andreas Fault, subduction melange will be educted along those parts of the margin having a more northerly trend than the relative motion once the ridge is subducted. Also, as North America moves southwards over the Mendocino transform, blueschists, under-plated on North America by the subducting Juan de Fuca plate, are carried south across the transform and educted by the emerging Pacific plate without being heated by the ridge. Emergence of Pacific plate from beneath the continental margin caused small-scale rifting and fragmentation of the margin. Formation of coastal sedimentary basins correlates with the expansion of the eduction regime as the Mendocino and Rivera triple junctions migrated along the continental margin. After coastal California joined the Pacific plate (inception of movement on San Andreas fault, ~5 Ma), eduction was restricted to that portion of the margin between the north end of the San Andreas system at Point Arena and the Mendocino transform (then both 450 km southeast of their present positions relative to North America). As coastal California moves northwest it traps educted Franciscan rocks in the wedge-shaped gap between the northern part of the San Andreas Fault and the former North American coast. The boundary between the North American and Pacific plates is the vertical San Andreas Fault near the surface but becomes a shallowly east-dipping discontinuity at depth. This explains the dearth of seismicity deeper than 15 km along the trace of the San Andreas Fault, and north of its northernmost surface exposure at Point Arena. Passage of the spreading East Pacific Rise beneath North America accounts for Neogene block faulting, uplift of the Colorado Plateau, and northeastward migration of bimodal volcanism across the southwest U.S.A. Southwestward movement of North America across the Mendocino transform shuts off Cascade volcanism and lifts up the Sierra Nevada block because the American plate moves from a cool subduction regime above the steeply dipping Juan de Fuca plate to a hot, eduction regime above the shallowly lying Pacific plate.

Miocene peralkaline volcanism in west-central British Columbia — Its temporal and plate-tectonics setting

The 600-km-long Anahim volcanic belt of upper Miocene-Quaternary alkalic and peralkalic volcanic centers trends east-west along approximately lat 52°N in British Columbia, in contrast to the Miocene Pemberton volcanic belt of calc-alkalic centers and the Pliocene-Quaternary Garibaldi volcanic belt of calc-alkalic centers, which follow the northwest-trending continental margin. Anahim belt rock types range from alkali basalt and nephelinite, found as small cinder cones and flows, to oversaturated (and undersaturated) peralkalic varieties found in evolved central volcanoes and in their erosion-exposed roots. In contrast to the usual subduction-related calc-alkaline volcanism in the Pemberton and Garibaldi belts, volcanic activity in the Anahim belt has been linked with lithospheric fracturing above the northern edge of the subducted Juan de Fuca plate or interpreted as an edge effect of the subducted plate in the mantle. Available isotopic ages from the oldest centers in the Anahim belt become younger eastward at a rate of 2 to 3.3 cm/yr, suggesting that volcanic activity there may well be related to a mantle hot spot beneath British Columbia. Volcanic chemistry and isotopic composition do not distinguish between either a rift or a hot-spot setting.

Miocene Tholeiitic Basalts of Coastal Oregon and Washington and Their Relations to Coeval Basalts of the Columbia Plateau

Research Article| February 01, 1973 Miocene Tholeiitic Basalts of Coastal Oregon and Washington and Their Relations to Coeval Basalts of the Columbia Plateau PARKE D. SNAVELY, JR.; PARKE D. SNAVELY, JR. 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar NORMAN S. MACLEOD; NORMAN S. MACLEOD 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar HOLLY C. WAGNER HOLLY C. WAGNER 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information PARKE D. SNAVELY, JR. 1U.S. Geological Survey, Menlo Park, California 94025 NORMAN S. MACLEOD 1U.S. Geological Survey, Menlo Park, California 94025 HOLLY C. WAGNER 1U.S. Geological Survey, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (2): 387–424. https://doi.org/10.1130/0016-7606(1973)84<387:MTBOCO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation PARKE D. SNAVELY, NORMAN S. MACLEOD, HOLLY C. WAGNER; Miocene Tholeiitic Basalts of Coastal Oregon and Washington and Their Relations to Coeval Basalts of the Columbia Plateau. GSA Bulletin 1973;; 84 (2): 387–424. doi: https://doi.org/10.1130/0016-7606(1973)84<387:MTBOCO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement.Tholeiitic basalt flows and breccias of Miocene age in western Oregon and Washington form three distinct stratigraphic units. Each unit was erupted from coastal vents marked by dikes and sills of the same composition as associated extrusive rocks. The three coastal basalt units are interbedded with predominantly marine sedimentary rocks of middle to late Miocene age. These units are here named, from older to younger, the Depoe Bay Basalt, Cape Foulweather Basalt, and basalt of Pack Sack Lookout. The three units can be distinguished by their petrographic characteristics. The Depoe Bay Basalt is nonporphyritic; Cape Foulweather Basalt has sparse large labradorite phenocrysts; and Pack Sack basalt has labradorite phenocrysts with numerous pyroxene and glass inclusions as well as augite and olivine phenocrysts. Chemical analyses of basalts from these three units show that each has a distinct and uniform composition. The Depoe Bay Basalt is characterized by high SiO2 content; the Cape Foulweather Basalt has high content of total iron, TiO2, and P2O5; and Pack Sack basalt is marked by relatively high MgO and CaO content.The Depoe Bay Basalt, Cape Foulweather Basalt, and basalt of Pack Sack Lookout on the coast occur in the same stratigraphic order and are essentially the same ages as three basalt units that erupted on the Columbia Plateau. The plateau-derived units are the Yakima and late-Yakima petrographic types of Waters (1961) and the Pomona flow of Schmincke (1967). The virtual identity in chemical composition of the Depoe Bay Basalt and Yakima-type basalt, the Cape Foulweather Basalt and the late-Yakima–type basalt, and the Pack Sack basalt and the Pomona basalt flow indicate that each pair is consanguineous.Fissure vents for the plateau basalt are located in eastern Oregon and Washington and western Idaho more than 500 km east of the coastal vent areas. Thus, a regional mechanism of magma generation or emplacement is required. Three models of magma genesis considered in this report are: (1) partial melting of the subducted Juan de Fuca plate; (2) partial melting along a nearly horizontal shear zone at the base of the American plate; and (3) partial melting within the asthenosphere and fractionation during ascent of the magma. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.