Crustal structure of the Cascadia subduction zone, southwestern British Columbia, from potential field and seismic studies

1997 ◽  
Vol 34 (3) ◽  
pp. 317-335 ◽  
Author(s):  
Ron M. Clowes ◽  
David J. Baird ◽  
Sonya A. Dehler
Keyword(s):  
Subduction Zone ◽  
Potential Field ◽  
Vancouver Island ◽  
Strike Slip ◽  
High Density ◽  
Pacific Rim ◽  
Cascadia Subduction Zone ◽  
Crustal Material ◽  
Juan De Fuca Plate ◽  
Juan De Fuca

The northern Cascadia subduction zone is a region of convergence between the oceanic Explorer and northern Juan de Fuca plates and the continental North American plate. Potential field and new seismic reflection data coupled with previous seismic results and geology enable derivation of a series of density – magnetic susceptibility cross sections and a block density model from the ocean basin to the volcanic arc from 2.5- and 3-dimensional interpretations. The lateral extent and thickness of the accreted wedge vary significantly along the zone. The narrow, metasedimentary Pacific Rim terrane lies immediately west of Wrangellia and extends the length of Vancouver Island, consistent with its emplacement by strike-slip faulting following the accretion of Wrangellia. The ophiolitic Crescent terrane is a narrow slice lying seaward of the Pacific Rim terrane but not extending northward beyond the Juan de Fuca plate. In this region, the Crescent terrane was emplaced in a strike-slip or obliquely convergent style during the latter stages of emplacement of Pacific Rim terrane. Below the accreted terranes, the Explorer plate is shallower than Juan de Fuca plate, resulting in a thinner crust. High-density lower crustal material lies beneath the western edge of Vancouver Island, supporting interpretations of wide-scale underplating of Wrangellia. The shape of the boundary region between Wrangellia and the Coast belt to the east varies along strike and may be controlled by properties of preexisting plutonic rocks. The low-density Coast belt plutons extend throughout most of the crust and are underlain by a lowermost crustal high-density layer, which may be indicative of fractionation accompanying magma generation.

scholarly journals Amphibious surface-wave phase-velocity measurements of the Cascadia subduction zone

2019 ◽  
Vol 217 (3) ◽  
pp. 1929-1948 ◽  
Author(s):  
Helen A Janiszewski ◽  
James B Gaherty ◽  
Geoffrey A Abers ◽  
Haiying Gao ◽  
Zachary C Eilon
Keyword(s):  
Phase Velocity ◽  
Subduction Zone ◽  
Upper Mantle ◽  
Seismic Data ◽  
Cascadia Subduction Zone ◽  
Crust And Upper Mantle ◽  
Juan De Fuca Plate ◽  
Wave Phase Velocity ◽  
Juan De Fuca ◽  
S Period

SUMMARY A new amphibious seismic data set from the Cascadia subduction zone is used to characterize the lithosphere structure from the Juan de Fuca ridge to the Cascades backarc. These seismic data are allowing the imaging of an entire tectonic plate from its creation at the ridge through the onset of the subduction to beyond the volcanic arc, along the entire strike of the Cascadia subduction zone. We develop a tilt and compliance correction procedure for ocean-bottom seismometers that employs automated quality control to calculate robust station noise properties. To elucidate crust and upper-mantle structure, we present shoreline-crossing Rayleigh-wave phase-velocity maps for the Cascadia subduction zone, calculated from earthquake data from 20 to 160 s period and from ambient-noise correlations from 9 to 20 s period. We interpret the phase-velocity maps in terms of the tectonics associated with the Juan de Fuca plate history and the Cascadia subduction system. We find that thermal oceanic plate cooling models cannot explain velocity anomalies observed beneath the Juan de Fuca plate. Instead, they may be explained by a ≤1 per cent partial melt region beneath the ridge and are spatially collocated with patches of hydration and increased faulting in the crust and upper mantle near the deformation front. In the forearc, slow velocities appear to be more prevalent in areas that experienced high slip in past Cascadia megathrust earthquakes and generally occur updip of the highest-density tremor regions and locations of intraplate earthquakes. Beneath the volcanic arc, the slowest phase velocities correlate with regions of highest magma production volume.

Upper mantle structure of the northern Cascadia subduction zone

1995 ◽  
Vol 32 (1) ◽  
pp. 1-12 ◽  
Author(s):  
M. G. Bostock ◽  
J. C. Vandecar
Keyword(s):  
British Columbia ◽  
Subduction Zone ◽  
Upper Mantle ◽  
Puget Sound ◽  
Cascadia Subduction Zone ◽  
Mantle Structure ◽  
Data Set ◽  
Juan De Fuca Plate ◽  
Upper Mantle Structure ◽  
Juan De Fuca

Previous knowledge of the structure of the Cascadia subduction zone north of the Canada–United States border has been derived from a variety of geophysical studies that accurately delineated the downgoing Juan de Fuca plate from the offshore deformation front to depths of ~50–60 km beneath south-central Vancouver Island and the Georgia Strait. Little is known, however, of the structure of the Cascadia subduction zone farther westward and to greater depths in the upper mantle. We have assembled a set of some 1100 teleseismic traveltimes from events recorded on the Western Canadian Telemetered Network to augment a previously existing data set recorded on the Washington Regional Seismograph Network. The composite data set is inverted for upper mantle structure below Washington, Oregon, and southwestern British Columbia. We analyze the new northern portion of the model between 48.5–50°N and 118–127°W, which provides the first images of the deep slab structure in this region. The model is parameterized using splines under tension over a dense grid of knots. The nonlinearity of the inverse problem is treated by iteratively performing three-dimensional ray tracing and linear inversion. Resolution tests performed with a synthetic slab model indicate that the deep structure is resolved by the data north to at least 50°N. The inversions are characterized by a quasi-planar, high-velocity body inferred to represent the thermal and compositional anomaly of the subducted Juan de Fuca plate. This body exhibits velocity deviations of up to 3% from the background reference model and extends to depths of at least 400–500 km. The depth contours of the slab in the upper mantle mimic those of the shallow slab by changing strike, in the latitude range 48.0–48.5°N, from north–south in Washington to northwest–southeast in southern British Columbia. This forces the development of two arch-type structures: a main arch observed in previous studies trending east–west over Puget Sound and a possible second arch extending northeasterly from the Georgia Strait into the British Columbia interior. A steepening of the deep slab dip from British Columbia south towards Puget Sound and complexity in the evolution of the arches in depth may be the result of a change in plate motions at 3.5 Ma associated with the detachment of the Explorer plate.

Summary of Coastal Geologic Evidence for past Great Earthquakes at the Cascadia Subduction Zone

1995 ◽  
Vol 11 (1) ◽  
pp. 1-18 ◽  
Author(s):  
Brian F. Atwater ◽  
Alan R. Nelson ◽  
John J. Clague ◽  
Gary A. Carver ◽  
David K. Yamaguchi ◽  
...  
Keyword(s):  
British Columbia ◽  
North America ◽  
Subduction Zone ◽  
Forest Soils ◽  
Pacific Coast ◽  
Cascadia Subduction Zone ◽  
Level Change ◽  
The Past ◽  
The Pacific ◽  
Juan De Fuca

Earthquakes in the past few thousand years have left signs of land-level change, tsunamis, and shaking along the Pacific coast at the Cascadia subduction zone. Sudden lowering of land accounts for many of the buried marsh and forest soils at estuaries between southern British Columbia and northern California. Sand layers on some of these soils imply that tsunamis were triggered by some of the events that lowered the land. Liquefaction features show that inland shaking accompanied sudden coastal subsidence at the Washington-Oregon border about 300 years ago. The combined evidence for subsidence, tsunamis, and shaking shows that earthquakes of magnitude 8 or larger have occurred on the boundary between the overriding North America plate and the downgoing Juan de Fuca and Gorda plates. Intervals between the earthquakes are poorly known because of uncertainties about the number and ages of the earthquakes. Current estimates for individual intervals at specific coastal sites range from a few centuries to about one thousand years.

An assessment of the megathrust earthquake potential of the Cascadia subduction zone

1988 ◽  
Vol 25 (6) ◽  
pp. 844-852 ◽  
Author(s):  
Garry C. Rogers
Keyword(s):  
United States ◽  
Subduction Zone ◽  
Subduction Zones ◽  
Oceanic Lithosphere ◽  
The United States ◽  
Cascadia Subduction Zone ◽  
The South ◽  
The North ◽  
The Pacific ◽  
Juan De Fuca

The active tectonic setting of the southwest coast of Canada and the Pacific northwest coast of the United states is dominated by the Cascadia subduction zone. The zone can be divided into four segments where oceanic lithosphere is converging independently with the North American plate: the Winona and the Explorer segments in the north, the larger Juan de Fuca segment that extends into both Canada and the United States, and the Gorda segment in the south. The oceanic lithosphere entering the Cascadia subduction zone in all segments is extremely young, less than 10 Ma. Of the other six zones around the Pacific where young (< 20 Ma) lithosphere is being subducted, five have had major thrust earthquakes (megathrust events) on the subduction interface in historic time. An estimation based on potential area of rupture gives maximum possible earthquake magnitudes along the Cascadia subducting margin of 8.2 for the Winona segment, 8.5 for the Explorer segment, 9.1 for the Juan de Fuca segment, and 8.3 for the South Gorda segment. Repeat times for maximum earthquakes, based on the ratios of seismic slip to total slip observed in other subduction zones, are predicted to be up to several hundred years for each segment, well beyond recorded history of the west coast, which began about 1800. Thus the lack of historical seismicity information provides a few constraints on the assessment of the seismic potential of the subduction zone.

Monitoring temporal change in conductivity in the central Vancouver Island region, an area with past large earthquakes

1992 ◽  
Vol 29 (4) ◽  
pp. 601-608 ◽  
Author(s):  
D. R. Auld ◽  
S. E. Dosso ◽  
D. W. Oldenburg ◽  
L. K. Law
Keyword(s):  
Temporal Change ◽  
Vancouver Island ◽  
Crust And Upper Mantle ◽  
Juan De Fuca Plate ◽  
Juan De Fuca ◽  
Elevation Changes ◽  
Robust Processing ◽  
Processing Techniques ◽  
Large Earthquakes

Two major earthquakes, magnitude 7.0 in 1918 and magnitude 7.3 in 1946, have occurred this century in the central region of Vancouver Island, British Columbia, Canada. Levelling data in the region indicate relative uplift of 4 mm/year from 1977 to 1984, followed by subsidence at approximately the same rate over the next 2 years. In response to the observed elevation changes, a program was initiated to investigate if temporal changes in the geoelectrical conductivity might be associated with earthquake occurrence. Beginning in 1986, magnetotelluric (MT) data have been measured annually at a number of sites on central Vancouver Island to monitor the long-term variability of the conductivity of the crust and upper mantle in the region. Robust processing techniques now used in the analysis of MT data enhance the possibility of detecting changes in the conductivity.Past studies involving the monitoring of MT stations have considered temporal change only in terms of the measured responses. However, formulating the inverse problem of constructing conductivity–depth models that vary minimally from year to year allows quantitative investigation of the changes required in the models to accommodate the yearly variations in the data. This provides a method of evaluating the processes and depths involved in observed changes in the data. Our modelling study indicates a small but systematic yearly decrease in conductivity from 1987 to 1990 localized in a conductive zone overlying the subducting Juan de Fuca Plate.

The northern Cascadia subduction zone at Vancouver Island: seismic structure and tectonic history

1990 ◽  
Vol 27 (3) ◽  
pp. 313-329 ◽  
Author(s):  
R. D. Hyndman ◽  
C. J. Yorath ◽  
R. M. Clowes ◽  
E. E. Davis
Keyword(s):  
Continental Shelf ◽  
Subduction Zone ◽  
Deep Ocean ◽  
Vancouver Island ◽  
Ocean Basin ◽  
Cascadia Subduction Zone ◽  
Accretionary Wedge ◽  
Seismic Structure ◽  
Tectonic History ◽  
Wide Range

The structure and Tertiary tectonic history of the northern Cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the deep sea, combined with adjacent land multichannel seismic data and results from a wide range of other geophysical and geological studies. The top of the downgoing oceanic crust is imaged for a remarkable distance downdip from the deep ocean basin to a depth of 40 km beneath Vancouver Island. The reflection depths are in good agreement with seismic refraction models and Benioff–Wadati seismicity. Two broad reflective bands imaged as dipping gently landward at depths of about 15 and 30 km on the land lines merge to a single reflector band offshore. They may represent underplated oceanic material or, alternatively, they may not be structural but may be zones of contrasting physical properties, perhaps representing trapped fluid. Two narrow terranes, the Mesozoic marine sedimentary Pacific Rim Terrane and the Eocene marine volcanic Crescent Terrane, have been thrust beneath, and accreted to, the margin in the Eocene, about 42 Ma, near the start of the present phase of subduction. They provide a landward-dipping backstop to the large sediment wedge accreted since that time. The deformation front is characterized by mainly landward-dipping thrust faults that cut close to basement. This result and the mass balance of the incoming sediment compared with that present in the accreted wedge suggest that there is little subduction of sediment into the mantle. The Tofino Basin sediments, up to 4 km in thickness, have been deposited on the continental shelf over the accreted terranes and the developing accretionary wedge.

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

1987 ◽  
Vol 24 (1) ◽  
pp. 31-51 ◽  
Author(s):  
R. M. Clowes ◽  
M. T. Brandon ◽  
A. G. Green ◽  
C. J. Yorath ◽  
A. Sutherland Brown ◽  
...  
Keyword(s):  
Large Scale ◽  
Sedimentary Rocks ◽  
Indirect Evidence ◽  
Vancouver Island ◽  
Late Eocene ◽  
San Juan ◽  
Juan De Fuca Plate ◽  
Reflective Layer ◽  
Juan De Fuca ◽  
Accreted Terranes

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.

Biostratigraphic, strontium isotopic, and geologic constraints on the landward movement and fragmentation of terranes within the Tofino Basin, British Columbia

2012 ◽  
Vol 49 (7) ◽  
pp. 819-856 ◽  
Author(s):  
Marjorie J. Johns ◽  
Julie A. Trotter ◽  
Christopher R. Barnes ◽  
Y. Roshni Narayan
Keyword(s):  
Late Eocene ◽  
Pacific Rim ◽  
Magnetic Data ◽  
Juan De Fuca Plate ◽  
Early Oligocene ◽  
The Pacific ◽  
Juan De Fuca ◽  
History Of ◽  
Oceanic Sediment ◽  
Middle Pliocene

Significant advancements in understanding the complex evolution of the Tofino Basin at a convergent accretionary margin are enabled by combining contextual geologic information with new isotopic and paleontological data. A high-resolution Cenozoic chronostratigraphy of the basin is constrained by strontium isotope ages (36.9–1.3 Ma) of Late Eocene to Pleistocene foraminifers together with a revised biostratigraphy (foraminifers and ichthyoliths) from six offshore wells and outcrop samples, new specimen thermal alteration values, and existing well log data. These data are integrated with archival multichannel seismic and magnetic data to interpret offshore well positions with relation to sub-basins and structural highs of the Pacific Rim and Crescent terranes, and other accreted strata. Six regions of the Tofino Basin are defined based on structure and depositional differences during the Eocene to Holocene history of accretion and fragmentation of the Crescent terrane and it underthrusting the Pacific Rim terrane. Subsequent oceanic sediment accretions and deposition of overlying sediments up to about 4000 m thick resulted as the Juan de Fuca plate subducted beneath Vancouver Island. Observations include different fragmentations and landward movements of the Crescent and Pacific Rim terranes in the regions and two fault styles in the Ucluelet and Carmanah regions where six new sub-basins are defined. Results, especially for the Ucluelet and Carmanah sub-basins, indicate periods of deformation during the Late Eocene, Late Oligocene, Middle–Late Miocene, and post middle Pliocene, whereas the Early Oligocene and Early Miocene had periods of relatively slow and less disturbed deposition.

Sign in / Sign up

Export Citation Format

Share Document