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

R. D. Kurtz; J. M. DeLaurier; J. C. Gupta

doi:10.1038/321596a0

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

Canadian Journal of Earth Sciences ◽

10.1139/e92-052 ◽

1992 ◽

Vol 29 (4) ◽

pp. 601-608 ◽

Cited By ~ 2

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.

Download Full-text

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

Canadian Journal of Earth Sciences ◽

10.1139/e87-004 ◽

1987 ◽

Vol 24 (1) ◽

pp. 31-51 ◽

Cited By ~ 173

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.

Download Full-text

Lithoprobe, southern Vancouver Island: Seismic reflection sees through Wrangellia to the Juan de Fuca plate

Geology ◽

10.1130/0091-7613(1985)13<759:lsvisr>2.0.co;2 ◽

1985 ◽

Vol 13 (11) ◽

pp. 759 ◽

Cited By ~ 41

Author(s):

C. J. Yorath ◽

A. G. Green ◽

R. M. Clowes ◽

A. Sutherland Brown ◽

M. T. Brandon ◽

...

Keyword(s):

Seismic Reflection ◽

Vancouver Island ◽

Juan De Fuca Plate ◽

Juan De Fuca

Download Full-text

Structure and Tectonics of the Continental Slope West of Southern Vancouver Island

Canadian Journal of Earth Sciences ◽

10.1139/e74-112 ◽

1974 ◽

Vol 11 (9) ◽

pp. 1187-1199 ◽

Cited By ~ 11

Author(s):

Sandra M. Barr

Keyword(s):

Continental Slope ◽

Vancouver Island ◽

Juan De Fuca Ridge ◽

Spreading Center ◽

Source Depth ◽

Northern Margin ◽

Juan De Fuca Plate ◽

Juan De Fuca ◽

Diffuse Zone ◽

Oceanic Basement

The lower continental slope west of southern Vancouver Island consists of a series of ridges formed by folding, faulting, and uplift of Cascadia Basin deposits; underlying oceanic basement, at least initially, is not involved in this deformation. The middle and upper continental slope has probably formed by the same process, combined with deposition of overlying material coming directly from the continent. This compressive deformation is postulated to be a result of underthrusting of the America Plate by the Juan de Fuca Plate. Linear magnetic anomalies produced at the Juan de Fuca spreading center can be traced under the slope for at least 40 km, further evidence for underthrusting. Anomaly source depth calculations indicate that oceanic basement dips beneath the continental slope at an angle of more than 10°. A diffuse zone of earthquake epicenters extending northeast from the northern tip of Juan de Fuca Ridge may mark the present northern margin of the Juan de Fuca Plate.

Download Full-text

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

Canadian Journal of Earth Sciences ◽

10.1139/e17-028 ◽

1997 ◽

Vol 34 (3) ◽

pp. 317-335 ◽

Cited By ~ 34

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.

Download Full-text

The Vancouver Island Seismic Project: a co-CRUST onshore–offshore study of a convergent margin

Canadian Journal of Earth Sciences ◽

10.1139/e83-065 ◽

1983 ◽

Vol 20 (5) ◽

pp. 719-741 ◽

Cited By ~ 25

Author(s):

R. M. Ellis ◽

G. D. Spence ◽

R. M. Clowes ◽

D. A. Waldron ◽

I. F. Jones ◽

...

Keyword(s):

Velocity Structure ◽

Deep Ocean ◽

Crustal Thickness ◽

Vancouver Island ◽

Seismic Structure ◽

Secondary Phases ◽

Juan De Fuca Plate ◽

Deep Crust ◽

Air Gun ◽

Juan De Fuca

The seismic structure of the British Columbia continental margin has been investigated using four reversed refraction profiles. The profile across strike extended 350 km from the volcanic arc on the continent to the deep ocean of the Juan de Fuca Plate; the three profiles along strike were located on Vancouver Island, on the continental shelf, and in the deep ocean on the Juan de Fuca Plate. Interpretation of the profile along Vancouver Island yields a well constrained model for the upper crust with velocity increasing from ~5.3 km/s at the surface to ~6.4 km/s at 2 km depth to ~6.75 km/s at 15.5 km depth where the velocity increases sharply to ~7 km/s. The velocity structure of the deep crust and the crustal thickness are poorly constrained. Four possible velocity functions, based on ambiguous first arrivals and (or) secondary phases interpreted as Moho reflections, are presented. The preferred one includes a deep crustal low velocity zone with a crustal thickness of 37 km; models with a constant 7.1 km/s deep crust require thicknesses of 52 km. Preliminary results from the profile across strike show the dip of the basement towards the continent steepens from approximately 1.4° immediately west of the continental rise to ~4° beneath the rise. Sediment velocities increase as the sedimentary layer thickens towards the shelf. The Moho, with velocity near 8 km/s, appears to dip at similar angles in this region; the dip is ~6° from the edge of the shelf to the central portion of Vancouver Island; here there is an abrupt thickening of the continental crust by about 10 km with a flat-lying Moho to the east. This suggests a contact between subducting oceanic Moho and continental Moho. A small positive velocity gradient is required in the mantle.Two short reflection lines, one using explosives and the other a large air gun fired in an inlet, were recorded on a land-based multichannel reflection system. These were run to test the feasibility of obtaining coherent reflections to upper mantle depths in this complex geological environment, and of acquiring deep reflection data in coastal areas with an air-gun source. The preliminary explosion section showed reflections near 4.4 and 6.8 s. The depths of these reflections correspond closely to the 15.5 km crustal refractor and the top of the subducting oceanic lithosphere, respectively. Dip on the deeper reflector is close to that estimated from the refraction profile. Without stacking or velocity filtering, the air-gun recordings on a line adjacent to the explosion profile show arrivals of energy at similar times.

Download Full-text