Cascadia subduction zone
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The Cascadia subduction zone is a very long sloping fault that stretches from northern Vancouver Island to northern California.
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[edit] Geography
The zone separates the Juan de Fuca, Explorer, Gorda and the North American Plate. Here, the oceanic crust of the Pacific Ocean is pushed toward and beneath the continent at a rate of 40 mm/yr.
The width of the Cascadia subduction zone fault varies along its length, depending on the temperature of the subducted oceanic plate, which heats up as it is pushed deeper beneath the continent. As it becomes hotter and more molten, it eventually loses the ability to store mechanical stress and generates earthquakes.
[edit] Earthquakes
Because of the very large fault area, the Cascadia subduction zone can produce very large earthquakes, magnitude 9.0 or greater, if rupture occurred over its whole area. When the "locked" zone stores up energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture. Thermal and deformation studies indicate that the locked zone is fully locked for 60 kilometers downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding. (Nedimovic, et al., 2003)
In 1999, a group of Continuous Global Positioning System sites registered a brief reversal of motion of approximately 2 centimeters over a 50 kilometer by 300 kilometer area. The movement was the equivalent of a 6.7 magnitude earthquake. (Dragert, et al., 2001) The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures. (Rogers & Dragert, 2003)
The last known great earthquake in the northwest was in January of 1700, the Cascadia Earthquake. Geological evidence indicates that great earthquakes may have occurred at least seven times in the last 3,500 years, suggesting a return time of 400 to 600 years. There is also evidence of accompanying tsunamis.
A future rupture of the Cascadia Subduction Zone will cause widespread destruction throughout the Pacific Northwest.
Other similar subduction zones in the world usually have such earthquakes every 100–200 years; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip.
[edit] Another Interpretation
A different interpretation is offered by Lyatsky (1996, 2006); the summary below is taken from the latter reference. The Cascadia subduction zone off western North America is quite unusual in the Circum-Pacific region, as it lacks some of the main geological and geophysical attributes from which subduction zones are typically identified. In some conventional tectonic models, this subduction zone runs from northern California to central British Columbia; the oceanic Juan de Fuca lithospheric plate is subducting along it beneath the North American continent. Remarkably, though, at the foot of the continental slope, this subduction zone has no deep-water bathymetric trench. Although this region is earthquake-prone, no thrust earthquakes are taking place: analysis of fault-plane solutions and young geologic structures suggests strike-slip and normal faulting, apparently with north-south compression in the continental crust and extension at deeper lithospheric levels. Along the western continental margin in the U.S. and Canada, the measured modern vertical movements of different continental crustal blocks differ greatly and fail to fit any simple two-dimensional flexural model as expected at subduction zones.
Instead, steep crustal-scale faults of Mesozoic and Cenozoic ages are seen from the geological and geophysical data to continue from the continental interior far outboard into the submerged continental-margin zone offshore western Canada. The diffuse plate boundary off western Canada seems to be marked by a zone of interlocked continental, transitional and oceanic crustal blocks, tens of kilometers wide, controlled by strands of the continental Fairweather-Queen Charlotte-Wallowa fault system.
Both the southern (Gorda, off northern California) and northern (off Vancouver Island) parts of the oceanic Juan de Fuca plate have contorted magnetic stripes and diffuse earthquake seismicity patterns. Reflection and refraction seismic data show the northern Juan de Fuca plate's crust is strongly faulted and has variable thickness. But, whereas the Gorda segment of the Juan de Fuca plate is commonly recognized to be squishing and probably not subducting, oceanic-plate rigidity and normal subduction continue to be conventionally assumed for the Vancouver Island segment, where the evident oceanic-crust disruption is by far the greater.
Assuming lithospheric-plate rigidity, a plate-interaction model with right-lateral movements off Queen Charlotte Islands, a plate triple junction with sea-floor spreading off Queen Charlotte Sound, and subduction off Vancouver Island, has for three decades guided geophysical data interpretations and seismic-hazard assessments. However, in the absence of solid geological constraints geophysical interpretations are often non-unique, and combining various types of data suggests an alternative.
The WNW-ESE-trending Early Tertiary Olympic-Wallowa crustal weakness zone is known from field mapping and geophysical studies to begin at least in Idaho and run diagonally across northeastern Oregon and the entire Washington state. It is expressed on the ground as a well-known regional set of topographic lineaments and faults. From geophysical and geological studies, it seems to partly control the shallow and deep structure of the Columbia Plateau. Farther west, it separates the North and South Cascade geologic provinces. The Tertiary Juan de Fuca graben and today’s Juan de Fuca Strait, between the Olympic Peninsula and southern Vancouver Island, lie within this major structural zone; the graben contains a deep, elongated Tertiary sedimentary basin.
Fault strands of the Olympic-Wallowa structural zone are evident from geologic field mapping on the Olympic Peninsula and Vancouver Island, and they are seen from geophysical and geological evidence to continue into the Tofino Basin on the continental shelf west of Vancouver Island. Some of these deep, steep faults on the shelf controlled volcanic eruptions, as evidenced by known elongated Tertiary volcanic-rock bodies along them. These volcanic rocks are assigned to the Crescent Formation, which is thought from various lines of evidence to have been extruded in a rift setting.
Unlike offshore Washington, no subduction-related accretionary mélange of pervasively sheared or ground-up rocks has been drilled off Vancouver Island. Poorly imaged quasi-compressional structures in seismic profiles on the continental slope off Vancouver Island are probably due to sediment slumping, and many well-resolved faults are steep (quite in contrast to the east-dipping thrusts with intra-sedimentary detachments clearly imaged in seismic profiles and drilled on the continental slope off Oregon).
The Cascadia zone's subduction is often recognized to be decaying off Oregon and Washington state, and from this analysis it no longer seems to be occurring in Canada.
No significant young compressional structures are found in the rock record on Vancouver Island: big Tertiary faults, some inherited from the Mesozoic, are straight and steep; some of them disrupt seismic-reflection patterns at depth. Offshore to the west, thick crust (10 to >20 km in seismic refraction and gravity models) and the absence of magnetic stripes under in a broad zone along the continental margin suggest that the underlying crustal blocks in the Tofino and Winona basins are continental. Indicating an extensional tectonic regime, in seismic reflection sections a huge, steep, listric, NNW-SSE-trending, west-dipping normal fault separates some of these foundered blocks from the continental shelf.
Several tens of kilometers outboard to the west, the outer edge of the Winona Basin is marked by the subparallel Revere-Dellwood fault, which truncates the magnetic stripes abruptly and west of which the crust is by all parameters oceanic.
Likewise, no magnetic stripes are found in a zone several tens of kilometers wide at the base of the continental slope west of Queen Charlotte Sound. Ocean-floor volcanic mounds in that magnetically blank zone are small and scattered, their geochemistry is inconsistent with sea-floor spreading, and gravity anomalies in the free-air and isostatic maps are minor. These observations rule out the commonly assumed existence in that area of sea-floor spreading and a plate triple junction.
The broad NW-SE-trending fault system, in which the Winona and Tofino basins lie, runs past Queen Charlotte Sound and Islands, where some of its strands bound the step-like Queen Charlotte bathymetric terrace formed by a foundered continental-crust block (with a Moho up to 20 km deep). Off the Alaska panhandle, this fault system merges with the well-known, broad Fairweather fault zone which forms the North American plate boundary offshore but begins in the north inside the Alaskan continental interior. It runs for hundreds of kilometers west of southeastern Alaska. Subparallel and slightly anastomosing fault strands of the Fairweather system (also called by some workers Chichagof-Baranof), although they are partly covered by young sedimentary deposits on the ocean floor, are well imaged in seismic reflection profiles. Their southeastward continuations are the recognized strands of the Queen Charlotte fault system off British Columbia.
The Fairweather-Queen Charlotte fault zone meets the Olympic-Wallowa zone of crustal weakness on the continental shelf and slope off southern Vancouver Island.
[edit] Volcanoes
The volcanoes within the subduction zone include:
- Mount Silverthrone
- Mount Meager
- Mount Cayley
- Mount Garibaldi
- Mount Baker
- Glacier Peak
- Mount Rainier
- Mount St. Helens
- Mount Adams
- Mount Hood
- Mount Jefferson
- Three Sisters
- Newberry Volcano
- Mount Mazama
- Mount McLoughlin
- Medicine Lake Volcano
- Mount Shasta
- Lassen Peak
[edit] See also
- Cascadia
- Cascadia Earthquake
- Cascade Mountains
- Cascade Volcanic Arc
- Geology of the Pacific Northwest
- North Cascades National Park
- Plate tectonics
- Subduction zone
[edit] References
- Atwater, BF (1987). "Evidence for great Holocene earthquakes along the outer coast of Washington State". Science 236 (4804): 942-44.
- Lyatsky, H.V., 1996. Continental-Crust Structures on the Continental Margin of Western North America; Springer-Verlag, 352 p.
- Lyatsky, H.V., 2006. Frontier next door: geology and hydrocarbon assessment of sedimentary basins offshore western Canada; Recorder (Canadian Society of Exploration Geophysicists), v. 31, no. 4, p. 66-75.
- Nedimovic MR, Hyndman RD, Ramachandran K, Spence GD (2003). "Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface". Nature 424 (6947): 416-20. PMID 12879067.
- Dragert G, Wang K, James TS (2001). "A silent slip event on the deeper Cascadia subduction interface". Science 292 (5521): 1525-8. PMID 11313500.
- Rogers G, Dragert H (2003). "Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip". Science 300 (5627): 1942-3. PMID 12738870.
[edit] External links
- Giant Earthquakes Beneath Canada's West Coast. Geological Survey of Canada - Pacific Division. Retrieved on 2006-06-10.
