Coast Plutonic Complex

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The Coast Plutonic Complex is the single largest contiguous granite outcropping in the world, extending approximately 1,800 kilometers on the coast of British Columbia, southwestern Yukon and southeastern Alaska. It is a large batholithic complex and is built on unusual island arc fragments, oceanic plateaus and continental margin assemblages accreted between the Triassic and the Cretaceous periods.^[1]

The Coast Plutonic Complex largely forms the Coast Mountains. This is a mountain range approximately 1600 kilometer long and 200 kilometers wide extending south from the Alaska Panhandle and covering most of coastal British Columbia.^[2]

Contents 1 Coast Range Arc 2 Cascade Volcanic Arc 3 See also 4 References

[edit] Coast Range Arc

During the Jurassic-to-Eocene periods, subduction of the Farallon Plate along the new continental margin created a new volcanic arc known as the Coast Range Arc. It was among the largest continential volcanic arcs in the world, extending from the US state of Washington to the Alaska Panhandle.^[3] Rising magma from the subducting Farallon Plate created new volcanoes along the continential margin. The Coast Range Arc volcanoes have long since been eroded away, leaving the granitic rocks that intruded and cooled at depth beneath the old volcanoes. These large granitic rocks now form the Coast Plutonic Complex.

One of the major events during the Coast Range Arc was between 80 and 90 million years ago when a huge rift of the Farallon Plate occurred. This rifting created a new tectonic plate known as the Kula Plate. It is unknown why such a large rupture occurred. Some geologists believe it was some fundamental change in convection within the Earth's mantle caused the rifting event, while others believe the huge oceanic plate became mechanically unstale as it continued to subduct beneath the Pacific Northwest.^[3] The Kula Plate once again continued to subduct beneath the continential margin, supporting the Coast Range Arc.

About 60 million years ago, the Coast Range Arc subduction began to cease as the rapid movement of the Kula Plate became parallel with the Pacific Northwest, creating a transform plate boundary. During this passive plate boundary, the Kula Plate began subducting underneath Alaska and southwestern Yukon. Rising magma from the subducting Kula Plate created new volcanoes along the northern section of the Coast Range Arc. Remnants of these volcanoes exist in southwestern Yukon, such as the Bennett Lake Volcanic Complex, Montana Mountain, Mount Nansen and the Mount Skukum Volcanic Complex.

[edit] Cascade Volcanic Arc

Over the last 37 million years, the southern portion of the Coast Plutonic Complex has been poked through with volcanoes to form the Garibaldi Volcanic Belt. This a northern extension of the Cascade Volcanic Arc in the United States (which includes Mount Baker and Mount St. Helens) and contains the most explosive young volcanoes in Canada. It formed due to subduction of the Juan de Fuca, Gorda and Explorer plates at the Cascadia subduction zone.^[4] This is a 680 mi (1,094 km) long fault, running 50 mi (80 km) off the west-coast of the Pacific Northwest from northern California to Vancouver Island, British Columbia. The plates move at a relative rate of over 0.4 inches (10 mm) per year at a somewhat oblique angle to the subduction zone.

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 (about 40 miles) downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding.

Unlike most subduction zones worldwide, there is no oceanic trench present along the continental margin in Cascadia.^[5] Instead, terranes and the accretionary wedge have been uplifted to form a series of coast ranges and exotic mountains. A high rate of sedimentation from the outflow of the three major rivers (Fraser River, Columbia River, and Klamath River) which cross the Cascade Range contributes to further obscuring the presence of a trench. However, in common with most other subduction zones, the outer margin is slowly being compressed, similar to a giant spring. When the stored energy is suddenly released by slippage across the fault at irregular intervals, the Cascadia subduction zone can create very large earthquakes such as the magnitude 9 Cascadia earthquake of 1700.

The volcanoes of the Cascade Arc share some general characteristics, but each has its own unique geological traits and history. Mount Meager in British Columbia, which erupted about 2,350 years ago, is generally considered the northernmost member of the arc. A few isolated volcanic centers northwest of Mount Meager such as Silverthrone Caldera, which is a circular 20km wide, deeply dissected caldera complex, may also be the product of Cascadia subduction, but geologic investigations have been very limited in this remote region. About 5-7 million years ago, the northern end of the Juan de Fuca Plate broke off along the Nootka Fault to form the Explorer Plate, and there is no definitive consensus among geologists on the relation of the volcanoes north of that fault to the rest of the Cascade Arc. When the Cascade Volcanic Arc resumed 4-5 million years ago after reorganization of the Explorer Plate, there were some apparent changes along the northern end. Where the northern end of the arc originally extended due north from the modern-day location of Glacier Peak - into the Chilliwack Batholith and the Pemberton Volcanic Belt in Canada, it now headed northwest into the Mount Baker - Garibaldi Volcanic Belt. This apparently reflects a steepening of the subduction zone on the northern end of the Juan de Fuca Plate. At the same time, the Juan de Fuca Plate assumed a more easterly-directed sense of motion relative to the continent. However, the Pemberton Volcanic Belt is usually merged with the Garibaldi Volcanic Belt and so is usually part of the arc.^[6]

[edit] See also

• Geography of British Columbia
• Geography of the Yukon
• Geography of Alaska
• Geology of the Pacific Northwest
• Volcanism in Canada

[edit] References