Convergent Margin: Cascades
Item
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Title
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Convergent Margin: Cascades
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Description
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Convergent Margin: Cascades
Tectonic Magmatic Setting
-Volcanic arc formed due to subduction along Cascade (O-C convergence) subduction zone
-Subduction of Juan de Fuca and Gorda plate beneath North American Plate (CM Figure 1)
-plate moves 0.4 inches per year
-large fault area (over 680 miles) produces violent earthquakes (CM Figure 3)
Landforms
-no oceanic trench - terrain and accretionary wedge uplift (plus heavy river sedimentation)
-Cascade volcanic arc consists of stratovolcanos, shield volcanoes, lava domes, and cinder cones—over 20 major volcanoes (CM Figure 2)
-Cascade Range collection of volcanic island arcs, deep ocean sediments, basaltic ocean floor, and parts of old continents that were carried into the North American plate by tectonic movement. Once these materials met North America they were shoved atop the plate and erosion began once above sea level. Volcanic eruptions also covered older rock with lava and ash.
Magma Generating Process
-Linear volcanic arc forms when subducting Juan de Fuca plate is dewatered and the volatiles released into overriding North American plate lower melting temperature, allowing formation of magmas and volcanic arc along linear subduction zone (CM Figure 4)
-Volcanic rocks consist primarily of tholeiitic and high-alumina basalt and basaltic magmas erupt from shield volcanos and cinder cone after magma travels through continental crust (CM Figure 5)
-some hornblende, pyroxene andesite, and dacites indicate existence of silica rocks from stratovolcanos
Geochemistry
-Geochemical graphs and Harker diagrams of various major elements vs. silica content are characteristic of subduction zone magma generation.
-The increasing trend of incompatibles such as alkalis and decreasing trend of compatibles such as MgO and FeO occurs from melting of upper mantle and crust when subducting plate is dewatered.
-Incompatible alkalis increase due to assimilation and other melting processes that incorporate constituents of surrounding rocks. Incompatibles are released from source rock first and continental crust such as North America is rich in incompatibles from previous magma generation.
-Compatibles weight percentage will steadily decrease as the the influx of incompatible elements during melting changes the overall proportion of composition in the magma.
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Creator
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John Onorati
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