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Geology of Mount Shasta

Figure 4

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Figure 4: Schematic cross-section of the Cascadia subduction zone at the latitude of Mount Shasta. The dip of the Gorda plate is approximated from cross-sections by Trehu and others (1995), and assumed to remain constant farther than 60 kilometers east of the surface trace of the subduction zone. Water released from the subducting Gorda plate (blue stipple) promotes partial melting of asthenospheric mantle beneath North America to produce buoyant mafic magmas (purple stipple). Some of these magmas, in turn, rise into the North American crust where they either reach the surface or interact with crustal rocks to produce andesitic and dacitic magmas (pink stipple).

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