Stage G - Cordilleran Mountains

Cordilleran Mountain Building
The existing subduction zone under the island arc is no longer active and the mountain on the edge of West continent has become a peneplain, but East continent and West continent are still being driven together by other forces; therefore, another subduction zone has begun to form. It could begin anywhere within the ocean basin and form another island arc, and it could dip in any direction. In this model, decoupling occurs dipping east under the edge of the East continent, forming a Cordilleran (volcanic arc) type of mountain building.
The processes of trench formation, subduction and fractional melting of the oceanic crust, melange deposition, and blueschist metamorphism are the same here as for an island arc orogeny; however, this tectonic activity occurs along an old divergent continental margin which, like all rifted margins, has accumulated a thick wedge of DCM sedimentary rocks. Thus, the rising intermediate-to-felsic batholithic magmas now inject into the thick wedge of continental margin sediments, heating them to very high-grade Barrovian metamorphism (amphibolite to granulite facies). If the sediments are limestones and quartz sandstones, the metamorphic rocks will be marbles and quartzites. Less mature sandstones and shales will form slates, phyllites, schists, and gneisses. It is also quite likely that the basement batholiths under the divergent continental margin will be metamorphosed into gneisses and migmatites.
Along with the metamorphism, the old divergent continental wedge of sediments and invading batholiths plus superposed volcanoes are uplifted along major thrust faults until they form towering mountains. The Andes in South America and the Cascades in Washington, Oregon, and northern California are mountains of this type.
Inland from the volcanic front, in the backarc region, backarc spreading occurs. Heat rising from above the subduction zone creates a small convection cell that stretches the continental crust so that normal faults develop into deep graben. Superficially this may seem like an axial rift but it forms under very different conditions and processes. 
The graben fills with a great complex of deposits including coarse clastic sediments in alluvial fans and braided rivers and intermediate to felsic volcanics rising from the subduction zone. Because the source land composition is so variable (divergent margin rocks, suture zone rocks, metamorphics, volcanics, and, when erosion is deep enough, felsic and intermediate batholithic rocks) the sediments eroded from it are rich in quartz and (many kinds of) lithics, plus lesser amounts of feldspar (sodic plagioclase and orthoclaseQFL diagram, blue field).
The volcanics in the backarc Basin begin as mafic rocks (basaltscoria, etc.), but slowly turn into intermediate (andesite), and finally felsic (rhyolite) rocks. In the latter stages/ granite dikes or stocks (small batholiths) may also invade the now mostly filled graben. 

Contributed by Lynn Fichter 

Friday, August 24, 2018
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