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Sedimentary rock units originally formed in subduction trenches are often found tectonically underplated directly below magmatic arc crustal sections along some segments of the ancient convergent margin of the North American Cordillera. During and immediately after tectonic underplating - which takes place during ultra-shallow subduction - magmatic arcs shut off completely or migrate suddenly inboard, thus leaving the underplated sections in the new forearc of the subduction system. A good modern equivalent is found in Southern Mexico where the Cocos plate subducts at shallow angle under North America. The process is episodic and corresponds to events of sudden trench inboard migration relative to the upper plate. If the trench sequence was dominated by quartz-rich material, the exposed rocks are schists; they display an inverse pressure-temperature path, suggesting that the crust collapsed and were exhumed immediately after the completion of this ablative process (Salinas type). If rich in feldspar, the trench-derived metasedimentary rocks are gneisses and display evidence for thermal relaxation-related heating and in some cases, partial melting (Skagit type). Feldspar-rich rocks presumably have a higher strength that precludes a quick gravitational collapse of the section. In both cases, this process leads to the complete reorganization of the crust with the addition of melt fertile, first cycle sedimentary materials in the deep crust of subduction systems. |
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