Evolving Subduction Zones in the Western United States, as Interpreted from Igneous Rocks

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Science  19 Nov 1971:
Vol. 174, Issue 4011, pp. 821-825
DOI: 10.1126/science.174.4011.821


Variations in the ratio of K2O to SiO2 in andesitic rocks suggest early and middle Cenozoic subduction beneath the western United States along two subparallel imbricate zones dipping about 20 degrees eastward. The western zone emerged at the continental margin, but the eastern zone was entirely beneath the continental plate. Mesozoic subduction apparently occurred along a single steeper zone.