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The tectonic significance of high-K₂O volcanism in the Sierra Nevada, California

¹ Department of Earth and Environmental Sciences, California State University–Fresno, 2345 E. San Ramon Ave., MS/MH24, Fresno, California 93720, USA
² Department of Earth Science, University of California–Santa Barbara, Santa Barbara, California 93106, USA

K₂O contents have long been recognized as a potential indicator of tectonic processes, and in the Sierra Nevada, California, high-K₂O volcanism has been attributed to lithosphere root delamination. However, new data from the central Sierra suggest a very different control: K₂O concentrations can be explained by variations in the degree of partial melting in the mantle, where high-K₂O volcanics are derived from low-degree partial melts of mantle lithosphere. Field evidence in the central Sierra further suggests that the pulse of high-K₂O volcanism there was synchronous with the development of a pull-apart structure along a series of right-stepping dextral transtensional faults at the onset of Walker Lane transtensional faulting. In our alternative interpretation, high-K, low-degree partial melts were tapped by the inception of transtensional stresses, recording the birth of a plate boundary. We speculate that high-K₂O lavas in the southern Sierra are similarly related to the onset of transtensional stresses, not delamination. A regional southward increase in incompatible element contents and decrease in erupted volumes are also consistent with a model for crustal thickness controls on magmatism. Depth-integrated density models show that dry mafic magmas beneath thick crust have insufficient buoyancy to erupt, but low-degree partial melts carry sufficient volatiles to allow eruption; as with K₂O, degree of partial melting, not source-region heterogeneity, controls water contents and buoyancy.

Key Words: Sierra Nevada • volcanism • Basin • Range • transtension • delamination • lithosphere

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