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

• Published in: Geology (Boulder) 2007-10, Vol.35 (10), p.923-926
• Main Authors: Putirka, Keith, Busby, Cathy J
• Format: Article
• Language: English
• Subjects: Alpine County California; Basin and Range Province; basin range structure; basins; buoyancy; California; crust; delamination; eruptions; faults; field studies; igneous and metamorphic rocks; igneous rocks; lithosphere; magmatism; Mono County California; North America; partial melting; Petrology; potassic composition; pull-apart basins; Sierra Nevada; Sonora Pass; stress; Structural geology; tectonics; thickness; transtension; Tuolumne County California; United States; volcanic rocks; volcanism
• ISSN: 0091-7613
• DOI: 10.1130/G23914A.1

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.