HR: 0800h
AN: V51C-0724    [Abstracts]
TI: Crustal and Tectonic Controls on the Distribution and Composition of Large Silicic Calderas in Arc Settings
AU: * Hughes, G R
EM: gwyneth@stanford.edu
AF: Stanford University, 450 Serra Mall Braun Hall, Building 320, Stanford, CA 94305, United States
AU: Mahood, G A
EM: mahood@stanford.edu
AF: Stanford University, 450 Serra Mall Braun Hall, Building 320, Stanford, CA 94305, United States
AB: The study of silicic magmatism in arc settings is key to understanding felsic plutonism and thus how continental crust forms and persists. We undertook a global compilation of silicic calderas in arc settings in order to: (1) determine whether the distribution and composition of silicic magmatism at convergent margins is related to tectonic and crustal factors suggested by previous studies; and (2) examine the spatial relationship between silicic volcanism in arcs and the associated "main volcanic axis," defined by stratovolcanoes of intermediate composition. Geological and tectonic characteristics were compiled for 91 arc-related silicic calderas younger than 2 Ma and larger than 5 km in diameter. A measure of "caldera density" was calculated for each arc that quantified the number of silicic calderas per kilometer of arc length. In addition, an arc-normal distance was measured between each caldera and the associated main volcanic axis. There is a positive correlation between caldera density and trench-normal convergence rate, a trend likely related to magmatic flux. In addition, silicic calderas tend not to form in volcanic arcs with intense backarc extension. Composition of the caldera forming eruption is related to both the thickness and nature of the underlying crust. Thin, young or oceanic crust yields almost exclusively dacitic calderas. Rhyolitic calderas are dominantly located on continental crust thicker than 25 km and Mesozoic or older in age. Additionally, 77% of rhyolitic calderas formed under local extension, suggesting that this tectonic regime favors magmatic evolution. Calderas in general do not preferentially form at a specific distance behind the arc, but in arcs on young unevolved crust, calderas tend to occur within 10 km of the main volcanic axis; on older continental crust, calderas tend to be distributed over a wide area behind the axis. This trend is likely related to the difference in the overall width of the two types of arcs. The results of this study suggest that rate of magmatic input and the nature and stress regime of the underlying crust fundamentally control the distribution and character of silicic magmatism in arc settings.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting