HR: 15:45h
AN: V23C-08 INVITED     [Abstracts]
TI: What Controls Space-Time Patterns of Magmatism in Western North America: Plate Tectonics, Delamination, or Convection?
AU: * Glazner, A F
EM: afg@unc.edu
AF: University of North Carolina, Dept. of Geological Sciences, CB# 3315, Chapel Hill, NC 27599-3315, United States
AB: Mesozoic and Cenozoic magmatism in western North America is commonly explained by shallowing and steepening of subduction along the west coast of North America, and progressive destruction of the subduction system by development of the San Andreas transform fault system. This hypothesis makes several specific predictions about space-time patterns of magmatism, including eastward and westward sweeps, development of slab-window magmatism, and progressive northward extinction of an ancestral Cascade arc. However, analysis of space-time patterns using the NAVDAT database indicates that these predicted patterns are curiously obscure in the magmatic record, although other unexplained patterns are strong. Animation of about 29,000 Cenozoic U.S. points from NAVDAT (www.navdat.org) demonstrates that: (1) calc- alkaline, intermediate volcanism is poorly linked to the subduction system; (2) there is little evidence for slab- window magmatism; (3) there was no ancestral Cascade arc south of Oregon until ca. 10 Ma; (4) magmatism shifted from primarily silicic to dominantly basaltic throughout the Miocene; and (5) magmatism was clearly migratory in several directions in ways that cannot be explained by plate-tectonic processes, at length scales ranging from 1000s to 10s of km. Space-time patterns that cannot be readily linked to plate-tectonic control include: (1) a silicic sweep from Montana into Nevada from 50 to 20 Ma; (2) a clockwise sweep around the Colorado Plateau from New Mexico to southern Nevada from about 30 to 15 Ma; (3) a burst of magmatism at about 16 Ma in northern Nevada, followed by outward sweeps to Yellowstone, Oregon, and the Sierra Nevada; (4) progressive encroachment of basaltic magmatism onto the Colorado Plateau, and (5) several local migrations, including from Phoenix north onto the Colorado Plateau and from the San Francisco Bay area north to the Geysers geothermal field. These migrations typically occurred at 20-50 mm/yr. Possible origins include convective upwelling related to extension, local, migrating delamination of the North American lithosphere or subducted Farallon plate, edge-driven convection at lithospheric discontinuities, and forced convection as the subducted Mendocino fracture zone scraped under North America.
UR: http:www.navdat.org
DE: 3640 Igneous petrology
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8158 Plate motions: present and recent (3040)
DE: 8178 Tectonics and magmatism
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly