HR: 0800h
AN: T21B-0522    [Abstracts]
TI: Global and Local Variations in Slab Depth Beneath arc Volcanoes
AU: * Syracuse, E M
EM: syracuse@bu.edu
AF: Boston University, Department of Earth Sciences, 685 Commonwealth Ave., Boston, MA 02215 United States
AU: Abers, G A
EM: abers@bu.edu
AF: Boston University, Department of Earth Sciences, 685 Commonwealth Ave., Boston, MA 02215 United States
AB: It is commonly accepted that the volcanic front at subduction zones occurs above where the slab reaches depths of 100-120 km. To test this assertion, we reanalyzed a variety of teleseismic and local seismicity catalogs for global variation, and have found that the actual range in slab depths beneath the arc front is from 60 to 225 km, with an average depth of 110 km. We have compiled data for 33,000 km of the global arc system, incorporating depths and other subduction parameters for 732 arc volcanoes. Slab dips vary globally from 20 to 70 degrees, convergence velocities range from 10 to 140 km per Ma, not including back-arc spreading, and incoming seafloor ages vary from 0 to 160 Ma. With the exception of some arcs with complex Quaternary histories, we have found that slab depth varies inversely with respect both to convergence velocity and to descent rate, at least when averaged along strike over arc segments, in agreement with the similar measurements of England et al. (2004). However, over short distances some arc segments exhibit abrupt variations in arc-slab geometry. The most notable cases are the 90-km increase in depth from Sumatra to Java, which occurs over an along-strike distance of 300 km, and the 60-km increase in depth from Nicaragua to Costa Rica, which occurs in 50 km. Several smaller variations occur in the Aleutians and Central America, among groups of 4-10 volcanoes. Most of these local variations correspond with offsets in the arc, not slab contortions, indicating that upper-plate tectonic processes may exert considerable control on the ascent of magmas from the mantle, at least over distances of a few hundred km. These data show that the geometry of the mantle melting region beneath volcanic arcs must vary considerably.
DE: 8150 Plate boundary--general (3040)
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting