HR: 11:05h
AN: V41D-04    [PDF]
TI: Position and Life Span of Volcanic Arcs - What Link to Present-day Kinematic Parameters in Subduction Zones ?
AU: * Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: Max W. Schmidt, ETH, Zuerich, 8092 Switzerland
AU: Poli, S
EM: stefano.poli@unimi.it
AF: Stefano Poli, Dip. Scienze della Terra, Via Botticelli 23, Milano, 20133 Italy
AB: Since slabs can be located at depth by geophysical methods, correlations are sought between the position of volcanic arcs above slabs and kinematic subduction parameters (subduction angle, velocity, burying rate...). However, the only (rough) correlation found so far, is between arc width and subduction angle, reflecting the projection of a slab depth-interval to a surface width. Volcanic arcs result from fluid and/or melt release from the slab (which occurs more or less continuously to $>$200 km depth), in most cases in combination with mantle wedge melting. Volcanic arcs may simply form above a zone with a sufficient extent of melting in the mantle and thus dominantly reflect the thermal structure in the mantle wedge which in turn mostly depends on mantle wedge convection. We argue that on geological time scales, pathways to actual (observable) subduction zone parameters are not unique as kinematic constraints in nature are fuzzy. Subduction systems do not converge to a steady state, implying that mantle wedge convection, the resulting thermal regime, loci of metamorphic reactions (i.e. fluid/melt release from the slab), and their surface manifestations, evolve continuously. As a consequence, present day kinematic parameters are not simply linked to the geometry of the volcanic arc (e.g. position of the volcanic front). An evolution of kinematic subduction parameters and their volcanic surface expression with time is documented in back arc basins which have an average life span of 11$\pm$6 Ma. About 48% of present intraoceanic subduction zones have or have had a Neozoic back arc and therefore they are highly dynamic systems strongly diverging from a steady state system with fixed kinematic boundaries. Other geologically short lived phenomena are aseismic ridge, oceanic plateau, and mid-ocean ridge subduction which modify buoyancy forces, slab geometry, and thermal structure within a subduction zone and thus cause changes in volcanic activity and melt type. In some continental subduction zones (e.g. SVZ, Sumatra), the volcanic arc simply locates on a major fault which apparently provides pathways for melt arriving at the base of the crust. We conclude that the position and life span of volcanic arcs is controlled by thermal and flow fields that evolve over geological time scales and as a consequence, any attempt to understand volcanic arcs mainly through a correlation with present day kinematics becomes futile. Only a time-integrated kinematic analysis is, on principle, able to provide a feasible solution to the volcanic arc dilemma.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8155 Plate motions--general
DE: 8400 VOLCANOLOGY
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting