HR: 1330h
AN: T32A-0904    [PDF]
TI: Fate of an Oceanic Island-arc at the Collision Zone: Insight From a Modern Case at the Izu Collision Zone, Central Japan
AU: * Aoike, K
EM: bluepond@jamstec.go.jp
AF: Center for Deep Earth Exploration, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, Kanagawa, 247-0061 Japan
AB: Arc-arc or arc-continent collision zone, which separates an arc crust into materials to be left on the earth_fs surface and to be returned to the mantle, is regarded as the final disposal place of the subduction factory product. Mass balance across the collision boundary, therefore, should have great significance in the processes of continental growth and mantle evolution. The Izu Collision Zone (ICZ) located at the conjunction of the Honshu arc and the Izu-Bonin arc (IBA), is a place of ongoing orthogonal arc-arc collision, where the middle to upper crust of the northern IBA is exposed on land as accretionary terranes. The IBA-ICZ system is much advantageous for elucidating the mass balance, because that the product is simple and fresh and the flow from manufacture to disposal is quite clear. Across arc variation of buoyancy and rheological state of the lithosphere controlled principally by geothermal gradient would regulate the regime of collision tectonics. Crust-scale accretion is taking place associated with conspicuous crustal shortening and thickening in the central ICZ where the active arc that is about 100 km wide and 20 km thick is colliding. By contrast, almost the whole arc crust is subducting, leaving the off-scraped sediments as accretionary prisms in the eastern and western areas where the inactive forearc and backarc, totally 200 km wide and averagely 14 km thick, are colliding. Based on the land geology and existing seismic structure, crustal volume of the accreted IBA is estimated for the line along the axis of the ICZ and another line passing through the Tanzawa Terrane situated eastward 30 km apart from the axis. The estimation indicates that the volume of the Tanzawa line is significantly smaller (16 %) than that of the axis, in spite of being very close. This difference is explainable, if the Philippine Sea Plate slab including expected aseismic part is accompanied with 7 km thick subducted arc crust. This result of calculation implies that the lower crust delamination is occurring beneath the Tanzawa Terrane and the major part of the central ICZ. As a consequence, survival rate of an arc crust in an orthogonal arc-arc collision zone like the ICZ can be estimated at about 70 % of the active arc and 30 % of the whole crust. Furthermore, taking a preexisting 6 km thick oceanic crust into account, the survival rate of arc magmatic products in this pattern of collision can be calculated at about 50 %.
DE: 1744 Tectonophysics
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 9320 Asia
DE: 9604 Cenozoic
SC: Tectonophysics [T]
MN: 2003 Fall Meeting