HR: 09:30h
AN: T41A-07    [PDF]
TI: Some Comments on Tectonic Erosion and Sediment Subduction in Convergent Margins
AU: * Clift, P D
EM: pclift@whoi.edu
AF: Woods Hole Oceanographic Institution, MS#22, Woods Hole, MA 02543 United States
AU: Vannucchi, P
EM: paolav@geo.unifi.it
AF: Universit… di Firenze, Via La Pira, 4, Firenze, 50121 Italy
AB: Subduction plate boundaries at which tectonic erosion removes material from the overriding plate account for 57% of the total length of the global subduction system are favored where convergence rates exceed about 6 cm/yr and where the sedimentary cover is less than 1 km. Accretion conversely preferentially occurs in regions of slow orthogonal convergence ($<$7.5 cm/yr) and/or trench sediment thicknesses $>$1 km. The slope gradients and taper angle of accretionary plate margins correlates with plate convergence rates, while erosive margin slopes appear to be independent of this. Rates of trench retreat do not appear to correlate with any simple characteristic of the plate interaction, but are largely a function of the history of seamount or ridge collisions. 55% of the total continental crustal material subducted globally below forearc regions in the modern ocean comprises material removed by tectonic erosion from the over-riding plate, 45% trench sediment. Even in accretionary margins a median of only 17% of the incoming sedimentary mass is accreted over time scales of 10 my or greater, resulting in long-term net loss of continental crust along convergent margins. Average magmatic productivity in the active margins must exceed 90 km$^{3}$/my to preserve the current volume of the continental crust. Geological arguments indicate that magmatic accretion rates must be faster in oceanic arcs (91-149 km$^{3}$/my) and less in the continental arcs (27-135 km$^{3}$/my). Mass balance arguments in oceanic arcs require that their crustal thicknesses must be $<$36 km and the trench retreat rate $<$8 km/my in order to maintain long term growth. Continental growth is principally achieved through the collision of oceanic island arcs to continental margins. Although oceanic arcs are chemically distinct from continental crust, the collision process involves the loss of mafic and ultramafic lower crust and the emplacement of voluminous, high silica, light rare earth element enriched melts, transforming the net composition into something more continental in character.
DE: 1020 Composition of the crust
DE: 3025 Marine seismics (0935)
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 8105 Continental margins and sedimentary basins
DE: 8155 Plate motions--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting