EVIDENCE FOR, AND AMOUNTS OF, CONTINENTAL CRUSTAL MATERIAL RECYCLED TO THE MANTLE AT CENOZOIC SUBDUCTION ZONES

INTRODUCTION: At subduction zones (SZ) two processes govern the movement of continental material toward the mantle within the subduction channel that tectonically separates the upper and lower plates:

(1) sediment subduction--the insertion of oceanic sediment beneath the seaward edge (backstop) of the upper plate's framework of older rock and the movement of subducted material toward the mantle, and (2) subduction erosion--the detachment of rock and sediment from the upper plate and the transport of this material toward the mantle.

SUBDUCTED SEDIMENT: Good estimates of sediment subduction are available for margins except those with large (> ~40 km) accretionary prisms--these border ~20 % of all SZ. For SZ edged by small to moderate width (5-40 km) frontal prisms (80 %), the efficiency of sediment subduction is ~80-100 %. Globally, the average volume of subducted ocean floor sediment is ~1 km3/yr (solid volume), which=~30 km3/Myr for each km of oceanic SZ.

SUBDUCTION EROSION: Long-term (> ~15 Myr) crustal thinning linked to subduction erosion has been estimated for the Japan, Izu-Bonin, Mariana, Tonga, South Sandwich, Chile, Peru, Costa Rica, Guatemala, and Mexico margins. Erosion rates are based best on the occurrence beneath the outer forearc of deeply (4-5 km) submerged nearshore deposits. Because crustal scale extension is uninvolved, neritic to subaerial deposits at near trench locations can be attributed to tectonic erosion of ~10-12 km of formerly underlying crust. The global, long-term average rate of subduction erosion is estimated at ~1.5 km3/yr, or ~40 km3/Myr/km of SZ margin (range=28-62).

IMPLICATIONS: Continental debris moving down the subduction channel thus globally averages ~70 km3/Myr/km of SZ. The typical thickness of this material is ~1 km, 60 % of which is eroded upper plate material and 40 % subducted oceanic sediment. Subduction erosion supplies the greater amount, in particular where bathymetric highs are subducted. The active seismogenic zone runs through the channel, the composition of which must affect seismicity. The estimated rate of recycling is higher than that of arc productivity (~30 km3/Myr/km), which we suggest has been underestimated by a factor of 2 or 3. Eroded forearc material injected into the mantle should affect arc magmatism and its geochemical signature.