SUBDUCTION EROSION: ITS ROLE IN ANDEAN ARC MAGMATISM, CRUSTAL GROWTH AND MANTLE EVOLUTION

Subduction erosion, which occurs at all convergent plate boundaries, is an important subduction-factory related process for chemical recycling, responsible globally for the transport of ~1.7 Armstrong Units (1 AU=1 km³/yr) of crust back into the mantle. Rates of subduction erosion, which range up to ≥440 km³/km/my, vary spatially and temporally as a function of convergence rates, subduction angle, sediment supply to the trench, and subduction of buoyant features such as seamount chains and oceanic spreading ridges. The paucity of pre-Neoproterozoic blueschists suggests that global rates of subduction erosion were probably greater in the remote past, perhaps due to higher plate convergence rates.

Crust removed from the forearc wedge by subduction erosion may be underplated below the wedge (relamination), or carried deeper into the mantle source of arc magmas and either melted in bulk to produce andesites or incorporated into the mantle source of arc basalts by dehydration of the subducted slab and the transport of their soluble components into the overlying mantle wedge. Along the Andean convergent plate margin, where Rutland (1971; Nature 233: 252-255) first proposed subduction erosion, a strong case can be made for the distinctive crustal isotopic characteristics of some Andean olivine-bearing basalts (⁸⁷Sr/⁸⁶Sr ≥0.7050; ε_Nd ≤-2; ε_Hf ≤+2) to be due to incorporation into the sub-arc mantle wedge of a small quantity of subducted crustal components. However, most subducted crust and sediment is transported deeper into the mantle and neither underplated below the forearc wedge nor incorporated in Andean arc magmas. Rates of subduction erosion are estimated for northern Chile at 50–70 km³/km/my since ~150 Ma, central Chile at 115 km³/km/my since ~30 Ma and southernmost Chile 30–35 km³/km/my since ~15 Ma. In central and southern Chile these rates exceed Pleistocene to Holocene extrusion rates, suggesting that here the Andean crust is shrinking due to subduction erosion.