Arc-Continent Collisions, Sediment Recycling and the Maintenance of the Continental Crust

Subduction zones are both the source of most new continental crust and the locations where crustal material is returned to the upper mantle. Globally the total amount of continental crust and sediment subducted below forearcs currently lies close to 3.0 Armstrong Units (1 AU = 1 km3 a-1), of which 1.65 AU comprises subducted sediments and 1.33 AU tectonically eroded forearc crust, compared with an average of c. 0.4 AU lost during subduction of passive margins during Cenozoic continental collision. Margins may retreat in a wholesale, steady-state mode, or in a slower way involving the trenchward erosion of the forearc coupled with landward underplating, such as seen in the central and northern Andean margins. Tephra records of magmatism evolution from Central America indicate pulses of recycling through the roots of the arc. While this arc is in a state of long-term mass loss this is achieved in a discontinuous fashion via periods of slow tectonic erosion and even sediment accretion interrupted by catastrophic erosion events, probably caused by seamount subduction. Crustal losses into subduction zones must be balanced by arc magmatism and we estimate global average melt production rates to be 96 and 64 km3 Ma-1 km-1 in oceanic and continental arc, respectively. Critical to maintaining the volume of the continental crust is the accretion of oceanic arcs to continental passive margins. Mass balancing across the Taiwan collision zones suggests that almost 90% of the colliding Luzon Arc crust is accreted to the margin of Asia in that region. Rates of exhumation and sediment recycling indicate that the complete accretion process spans only 6-8 Ma. Subduction of sediment in both erosive and inefficient accretionary margins provides a mechanism for returning continental crust to the upper mantle. Sea level governs rates of continental erosion and thus sediment delivery to trenches, which in turn controls crustal thicknesses over 107-109 years. Tectonically thickened crust is reduced to normal values (35-38 km) over time scales of 100-200 Ma.