THE INTRA-OCEANIC ALEUTIAN, MARIANA, AND TONGA SUBDUCTION ZONES FORMED SEPARATELY BUT SINCE INCEPTION AT ~45-55 MA BUILT AT HIGH PRODUCTION RATES VOLUMETRICALLY SIMILAR AND LARGE ARC MASSIFS—WHY?
LONG-TERM RATES: With corrections applied for removed material but not for porosity, since 50-55 Ma the Mariana SZ generated at the once contiguous Palau-Kyushu, West Mariana, and Mariana Ridges a bulk magmatic mass of ~7500 km3/km at a long-term production rate of ~145 km3/Myr/km (Takahashi et al., 2007, Geology, v. 35). Although less constrained by field and age data, in ~50 Myr the Tonga SZ produced at the Tonga and Lau Ridges an arc volume of ~6000 km3/km at a long-term rate of ~120 km3/Myr/km. Beginning at or somewhat before 50 Ma the Aleutian SZ constructed a magmatic volume of ~7500 km3/km at a long-term rate of ~150 km3/Myr/km (Jicha et al., 2006, Geology, v. 34).
INITIAL RATES: Island mapping and offshore studies document that the great bulk (80+%) of arc magmas generated above the Aleutian, Mariana, and Tonga SZs accumulated during the first 5-10 Myr of SZ magmatism. The corresponding rate of initial magmatic growth was ~400-500 km3/Myr/km, equivalent to that of a slow spreading center.
INFERENCES: We infer the prodigious rate of initial magmatism as a signature flare-up from a new SZ. Rapid building of a consequent magmatic massif 200-250-km wide and 20-30-km thick reflects decompression melting of asthenosphere ascending into a rift-widening suprasubduction zone setting behind a retreating (rolling back) trench axis. The massive, short-lived outpouring dominates calculations of long-term rates and accounts for the similar and large volumetric masses of Pacific-rim intra-oceanic arcs.