2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 89-15
Presentation Time: 11:45 AM

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?


SCHOLL, David, Geology and Geophysics, University of Alaska Fairbanks and U.S. Geological Survey, 345 Middlefield Rd, MS 999, Menlo Park, CA 94025 and STERN, Robert, Geosciences Department, University of Texas at Dallas, Box 830688, 800 W. Campbell Rd, Richardson, TX 75080, dscholl@usgs.gov

INTRODUCTION: When geophysical images of Pacific-rim intra-oceanic arcs became available it was recognized that their long-term rate of magmatic growth was at least twice that thought typical (30km3/Myr/km of arc) of arcs (Dimalanta et al., 2002, EPSL v, 202). Improved imagery and age data have since increased estimates of long-term growth to ~130-150 km3/Myr/km reflecting much higher outbursts during the initial 5-10 Myr of subduction zone (SZ) magmatism.

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.

Handouts
  • SCHOLL SESSION 89-15 CENTRE-WEST 121, 1145AM copy.pptx (25.7 MB)