Tectonic and Magmatic Controls on Backarc Basin Sedimentation

Abstract

Previous models for intraoceanic backarc basin sedimentation are static in that they focus on a long-lived, stationary, magmatic source along the arc axis. These models are adequate in the case of backarc rifting, but a more dynamic model is needed in the case of forearc rifting. We present a revised model for backarc basin sedimentation that reflects progressive shifts in volcanism and associated depocenters through the history of a backarc basin developed by forearc rifting. This model is based on recent Ocean Drilling Program results from the Lau Basin and a reexamination of the sedimentary sequences recovered on Deep Sea Drilling Project legs across the Mariana arc and backarc system. We report new petrographic data for volcaniclastic sand from the Mariana forearc and backarc regions and compare them to data collected from other backarc basins. The proportions of colorless, brown and black glassy volcanic fragments within coarse, sandy intervals associated with these arc systems suggest that the early rift phase of backarc basin formation is charac- terized by felsic (bimodal) volcanism and that volcanic centers become progressively more intermediate with time. This trend is consistent with the geochemical evolution of arc lavas. In the case of forearc rifting, this compositional trend may coincide with a shift in volcanism from the protoremnant arc, across the nascent backarc basin, toward the fron- tal arc.