Geodynamic evolution of a forearc rift in the southernmost Mariana Arc

The southernmost Mariana forearc stretched to accommodate opening of the Mariana Trough backarc basin in late Neogene time, erupting basalts at 3.7-2.7 Ma that are now exposed in the Southeast Mariana Forearc Rift (SEMFR). Today, SEMFR is a broad zone of extension that formed on hydrated, forearc lithosphere and overlies the shallow subducting slab (slab depth30-50km). It comprises NW-SE trending subparallel deeps, 3-16km wide, that can be traced approximate to 30km from the trench almost to the backarc spreading center, the Malaguana-Gadao Ridge (MGR). While forearcs are usually underlain by serpentinized harzburgites too cold to melt, SEMFR crust is mostly composed of Pliocene, low-K basaltic to basaltic andesite lavas that are compositionally similar to arc lavas and backarc basin (BAB) lavas, and thus defines a forearc region that recently witnessed abundant igneous activity in the form of seafloor spreading. SEMFR igneous rocks have low Na-8, Ti-8, and Fe-8, consistent with extensive melting, at approximate to 23 +/- 6.6km depth and 1239 +/- 40 degrees C, by adiabatic decompression of depleted asthenospheric mantle metasomatized by slab-derived fluids. Stretching of pre-existing forearc lithosphere allowed BAB-like mantle to flow along the SEMFR and melt, forming new oceanic crust. Melts interacted with pre-existing forearc lithosphere during ascent. The SEMFR is no longer magmatically active and post-magmatic tectonic activity dominates the rift.