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The southernmost Mariana forearc stretched to accommodate opening of the Mariana Trough backarc basin in late Neogene time, erupting basalts now exposed in the SE Mariana Forearc Rift (SEMFR) 3.7 – 2.7 Ma ago. Today, SEMFR is a broad zone of extension that formed on hydrated, forearc lithosphere and overlies the shallow subducting slab (slab depth ≤ 30 – 50 km). It comprises NW-SE trending subparallel deeps, 3 - 16 km wide, that can be traced ≥ ~ 30 km 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 Na8, Ti8, and Fe8, consistent with extensive melting, at ~ 23 ± 6.6 km depth and 1239 ± 40oC, 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 SEMFR and melt, forming new oceanic crust. Melts interacted with preexisting forearc lithosphere during ascent. SEMFR is no longer magmatically active and post-magmatic tectonic activity dominates the rift.