LAUSR

Extension within the southern Mariana margin occurs both normal to and parallel to the trench. Trench-normal extension takes place along focused and broad backarc spreading axes forming crust that is passively accreted to the rigid Philippine Sea plate flank to the northwest. To the southeast, trench-parallel extension has split apart the Eocene-Miocene forearc terrain accreting new crust diffusely over a 150–200 km wide zone forming a pervasive volcano-tectonic fabric oriented at high angles to the trench and the backarc spreading center. Earthquake seismicity indicates active extension over this forearc region and basement samples date young although waning volcanic activity. Such diffuse formation of new oceanic crust and lithosphere is unusual; in most oceanic settings extension rapidly focuses to narrow plate boundary zones—a defining feature of plate tectonics. Diffuse crustal accretion has been inferred to occur during subduction zone infancy, however. We hypothesize that in a near-trench extensional setting, the continual addition of water from the subducting slab creates a weak overriding hydrous lithosphere that deforms broadly. This process counteracts mantle dehydration and strengthening proposed to occur at mid-ocean ridges that may help to focus deformation and melt delivery to narrow plate boundary zones. The observations from the southern Mariana margin suggest that where lithosphere is weakened by high water content narrow seafloor spreading centers cannot form. These conditions likely prevail during subduction zone infancy, explaining the diffuse contemporaneous volcanism inferred in this setting. Plain Language Summary The edges of plates above subduction zones deform diffusely unlike the focused boundaries predicted by plate tectonics to delimit oceanic lithosphere. This diffuse deformation has been mapped in a presently active rift within the overriding plate behind the southern Mariana trench and appears to be due to weakening effects of water released by the subducting plate. The southern Mariana margin may represent an active analog of the broad and diffuse deformation and volcanism inferred to occur at the earliest stages of subduction zone formation. The observations also suggest that mantle dehydration thought to accompanymelting at mid-ocean ridges may be an important process in forming the focused plate boundary zones characteristic of plate tectonics. Such dehydration apparently cannot occur along the upper plate edges of subduction zones due to continually dewatering subducting slabs.