Recent volcanic and tectonic evolution of the southern Mariana arc

The Mariana Trough is the backarc basin of the Mariana arc system. It opens with slow to intermediate spreading rates, and for most of its length has a spreading axis with a deep axial graben flanked by abyssal hill fabric. South of 14°N, however, it shallows from ∼5000 m to ∼4000 m and the spreading axis changes from an axial graben to the broad, smooth-surfaced Malaguana-Gadao Ridge (MGR) ∼3000 m deep, a morphology more typical of fast-spreading ridges with high magma supply rates. A 6-channel seismic-reflection profile reveals a magma chamber reflector with a highly negative coefficient of refraction of -0.41 that could be produced by bubbles within the magma body. Submersible observations also reveal recent explosive volcanic activity, and lava samples likely have high volatile contents. Therefore, the MGR is receiving an excess of volatile-rich magma. The descending slab of the Pacific Plate lies beneath the MGR at a depth consistent with arc magma generation, so the MGR could be entraining arc magma or decompression melting mantle hydrated by subduction. New swath-mapping sonar surveys of the region also show that spreading continues south and west of the MGR in a region distinguished by numerous small volcanic cones. Some of these volcanoes have coalesced into a high-relief ridge that may represent a spreading axis supported by arc volcanism alone. Extension continues further west where it forms a ∼5000 m deep amagmatic graben within the West Mariana Ridge, which also appears to lie in the forearc. If spreading in the southern Mariana Trough is propagating southwestward, then features observed along the spreading axis could reveal how the southern Mariana Trough has evolved with time, suggesting that backarc extension can begin with amagmatic rifting of the forearc. As the forearc graben widens it would intersect the volcanic arc, to first produce small arc volcanoes and then an inflated spreading axis with the onset of decompression melting. With further widening of the basin the arc volcanoes come to lie trenchward of the spreading axis while backarc spreading is sustained by decompression melting alone, resulting in the typical Mariana Trough morphology north of 14°N.