Malaguana-Gadao Ridge: Identification and implications of a magma chamber reflector in the southern Mariana Trough

Six-channel seismic reflection data reveal a magma chamber reflector beneath the Malaguana-Gadao Ridge, the southernmost segment of the spreading center in the Mariana Trough. For most of its length the spreading center in this active back-arc basin is morphologically similar to slow spreading mid-ocean ridges, having a deep central graben flanked by a zone of abyssal hill fabric. This southernmost segment, however, has a broad, smooth cross section, lacks a deep central graben, and is thus similar in morphology to fast spreading ridges (e.g., the East Pacific Rise). We identify a magma chamber at 1.5 s two-way travel time below the crest of the ridge. Observations from remotely operated vehicles along the ridge reveal not only fresh pillows, lobate, and sheet lava flows but also an abundance of volcaniclastic debris and intense hydrothermal activity. These observations, together with the ``fast spreading{\{}''{\}} morphology of the ridge, suggest that this segment has a considerably higher magma supply than is typical in the Mariana Trough. We suggest that the volcanic arc or enhanced melting of a hydrated mantle is supplying volatile-rich magma as evidenced by a highly negative coefficient of reflection, -0.42, for this MCR and the presence of evolved, highly vesicular lava and volcaniclastic materials. The southeastern Mariana back-arc basin spreading ridge does not compare readily with mechanical models for global mid-ocean ridge data sets because of marked asymmetry in both volcanism and deformation that may be the consequence of slab-related geometry in this part of the convergent margin system.