Diffuse Crustal Accretion at the Southern Terminus of the Malaguana-Gadao Ridge, Mariana Trough

Abstract:

The mode of extension and crustal accretion in backarc basins is strongly affected by proximity to the arc volcanic front. The factor that likely has the strongest control on these processes is mantle water content. At Mid-Ocean Ridges, the small amount of water in the mantle is efficiently extracted into the melt, dehydrating the residual material and increasing the viscosity and strength of the lithosphere. This may aid in focusing melt generated over a broad (~200+ km wide) zone in the mantle toward a narrow zone of crustal accretion ~1-2 km wide. In the near-arc setting, the continuous flux of water into the mantle wedge should oppose lithospheric dehydration and inhibit strengthening of the lithosphere, which may allow deformation, volcanism, and crustal accretion to occur over a broad area instead of along a narrow axis. A possible example of this process can be observed at the southern terminus of the Malaguana-Gadao Ridge, a backarc spreading center in the Southern Mariana Trough, at the southern end of the Izu-Bonin-Mariana convergent margin. The spreading axis, which forms an axial high in this area, abruptly terminates at 143˚20'E, 12˚37'N and is replaced by a broad zone of active volcanism and tectonism characterized by short volcanic ridges, volcanic cones, and low-relief grabens. This study uses deep-towed and ship multibeam sonar, gravity, and magnetics data collected during an early 2012 cruise on R/V Thomas G. Thompson (TN273) along with available geophysical and geochemical data in the Southern Mariana Trough to gain insight into the nature of the diffuse crustal accretion process. Evidence of a similar transition from organized to "disorganized" spreading can also be observed at Valu Fa Ridge in the southern Lau basin and other backarc spreading centers. This suggests that this process is not unique to the Southern Mariana Trough, and may be an important mode of crustal accretion in a variety of backarc settings where there is extension in close proximity to the arc.