Asymmetric seafloor spreading of the southern mariana trough back-arc basin

Abstract

We compiled multi-narrow beam bathymetric data and geomagnetic field data obtained by a series of JAMSTEC research cruises in the SouthernMariana Trough back-arc basin, where is selected as one of three integrated target sites for the Japanese TAIGA Project. The bathymetric data are used to trace the non-transform offsets that define the ridge segments at the off-axis, and to characterize the seafloor morphology signatures from the bathymetry profiles across the spreading axes of two ridge segments. The geomagnetic field data are used to derive the crustal magnetization distribution and to estimate the spreading rate of the southern segment. Both of the spreading rate and the seafloor deepening rate of the southern segment support highly asymmetric seafloor spreading; much faster spreading in the west side of the spreading axis compared to the east side (trench side).We estimated the full spreading rate as 46 km/Myr with its half rate of 33 km/Myr for thewest side and 13 km/Myr for the east side. In contrast to the southern segment, our results indicate that the northern segment has a different style of the asymmetric seafloor spreading; that is accompanied by an obvious trace of a ridge jump to the trench side. The local symmetry axis in the bathymetry profiles locates at a distance of 18 km to the west fromthe spreading axis, suggesting that it is the failed spreading axis due to the ridge jump. The location of this failed spreading axis coincides with the center of the bull’s eye feature in the Mantle Bouguer anomaly, suggesting that the ridge jump to the trench side with an increase in the magma supply. We propose that the influence of the low viscosity region in the mantle wedge due to hydration driven by water release from the subducting slab leads to the highly asymmetric seafloor spreading; the low viscosity mantle would preferentially captures the mantle upwelling zone beneath the spreading axis as the spreading axis has been kept in the area closed to the lowviscosity region in themantlewedge, resulting in the highly asymmetric seafloor spreading. Further, the different styles of the asymmetric seafloor spreading between the northern segment and the southern segment probably show evidence that the influence varies with the distance from the low viscosity region in the mantle wedge.