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 Southern Mariana 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 offaxis, 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 the west 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 from the 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 low viscosity region in the mantle wedge, 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.

Comparative Analysis of Microbial Communities in Iron-Dominated Flocculent Mats in Deep-Sea Hydrothermal Environments

“…1a and d). This field was found in the year 2010 during the YK10-10 cruise by the DSV Shinkai6500 (52,79,80). Orange chimneys approximately 10-m high were observed throughout the site (Fig.…”

Diffuse Extension of the Southern Mariana Margin

“…This is shown by basement fabric, which conforms with the shape of the spreading axes, the paucity of teleseismic earthquake activity, and the occurrence of apparent linear magnetic isochrons recorded on the Philippine Sea flank of the MGR (Seama & Okino, 2015), although available data do not sufficiently resolve the magnetic character of the DSZ flank. This is shown by basement fabric, which conforms with the shape of the spreading axes, the paucity of teleseismic earthquake activity, and the occurrence of apparent linear magnetic isochrons recorded on the Philippine Sea flank of the MGR (Seama & Okino, 2015), although available data do not sufficiently resolve the magnetic character of the DSZ flank.…”

“…Broadly, the southern Mariana basin crust has been estimated from gravity data to be on average about 6-7 km thick (Kitada et al, 2006) and our local analysis suggests possibly a 1.7 km thinner crust on the Mariana platelet compared to the Philippine Sea flank of the margin. At current opening rates of about 45 mm/yr determined geodetically on Guam (Kato et al, 2003) and magnetically on the MGR (Seama & Okino, 2015), the present~200 km width of the southern basin between Guam and the West Mariana Ridge could have formed in about 4.5 Myr. This thinning was achieved by rifting apart the preexisting Eocene-Miocene forearc so that the present-day crustal thickness in the basin represents predominantly newly accreted volcanic material.…”

“…South of 14°22 0 N, the slab lies directly beneath the backarc spreading axis, facilitating its hydration, whereas to the north the slab dips more steeply and is located east of the spreading center (Kitada et al, 2006). Ridge-like topographic features flanking the MGR may represent abyssal hill-type fabric, although some of the more prominent ridges may have formed by ridge jumps or rapid ridge propagation events (e.g., Perram et al, 1993) that caused ridge axes to be cut off and abandoned on the Philippine Sea plate (Seama & Okino, 2015). The northern flank fabric domain on the Philippine Sea plate conforms with the shape of the MGR/DSZ axis and appears to passively record the crustal fabric from the West Mariana Ridge, the rifted remnant arc, to the present zones of accretion on the MGR/DSZ.…”

“…There are second-order differences in the character of the crust accreted on the MGR versus DSZ, however. Ridge-like topographic features flanking the MGR may represent abyssal hill-type fabric, although some of the more prominent ridges may have formed by ridge jumps or rapid ridge propagation events (e.g., Perram et al, 1993) that caused ridge axes to be cut off and abandoned on the Philippine Sea plate (Seama & Okino, 2015). Discontinuities and changes in trend between sets of these lineated ridges are probably nontransform ridge segmentation boundaries.…”