Faulting and volcanism in the axial valley of the slow‐spreading center of the Mariana back arc basin from Wadatsumi side‐scan sonar images

Abstract

[1] We analyzed in detail the geology of the median valley floor of the Mariana Basin slow-spreading ridge using sea surface geophysical data and a high-resolution deep-tow side-scan sonar survey over one spreading segment. Analysis of surface magnetic data indicates highly asymmetric accretion, with the halfspreading rate on the western side of the basin being two to three times larger than on the eastern side. Surface magnetic and reflectivity data together suggest that asymmetric spreading is accomplished through eastward ridge jumps of $10 km of amplitude. Deep-tow backscatter data indicate along-axis variations of the volcanic processes with the emplacement of smooth and hummocky flows at the segment center and end, respectively. This variation likely relates to changes in the effusion rate due to the deepening or even disappearance of the magma chamber toward the segment end. Concerning tectonic processes, we find a power law distribution of the fractures, with an exponent of 1.74. This suggests that within the inner valley floor, fracture growth prevails over fracture nucleation and coalescence and that fractures accommodate less than 8% of the strain. According to our calculation based on a ratio of 0.02 to 0.03 between the vertical displacement and the length of faults, the amount of tectonic strain accommodated in the inner valley floor would consistently be $1.1-3.4%. Data also show two distinct sets of fractures. One trend is parallel to the rift direction at the segment center ($N160°E) and perpendicular to the plate separation direction. Another set trends $17°oblique to this direction ($N175°E) and is located over the eastern part of the valley, in the vicinity of a major bounding fault also trending $N175°E, that is, obliquely to the direction of plate motion. We modeled the stress field near a major fault that is oblique to the regional stress field associated with plate separation using a three-dimensional boundary element approach. We found that the orientation of the predicted fissuring near the oblique fault is locally rotated by $15°due to a flexure of the bending plate close to this fault.

Philippine Sea Plate inception, evolution, and consumption with special emphasis on the early stages of Izu-Bonin-Mariana subduction

“…Many authors have studied the morphological, geophysical, and petrological characteristics of this basin (Karig 1971b;Baker et al 1996;Yamazaki and Murakami 1998;Stüben et al 1998;Martinez et al 1995Martinez et al , 2000Deschamps and Fujiwara 2003;Deschamps et al 2005). The asymmetric (full) spreading rate varies from 20 mm/year in the north to 40 mm/year in the south (Asada et al 2007).…”

Deformation pattern in a massive ponded lava flow at ODP‐IODP Site 1256: A core and log approach

“…[3] Many studies on faults and modes of faulting at mid-ocean ridges have been carried out in the last decades [e.g., Kent et al, 1996;Macdonald et al, 1996;Poliakov and Buck, 1998;Buck et al, 2005;Deschamps et al, 2005]. Normal faulting is not common on fast and superfast spreading ridges within ±3 km of the ridge axis because the lithosphere is too thin and weak [e.g., Edwards et al, 1991;Carbotte and Macdonald, 1994].…”

A submersible study of the Mariana Trough back-arc spreading center at 17.DEG.N

“…On the other hand, small ridges around the axial portion of the median valley and areas toward the segment end are associated with sidescan imagery of bumpy surfaces (Deschamps et al, 2004;Deschamps et al, 2005;Asada et al, 2007). The bumpy sidescan images suggest pillow mounds and ridges, and the smooth surfaces suggest sheet-like lava flows or lobate flows.…”

“…Detailed tectonic evolution will be a key to understanding this. However, the details of magnetic stripe patterns are still not clear (Iwamoto et al, 2002;Deschamps et al, 2005); because the trough is situated in low latitudes and the survey line interval is sparse. Therefore, acquisition of additional magnetic data is significant.…”