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

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 half-spreading 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.