Thematic Article A bathymetric overview of the Mariana forearc

Bathymetric data at a 200-m contour interval for the entire Mariana forearc, from south of 13°N to 25°N, permits the ®rst comprehensive overview of this feature. The Mariana forearc represents a sediment-starved end-member. The forearc in its southern and central sections is divisible into a structurally complex eastern province and a less-deformed western province. Despite the absence of an accretionary complex the Mariana forearc has a well-de®ned outer-arc high; this probably results from a greater concentration of low-density serpentinized mantle lithosphere beneath the outer forearc relative to the inner forearc. This serpentinization gradient is coupled with differing deformational styles of thinner and more brittle lithosphere beneath the outer forearc compared to thicker and more ductile lithosphere beneath the inner forearc. The bathymetric data also support models calling for extension along-strike of the forearc, re ̄ecting an increase in arc length accompanying the crescent-shaped opening of the Mariana Trough back-arc basin. Both northeast and northwest ridges and grabens can be identi®ed, with the latter restricted to the southern part of the forearc and the former widely distributed in the central and northern forearc. Northeast-oriented extensional structures are supplanted northward by long, linear northwest-trending structures that are interpreted as left-lateral strike±slip faults. These variations in deformation along-strike of the forearc manifest a transition from nearly orthogonal convergence in the south to highly oblique convergence in the north.