Deformation paths in the shallow levels of an accretionary prism: The Eocene Shimanto belt of southwest Japan
Detailed structural analysis of coherent turbidites in a part of the Eocene Shimanto belt on the Muroto Peninsula, Shikoku Island, Japan, reveals a structural evolution that is typical of the shallow levels of accretionary prisms in style and sequence of deformational fabrics, but anomalous in that it includes both a marked rotation in shortening direction and a major out-of-sequence thrust. Deformation throughout a history of progressive lithification and burial was dominated by subhorizontal shortening and seaward-vergent shear. Structures evolved from predominantly mesoscale thrusts and folds to larger, more brittle faults, and finally to kilometer-scale, amplified fault-bend folds with an associated pressure-solution cleavage. This structural progression is consistent with increasing strength of the rocks, decreasing fluid pressures, and increasing temperature and pressure as the sediments were dewatered, lithified, and accreted to the prism. In contrast to this straightforward structural evolution, the prism-scale deformation of the Eocene Shimanto belt is difficult to reconcile with simple models of prism developmentthat assume orthogonal subduction and seaward-stepping imbrication. A 50° counterclockwise rotation in the shortening direction and a major out-of-sequence (or intraprism) thrust are recorded by two distinct structural domains. In both, the earliest structures are clastic dikes and faults indicative of north-south shortening. One domain is characterized by east-striking, regional-scale folds and cleavage that are transected by an east-northeast-trending cleavage; however, the other is characterized by northeast-striking folds and cleavage that are transected by a slightly more north-striking cleavage. The different deformation histories of the two do-mains require that they be separated by a major intraprism fault. This fault carries tectonic melange containing blocks of pillow basalt and chert, indicating that it rooted in the decollement zone beneath the prism. The tectonostratigraphy and deformation histories of the two domains suggest that both before and after the out-of-sequence fault developed, higher structural levels of the prism experienced significant subhorizontal shortening, whereas lower structural levels bypassed this zone of deformation.