DEFORMATION AT THE TOE OF AN ACTIVE ACCRETIONARY PRISM - SYNOPSIS OF RESULTS FROM ODP LEG-131, NANKAI, SW JAPAN

Toes of accretionary prisms record the initial deformation of wet sediments during accretion at a convergent plate margin. ODP Leg 131 succeeded in coring the toe of the Nankai prism, SW Japan, together with the basal decollement, the underthrust sediments and the basaltic ocean basement. An unprecedented structural geological inventory has been compiled. Gravitational deformation is important in the slope-apron deposits, but tectonic stresses, too, are transmitted to these levels. Core-scale deformation bands, due to heterogeneous bulk-shortening of the prism, are exceptionally well-developed at Nankai, and range through kink-bands, shear zones and faults. Anomalous frequencies indicate localized high-strain zones. Stress-inversion analysis gives a principal compression close to the plate convergence vector. The frontal thrust of the prism is a 26 m thick zone of breccia/scaly fabric, with a 309 m net slip. It may well be currently active, and accommodating almost half of the present convergence strain. The basal decollement, a 19 m thick breccia zone, separates the deforming, overconsolidated sediments of the prism from the virtually undeformed materials below. These are highly underconsolidated and probably overpressured. Anomalous fluid pressures localize several of the prism structures. Unlike some other prisms, drainage of the Nankai prism is dominantly pervasive rather than controlled by major structures.