Deformation Structures at Site 808, Nankai Accretionary Prism, Japan

Abstract

The toe of the Nankai accretionary prism, south Japan, was cored at Ocean Drilling Program Site 808, and a range of exceptionally well-developed deformation structures was revealed. This paper documents their appearance at the optical microscopic scale. Bedding fabrics, largely because of gravitational consolidation, perhaps with some tectonic contribution, increase in intensity with burial depth, but this is irregular, partly because of the effects of bioturbation. Small folds in the slope apron sediments are thought to represent gravitational mass movement, but rare similar structures now deeply buried in the prism may have the same origin. The most common structures are grouped into a family termed deformation bands. These include true kinks, shear zones, and very narrow zones of phyllosilicate-preferred orientation called faults, but range through many structures with intermediate attributes. Despite this gradational aspect, the main types of deformation bands are thought to originate independently, according to the mechanical conditions operative at the time, rather than by a progressive evolution. The shear zones and faults show complex substructures and interrelationships and are dominant in the Nankai prism at depths not previously drilled. Whereas features such as the vein structures occurring at 640 mbsf, a hydraulic breccia at 800 mbsf, and subtle textures in the decollement zone owe their origin primarily to overpressuring, the deformation bands reflect focused tectonic deformation in the prism, a direct result of plate convergence.