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 décollement zone owe their origin primarily to overpressuring, the deformation bands reflect focused tectonic deformation in the prism, a direct result of plate convergence.

Microstructural Analysis of Sites 891 and 892: Implications for Deformation Processes at the Frontal Thrust and an Out-of-Sequence Thrust

“…Discontinuous kink bands with variable displacement direction are developed in several moderately deformed samples just above the 66-mbsf fault. We agree with Maltman et al's (1993) conclusion that these kink bands accommodate strain associated with displacement on the shear zones. The kink bands would therefore begin to form after the conjugate fabrics and continue to develop with continued displacement along the conjugate shear zones and may indicate a higher degree of strain along shear zones nearer the fault.…”

“…Deformation bands with an internal fabric of grains aligned at an angle to the trend of the band and up to 90° to the compaction fabric are consistent with previous description of kink bands Maltman et al, 1993). Kink bands are narrow (0.04-0.1 mm in width), have distinct boundaries in clay-rich layers, but are broad with indistinct boundaries in sands.…”

“…Microstructures observed in Cascadia wedge sediments largely fall into the classification scheme of the Maltman et al (1993) for Nankai wedge sediments. We provide brief descriptions following their scheme and describe other structural fabrics that do not fit well into their categories.…”

“…Clast formation may also be favored near the fault zone where both high strain localization and fluid transport occur, thus producing a greater number of clasts. Maltman et al (1993) and Byrne et al (1993) describe a hydraulic breccia from the Nankai accretionary wedge that appears to be the initial stage of clast formation, without disruption by further deformation. The breccia consists of angular clasts thought to be formed by dilation associated with high pore pressure.…”

Microstructural Analysis of Deformation Bands from Site 860, Chile Margin

“…Microstructural studies of deformation features in forearcs are important in constraining the conditions of deformation and the interrelationships of deformation, sediment lithification, diagenesis, and fluid flow (Agar et al, 1989;Prior and Behrmann, 1990;Knipe et al, 1991;Byrne et al, 1993;Maltman et al, 1992Maltman et al, , 1993. In this paper we describe deformation bands from Site 860 and their implications for the evolution of the forearc.…”