Microstructural Analysis of Deformation Bands from Site 860, Chile Margin

Abstract

Deformation bands, with good macroscopic and microscopic indicators of shear are the clearest macroscopic manifestation of accretionary deformation from ODP Site 860. Backscattered electron SEM studies of deformation bands show that these structures are characterized by low porosity, an increase in matrix:grain ratios and grain-size reduction. Locally grain alignment fabrics are observed and some deformation bands show evidence of injection of foreign material and diagenetic changes. Strain magnitudes from space and shape fabrics are small, both inside and outside deformation bands. We propose that deformation bands develop during transient cycles of dilation, shear and fluid flow associated with accretionary deformation.

The damage is done: Low fault friction recorded in the damage zone of the shallow Japan Trench décollement

“…Margins of bands vary from sharp outlines of opaque mineralization to gradational boundaries. Similar structures have been recognized at the Japan Trench as "dark seams" [Chester et al, 2013a], at the Nankai margin [e.g., Lundberg and Karig, 1986;Maltman et al, 1993;Maltman, 1998;Ujiie et al, 2004;Lewis et al, 2013], and Chile margin [e.g., Rochford et al, 1995].…”

“…BSE images show that the bands contain weakly aligned grains (Figure 3b), except where the phyllosilicate fabric is deflected around larger grains, similar to "kink-like bands" observed by Lewis et al [1997]. Here we group the microstructures displaying Journal of Geophysical Research: Solid Earth 10.1002/2015JB012311 any form of preferred orientation, whether kink-like [e.g., Lundberg and Moore, 1986;Maltman et al, 1993;Lewis et al, 1997;Vannucchi and Tobin, 2000] or deformation bands [e.g., Karig and Lundberg, 1990;Rochford et al, 1995;Vannucchi and Tobin, 2000;Ujiie et al, 2004]. Displacement or truncation of piercing points, most commonly microfossils, is observed for a small number of microfractures or zones of clustered microstructures that are classified as shear microfractures and shear zones, respectively (Figure 3c).…”

The Maximum Effective Moment Criterion (MEMC) and Its Implications in Structural Geology

“…Roughly planar microscope bedding bands that typically develop in argillaceous sediments at the toe of accretionary prisms are called deformation bands such as in the Nankai accretionary margin in Japan (Ujiie et al 2004) and convergent plate margins in Chile and Costa Dica (Rochford et al, 1995). The origin of deformation bands is controversial.…”