From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, February 5, 2010
Small-scale petrographic features in the muds of the Nankai accretionary prism
reveal the onset of strain localization in early diagenesis
Dr. Kitty Milliken and Rob Reed
Bureau of Economic Geology
In the Nankai accretionary prism and its associated slope and forearc basin sediments early (pre-lithification) mechanical modification of mudrock fabrics involves development of preferred alignments of detrital phyllosilicates and other elongate or platy particles and the loss of intergranular porosity. Across a range of depth from the modern slope surface to 600 m deep and in structural settings ranging from the mega-splay fault (Sites C0004 and C0008) to the frontal thrust (Sites C0006 and C0007), nearly all samples display bedding-parallel phyllosilicate alignment, bioturbation-related particle alignments, or both. Deformation bands are also widely distributed and are somewhat more abundant near the toe of the prism (60 percent of samples at Sites C0006 and C0007) than in the region of the mega-splay (43 percent of samples at Sites C0004 and C0008).
Subtle compositional contrast between deformation bands and surrounding host rocks, as seen in BSE and WDS X-ray mapping, is consistent with the compactional loss of micropores within the bands. The minimal level of compositional contrast supports the dominance of mechanical over chemical processes in the formation of these localized deformation features. Direct observation by field-emission SEM imaging of Ar-ion-milled cross-sections shows that collapse of the larger (> 2 µm) pores, many localized at the margins of the larger silt-size particles, reduces porosity within the bands by about 5 percent compared to the adjacent host rock. Despite the clear role of shear, evidence for particle comminution is equivocal, possibly reflecting the role of high porosity and the low degree of lithification in allowing the development of preferred alignment through particle rotation.
These results support previous studies showing that spatial localization of mechanical diagenesis and the consequent development of microscale heterogeneities in fabric anisotropy (potentially affecting permeability) are affiliated with very early stages in the diagenetic history of mud in the accretionary prism depositional setting.