Smectite swelling in the Miura–Boso accretionary prism: Possible cause for incipient décollement zone formation

Abstract

An analysis of the mineralogy of the Miura–Boso accretionary prism, which consists of off-scraped upper Miocene strata exposed on the southern parts of the Miura and Boso peninsulas in Central Japan, revealed homogeneous mineral composition throughout all levels of silt layers hosting deformation structures associated with sediment accretion. XRD analyses showed that the sediment consists of randomly ordered mixed layer illite–smectite (I–S) with low illite content (> 40 wt.%), quartz (30 wt.%), illite (10–20 wt.%), plagioclase (10 wt.%), kaolinite (+ chlorite) (5 wt.%), and calcite (0–10 wt.%). The observed mineralogical assemblage indicates that location of the décollement formation was not constrained by mechanical heterogeneities associated with mineralogical variations, but was controlled by spatial variations in effective pressure. Within the over-consolidated sediment located above the incipient décollement zone, the occurrence of abundant smectite with a shape-preferred orientation gave rise to smectite swelling, producing pressures in the order of ~ 3.0 MPa. The combined effect of high swelling pressure and high pore pressure resulted in an overpressured zone at the base of the over-consolidated zone, facilitating the development of a décollement zone within the prism.