Nankai Trough
The detrital clays, primarily smectite, display variation over time and location in the Nankai Trough as well as the Shikoku basin. At depth there is an increase in the smectite clay content in the sediments, inferring that there has been a change in the deposition source rock (Steurer et al., 2003). Furthermore, there is a geothermal alteration of the smectite, converting it to illite clay (Steurer et al., 2003).