Structural styles across the Nankai accretionary prism revealed from LWD borehole images and their correlation with seismic profile and core data: Results from NanTroSEIZE Stage 1 expeditions

摘要

Four drill sites of IODP NanTroSEIZE Stage 1 Expedition transected the Nankai Trough, offshore SW Japan, from the deformation front to the Kumano fore-arc basin. Borehole resistivity images from the logging-while-drilling (LWD) data were analyzed to extract orientations of faults, fractures, and bedding planes to examine the structural styles. On the basis of these features, drilling intervals were classified into fore-arc basin deposits, surface slope sediments, and deformed accretionary wedge, and these can be compared with characteristics from seismic profiles and core structural data. Bedding orientations identified in these three data sets are generally comparable, but the difference in resolution between the data sets produces different results in interpretation where geology is highly deformed or includes finer internal structures. Faults can also be correlated between these three data sets, but the differences in their appearance require special attention for accurate correlation. Many faults imaged in seismic profiles actually consist of microfracture systems, as shown in cores, that can also be identified in borehole images. Some clear faults in seismic profiles cannot be identified in borehole images, probably because of their minimal resistivity contrast with the surrounding rocks or a more complex fault zone at the borehole scale. These results suggest that these three data sets can be used to extract not only the general structure but also different styles of deformation at different scales from core samples (mm to cm), to LWD (mm to 10 m), to seismic (10 m to tens of km). This correlation requires a deep understanding of the resolution and shortcomings of each methodology.