6:15 PM - 7:30 PM
[MIS32-P07] Structures characterizing the megasplay fault across shallow slope sediments of the Nankai accretionary prism
Based on CT scan images, seven intervals were selected from Core 10T (three from section 5, one from core catcher) and from Core 11T (three intervals). For each interval, a slab was cut, dried, epoxied and polished before being analysed by XRF scanner for compositional mapping. Polished thin sections were made from the most deformed-looking parts of the slabs.
XRF scanning analysis brings two main results: At least three gently dipping planar surfaces outlined by iron sulphide (most likely pyrite) cross the cores. It is not clear whether these surfaces are faults or not but, in one case (11T-1, 72-83 cm), the surface separates clays with different Ca contents, suggesting some offset. The presence of iron sulphide along the surfaces may be related to fluid flow having allowed recrystallization of this mineral species. Shipboard observation revealed three intervals characterized by weakly marked planar fabrics. Two of these intervals (338-C0022B-10T-5, 49-51 cm and 11H-1, 18-19 cm) are characterized by a depletion in Ca and Sr and an enrichment in K with respect to the surrounding sediments.
Lastly, preliminary optical microscope observation indicates that some incipient non-coaxial shear deformation (asymmetrical sand lenses and composite planar fabric) affected the intervals with planar fabrics. Put together, these results indicate that deformation so far recognized in C0022B cores is weak to moderate. This is not an unexpected result given (1) the young age of the sediments and (2) the fact that samples likely come from the damage zone a few meters and not from the core zone.
We thank the crew of D/V Chikyu and all drilling personnel who contributed to the overall success of Expedition 338. We also thank Kyuichi Kanagawa, Takehiro Hirose and Kentaro Hatakeda for constructive discussions and technical help. This research used samples and data provided by the Integrated Ocean Drilling Program (IODP). Funding for this post- expedition research was provided by CDEX, JAMSTEC.