Abstract:
Deformation processes in sediments are directly controlled by the state of in situ effective stresses, the mechanical-, physical- and geochemical properties of the materials of the fault zone and surrounding wall rocks, and time. Measurements of these properties are needed for understanding of the process of earthquake generation. The recent drilling expeditions to Nankai Trough, by the Integrated Ocean Drilling Program demonstrate the technical difficulties involved in subduction zone drilling. At present, scientific drilling operations have successfully penetrated the decollement only at shallow depths in the Nankai accretionary prism. These data provide the only present access to future seismogenic zone materials. In the frontal region, the strata are being partitioned into accreting and subducting packages, some of which may ultimately pass into the seismogenic zone. Sediments deposited and locally cemented within the Shikoku Basin are partitioned by the basal decollement and they follow distinctive deformation pathways across the margin. Our hypothesis is that enhanced strength in the underthrust package allows it to carry excess pore fluids deep into the subduction zone, potentially to be released rapidly and seismogenically. The objectives for mechanical testing are to probe the yield and failure surfaces of these sediments, as well as their post-failure deformation behavior. This information is useful for making predictions about sediment response to accretion, underplating, and slip along the decollement. Reconsolidation tests have been carried out on Ocean Drilling Program cores collected from the reference site seaward of the active Nankai decollement zone off the southeast coast of Japan (ODP Site 1173). We have conducted two tests each from two subsamples from within Lower Shikoku Basin, approximately 30 m above, and approximately 85 m below the proto-decollement. This allows documentation of changes in mechanical strength and stress history across the proto-decollement horizon within the same lithostratigraphic unit. The initial porosities for the deeper and shallower samples are 43-44% and 56-57%, respectively. The results suggest that the shallower samples are significantly stiffer than the deeper ones, which is reflected by higher effective yield stress and higher elastic modulus in the shallower samples. The results regarding the elastic-plastic behavior is more challenging to interpret: One sample from above the decollement experienced brittle failure at an effective vertical stress of 8.7 MPa, whereas one sample from below the decollement show evidence on cement destruction and plastic deformation up to the maximum stress of 9.5 MPa. These two stress levels also are the highest effective vertical stress of samples. The results will contribute to our model for the evolution of deformation in sediments in the Nankai accretionary prism They will also allow us to project the mechanical behavior from shallow to greater depths.