Strength, textures, microfabrics and acoustic properties of active plate margin sediments on- and offshore SW Japan
Earthquakes are the violent expression of plate motion caused by a sudden release of elastic energy along fault planes known from a variety of tectonic settings. The seismically most active regions on Earth are the subduction zones, where lithospheric plates descend. Especially the Nankai subduction zone southeast of the Japanese islands of Honshu and Shikoku is known for the repeated occurrence of strong earthquakes and devastating tsunamis. The large-scale processes of stick-slip instabilities responsible for earthquake generation are vastly studied, but at least in parts not well understood. This thesis aims to investigate the fundamental processes of faulting and earthquake generation in a setting of converging lithospheric plates. Deformation processes in front of the seismogenic zone and in the accretionary wedge are in the main focus of this work. The question whether the rocks of the Nankai accretionary prisms are capable to produce surface breaks and related tsunamis by brittle faulting, or whether these rocks are capable of distributing deformation within large volumes (slow, stable slip) will be answered. For this purpose, deformation experiments of drill cores from the Nankai accretionary prism were conducted to provide information on the geotechnical properties of the near-surface sediments. Analogue cases exposed along the coast line of Shikoku Island and Boso Peninsula cover those parts in the accretionary prism not accessible by deep sea drilling, so far. Microstructural studies will connect the laboratory data to the natural case. The first study provides a unique dataset of sonic velocities measured during triaxial deformation under laboratory conditions. The slity clay and clayey silt samples were obtained from IODP Expeditions 315, 316 and 333. Seismic time series analysis helped to increase the data quality and to identify erroneous first arrival picks identified by trace-by-trace picking. During axial deformation, the compressional and shear wave velocities (Vp and Vs) range between 1300 – 2200 m/s and 150 -800 m/s, respectively. Different velocities were measured for the tectonic settings. Samples from the frontal thrust (accretionary prism toe) show higher Vp compared to the samples from the megasplay fault settings, whereas lowest Vp were measured for the incoming plate sediments, being characterized by higher clay contents compared to the accretionary prism sediments. Generally, Vp increases slightly with increasing effective confining pressure and effective principal stress. Similar observations were made for Vs, irrespective of tectonic setting and composition. Shear, bulk and elastic moduli were calculated from the velocities to differentiate between tectonic settings. The shear moduli ranges between 0.2 – 1.3 GPa and the bulk modulus between 3.8 – 8.4 GPa, while the elastic moduli, ranges between 0.5 – 3.8 GPa pointing to over-consolidation of the accretionary prism toe samples and normal consolidation of the footwall of the megasplay fault and incoming plate. The results of Rietveld refinement-based synchrotron texture analysis and microfabric studies of the slity clay and clayey silt samples are reported in the second part of the thesis. Crystallographic preferred orientation (CPO) of illite, kaolinite and smectite basal planes is bedding-parallel and increases with depth. Bedding parallel orientation of the basal planes of the phyllosilicates is preserved in samples tilted by tectonic deformation, indicating progressive burial and compaction as texture forming processes. Shape preferred orientation (SPO) of micropores and detrital illites investigated by Scanning Electron micrographs indicate natural compaction as well. Samples deformed in triaxial experiments differ from these observations as basal planes of phyllosilicates and calcite are oriented almost always perpendicular to the experimental shortening direction. SPO analyses indicate reorientation or flattening of the micropores due to experimental deformation. From these data we show that even in water-rich fine-grained and phyllosilicate rich sediments synchrotron x-ray radiation allows to quantify textures. In the third part, the results of triaxial deformation tests of mudstone samples derived from the tectonically exhumed accretionary prisms belonging to the Boso and Shimanto Belts, representing pre-exhumation burial depths up to 9000 m are presented. To investigate the geomechanical properties of these rocks, pressure stepping tests, constant confining pressure tests and a cyclic loading tests were conducted. To determine the effective shear parameters angle of internal friction and cohesion experimental results were used. Friction angles between 30 and 50° are rather high. The sample cohesion for Boso and Shimanto ranges between 2 and 6 MPa and 13 and >30 MPa, respectively. Due to structural and compositional similarities to the Nankai accretionary prism, the results can be used to estimate the geomechanical properties of the deeper parts of the Nankai accretionary prism, which are not accessible by deep sea drilling. The data shows that the forearc rocks are relatively strong. Diagenesis and/ or low-grade metamorphism increase the cohesion. From the data we conclude that stresses up to 18 MPa can be transmitted to the updip limit of the seismogenic zone and between 5 to 13 MPa to the actively deforming frontal prism. In the article of Stipp et al. (2013), to which I contributed as co-author, whole-round core samples drilled during IODP Expedition 315, 316 and 333 were experimentally deformed in a triaxial cell having been drilled in depths between 28 – 128 m below sea floor (mbsf). During undrained experimental deformation, at 400 to 1000 kPa confining pressure, between 0.001 – 9.0 mm/min displacement rate, up to ~64% axial compressive strain was reached. Although composition and grain size distribution of the slity clay and clayey silt samples is rather similar, geomechanical data lead to the distinction of two ‘rheological groups’. (1) Structurally weak samples, showing peak deviatoric stress conditions after few percent of compressional strain, continuous stress decrease after peak conditions and contractant behavior (increasing pore pressure) and (2) structurally strong samples, showing continuous strength increase or weaken only moderately at much higher strength levels and higher strains. Strong samples from the accretionary prism toe are overconsolidated, while the weak samples from further upslope are normally consolidated, similar to the samples from the incoming plate. The mechanical state of the décollement can be inferred from the results of the triaxial tests. Up to 18 MPa can be transmitted to the updip limit of the seismogenic zone and up to 13 MPa to the backstop of the active deforming prism. Overconsolidated frontal thrust sediments are able to stable slip and strain partitioning resulting in folding. The middle part of the accretionary prism tends to creep under cyclic loading. Normal consolidated sediments of the megasplay setting are capable to unstable slip and brittle faulting and thus can produce surface breaks. Crystallographic preferred orientation and shape preferred orientation may act as runaway structures transmitting strains to the (near) sea floor.