Cementation affects the mechanical properties that control sediment strength and deformation. A small volume of grain coating cement can greatly increase sediment strength. Therefore, even minor cementation may affect consolidation in basins and control deformation in accretionary margins. The effects of grain-coating silica cement on the physical properties of sediment on the Philippine Sea plate as it approaches the Nankai Trough subduction zone in the central and southwestern portions of the Shikoku Basin were documented by previous work on samples from Deep Sea Drilling Project Site 297 and Ocean Drilling Program Sites 1173 and 1177. At these sites, a small amount of glass disseminated throughout hemipelagic sediment is altered to a silica gel upon burial. The gel coats grain contacts, and inhibits sediment consolidation. The cemented sediment has anomalous porosity, seismic velocity, and rigidity. With further burial, onset of tectonic deformation, and increasing temperature, cement dissolution and mechanical breakdown leads to dramatic reduction in rigidity and collapse of the sediment framework (i.e. porosity loss). How do differences in sediment thermal history, fluid flow, and pore water chemistry between sites control shifts in the location and extent of the cemented zone? How does the incorporation of cemented units with transient properties into the margin wedge influence the nature and distribution of deformation? Toward answers to these questions, the silica cement distribution at IODP Sites C0011 and C0012 will be determined, and multicomponent reactive transport modeling for Sites C0011, C0012, 1173, and 1177 will be performed. The shear-wave velocity of samples from NanTroSEIZE drilling sites C0011 and C0012 will be determined to locate regions of anomalous strengthening. The four sites selected for examining sediment-pore water interactions provide a range of sediment accumulation and thermal histories. The results will allow examination of the effects of fluid flow rate and thermal state on the vertical location and extent of silica cementation in the Shikoku Basin sediments. The proposed investigation addresses one of the main goals of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE); to determine how geologic differences affect mechanical properties, permeability, fluid flow, pore pressure, shear strength, and earthquake rupture processes within the Nankai margin. The proposed study may transform our understanding of accretionary margin processes by shedding light on a previously underappreciated control on wedge deformation. By examining processes that control cementation of sediment entering a subduction zone, the proposed research has societal relevance as it advances understanding of the mechanisms at play in potentially hazardous seismogenic margins. Deformation features control fluid drainage through a margin wedge. Therefore, sediment cementation and deformation impact margin hydrogeology and fluid pressure, which are related to strain accumulation and seismicity on the plate interface. The proposed research will be of interest to seismologists, geochemists, and hydrogeologists. This work enhances human resources by funding a graduate student at New Mexico Tech (NMT), a Hispanic-Serving Institution. The proposed project will enhance facilities used for both research and teaching at NMT. Outreach efforts include dissemination of seismogenic zone processes through The SMILE program at Oregon State University (http://smile.oregonstate.edu/), and through an ongoing Adult Education program at COAS (http://literacyworks.org/ocean/). In addition, the Pi?fs will work with the COSEE?]Pacific Partnerships (www.coseepacificpartnerships.org) by participating in one of their ?gscience pub nights?h in Newport, OR discussing earthquake related processes around the Pacific Rim.
We have examined what controls the chemical alteration of volcanic ash as it is buried and heated within marine sediments. Because some of the chemical products of this ash alteration can cement sediment grains together, this process can alter sediment strength. The sediments examined in this study are approaching and entering a subduction zone, where hazardous earthquakes and tsunami may be generated. The deformation of sediment in a subduction zone can affect fluid pressures in the system; fluid pressures, in turn, affect fault strength and behavior. Therefore, understanding deformation in subduction zones is necessary to develop societally relevant seismic and tsunami hazard assessments. In this study, we determine sediment composition and physical properties to identify locations of silica cementation, and we develop numerical models to examine how the thermal history and the chemistry of water in the pore space between sediment grains control the precipitation and dissolution of silica cement in marine sediments. We find that silica cementation of hemipelagic sediment occurs throughout Shikoku Basin (offshore Japan), at sites separated by more than 400 km. Thus, this cementation that affects sediment physical properties may influence the nature of sediment deformation along a wide swath of a subduction zone. We have identified the most likely reactants, products, and reaction network controlling silica diagenesis in shallow hemipelagic sediments with disseminated volcanic ash. In this system, the silica released from ash dissolution is consumed by amorphous silica cement and clinoptilolite formation. The distribution of silica cement in the system is controlled by the thermal history of the sediments. Because volcanic ash is common in sediments approaching subduction zones, these processes may affect sediment strength at other locations.
