4:35 PM - 4:50 PM
[SSS02-21] Physical property and Microstructure across the Seismic reflectors in the Upper plate of the Costa Rica Subduction zone
Keywords:Costa Rica subduction zone, seismic reflector, IODP, Costa Rica Seismogenesis Project, physical property, microstructure
The microstructures of the samples observed through the microscope tend to develop dense and cohesive textures in the low porosity sediments of Unit 2 and 3, and particle size changes across several unconformities. Particle size decrease with depth in Unit 2 and finer sediments form well-sorted structures that may account for the porosity decrease. The cross correlation between measured particle size and shipboard porosity show negative correlation especially at Unit 2, indicating that larger sized particles form smaller or fewer pores which may be due to finer particles filling pores between larger particles. The microstructures of the sediments occasionally exhibit foliated fabrics that are partially distributed. From the X-ray fluorescence and X-ray diffraction measurements, Al, K, Ti tend to concentrate in the higher porosity sediments of Unit 1, whereas Si, Ca, P, Mg, Na, and Mn concentrate in the lower porosity sediments of Unit 2 and 3, which are consistent with the transition in mineral composition marked by the increase in analcime and chlorite and the decrease in clay abundance in Unit 2 and 3. The crossplots between porosity and element concentration show negative correlations in Mg, Na, and Mn with porosity, suggesting that the minerals rich in these elements may relate with the consolidation and/or form the pore-filling materials or cements in Unit 2 and 3.
The depositional effect of grain size sorting, the tectonic effect of formation of foliated fabrics, and the geochemical effect of mineral precipitation and/or cementation are indicated to be the factors that characterize the unconformity, consolidation state and the seismic reflectors. In our presentation, we further investigate each effect to constrain the geologic processes occurring across the unconformity and seismic reflectors that form the upper plate structure of the Costa Rica subduction zone.