Physical property and Textural transition across the Unconformity and Major Seismic Reflectors in the Upper plate of the Costa Rica Subduction zone offshore Osa Peninsula

Abstract:

At the Costa Rica subduction zone offshore Osa Peninsula, the Cocos plate and Cocos Ridge subduct under the Caribbean plate along the Middle America Trench, creating active seismicity. In this region, the Caribbean plate is characterized by a well-consolidated, high velocity framework material beneath the slope sediments, but the nature of the upper plate material is yet unknown. During Integrated Ocean Drilling Program (IODP) Expedition 334 and 344, the unconformity between the slope sediments (Unit 1) and upper plate material (Units 2 and 3) consisting of lithic sedimentary units was penetrated at mid-slope Site 1380. In the current study, to characterize the compaction behavior of the upper plate material, we investigate the physical properties, texture and composition of the sediments at Site 1380 by conducting microstructural observations, resistivity measurements, particle size analyses, X-ray fluorescence and X-ray diffraction analyses.

The microstructures of sediments observed through the microscope tend to develop dense and cohesive textures in low porosity sediments, and particle size changes across several unconformities. In particular, the small particle-sized lithic fragments compose larger bodies and form cohesive structures. 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. From the results of XRF and XRD analyses, we found that 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. The higher concentration in Mg, Na, Mn, Si may be due to minerals such as chlorite, serpentine, amphibole, and sodium manganese. 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 in Unit 2 and 3.