Texas A&M University, United States

Ocean Drilling Program Site 1224 is located about halfway between California and Hawaii in the eastern Pacific and was drilled, cored, and logged during Leg 200. The upper oceanic crust ( approximately 45-50 Ma) at this site was previously considered to be simple and uniform. The core and logging data acquired at this site, however, reveal five distinct units differentiated by petrophysical property variations in the approximately 145-m-thick basement. In addition, a hydrothermal vein exists near the bottom of the basement that penetrates the drilled location. For the first time, this study provides seismic evidence of the five-layer structure of the upper oceanic crust around the site. Through advanced high-resolution seismic processing, the boundaries of these logging units can be clearly identified on the processed seismic section and extended laterally away from the drill hole. The fault systems around the site can also be imaged clearly. Relative to the seafloor reflection, the hydrothermal vein has a unique reversed polarity seismic signature. Seismic analysis also indicates that the hydrothermal vein remains open to nearby faults connected to the seafloor. This finding provides geophysical evidence of hydrothermal circulation in the upper oceanic crust that may host observed microbial activity at the site.