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 (~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 ~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.
1Sun, Y.F, Kasahara, J., Stephen, R., Baechle, G.T., Eberli, G.P., Hoskins, H., Nakamura, M., and Teng, Y.C., 2006. Shallow seismic structure of eastern Pacific Ocean crust at ODP Site 1224. In Kasahara, J., Stephen, R.A., Acton, G.D., and Frey, F.A. (Eds.), Proc. ODP, Sci. Results, 200 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/200_SR/006/006.htm>. [Cited YYYY-MM-DD]
2Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades NY 10964, USA. sunyf@ldeo.columbia.edu
3Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunko-ku, Tokyo 113-0032, Japan.
4Department of Geology and Geophysics, Woods Hole Oceanographic Institution, MS 24, 360 Woods Hole Road, Woods Hole MA 02543-1524, USA.
5Comparative Sedimentology Laboratory, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami FL 33149, USA.
6Femarco, Inc., New York NY 10025, USA.
Initial receipt: 25 May 2004
Acceptance: 9 September 2005
Web publication:
28 March 2006
Ms 200SR-006