Leg 170

Ocean Drilling Program scientists probe recycling of oceanic crust

December 15, 1996
PANAMA -- Ocean Drilling Program (ODP scientists representing nine countries have concluded a two-month expedition in the South Pacific investigating how much of the sediment on the incoming Cocos plate is scraped off and accreted to the margin of Costa Rica and how much is carried to greater depths in the Earth.

"We also wanted to find the pathways of fluids that move through this system," says Dr. Eli Silver, director of the Institute of Tectonics at the University of California-Santa Cruz and ODP co chief scientist for this expedition. "This offers us a chance of studying this process unencumbered with the rapid changes in rates of sediment input to the trench floor that characterize many other regions."

The research team and ship's crew drilled holes in five different sites near Costa Rica. Initial results indicate that a site 400 m landward from the toe of the continental slope has accreted about 10 percent of the incoming sediment, but at a second site, 1600 m landward of the toe, essentially all of the incoming sediment went smoothly under the slope.

The differences in these two sites happen over a short distance and a very short geologic time (a few thousand years). The difference in response between these sites indicates episodic accretion events as part of the initial subduction process, something that has not been previously documented over such short time intervals.

"We speculate that fragments of the lower plate are scraped off when fault scarps attempt to pass beneath the toe of the slope," explains Silver. "These faults are seldom considered remarkable, but they may be responsible for 'deciding' which bits of the ocean plate remain at the surface and which descend to greater depths."

The scientists had also hoped to discover why the sea floor near Costa Rica has some of the lowest hear flow measured anywhere on Earth.

"We suspected that this widespread zone of low heat flow is being chilled by unusually high rates of sea-water flow," says Silver. "In our reference site we found evidence for such flow from geochemical indicators in the pore waters. They imply a source of seawater beneath the sediments, suggesting communication with the surface. Cold ocean bottom water percolating through those same fault zones may chill the upper part of the crust sufficiently to produce the very low temperatures measured." The Ocean Drilling Program is funded by the U.S. National Science Foundation, Canada, Australia, South Korea, the European Science Foundation Consortium, Germany, France, Japan, and the United Kingdom to investigate such topics as earth's history and evolution, climate change, and formation of the ocean crust.

Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES), an international group of scientists, provides scientific planning and program advice. Joint Oceanographic Institutions, Inc., a nonprofit consortium of 10 major U.S. oceanographic institutions, manages the program.

Texas A&M University, science operator, operates and staffs the drill ship that retrieves core samples from strategic sites in the world's oceans. Lamont-Doherty Earth Observatory of Columbia University is responsible for downhole logging.

U.S. members of JOIDES are: University of California at San Diego, Columbia University, University of Hawaii, University of Miami, Oregon State University, University of Rhode Island, Texas A&M University, University of Texas at Austin, University of Washington, and Woods Hole Oceanographic Institution. The European Science Foundation Consortium consists of Belgium, Denmark, Finland, Iceland, Italy, the Netherlands, Norway, Spain, Sweden, Switzerland, and Turkey.

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Scientist's Contacts:

Co-Chief Gaku Kimura
Department of Earth Sciences
University of Osaka Prefecture
College of Integrated Arts and Sciences
1-1, Gakuen-cho
Sakai-shi 593
Japan
Internet: gaku@cias.osakafu-u.ac.jp
Work: (81) 722-52-1161, ext 3735
Fax: (81) 722-55-2981

Co-Chief Eli A. Silver
Earth Sciences Department
University of California, Santa Cruz
Santa Cruz, CA 95064
U.S.A.
Internet: silver@earthsci.ucsc.edu
Work: (408) 459-2266
Fax: (408) 459-3074

Dr. Jamie Allan
Ocean Drilling Program
(979) 845-0506
james_allan@odp.tamu.edu