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Scientific ocean drilling and continental margins: understanding the fundamental transition from continent to ocean.

Extending from the beach to the base of the continental rise, continental margin waters are the "ocean" most familiar to Earth's human population. They are a popular recreational site, and fish from these waters sustain much of the global population. Most of the remaining hydrocarbons that fuel modern civilization's activities are expected to be found beneath these waters, associated with the thick sediments that line many continental edges. Climate researchers are now concentrating on continental margins, because their sediments hold vital historical clues for helping us unravel global temperature changes and associated sea-level fluctuations. However, we still know little about the most fundamental crustal transition on the earth's surface: that from continent to ocean. The study of plate tectonics has rewritten Earth's geological history as a story of continents moving across the surface of a (presumed) rigid sphere through time, but it has not yet provided details of their interactions, the critical link in understanding the nature of ocean-continent boundaries (OCBs). Drilling, along with detailed geological and geophysical surveys, must fill that gap.

Ascertaining the geological history of continental margins has been a priority of scientific ocean drilling for many years. Drilling transects across margin pairs are now recognized as critical to properly describe the competing models of intracontinental extension, in particular the roles of thoroughgoing crustal detachment faults in margin formation and subsidence. The Atlantic Ocean is an obvious place for ODP to attack this important theme, because conjugate "passive" continental margins (defined as those where continental and oceanic crusts are fused together) are better developed and more accessible around the Atlantic than anywhere else. The birth of various Atlantic margin pairs has occurred at different times: about 50 million years ago north of Iceland, 130 million years ago between southern South America and Africa, 180 million years ago between North America and Africa, and 110 million years ago for eastern Canada and the Iberian Peninsula. (Even younger margin pairs, for example in the Red Sea, may be addressed in the future.) ODP has chosen two of these pairs as prime examples of volcanic and nonvolcanic end-members of continental fragmentation and ocean-basin formation: southeast Greenland-Norway and Iberia Abyssal Plain-eastern Canada, respectively.

The Eastern Canada-Iberia "Nonvolcanic" Transect

The margins off the Grand Banks and Iberia are logical drilling candidates for several reasons. They have been intensively studied using a variety of marine geophysical and geological techniques, including coring, dredging, bottom and subbottom sound profiling, and submersible diving. As a result, their prebreakup reconstruction is well understood. Breakup-related crustal structures, the key to these margins' early history, are buried under just 2 to 3 kilometers of sediments, making basement rock accessible to ocean drilling. In addition, their locations relative to other thematically important ODP study sites allow convenient repeated access by JOIDES Resolution, and return visits of the drill ship are essential for successful margin drilling, because the research targets are deep and technically challenging.

ODP has just commenced a systematic approach to drilling in passive margins in the North Atlantic with Leg 149 (March to May 1993), which included a transect across part of the Iberia Abyssal Plain (IAP) west of Portugal. The shipboard scientific party encountered faulted blocks composed of rocks of continental affinity separated from normal Atlantic Ocean seafloor basaltic volcanic crust by a broad zone containing both exhumed, faulted oceanic crust and altered plutonic igneous rock known as peridotite. The peridotite forms a ridge that extends for more than 100 kilometers and delimits the approximate ocean-continent boundary along this margin.

The northern Newfoundland Basin (NB) is the conjugate to the Iberia Abyssal Plain. Available geophysical data suggest that the Newfoundland Basin contains a zone approximately 150 kilometers wide of thinned continental crust separating known Grand Banks continental crust from known oceanic crust seaward of a mid-Cretaceous period (about 118 million years ago) isochron, a magnetic anomaly known as MO. This crustal transition is much like that postulated for the Iberia Abyssal Plain. This zone constitutes one of the largest areas of enigmatic seafloor in the North Atlantic. The Newfoundland Basin may also be characterized by a peridotite ridge. However, the Newfoundland Basin differs significantly in one major way--it exhibits a well-defined geological unconformity that caps and occasionally truncates underlying crust out to the interpreted ocean-continent boundary 20 to 40 kilometers west of magnetic anomaly MO. The strong development, relative flatness, and wide areal extent of "U" suggest that it was eroded at or near sea level during the Iberia-Grand Banks breakup.

The first-priority issue for proposed ODP drilling in the Newfoundland Basin is to ascertain the origin of the "U" unconformity and the nature of underlying crust. If the wide transition zone in the Newfoundland Basin proves to be floored by continental crust that has thinned, faulted, and eroded in a subaerial environment, a fundamental new class of crust will be documented that must be accounted for in future models of continental breakup. Drilling in the Newfoundland Basin will also provide the crucial geological control for understanding the early history of this part of the North Atlantic, particularly when used in conjunction with results from the Iberia Abyssal Plain, the other half of the conjugate pair.

What the Future Holds

Continental margin drilling represents a long-term, multinational commitment. Completing the volcanic and nonvolcanic transects as presently defined will take multiple drill ship expeditions over a period of years. This will cost tens of millions of dollars, because continental margin holes require multiple nested metal liners to promote stability for deep penetration. Furthermore, thick sediments present safety hazards because of their potential to contain overpressured fluids and gases. JOIDES Resolution or her successor will eventually need to be equipped with complicated and expensive blowout-prevention capabilities, similar to those now used in the oil and gas industry. Despite these inherent costs and the remaining engineering difficulties, ODP must meet the margin challenge if we are ever to understand the essence of the global jigsaw puzzle that we call home.

James A. Austin, Jr., first recollects seeing the Woods Hole Oceanographic Institution as a toddler, staring through the railing of the ferry bound for his parents' summer home on Martha's Vineyard. About 18 years later, he was admitted to the MIT/WHOI Joint Program in Oceanography, from which he emerged (relatively unscathed) with his doctorate at the end of 1978. Since that time, he has been a research scientist at the University of Texas at Austin. However, New England still calls, that summer home on the Vineyard still exists, and the ferries from Woods Hole still run, so with luck he will never get too far from his oceanographic roots.
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Title Annotation:25 Years of Ocean Drilling
Author:Austin, James A., Jr.
Date:Dec 22, 1993
Previous Article:Fluid composition in subduction zones.
Next Article:When plates collide: convergent-margin geology.

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