Eavesdropping at Continental Edges

Abstract

Scrolls of seismic records, looking to the uninitiated like TV screens full of static, were tacked everywhere around the conference room at Woods Hole (Mass.) Oceanographic Institution (WHOI). Forty-eight earth scientists from four countries gathered recently to compare these 3to 18-foot-long records of the earth's response to probing with sound waves. The seismic profiling method that generated these records was first developed 60 years ago, but with technological advances has more recently been embraced by academic scientists as a powerful tool for mapping the underlying structure of the crust and upper mantle. One purpose of the WHOI meeting was to discuss the strengths and weaknesses of the various profiling techniques that are currently available. in a larger sense, these geologists and geophysicists had come to learn about the evolution of continents by hunting for common patterns in seismic sections of continental margins, the transition regions between the continents and the ocean floor. According to the theory of plate tectonics, continents are embedded (along with oceanic crust) in about a dozen plates that float, like rafts, on a viscous underlayer in the mantle. Propelled by the formation of new seafloor, the drifting plates periodically collide and grind together, creating mountain ranges and deep sea trenches. When plates separate, oceans are born and continents can be rifted and fragmented. Many scientists believe, for example, that the present Atlantic Ocean was created 150 million years ago when a large land mass split into the continents of Africa, America and Eurasia. Plate tectonics was a great event in the history of geology, says Jack Oliver of the Consortium for Continental Reflection Profiling (COCORP), which operates through Cornell University in Ithaca, N.Y., and has been instrumental in seismically charting the North American continent. But at the same time, plate tectonics is based largely on data for the oceanic areas, and it tells us mostly about what's going on in the oceans. We're developing a story of continental tectonics that will mesh with plate tectonics, but it will tell us in more detail about the kinds of rocks we see in the continents. A key to understanding the processes that shape continents may lie with the layers of rock buried deep in the crust and upper mantle at continental margins. Because these layers are well beyond the reach of drilling, geophysicists have turned to seismic profiling to probe the earth's architecture. Seismic reflection profiling was first developed by oil companies in the 1920s to map the unseen layers of sediment and rocks in the upper crust. Sound waves are generated at the surface either with large truck-mounted vibrators on land or with air guns or explosive charges at sea. These sonic signals, propagating through the crust, are reflected back whenever they encounter an interface between different kinds of rock-sandstone and carbonates, for example. By collecting, sorting and adding up all the reflected sound waves, a picture, or seismic section, of the earth's vertical structure is constructed in much the same way medical ultrasonic scanning is used to depict the interior of the human body. The petroleum industry which is, of course, interested in looking for oil only at depths to which it can drill -tends to take seismic data of relatively shallow layers, down to only about 10 kilometers. By the 1960s, however, geophysicists began to notice reflections from much deeper layers, around 30 km. These signals were written off at first as being artifacts of the seismic technique, but gradually scientists began to realize that the deep reflectors in the lower crust and upper mantle were real. It's become clear that observations of Ridge-rift New sea floor ' Continental marNorth America Africa '2 gins, the transition . | 2regions between 3 the continental crust , and the ocean floor, o are created after a n continents are torn O or rifted apart. As Continental Oceanic crust . the rift widens and c margin rSediments | (Ridge-rift Opening Atlantic 3 new seafloor is C ~ ~ ~ ~ ~~~8 born, thick wedges $ *... of sediment are de' i _ posited on the margins from inland erosional debris.