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Sykes’s study was facilitated by the rapidly accumulating collection of seismograms, readily available on photomicrofiche, from the new World Wide Standardized Seismographic Network (WWSSN) set up under Project Vela Uniform. These high-quality seismometers had good timing systems, fairly broad bandwidth, and a nearly uniform response to ground motions, and they were installed and permanently staffed around the world at recording sites with relatively low background noise levels (53). The high density of stations allowed smaller events to be located precisely and their focal mechanisms to be determined more rapidly and accurately than ever before. One result was much more accurate maps of global seismicity, which clearly delineated the major plate boundaries, as well as the Wadati-Benioff zones of deep seismicity (Figure 2.10).
Subduction of Oceanic Lithosphere
If the Earth’s surface area is to remain constant, then the creation of new oceanic crust at the ridge crests necessarily implies that some old crust is being recycled back into the mantle. This inference was consistent with the theories of mantle convection that attributed the volcanic arcs and linear zones of compressive orogenesis to convective downwellings (54), which David Griggs had discussed as early as 1939, calling it “a convection cell covering the whole of the Pacific basin, comprising sinking peripheral currents localizing the circum-Pacific mountains and rising currents in the center” (55). Griggs belonged to a growing group of “mobilists” who espoused the view that the Earth’s solid mantle is actively convecting like a fluid heated from below, causing large horizontal displacements of the crust, including continental drift (56). The alternative, expanding-Earth hypothesis states that the planetary radius is increasing, perhaps owing to radioactive heating or possibly to a universal decrease in gravitational strength with time, and that seafloor spreading accommodates the associated increase in surface area (57). Thus, new oceanic crust created at the spreading centers does not have to be balanced by the sinking of old crust back into the mantle.
Because of this controversy, as well as the geologic complexity of the problem, subduction was the last piece of the plate-tectonic puzzle to fall into place (58). While the system of oceanic ridges and transform faults fit neatly together in seafloor spreading, the compressional arcs and mountain belts juxtaposed all types of active faulting, which continued to baffle geologists. Benioff had pointed out the asymmetric polarity of the island arcs, correctly proposing that the deep oceanic trenches are surface expressions of giant reverse faults (59). Robert Coats used this idea to account for the initial formation of island arcs such as the Aleutians and the geochemical data bearing on the development of the