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Active Tectonics: Studies in Geophysics
FIGURE 2.1 Global seismicity. Earthquakes tend to be concentrated along relatively narrow belts that define the boundaries of active lithospheric plates. However, some earthquakes also occur well within the plates thus defined. Examples include southeast Asia and the intermontane western United States (after Barazangi and Dorman, 1968).
whole story as far as active tectonics is concerned. In many parts of the world, within both ocean basins as well as continental interiors, there are concentrations of earthquake and volcanic activity that seem divorced from plate-tectonic precepts. Hawaii is a dramatic example. The volcanoes of the Hawaiian Islands-Emperor Seamount chain are generally believed to result from upwelling of magmas from a source beneath the Pacific lithospheric plate. Although the geometry of the volcanic chain appears to be due to the motion of this plate over the underlying mantle “hot spot,” the hot spot itself is arguably a phenomenon independent from the overlying global plate framework.
Regions such as southeast Asia and the western United States, where plate boundaries cut into continents, seem especially prone to intraplate tectonics. The volcanic and seismic activity in such areas is conspicuous (Figure 2.1) and often dramatic (Figure 2.2). The semantics of whether tectonic activity in such areas should be be considered truly intraplate or treated as some kind of distributed effect of a distant plate boundary largely begs the issue. Regardless of nomenclature, the fundamental causes of many such phenomena remain unclear, and their place in the plate-tectonic framework unresolved.
The ancient Precambrian cores of the world’s continents contain special problems for understanding active tectonics. Although the most obvious manifestations of contemporary tectonics, such as seismicity, are decidedly less pronounced than along the plate boundaries, or even in the geologically younger intraplate regions, earthquakes do occur in the craton and near its periphery. In the United States, two of the most destructive earthquakes in history occurred not along the San Andreas Fault but in the nominally “stable” eastern United States near New Madrid, Missouri, in 1811–1812, and near Charleston, South Carolina, in 1886. Seismicity continues in many parts of the eastern United States, and faults of relatively recent geologic vintage have been identified (York and Oliver, 1976).
That parts of the cratonic interior are tectonically active in the present time should perhaps not be surprising in view of the geologic record. Features such as the Michigan Basin and the Adirondack Dome (Figure 2.3) are incontrovertible evidence that the cratons were subject to major vertical motions in the past that lack a clear connection to the plate-tectonic scenarios of those times. Geologic strata attest to many gentle inundations and uplifts of the interior platforms that reflect relative mo-