tral uplift over the magma chamber is flanked by a zone of subsidence, the overall pattern suggesting withdrawal of magma from one reservoir and injection into another (Larsen et al., 1985). An intriguing aspect of this uplift is that it occurs in an area where there have been no historical eruptions, although volcanic rocks in the rift attest to such activity as recently as 100,000 yr ago (Lipman and Mehnert, 1975). Whether this ground motion portends some future eruption or is part of some normal cycle of magma transfer at depth with little chance of breaking out at the surface has yet to be ascertained.

Geodetic observations have been linked to possible intracrustal magmatism in other parts of the Rio Grande rift (e.g., Reilinger et al., 1979), in Yellowstone National Park (Pelton and Smith, 1982), and, most recently, near Mammoth Lakes in eastern California (Savage and Clark, 1982; Castle et al., 1984). In the latter example, changes in elevation and horizontal strain have been interpreted to indicate magmatic resurgence of the Long Valley caldera by inflation of an approximately 10-km-deep magma chamber. This inflation may be responsible for a series of four magnitude-6 earthquakes in the area in 1980. Because there have been several explosive eruptions and extrusion of rhyolite domes in this area during the past 400 yr, and because Long Valley is less than 200 miles from major cities like Sacramento and San Francisco, deformation and seismic activity are being monitored in order to predict possible future activity.

The local and regional doming near Yellowstone is difficult to relate to standard plate-boundary processes. Magma injection is thought to result from a “hot spot” that can be traced into the North American continent along the Snake River Plain (e.g., Suppe et al., 1975). In this sense it is clearly an intraplate phenomena. On the other hand, it could be argued that the apparent magma uplifts near Mammoth Lakes and in the Rio Grande rift really represent plate-boundary processes. The former may be a relict of the subduction-related volcanism, which for the most part ceased when the West Coast converted from a convergent to a strike-slip margin. Likewise the New Mexico activity could well represent the beginnings of a new plate boundary, a rift that may evolve into a new ocean basin by splitting off the southwestern United States.

Semantics notwithstanding, such examples must be considered in order to understand intraplate phenomena. After all, neither lies upon a currently active plate boundary per se. More to the point, the geologic processes that they represent may well pertain to other “intraplate” phenomena whose association with similarly defunct or precursor boundaries is simply not yet so apparent.

CRUSTAL LOADING

Contemporary deformation of the stable interior is virtually synonymous in many minds with postglacial rebound. The broad doming of recently deglaciated parts of North America and Scandinavia has long been documented by geologic studies of warped beach terraces and geodetic measurements of continued uplift (Figure 2.7). This motion is perhaps the best understood, geomechanically speaking, of any type of intraplate deformation, although controversy still revolves around distinguishing those possible deep-earth rheologies that are most consistent with the observed rebound effects (e.g., Kaula, 1980).

An important aspect of recent studies of postglacial phenomena, especially sea-level changes, is that this is a global phenomenon, not restricted to the immediate area of glacial retreat. Concepts such as the collapse of a peripheral bulge (e.g., Walcott, 1972) have been re-

FIGURE 2.7 Postglacial rebound of Fennoscandia (Balling, 1980). Contours represent uplift in millimeters per year measured by precise leveling.



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