Active Tectonics Impact on Society (1986) / Chapter Skim
Currently Skimming:
2. Epeirogenic and Intraplate Movements
2. Epeirogenic and Intraplate Movements
Pages 30-44
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 30... ...
Even seemingly slow vertical motions may threaten river courses or seafront properties on socially relevant time scales, and the subtle strains accumulating elsewhere may portend future earthquakes or volcanoes in the least predictable places. Even a cursory glance at global earthquake activity makes clear that much of the world's tectonic activity is concentrated in relatively narrow belts (Figure 2.1~.
|
From page 31... ...
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.
|
From page 32... ...
Any consideration of the active deformation of intraplate interiors must, of course, recognize the importance of vertical motions associated with the retreat of the ice sheets since the last major continental glaciation. Studies of the contemporary uplift of Fennoscandia as documented by leveling observations and tilted beach terraces are now classic (e.g., Niskanen, 1939~.
|
From page 33... ...
. Plate interiors often contain isolated geologic structures that are difficult to relate to ancient plate boundaries.
|
From page 34... ...
It has proven extremely difficult to associate intraplate earthquakes, even large ones, with specific faults in areas like the eastern United States. The easternmost example of Table 2.1 is that of inferred coseismic deformation associated with the 1931 Valentine, Texas, earthquake, which is arguably still within the seismic regime of the active Rio Grande rift system (Ni et al., 1981~.
|
From page 35... ...
An earthquake in the eastern United States is likely to be felt over or damage a much larger area than an equivalent earthquake along the San Andreas Fault, for example (Figure 2.5~. Mitigating hazards from earthquakes in the stable interior remains one of least tractable yet most important problems in contemporary tectonics.
|
From page 36... ...
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 ter races and geodetic measurements of continued uplift (Figure 2.7~.
|
From page 37... ...
r ~ · W= ; . ~ = ~Pro cello Focies 1 I ~ HI ~ I I 2 RECENT VERTICaL CRUSTAL MOVEMENT Theoreti ca I Def lections I Observations Theoretical Sediment Load _ , 1 ~:~ DJ Deltaic Deposits [53 Strand Plain Deposits O Pleistocene Prairie Fm.
|
From page 38... ...
Although some of these apparent motions may be remnants of postglacial rebound, as in the Baltic Shield or the Great Lakes area of the United States, most lack a clear-cut neotectonic explanation. However, before ascribing these motions to some new, unheralded form of intraplate tectonics, it is important to recognize that there are major outstanding questions about the accuracy of the geodetic measurements on which most such studies are based.
|
From page 39... ...
Yet neither intraplate stress patterns nor seismicity are by any means simple, and the link, if any, to distant plate boundaries is more often obscure than not. One possible reason for complexity in intraplate tectonics is reactivation, i.e., the concept that present tectonics is guided by crustal heterogeneities formed during much earlier times.
|
From page 40... ...
In some respects, such an explanation seems a bit ad hoc, and skeptics might infer that the inconsistency between some geodetically measured rates and the more subdued geologic record is further reason to question the accuracy of the former. However, it should be remembered that rates of vertical motion on the order of a few millimeters per year are much smaller that the commonly accepted rates of a few centimeters per year for the hori ~?
|
From page 41... ...
reactivated during //; in early Orb it/`, ~ - , / rifting in Mesozoic //' No of He / Offset Magnetic Anomalies E=Earthquake; o=Alkalinerocks; ring dikes; K-Kimberlite FIGURE 2.12 Intraplate tectonics may be influenced by inherited tectonic structures. In this example, relict transform faults left behind from the opening of the Atlantic may serve as weaknesses that concentrate modern intraplate stresses, thus constituting potential foci of seismic activity (Sykes, 1978)
|
From page 42... ...
Yet much remains to be done. Funding agencies and investigators alike must realize that proper resolution of critical issues concerning active tectonics of plate interiors will require a longterm commitment because of the very nature of the processes involved.
|
From page 43... ...
. Recent vertical crustal movements from precise leveling data in southwestern Montana, western Yellowstone National Park, and the Snake River Plain, J
|
From page 44... ...
Correlation of tectonically deformed shorelines on the southern Atlantic Coastal Plain, Geology 5, 123-127. Woollard, G
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.