FIGURE 13.13 A, five patterns of fault offset versus time. B, contrasting slip rates for these five patterns over short, medium, and long intervals of time. Slip rates are in arbitrary distance/time units, and long-term deformation for all patterns has been arbitrarily set at 1.

slip rate or constant recurrence intervals would be, of course, quite misleading.

Thus, sufficient dating related to deformation histories is required to understand the character of faulting in different tectonic settings and thereby to anticipate more intelligently the future deformation over time intervals of concern to man. Dating of a single fault history without concern for related faults may be examining too small a component, for relatively constant tectonic activity may be unevenly distributed among a group of faults.

The definition of rates and knowledge of the constancy or variation of these rates through time permit quantitative ranking of tectonic activity, both for the purposes of scientific understanding and hazard assessment.

DATING METHODS IN FUTURE RESEARCH ON ACTIVE TECTONISM

All the dating methods discussed (Tables 13.1 and 13.2) have importance to studies of active tectonism. In the last decade, much improvement, refinement, and evaluation of the reliability of these methods have been associated with studies of active tectonism.

Because experimental methods such as 36Cl and 26Al are numerical and based on radioactive decay, some may consider such methods the most promising for future advances. But such methods also involve assumptions concerning nonradiometric processes—one assumes that the isotope measured both accumulated at a known rate and no subsequent leaching has occurred. These nonradiometric factors are difficult to evaluate rigorously and have similarities with, for example, the extremely complex process of soil development. For example, 10Be accumulation in soils is influenced by the clays in the soil, which generally increase in quality and may change mineralogically as the soil develops. The recent quantification of soil development using the Profile Development Index, as well as analyses of changes in individual soil components such as soil carbonate or clay, can provide useful but not precise dating control; soil development is nearly always applicable to the dating of active tectonism.



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