tonism, we know little about whether strain rates are uniform through time. Even with the simplifying assumption that strain and long-term slip rates are uniform, a fault scarp with evidence of recent movement can yield dramatically different predictions: (1) if the slip rate is fast, future movements are likely soon, or (2) if the slip rate is slow, future movements are unlikely soon.

Spreading rates at oceanic ridges and movement rates of crustal plates appear to be rather constant over the planning intervals of concern to man and his activities. Consequently, constant strain rates may be appropriate for some major tectonic features. For many individual faults, however, this assumption is probably not valid. Even for some faults in the boundary zone between crustal plates, slip rates have changed greatly. On the San Jacinto Fault southeast of Los Angeles, the slip rate for the past 730 ka has averaged about 9 mm/yr, whereas the rate between 0.4 and 6 ka averaged about one-fifth that (Table 13.3; Sharp, 1981). These large differences in rate may have resulted from differential movements between the Pacific and American plates being localized at times on the San Jacinto Fault and at other times on the nearby San Andreas Fault (Sharp, 1981).

On a fault in the Rio Grande rift near Albuquerque, New Mexico, the rates of deformation have decelerated over the last 400 ka. Recurrence intervals on this fault are quite long, averaging more than 100 ka (Figure 13.12 ).

For the total Basin and Range province encompassing the 700-km distance between the crests of the Wasatch Mountains of Utah and the Sierra Nevada of California, the overall rate of extension may be relatively constant. But Holocene and historical activity in the Basin and Range is spatially clustered in zones. Holocene tectonic events (Ms about 7 or larger) define an eastern and western zone; these zones are separated by a zone about 300-km wide encompassing the Nevada-Utah border in which no late Quaternary scarps are recognized (Wallace, 1981).

Grouping of events in time may also occur. Some segments of the Wasatch Fault zone have had three or more

TABLE 13.3 Variable Slip Rates Through Time, San Jacinto Fault, Southern California (from Sharp, 1981)

Time Interval

Slip Rate (mm/yr)

Change in Slip Rate Through Time

0–400

3.9±1.1

Twofold increase

400–6000

1.7±0.3

Fivefold decrease

0–730,000

 

Holocene (last 10 ka) offsets and exhibit slip rates during the Holocene of 1.3+0.1 m/ka, yet other sections of the Wasatch Fault zone have not been active in the Holocene (Schwartz et al., 1983). In addition, based on fission-track annealing ages, the uplift rate of the Wasatch Mountains for the past 10 Ma has been about one-third the Holocene rate, or about 0.4 m/ka (Naeser et al., 1983). Work in progress on deposits as old as 250 ka sheds light on the meaning of these rates (Machette, 1984). As dated by calcic-soil development, the slip rate has been on the order of 1 m/ka during the last 5 ka; this rate appears to have been more than 5 times greater than that over the past 250 ka, suggesting variable slip rates and temporal grouping of fault offsets (Machette, 1984).

Rates of fault slip or other deformation are dependent on both the deformation pattern through time and the time window of observation. To illustrate this point, Figure 13.13 shows the effect of short, medium, and long time windows on the slip rates for five patterns of deformation: accelerating, constant, decelerating, episodic-quiescent, and episodic-active. For convenience, the deformation patterns are shown as systematic, and the deformation rates are arbitrarily set at 1 for the longest interval. Depending on the time window, deformation rates for each pattern may differ by more than an order of magnitude. Also, although the long-term rate is arbitrarily set at 1, the short interval has rates that differ by more than 2 orders of magnitude (Figure 13.13B),

Deformation rates determined for differing time intervals will contribute to an understanding of deformation patterns through time in different tectonic settings. Few fault histories such as those shown by Figures 13.2, 13.3, and 13.12 have been determined. Deformation histories like that shown by lines for episodic-active and episodic-quiescent (Figure 13.13) have not been well documented, but, as discussed previously, some evidence suggests that these patterns of deformation have occurred.

Only by understanding the history of a fault can we better understand what can be expected in the future. Consequently, the deformation history needs to be defined by multiple dates. The simplifying assumption that tectonic events such as rates of faulting or rates of uplift are constant may be useful as a first approximation, but this assumption may be quite misleading in some tectonic environments. Few fault histories are well enough dated to know in what cases the assumption of constant slip rate is valid and in what cases it is not. By detailed documentation, we can construct predictive models appropriate to a given tectonic setting. If movements on a given fault are grouped in time, or if faults in an area alternate in activity, concepts such as constant



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