The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
Active Tectonics: Studies in Geophysics
Dating is a critical tool in the assessment of active tectonism. The two primary measures of active tectonism are the age and the amount of deformation of a stratigraphic unit, which together define rates of deformation. Although slip or other deformation rates may vary through time, such rates are still one of the most useful measures of active tectonism (Slemmons, 1977).
Dating of active tectonism is commonly accomplished by dating surficial or volcanic deposits. In addition to dating conventional stratigraphic units, materials such as calcite infillings may be deposited in a fault plane and dating of these infillings used to determine the history of faulting. With either conventional stratigraphic units or materials infilling a fault zone, if the material is unbroken its age provides a minimum age for the last faulting; and if it is broken, it provides a maximum age for the last faulting. Although individual fault offsets obviously occur in jumps, the overall slip rate can be determined from the amount of offset, especially for multiple events, divided by the geologic time interval involved.
In the past few decades, our ability to date geologically young deformation and associated deposits has improved greatly. In this time, dozens of dating techniques (Tables 13.1 and 13.2) have been either developed or greatly refined. As few as 10 yr ago, estimates of the age of Quaternary deposits were commonly in error by severalfold, and it was not uncommon for age estimates to have been off by a factor of 10. Although we have recently learned much about the ages of young deposits and deformation, we still have a long way to go, for one of the greatest constraints in our understanding active tectonism is accurate and reliable dating in order to define rates of past deformation and times of past earthquakes (Allen, Chapter 9, this volume).
An appreciation of the amount of dating control needed is illustrated in Figure 13.1, which shows one model of fault activity through time. Predictions based on such a model require multiple dates to determine (1) the slope of the line shown as “accumulation rate,” (2) the interval between fault movements, and (3) the time since the last movement. To define a fault history involving multiple events, at least one age is needed to define each event. This kind of model depends on the assumption of a constant accumulation or slip rate; dating control spanning different time intervals is needed to know if the assumption of a constant slip rate is warranted.
More than one age determination is required to establish reliable age control. Numerical ages are preferred, but relative-dating and correlation methods are important because they can provide age control in the absence
FIGURE 13.1 Diagram of the earthquake-generation process of Cluff et al. (1980) showing importance of time (horizontal scale) in earthquake history and prediction. In order to predict the time of a future earthquake based on this model, note how much dating control is needed to define holding time, accumulation rate (similar to slip rate), and elapsed time, even for this example in which accumulation rate is assumed to be constant.
of numerical techniques, or they can be used to evaluate the numerical ages, which can be subject to large nonanalytical errors. Surficial geologic studies and local time-calibrated stratigraphies are vital in the study of active tectonism, both to provide dating control and to evaluate the reliability of specific age estimates for tectonic events.
OVERVIEW OF DATING METHODS
About 26 different dating methods can be used in dating active tectonism. Table 13.1 categorizes these methods as primarily numerical, relative dating, or correlation (Table 13.1). The numerical methods are based on processes that do not require further calibration. Relative-dating methods are applied to a local sequence of deposits that differ in age but are similar in other characteristics, such as a sequence of glacial moraines or a flight of alluvial terrace deposits. Relative-dating methods provide information on the magnitude of age differences between stratigraphic units. If calibrated, relative-dating methods can be used to estimate numerical ages. This normally requires calibration by numerical methods and an understanding of the process being measured and its relevant history (e.g., temperature, precipitation). The correlation methods do not directly yield a numerical age, but if a feature can be correlated with an event whose age is known, such as a volcanic ash