in judging the reliability of these ages and relating ages to deformation of stratigraphic units.

Tephrochronology

A volcanic ash can provide a time-parallel marker whose age is as accurate as the best dating either at any of its occurrences or of the correlative volcanic rocks in its source area. Recognition of a given ash bed should be based on multiple criteria, especially the petrography and chemistry of the glass and phenocrysts, as well as stratigraphy, paleomagnetism, paleontology, and radiometric dating (Westgate and Gorton, 1981). Tephrochronology has proved of great value in dating active tectonics. For example, in southern California deposits containing the 0.7-Ma Bishop ash are laterally offset about 6.6 km on the San Jacinto Fault (Sharp, 1981), and deposits containing 0.6-, 0.7-, and 1.2-Ma volcanic ashes are uplifted and tilted along the Ventura Fault (Yeats, Chapter 4, this volume). Tephrochronology is also important in calibrating other relative-dating and correlation methods, such as soil development, uranium-trend dating, amino-acid racemization, thermoluminescence dating, and dating of faunal boundaries. These calibrated methods can then, in turn, be used to date active tectonism.

Improvements in tephrochronology have led to major revisions in Quaternary stratigraphy and age assignments. In the 1960s, the Pearlette ash (known present distribution from California to Iowa) was considered to be of a single age, and the stratigraphy and paleontology of older Quaternary deposits all the way from the midcontinent to the Rocky Mountains were founded on the assumption that the Pearlette ash represented one eruption of late Kansan age. Careful petrographic and chemical study of the Pearlette ash has shown that it actually includes ashes from three different and distinct eruptions from the Yellowstone area that differ in age by a factor of 3 (Huckleberry Ridge ash, 2 Ma; Mesa Falls ash, 1.2 Ma; and Lava Creek ash, 0.6 Ma; Izett, 1981).

Paleomagnetism

The orientation of the Earth’s magnetic field is recorded by the orientation of magnetic minerals at the time of deposition of many fine-grained sedimentary deposits. Dating control can be obtained if the paleomagnetic record determined from a sequence of late Cenozoic sediments or volcanic rocks can be correlated with the established paleomagnetic polarity time scale (Mankinen and Dalrymple, 1979; see Barendregt, 1981, for a review). For example, the change from the Matuyama Reversed-Polarity Chron to the present Brunhes Normal-Polarity Chron occurred about 730 ka. This change provides a global datum for the assessment of long-term tectonic rates and calibration of relative-dating methods.

The established polarity time scale also dates the age of the ocean floor outward from the ocean-ridge spreading centers. The rates of movement of crustal plates away from these spreading centers is based on the age and width of the normal and reversed polarity stripes of the ocean floors.

Within the Brunhes Normal-Polarity Chron, potential age control may be provided excursions or reversed-polarity subchrons that lasted a few thousand years; about five such events have been suggested. In addition, secular variation in the geomagnetic field with periodicities of thousands to tens of thousands of years may provide a basis for local correlation and dating. The record of secular variation is best studied in lacustrine and other environments of continuous fine-grained sediment deposition. Such sediment dated by paleomagnetic criteria may also record sediment deformation associated with nearby earthquakes.

Fossils and Artifacts

Fossils have been of limited value in dating young deposits because the amount of Quaternary evolutionary change has been small. Some organisms, such as the rapidly evolving microtine rodents, do show several changes each million years and can therefore be of great value in estimating long-term deformation rates. An example of dating active tectonics comes from east of San Francisco Bay where the Verona Fault offsets Livermore Gravel and is mapped within 60 m of the General Electric Test Reactor at Vallecitos (Herd, 1977). The age of this faulted gravel was poorly known until it was dated using small mammal faunas as about 500 ka (C.A.Repenning, U.S. Geological Survey, personnal communication, 1984).

The cyclic climatic changes of the Quaternary Period resulted in cyclic changes in plant and animal populations. Such plant or animal changes also provide a basis for dating active tectonism. For example, pollen assemblages representing climate considerably colder than at present may be used to infer a pre-Holocene age (>10 ka).

WHY DATING SPANNING DIFFERENT TIME INTERVALS IS NEEDED

Geologic prediction of future deformation requires enough dating control to understand if and how deformation has changed through time. For most active tec-



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