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Active Tectonics: Studies in Geophysics
Age Range and Optimum Resolution
Basis of Method and Remarks
RESOLUTION DEPENDS ON RECOGNITION OF FEATURE AND ACCURACY OF DATING THAT FEATURE
Based on physical properties and sequence of units, which includes superposition and inset relations. Depends on the establishment of time equivalence of units; deposition of Quaternary units normally occurs in response to cyclic climatic changes.
Requires volcanic ash (tephra) and unique chemical or petrographic identification and (or) dating of the ash. Very useful in correlation because an ash eruption represents a virtually instantaneous geologic event.
Depends on correlation of remnant magnetic vector, which includes polarity, or a sequence of vectors with a known chronology of magnetic variation. Subject to errors due to chemical magnetic overprinting and physical disturbance.
24. Fossils and artifacts
Depends on the availability of fossils, including pollen, and artifacts. Resolution depends on the rate of evolution or change of organisms or cultures and on calibration by other techniques. Subject to errors due to misidentification and interpretation.
25. Stable isotopes
Depends on correlation of the sequence of isotopic changes with an age-controlled master chronology. Oxygen isotopic record is useful in deep-sea and ice-cap cores and perhaps in cave deposits.
26. Tektites and microtektites
Depends on recognition and dating of glassy material (tektites) formed during impact of extraterrestrial masses. Tektites are scattered over large areas, such as the Australo-Asian tektite field, formed about 700 ka.
XXXX, nearly always applicable
XX, often applicable
XXX, very often applicable
X, seldom applicable
tonism illustrate powerful applications of this method. Along the San Andreas Fault 55 km northeast of Los Angeles at Pallet Creek, about 50 carbon-14 ages date 11 episodes of faulting in the 1700 yr prior to the 1857 historical rupture and define an average recurrence interval of about 145 yr (Figure 13.2; Sieh, 1984). For the south Boso Peninsula of Japan, carbon-14 dating defines four stepwise uplifts of land relative to sea level in the last 7000 yr (Figure 13.3). Shimazaki and Nakata (1980) concluded that the offset history supports a time-predictable recurrence model; that is, the larger the amount of the coseismic slip, the longer the interval before the next earthquake (Figure 13.3).
Carbon-14 ages may be in error by much more than the analytical uncertainty. Because of the extensive use of carbon-14 dating in studies of active tectonism, consideration should be given to the following three types of carbon-14 dating problems.
Fluctuations in Atmospheric Carbon-14 Based on carbon-14 dating of tree rings whose absolute age is known, carbon-14 ages deviate from actual ages by amounts that are significant for some tectonic studies. For example, in the interval from 5000 to 8000 yr ago, carbon-14 ages are about 500–900 yr too young (Klein et al., 1982). In the late Holocene, fluctuations in atmospheric carbon-14 introduce significant uncertainty in dating tectonic events (Figure 13.4). For example, a carbon-14 age of 150+20 ya (years before AD 1950) only defines an age in the interval from 0 to 295 calendar years before AD 1950 (Klein et al., 1982, Table 2).
Contamination with Old Carbon Independent of the age of the sample, the effect of contamination with old carbon is constant (Figure 13.5, upper left half). Regardless of whether a sample is 1 or 30 ka, incorporation of 10 percent “dead” carbon will make ages 800 yr too