Active Tectonics Impact on Society (1986) / Chapter Skim
Currently Skimming:
13. Dating Methods
13. Dating Methods
Pages 195-214
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 195... ...
Experimental isotopic-dating techniques such as Abe, 36C1, and °6A1 also offer potential for dating faulting and other deformation. However, refinement of relative-dating techniques, such as soil development, rock weathering, and progressive landform modification are likely, because of their general applicability, to provide much of the needled age control.
|
From page 196... ...
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.
|
From page 197... ...
A dating technique, whether it be primarily a numerical, relative-dating, or correlation method, may be converted to the other two categories of methods (Table 13.1~. For example, the relative-dating methods of amino-acid racemization or soil development can also serve either as local correlation techniques or, if calibrated by numerical dating, as numerical techniques.
|
From page 198... ...
TL precision better than indicated for ceramics in 400-10,000-year range. Dating methods analogous to '4C-dating are based on the cosmogenic isotopes (half-life in hyears in parentheses)
|
From page 199... ...
Requires relatively constant rate of sedimentation over time intervals considered. Numerical ages based on sediment thickness between horizons dated by other methods.
|
From page 200... ...
Z Z O ~ u: O Z O ~ ~ Z o cat Let at; 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.
|
From page 201... ...
was thought to date the Salmon Springs glaciation of northwest Washington State (Stuiver et al., 201 1978) until associated ash deposits were fission-track dated at 700 and 800 ha (Easterbrook et al., 1981~.
|
From page 202... ...
Lower part of figure shows cumulative uplift versus time; line through corners of stairstep plot shows that uplift, inferred to be caused by coseismic faulting, conforms to a time-predictable model. FIGURE 13.4 Carbon-14 age as a function of dendrochronological age for the past 1000 yr (from Stuiver, 1982~.
|
From page 203... ...
For active tectonism in semiarid and drier areas, 230Th/234U dating of carbonate may date fault movements. Soils may contain carbonate coats on the undersides of stones that can date faulted alluvial surfaces.
|
From page 204... ...
Potassium-Argon Dating The potassium-argon (K-Ar) method is limited in its applicability to dating active tectonics by its primary use with igneous rocks and general lack of resolving power for rocks younger than a few tens of thousands of years.
|
From page 205... ...
(1984) showed that i°Be is adsorbed onto clays in soils and systematically increases in abundance with soil age for at least the first 100 ka of soil development.
|
From page 206... ...
Weathering rinds on basaltic and andesitic stones from the B horizons of soils have yielded age information on middle and late Quaternary deposits at seven different areas in the western United States (Figure 13.9; Colman and Pierce, 1981~. Multiple measurements of rind thicknesses from a given stratigraphic unit are consistent; and, for a succession of deposits, rind
|
From page 207... ...
Local calibration by numerical dating indicates that rind thicknesses increase logarithmically with time. Based on a logarithmic increase in rind thickness and on the assumption that deposits with a certain relative moraine form and degree of soil development correlate with deposits of the Bull Lake glaciation at West Yellowstone (oxygen-isotope stage 6)
|
From page 208... ...
but are discussed here because they are a component of soil development. On the downwind and downthrown side of a fault offsetting a flat, former basin floor of the Rio Grande rift, New Mexico, fault movements were rapidly followed by deposition of eolian sand.
|
From page 209... ...
Numerical dating control is normally obtainable only at scattered localities, and extension of this dating control to sites of deformation depends on stratigraphic correlation over distances of tens to even hundrecls of kilometers. The age of stratigraphic units in Quaternary geologic successions is the subject of much current research.
|
From page 210... ...
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)
|
From page 211... ...
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.
|
From page 212... ...
For example, Abe 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.
|
From page 213... ...
Because the stratigraphy of many Quaternary deposits reflects the cycles of climatic changes, the ages of faulted or uplifted datums can be inferred if relations between stratigraphic units and climatic cycles are known. Study of active tectonism and Quaternary stratigraphy should proceed together also because evidence of tectonism and climatic change is commonly similar.
|
From page 214... ...
. Preliminary investigations of late Quaternary slip rates along the southern part of the Wasatch Fault zone, in Evaluation of Regional and Urban Earthquake Hazards and Risk in Utah, W
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.