--> Exploring Application of K-Ar Isotopic Analyses to Fault Gouges: Is Age of Faulting Disclosed?, by Muhammad Shafiqullah, George H. Davis, Paul E. Damon, J. Reed Glasmann, and Louis J. Demer; #91024 (1989)

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Exploring Application of K-Ar Isotopic Analyses to Fault Gouges: Is Age of Faulting Disclosed?

Muhammad Shafiqullah, George H. Davis, Paul E. Damon, J. Reed Glasmann, Louis J. Demer

K-Ar isotopic analysis of three samples of illitic clay in fault gouge and one sample of ultracataclasite appear to contribute critical information on the timing and the physico-chemical environment of a middle Tertiary detachment fault near Tucson, Arizona.

The submicron illitic clays from hanging-wall gouge zones along the middle Tertiary Catalina detachment fault yield concordant K-Ar dates of approximately 21 Ma. Analyses employing XRD and TEM reveal that the clays consist mainly of illite and chlorite with well-defined crystal habits. This illite-chlorite assemblage is interpreted to have formed authigenically from fluids in the temperature range of 150° to 200°C. These fault gouges were derived from Precambrian, Paleozoic, and/or Mesozoic protoliths. The 21 Ma age determination represents the minimum age of faulting. Abundant evidence for hot fluid circulation during faulting suggest that 21 Ma closely approximates the actual timing of faulting.

The ultracataclasite sample was collected from footwall rocks immediately beneath the low angle Catalina detachment fault. The ultracataclasite was formed by extreme comminution of mylonitic gneiss derived originally from Precambrian granite in a dry high-stress environment. Submicron quartz, feldspar, and biotite are present within exceedingly fine-grained flattened, and welded microbreccia fragments, as revealed by XRD, SEM, and TEM analyses, which yield a concordant 21.4 Ma K-Ar date.

Geologic evidence suggests that formation of the ultracataclasite preceded formation of the clayey gouge. The close similarity of the K-Ar dates implies that the detachment faulting along the Catalina fault system was geologically instantaneous.

Work to date demonstrates that K-Ar dating of fault gouge and cataclasite is a frontier of research that may assume significance and practical importance to structural geologists, regional tectonicists, and economic geologists in evaluating the chronology of tectonic events involving fault displacements.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.