PATTERNS OF COOLING IN BASEMENT ROCKS - A BOOTSTRAP METHOD TO MEASUREANOMALOUS SPATIAL-DISPERSION OF ZIRCON FISSION-TRACK AGES

Thermochronology depends on isotopic systems for which the age is rela ted to the time that a specified mineral cooled through its closure te mperature. In tectonic studies, it is often of interest to examine reg ional variations in cooling age. When the variation in age within a da ta set exceeds the analytical errors on the age determinations, many o ptions are available to model the spatial variation in age and to corr elate it with other data or with the predictions of hypotheses. For ex ample, trend surfaces [e.g. Davis J. C. (1986) Statistics and Data Ana lysis in Geology, 2nd edn. Wiley, New York] could be used to explain t he variation that exceeds experimental error. In the case of interest in this study it may appear that the variations in age originate entir ely from random analytical error. We show how geologically significant patterns that may be present in such apparently random data can be de tected. Our analysis is based on characterizing how the extreme ages ( oldest and youngest) are distributed among the sample localities. In p articular, we explore whether the extremes are more dispersed or more clustered than could be expected from a random assignment of ages to t he localities, as deduced from bootstrap simulations. This mode of ana lysis is non-parametric and requires no assumptions about the distribu tional form of the errors or the ages. The proposed analysis is applie d to 34 zircon fission-track ages from the central Appalachian Piedmon t, eastern U.S.A. Our results show that the older ages are concentrate d near the center of the sample region and are surrounded by younger a ges. This age pattern suggests that rocks now at the surface in the ce ntral part of the study area cooled first, followed by rocks located t oward the periphery of the area.