TOPOGRAPHY AND ISOTHERMS: IMPLICATIONS FOR LOW-T GEOCHRONOLOGY

In thermal models that are used to explain geochronological cooling age data it is generally assumed that the isotherms are planar surfaces at depth. However, at shallow crustal levels isotherms will follow the surface topography in a damped manner. Thus, for low-T geochronology it may be important to consider the curvature of isotherms underneath topography for a meaningful interpretation of the data. Stüwe et al (1993) have shown that the interpretation of apatite fission track ages and ages derived from the closure of the (U-Th)/He system may be significantly influenced by topography above 2 km, if the denudation rate is of the order of several millimeters per year or more. Their study was confined to the interpretation of a topography where the denudation rate is a constant. We have now expanded this study, in order to explore the influence of topography on low-T isotherms if the denudation rate is spatially variable, for example due to differential rain fall on the different sides of a range or due to asymmetry of the topography, for example at passive margins. We show that such asymmetric denudation should be recognizable in the record of low-T geochronological methods that date temperatures below about 150°C, if the denudation rate exceeds several millimeters per year. We expect to be able to document an increase in cooling rate with decreasing temperature on the high denudation side of a range and a decrease in cooling rate with decreasing temperature on the low denuding side of the range. Cooling ages may be twice as old on the slow denuding side than they are on the rapidly denuding side, even if the topography remains self similar through time.