Thermochronologic approaches to paleotopography

Although thermochronology cannot directly constrain paleoelevation, it can provide estimates of the form, location, and scale of paleotopographic relief, i.e., paleotopography, and its change through time. Unique thermochronologic perspectives on paleotopography come from 1) spatial patterns of surface and subsurface cooling ages or cooling histories that reflect either the influence of topography on subsurface isotherm warping, or spatially focused erosion (including incision), and 2) the age-elevation relationship in paleolandscapes that may be preserved in detrital cooling age distributions. This chapter reviews the fundamental theory and results of these approaches and several example applications. Case studies show examples of both decreasing and increasing topographic relief through time, at the orogen scale and across short ridge-valley wavelengths, and significant modification of local topographic features in glaciated and fluvial settings. In some cases, thermochronologic evidence for fluvial incision at short wavelengths has also been argued to be the result of surface uplift at very long wavelengths. Although not yet used for such purposes, detrital approaches also have the potential to reconstruct paleotopographic relief and paleohypsometry in paleocatchments. In all cases, paleotopographic interpretations from thermochronology require important assumptions or case-by-case support from other lines of evidence. Central issues pertinent to thermochronologic interpretations of paleotopography are the nature of the shallow crustal thermal field through which the samples cooled (including the influence of fluid flow) and the role of rock uplift gradients in modifying simple relationships between erosion and topography.