FROM THERMOCHRONOLOGY TO EROSION

Thermochronology constrains erosion rates and their spatial and temporal patterns by using temperature, and its systematic control on diffusion or annealing, as a proxy for crustal depth. Thermochronology integrates erosion histories over lengthscales of closure isotherm depths, which, for most erosion rates and thermochronometers, means averaging over 10⁵-10⁷ yr timescales, commensurate with timescales for potential climate-tectonic feedback response times. Inverting cooling ages for erosion rates requires constraints on closure depths, which are a function of closure temperature (T_c), crustal thermal field, and topography. Both T_c and crustal thermal field also depend in turn on erosion, both absolute rate and temporal and spatial variations. Because of these interdependencies, for steady erosion, closure depths are shallowest for both very slow and very fast (>~2 km/Myr) erosion rates and deepest for intermediate rates. For example, closure depths for apatite He and muscovite Ar systems can vary as much as 2 or 10 km, respectively, for steady erosion ranging from 0.1 to 5 km/Myr. Thus an explicit model coupling thermal field, T_c, and erosion rate can be critical.  For transient erosion, closure depths can move with respect to the surface nearly as rapidly as rocks are exhumed, complicating interpretations of erosion rates and their changes through time. This problem is most pronounced for higher-T_c systems.  Topography introduces an additional complexity to erosion rate inversions from thermochronology. For static topography, closure depths should be referenced to mean topography over a horizontal lengthscale commensurate with the closure depth. Changing topography further complicates erosion rate estimates in a number of ways, and resolving it requires more sophisticated sampling and interpretational strategies. Both bedrock and detrital approaches are used to constrain spatial and temporal patterns of erosion. Provocative results include observations of similarities in erosion, precipitation, and/or uplift patterns in orogens. These are consistent with, but do not necessarily demonstrate, dynamic coupling between climatic and tectonic processes. Deconvolving relative forcings, responses, and the nature and efficiency of coupling has been difficult to address using such observations.