16. A Guide for Interpreting Complex Detrital Age Patterns in Stratigraphic Sequences

Thermochronologic age trends in sedimentary rocks collected through a stratigraphic sequence provide invaluable insights into the provenance and exhumation of the sediment sources. However, a correct recognition of these age trends may be hindered by the complexity of many detrital thermochronology datasets. Such a complexity is largely determined by the complexity of the thermochronology of eroded bedrock that may record, depending on the thermochronologic system under consideration, cooling during exhumation, episodes of magmatic crystallisation, metamorphic mineral growth and/or late-stage mineral alteration in single or multiple source areas. This chapter illustrates how different geologic processes produce different patterns of thermochronologic ages in detritus. These basic age patterns are variously combined in the stratigraphic record and provide a key for the geologic interpretation of complex detrital thermochronology datasets. Grain-age distributions in sedimentary rocks may include stationary age peaks and moving age peaks. Stationary age peaks provide no direct constraint on exhumation, as they relate to episodes of magmatic crystallisation, metamorphic growth or thermal relaxation in the source rocks. Moving age peaks are generally set during exhumation and can be used to investigate the long-term erosional evolution of mountain belts using the lag-time approach. Post-depositional annealing due to burial produces age peaks that become progressively younger down section. The appearance of additional older age peaks moving up section may provide evidence for a major provenance change. When interpreting detrital thermochronologic age trends, the potential bias introduced by natural processes in the source-to-sink environment and inappropriate procedures of sampling and laboratory processing should be taken into account.