8. From Cooling to Exhumation: Setting the Reference Frame for the Interpretation of Thermochronologic Data

The reference frame for the interpretation of fission-track (FT) data is a thermal reference frame. Using thermochronology to constrain exhumation largely depends on understanding the linkage between this reference frame and Earth’s surface. The thermal frame of reference is dynamic, that is it is often neither stationary nor horizontal, as it is influenced by the shape of the topography, heat advection associated with rapid exhumation and mass redistribution across major faults. Here, we review the nomenclature and basic relationships related to cooling, uplift and exhumation and describe strategies to independently constrain the paleogeothermal gradient at the time of exhumation. In some cases, cooling may not be related to exhumation, but can be used instead to constrain the thermal evolution of the upper crust and the emplacement depth of magmatic rocks. In general terms, useful constraints on exhumation are often only directly provided by thermochronologic ages that are set during undisturbed exhumational cooling across the closure temperature isothermal surface. Thermochronologic ages from minerals crystallised at temperatures less than the closure temperature, e.g. in volcanic rocks and shallow intrusions, provide no direct constraint on exhumation.