An Introduction to Low-temperature Thermochronologic Techniques, Methodology, and Applications

Abstract

Low-temperature thermochronometers can be used to measure the timing and the rate at which rocks cool. Generally, rocks cool as they move towards Earth's surface by erosion or normal faulting (tectonic exhumation of the footwall), or warm as they are buried by sediments and/or thrust sheets, or when they are intruded by magma and associated hydrothermal fluids. Changes in heat flow or fluid flow can also cause heating or cooling. Apatite fission-track and apatite (U-Th)/He dating have low closure temperatures of ∼120°C and ∼70°C respectively, and are used to date cooling in the upper ∼3–4 km (∼1.8–2.4 mi) of Earth's crust. Age-elevation relationships from samples collected from different elevations along vertical transects or from wellbores are used to calculate exhumation rates and the time of onset of rapid exhumation. The spatial distribution of cooling ages can be used to map faults in basement or intrusive rocks where faults can be difficult to recognize. Cooling ages from detrital minerals in sedimentary rocks can be used to constrain provenance. If sedimentary samples reached temperatures high enough to reset the thermochronometers, then ages may provide information on the cooling history of the basin. Forward thermal modeling can be used to test proposed thermal history models and predict thermochronometer ages. Inverse thermal modeling finds a best-fit thermal history that provides a good statistical match to measured thermochronometer ages. Both types of thermal modeling may help constrain maximum temperature of the sample and time spent at that temperature. Thermochronometer ages can be used as constraints in basin modeling. Maturation of kerogen to petroleum in a sedimentary basin is controlled by the maximum temperature reached by the kerogen and the amount of time it spends at or near that temperature (i.e., the thermal history of the basin). The timing of tectonics and the formation of structures in a region influence the generation, migration, entrapment, and preservation of petroleum. Techniques such as low-temperature thermochronology that illuminate the relationship between time and temperature during basin evolution can be valuable in understanding petroleum systems. These techniques are especially powerful when multiple dating techniques (such as apatite fission-track, zircon fission-track, and apatite (U-Th)/He dating) are applied to the same sample and when they are combined with other thermal indicators such as vitrinite reflectance data.