Geochronology and Thermochronology in Orogenic Systems

An important part of modern tectonic research is determining the rates of deformational, thermal, and erosional processes that define the evolution of orogenic systems. Such endeavors fall into three categories: studies of the crystallization ages of rocks and minerals (geochronology); studies of the thermal history of rocks (thermochronology); and studies of the exposure ages of geomorphic surfaces (cosmogenic nuclide dating). This chapter focuses on the first two types of study; additional material regarding geochronology may be found in Chapter 3.10. Here a brief treatment of the basic concepts of radioactive decay and isotopic dating is followed by a discussion of how limited open-system behavior of radiogenic isotopes in minerals, as well as the partial annealing of radiation damage in mineral structures, may be used to explore the thermal histories of orogens. Subsequent sections describe specific applications of these techniques to a variety of problems encountered in orogenic systems, ranging from the frequency of magmatism to the timescales of landscape evolution. The final section suggests potential avenues for future research, both to improve the quality of geochronologic and thermochronologic data and to evaluate their capacity to constrain orogenic processes. Readers interested in learning more about such topics may consult isotope geology textbooks such as those by Faure (1986) or Dickin (1995).