Application of Fission-Track Thermochronology to Understand Fault Zones

Abstract

The timing and thermal effects of fault motions can be constrained by fission-track (FT) thermochronology and other thermochronological analyses of fault zone rocks. Materials suitable for such analyses are produced by fault zone processes, such as: (1) mechanical fragmentation of host rocks, grain-size reduction of fragments and recrystallisation of grains to form mica and clay minerals, (2) secondary heating/melting of host rocks as the result of friction and (3) mineral vein formation as a consequence of fluid flow associated with fault motion. The geothermal structure of fault zones is primarily controlled by three factors: (a) the regional geothermal structure around the fault zone that reflects the background thermotectonic history of the study area, (b) frictional heating of wall rocks by fault motion and consequent heat transfer into surrounding rocks and (c) thermal effects of hot fluid flow in and around the fault zone. The thermal sensitivity of FTs is briefly reviewed, with a particular focus on the fault zone thermal processes, i.e., flash and hydrothermal heating. Based on these factors, representative examples as well as key issues, including sampling strategy, are highlighted for using thermochronology to analyse fault zone materials, such as fault gouges, pseudotachylytes and mylonites. The thermochronologic analyses of the Nojima fault in Japan are summarised, as an example of multidisciplinary investigations of an active seismogenic fault system. Geological, geomorphological and seismological implications of these studies are also discussed.