Pressure-temperature (P-T) paths in structural terranes dominated by folds and thrust faults can be complex and can vary widely within a single metamorphic belt. The complexity and variability of P-T paths result from the advection of heat by folding and thrusting, conductive heat transfer during and after deformation, and the relatively short times for thermal relaxation within folds and thrusts.
In fold terranes, contrasting P-T paths are observed between anticlines and adjacent synclines. Cooling paths are observed within anticlines, whereas heating paths are observed within synclines. These different P-T paths result from the folding of isotherms during deformation and the subsequent relaxation of isotherms after deformation. In polydeformed fold terranes, noncoaxial folding results in complex P-T paths over small distances (less than 15 km) within a metamorphic belt because of the repetition of the process of folding and relaxation of isotherms during each folding event
In structural terranes dominated by thrust faults, P-T paths differ between major thrust sheets because heat is transferred from the hotter overriding sheet to the cooler lower plate during deformation. Heat transfer during thrusting causes the upper plate to cool and the lower plate to heat. In terranes with multiple thrust sheets, P-T paths can be complex and will depend upon the relative timing of thrust development and the rate at which heat can be transferred from one thrust sheet to the next
The P-T paths observed in both the fold and thrust terranes discussed here record relatively short time intervals of 105 or 106 yr. Because the mineral equilibria respond to small-scale thermal perturbations, there may be a wide variability in P-T paths over a small distance in a metamorphic belt Therefore, before P-T paths are used to reconstruct tectonic histories, both structural and petrologic investigations must be carried out.