The recent proliferation of quantitative geobarometers for common metamorphic rocks has produced a potentially powerful tectonic tool for metamorphic terranes. The geobarometric break across postmetamorphic faults could be useful for determining the structural throw of the fault. However, the metamorphic break will correspond to the structural throw on the fault only if the metamorphic conditions preserved in rocks near the fault are the P-T conditions of the rocks just prior to fault movement. In general, one-dimensional thermal models for tectonically thickened orogens do not produce metamorphic breaks across thrust faults, suggesting that anomalous conditions might be required to preserve postmetamorphic thrust faults (PMTF).
This study examined three classes of one-dimensional numerical thermal models in order to simulate the evolution of a PMTF. Class I models try to preserve the PMTF by rapid uplift following thrusting. Class II models allow for cooling and uplift between the metamorphic peak and thrusting. Class III models consider the effect of the underthrusting of a cool sheet beneath the thrust of interest. The relation between the geobarometric break across the fault and its structural throw is examined.
All three classes of models manage to produce and preserve metamorphic inversions, given the proper starting conditions. However, in all class I and class III models, as well as most class II models, rocks below the thrust heat and reach their metamorphic peak subsequent to thrusting. This implies that the metamorphic conditions preserved across a PMTF do not represent the P-T conditions at the time of thrusting. The models investigated here all produced barometric breaks less than or equal to the throw of the fault. This suggests that the geobarometric break could place a lower limit on fault throw. However, the class II models demonstrated the importance of upper plate cooling and uplift to the preservation of a metamorphic inversion. Preferential uplift and cooling of the warmer upper plate rocks prior to thrusting could produce a barometric break that exceeds the fault throw. The geobarometric break across a PMTF may be either greater or less than that which would correspond to the structural throw of the fault and is therefore likely to be a rather unreliable indicator of its structural throw.