Non-uniqueness, symmetries and invariance of thermal history modelling in sedimentary basins

Reconstructing thermal history of sedimentary basins is a non-unique inverse problem, because more than one set of controlling factors could produce exactly the same observations today. Using a forward modelling approach, we demonstrate that the multiple solutions (i.e. a set of parameters that promote a good agreement between predictions and observations) are convertible because the problem processes underlying symmetries. In particular, we propose an inverse method to construct additional solutions directly from an existing solution whilst leaving the predictions of thermal indicator data unaltered. Repetitive application of the approach allows additional solutions to be directly constructed from any arbitrary solution. Having these techniques adapted to forward models of vitrinite reflectance, we investigate the thermal history of the Sichuan Basin, southwest China. The modelling results confirm that a heat flow anomaly during Middel-Late Permian is genuine to the interpretation of the vitrinite reflectance beneath the basin at present as the anomaly appears in every possible thermal history discovered. The amplitude of the heat flow anomaly might be significantly smaller than previously thought since the vitrinite reflectance values underneath the basin might not have necessarily been produced by a short-lived, high heat flow event, but by a gradual increase in heat supply.