Raman spectroscopy allows thermal maturation of carbonaceous sediments to be determined. The technique has been employed on metamorphic samples exceeding temperatures of 270°C, but recently has been shown to be effective at lower temperatures. Thermal maturation techniques commonly depend on sample size, have varying efficacies at different temperatures and in different conditions. The underlying processes are not well understood, thus data interpretation can be ambiguous. Here we show the efficacy of Raman as a low-temperature thermal marker in a thrust belt. The Bornes region, in a French Subalpine chain, provides an opportunity to test the technique against published vitrinite reflectance data and thermal modelling for the first time. In doing so we show that Raman is an effective thermal marker to temperatures as low as 75°C, has a small error and is consistent with previous work. The Raman data allow us to postulate the relative thickness of the sedimentary succession across the chain, the timing of thermal maturation and the timing and thickness variations of the over-thrust Prealpine nappe. The work establishes Raman as a low-temperature thermal marker for correlation with other techniques to ensure effective and robust interpretation, that can readily be applied to fold–thrust belts in hydrocarbon provinces.