Raman Spectral Shifts in Naturally Faulted Rocks

Abstract

Raman spectral shifts of carbonaceous materials arising from faulted rocks produce varied and complex results. By analyzing faulted and adjacent non-faulted samples from three discrete localities in France, Italy, and Morocco, we assess changes in specific Raman parameters, including G-peak width, G-peak position, and peak intensity ratios. We consistently observe a shift in Raman parameters between faulted and non-faulted samples; however, the direction and magnitude of this shift varies. Raman peak intensity ratios are shown to both increase and decrease on fault planes. The majority of samples exhibit decreasing peak width and peak position in the faulted samples, but this is not consistent; in two samples an increase is observed, but the shift tends to be small. These data are compared to published Raman spectral shifts from experimental fault data. Our results suggest that fault zone deformation processes may measurably change the carbon nanostructure in faulted rocks. The inconsistent nature of Raman spectral shifts in the fault rock samples analyzed, and those published suggest that a complex set of factors control carbon nanostructure changes in fault rocks. These factors, although not discriminated here, likely include, background temperature, frictional heating, strain, carbonaceous material type, amongst others. We have shown that Raman Spectral shifts occur in faulted rocks with implications for how Raman data are used to predict maximum temperatures in faulted sedimentary rocks. We recognize the potential for a range of fault zone processes to modify Raman spectral shifts, however, systematic sampling across individual fault zones is critical.