Carbon ordering in an aseismic shear zone: implications for Raman geothermometry and strain tracking

Determining the burial and strain history of sedimentary rocks is important for understanding crustal behaviour. When rocks contain organic carbon, increased temperatures at depth can alter the molecular structure of the carbon. This change, known as carbon ordering, can be detected using Raman spectroscopy. As a result, Raman spectroscopy is increasingly used to estimate burial depths
and associated maximum temperatures in carbon-bearing rocks. It is known from experiments and natural samples that other factors can affect Raman-derived maximum temperatures, including frictional heating on fault planes and interaction with hot fluids. For faulted samples, a question remains as to whether it is purely frictional heating that causes carbon ordering or strain, or a
combination of the two. In this study, we use a mid-crustal shear zone to show that strain-related carbon ordering occurs in natural rock samples during aseismic shear strain. A traverse across the shear zone, whose relative strain we quantify with respect to the surrounding less deformed rock, shows a marked decrease in Raman D/G peak intensity ratios indicating greater carbon ordering within the shear zone. We interpret this as evidence for carbon ordering as a result of aseismic shear strain, rather than inflated temperatures, due to frictional heating commonly associated with seismic strain rates on faults. Our results have implications for the further development of Raman spectroscopy as a geothermometer (which may yield erroneous results in strained rock samples) and for understanding strain localisation processes in the Earth’s crust, and its associated rheological implications.