The Rheological Effects of Disordered Carbon in Strained Rocks: a Case Study in the Haut Giffre, French Alps.

Abstract:

Organic carbon is common in crustal rocks and can take a wide range of molecular structures. This nanostructure is altered as temperature increases, a process which is commonly known as carbon ordering. Carbon ordering can also occur without thermal input, in zones of high strain such as faults and shear zones. In organic-rich carbonates, carbon ordering involves a progression from kerogen-like amorphous carbon to graphitic carbon ‘sheets’, via a series of intermediate molecular structures.

Numerous studies, both experimental and field-based, have shown that carbon nanostructure can affect rheology. Experiments indicate that graphite on fault planes reduces the peak coefficient of friction, whilst some models suggest that strain-induced “wrinkles” in graphitic sheets may strengthen the material. Dispersed graphite within a rock promotes localised reaction softening and influences grain size. While the effects of graphite on rheology have been well-documented, many rocks do not contain pure graphite but rather an intermediate molecular structure, depending on peak temperature and strain conditions.

Raman spectroscopy is a common method for analysing organic carbon nanostructure. In this study we use Raman spectral parameters to constrain the relative progression of carbon ordering across multiple scales in naturally-deformed rocks from the French Alps. We analyse bulk samples through fault zones, as well as sub-mm scale Raman mapping of rock chips. We compare Raman results with microstructural analysis of the host calcite (optical microscopy and EBSD) and dispersed carbon particles (TEM). We combine Raman spectra and microstructural analysis to assess carbon ordering in localized shear zones.

Our work enables us to better constrain the effects of small-scale strain variations on Raman signal. Such strain variations may explain the wide distribution of values in bulk-sample Raman analysis. Our results develop previous work on crustal anisotropy induced weakness in graphite-bearing rocks, to assess the effects of non-graphitic carbon on rock deformation. Since a large proportion of sedimentary rocks in orogenic belts contain non-graphitic carbon, understanding the effect of intermediate carbon nanostructures on rheology is vital in the study of rock deformation.