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2026
ACL
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Hugues Raimbourg, Benjamin Moris-Muttoni, Romain Augier, Kristijan Rajič, Ismay Vénice Akker, Aurélien Canizarès, Emmanuel Le Trong, 'Impact of strain on carbonaceous matter crystallinity: Insights from Raman spectroscopy and microstructural analysis of strain gradients from exhumed accretionary complexes', Journal of structural Geology 203 (2026) doi:10.1016/j.jsg.2025.105560
Carbonaceous matter is a common component of metasedimentary rocks, mainly deriving from the diagenetic and then metamorphic evolution of the organic matter originally present in the sedimentary protolith. During burial, the maturity of carbonaceous particles increases by rearrangements of the aromatic carbon sheets and expulsion of heteroatoms, mostly as a result of temperature increase. However, additional external factors could enhance the maturity such as the pressure, fluid-rock interactions and/or deformation. In this work we explored the effect of strain at low metamorphic temperatures (in the range ∼200–320 °C) along natural strain gradients of variable scales, in three accretionary or collisional complexes (Shimanto and Kodiak accretionary complexes, Infrahelvetic Flysch Units), using Raman spectra of carbonaceous matter. In these examples, both ductile and brittle zones of localized deformation were observed, in the form of shear and breccia zones, respectively. Carbonaceous particles observed by TEM are ∼50–100 nm in size and are distributed throughout the rocks, enabling automated Raman spectroscopy profiles and maps to be carried out. Irrespective of the deformation process, the intensity ratio between D and G band (R1 ratio) of Raman spectra is systematically increased by up to ∼60 % in high strain zones compared to the least deformed, reference zones. From the comparison with a series of undeformed metamorphic rocks spanning the same temperature range, the R1 increase reflects the progressive organization of the carbonaceous matter towards higher maturity. In mm-scale shear bands, the increase in R1 can be unambiguously ascribed to localized strain, whereas in brecciated domains, localized temperature increase may also have contributed to R1 anomaly
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