EGU2020-9155
https://doi.org/10.5194/egusphere-egu2020-9155
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Carbon ordering in an aseismic shear zone: implications for crustal weakening and Raman spectroscopy

Lauren Kedar, Clare Bond, and David Muirhead
Lauren Kedar et al.
  • School of Geoscience, University of Aberdeen, Aberdeen, United Kingdom (l.kedar@abdn.ac.uk)

Multi-layered stratigraphic sequences present ample opportunity for the study of strain localization and its complexities. By constraining mechanisms of crustal weakening, it is possible to gain a sounder understanding of the dynamic evolution of the Earth’s crust, especially when applied to realistic, field-based scenarios. One such mechanism is that of strain-related carbon ordering. This is the process whereby the amorphous nanostructure of fossilized organic matter contained within the rock is progressively organized towards a more sheet-like structure, similar to that of graphite. One common method of studying this process is through Raman spectroscopy. This is a non-destructive tool which makes use of the relative positions and intensities of two key spectral peaks, where one peak represents graphitic carbon and the other disordered (or amorphous) carbon. The intensity ratio between these two peaks suggests the degree to which the carbon has progressed from its original kerogen-like structure towards that of graphite. This progression can be due to increasing temperature or increasing strain, and until now, these two contributory factors have been difficult to separate, particularly in field examples.

Previous field-based studies have focused on carbon ordering on fault planes, while experimental studies have monitored the effects of strain-related ordering in organic carbon on both fault surfaces and more distributed shear zones. These studies confirmed the occurrence of strain-related ordering at seismic rates, particularly in the form of graphitization of carbon. However, these experiments showed the effects of strain-related ordering at aseismic rates to be limited when distributed shear zones were considered, in part due to the geological timescales required to emulate true conditions.

In this study, Raman spectroscopy is used to compare the relative nanostructural order of organic carbon within a recumbent isoclinal fold formed of interbedded limestones and marls. The central, overturned fold limb forms a 170m wide, 1km long aseismic shear zone, with evidence of increased strain recorded in calcite grains relative to the upper and lower limbs. Raman spectroscopy intensity ratios (I[d]/I[g]) are compared across the fold, showing a marked 23% decrease in the overturned limb. Such a decrease in I[d]/I[g] suggests increased carbon ordering within the overturned limb, which in combination with evidence for increased strain in calcite, suggests that the carbon ordering here is derived directly from strain-related ordering. This has important implications. We infer, from previous studies, that strain-related carbon ordering encourages further strain partitioning in carbonaceous material, and may enhance zones of weakness in the rock. This ordering in aseismic shear zones has so far been unreported in nature, and so our field-based results are significant in supporting previous experimental evidence for this phenomenon. Our results also have implications for understanding dynamic crustal evolution, and will play an important role in the development of Raman thermobarometry, especially since current methods do not distinguish between strain-related and temperature-related ordering.

How to cite: Kedar, L., Bond, C., and Muirhead, D.: Carbon ordering in an aseismic shear zone: implications for crustal weakening and Raman spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9155, https://doi.org/10.5194/egusphere-egu2020-9155, 2020

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  • CC1: Comment on EGU2020-9155, Samuel Angiboust, 06 May 2020

    This is very interesting. In fact in our 2012 publication (journal of metamorphic geology, Angiboust et al., figure 5) about the exhumation of the Monviso ophiolite, we have noticed that RSCM temperatures decrease approaching the "upper shear zone", which suggests that retrogressive deformation also contributes to de-organization of graphite planes. Looking forward to see this work published. Cheers!

    • AC1: Reply to CC1, Lauren Kedar, 06 May 2020

      Thank you for your comment! Carbon ordering is a complex puzzle, so it's always exciting to hear evidence like this.

  • CC2: Comment on EGU2020-9155, Philip Groß, 06 May 2020

    Hi Lauren, we should continue discussion here I guess... great to have someone working on this important problem!

    Following up on Samuels question, do you already know if thrusting happend during prograde or retrograde conditions?

    • AC2: Reply to CC2, Lauren Kedar, 06 May 2020

      Hi Philip,

      I agree, good to continue the discussion! Previous work (Pfiffner 1993, Kirschner 1999, Austin et al. 2008) suggests that peak metamorphic conditions occurred at the same time as the onset of thrusting, but how much overlap there was between these events is unclear.

  • CC3: Comment on EGU2020-9155, Philip Groß, 06 May 2020

    Okay. What my question was aiming for was if the lower crystallinity at the fault is the product of deformation-related destruction of the CM or maybe deformation-induced recrystallisation of the CM at lower metamorphic conditions, which resulted in the lower crystallinity. I don't know if this could occur...

    • CC4: Reply to CC3, Philip Groß, 06 May 2020

      this should have been a reply...

    • AC3: Reply to CC3, Lauren Kedar, 06 May 2020

      Thanks for that, you make a very interesting point! We hadn't considered this previously, but it would be worth investigating for the fault plane samples.

      However, where more ductile fabrics dominate, we see a similar drop in I[d]/I[g], which can be more easily explained by increased 'crystallinity' (although all samples here yield relatively amorphous carbon) due to strain-related ordering.

      The plot thickens...

      • CC5: Reply to AC3, Philip Groß, 08 May 2020

        Interesting!

        Another question I have is if your observation also holds if you look at other spectrum parameters? That means, not looking at the intensity but rather area ratios, e.g.

        And did you already try to calculate temperatures from the data? It would be interesting to see to which temperature values your observation translates...