EGU21-7335, updated on 04 Mar 2021
https://doi.org/10.5194/egusphere-egu21-7335
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Carbon-oxygen isotope analysis of fault rocks in carbonates from different orogenic cycles in the Cantabrian Zone (N Spain): similarities and differences

Sergio Llana-Funez1, Manuel Ignacio de Paz-Álvarez1, Marco Antonio Lopez-Sanchez2, Stefano M. Bernasconi3, Juan Luis Alonso1, Edgar Berrezueta4, Ana Méndez5, and Heather Stoll3
Sergio Llana-Funez et al.
  • 1University of Oviedo, Geology, Oviedo, Spain (llanasergio@uniovi.es)
  • 2CNRS-Geosciences Montpellier, Montpellier, France
  • 3Geologisches Institut, ETH Zurich, Switzerland
  • 4Instituto Geológico y Minero de España, IGME, Oviedo, Spain
  • 5Scientific and Technical Resources, University of Oviedo, Spain

The isotopic carbon and oxygen isotope composition of carbonates (δ13C and δ18O), determined by temperature and the relative abundances of stable isotopes of both elements in water at the time the carbonate is precipitated, can be modified subsequently during geological processes that involve the recrystallization of carbonate. Temperature changes mostly affect δ18O, while additional sources of carbon have a greater impact on δ13C. Amongst the various processes that may alter the original isotopic signature of carbonate rocks are deformation processes, which can lead the dissolution and reprecipitation of carbonates during deformation, or the involvement of fluids of various origin during younger tectonic events.

Here, we present the results of isotope analysis in fault rocks from two distinct faults in the Cantabrian Zone (CZ) in northern Spain. It represents the foreland fold and thrust belt of the Variscan orogen in Iberia and is characterized by numerous and large thrust sheets that were emplaced during the Carboniferous. Subsequent rifting episodes in the Mesozoic and more recently Alpine North-South convergence produced the overprinting of some of the earlier Variscan structures. In all cases, brittle processes produced often similar-looking rocks as the fracturing occurred under upper crustal conditions, relatively close to the surface. Fluids involved during deformation on both cycles are likely to differ, so to evaluate alternative tools to distinguish the different cycles of fracturing in carbonates, a stable isotope analysis on carbon and oxygen was undertaken in two well-known structures in the region: the Somiedo nappe and the Ventaniella fault.

The Somiedo nappe is one of the largest thrust sheets in the Cantabrian Zone, with an estimated offset close to 20 km. The base of the thrust sheet is characterized by well-developed cataclasites and ultracataclasites that formed on Cambrian fine-grained dolostones. It has relatively minor vein activity associated, although the dolostones have been partially recrystallized. The Ventaniella fault is a dextral strike-slip structure cutting obliquely the Cantabrian Mountains. It runs for tens of kilometres inland and has an estimated offset of approximately 5 km. The fault zone in the studied area is characterized by the fracturing and dextral offset of Carboniferous micritic limestones and, more importantly, a relatively strong vein activity that formed a distributed network of calcite veins.

Cataclasite matrix and fragments, and associated veins were sampled for isotope analysis in the two fault zones. In both cases, the matrix has a signature which is intermediate between the undeformed rock and that of the veins. The fragments have a signature which is indistinguishable from the matrix, suggesting the reworking of the fault rock. The veins have a distinct pattern in both faults, but different from each other. Those related to the Ventaniella fault are mostly hydrothermal, with limited range in δ18O and δ13C, while the veins from the base of the Somiedo nappe have a larger range of δ13C, but limited δ18O variation.

How to cite: Llana-Funez, S., de Paz-Álvarez, M. I., Lopez-Sanchez, M. A., Bernasconi, S. M., Alonso, J. L., Berrezueta, E., Méndez, A., and Stoll, H.: Carbon-oxygen isotope analysis of fault rocks in carbonates from different orogenic cycles in the Cantabrian Zone (N Spain): similarities and differences, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7335, https://doi.org/10.5194/egusphere-egu21-7335, 2021.

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