Understanding the mechanical behavior of subduction megathrusts is intimately related to the characterization of their fluid regime. Stable isotopes help in characterizing the fluid regime that existed during megathrust development. Here we analyze the late Eocene-mid Miocene, erosive boundary between the European and Adriatic plates, now exhumed in the N. Apennines. 13C and 18O composition of 125 calcite veins and 83 host rocks have been analyzed from 4 transects of the 500 m-thick fault zone. Stable isotopes have been coupled with structural analysis. Field and microstructural observations suggest that the evolution of structures in the deeper portion of the outcropping megathust, correspondent to 5 km depth, was genetically linked to episodic high Pf, resulting in cyclic fluid-induced brittle deformation interspersed with aseismic ductile creep. δ18O and δ13C values vary from samples of host rock and veins. For the hangingwall rocks the δ18O values show a great variability for each transect if they relate to the previous accretionary prism, from -8.23 to -2.16‰ PDB. If they relate to the slope sediments the isotopic values within each transect are more uniform as well as shown for the footwall samples. The accretionary prism-related rocks are also characterized by a high variability of δ13C, from -1.28 to 2.29‰ PDB, as well as the slope sediment related rocks, from -1.82 to 0.92‰ PDB. Systematic variations of isotopic values are instead observed in the vein samples. In general they show a depletion in δ18O and a shift from mostly positive to negative δ13C values with respect to both the hangingwall and footwall. For the shear zone veins O- and C-isotopic composition tend to be more uniform irrespective of the host rock values. Different localities, though, have different linear trends of δ13C and δ18O values. In general the isotopic data are consistent with a fault architecture where the deformation results concentrated on an array of discrete faults.
Stable isotope constraints on fluid-pressure buildup cycles and fluid–rock interaction at the frontal part of the early Eocene-middle Miocene convergent system of the Northern Apennines (Italy) / Remitti, Francesca; Vannucchi, P.; Dallai, L.; Boschi, C.. - ELETTRONICO. - 89(53):(2008), pp. V31B-2139--. (Intervento presentato al convegno AGU Fall Meeting 2008 tenutosi a San Francisco (US) nel 15-19 dicembre 2008).
Stable isotope constraints on fluid-pressure buildup cycles and fluid–rock interaction at the frontal part of the early Eocene-middle Miocene convergent system of the Northern Apennines (Italy)
REMITTI, Francesca;
2008
Abstract
Understanding the mechanical behavior of subduction megathrusts is intimately related to the characterization of their fluid regime. Stable isotopes help in characterizing the fluid regime that existed during megathrust development. Here we analyze the late Eocene-mid Miocene, erosive boundary between the European and Adriatic plates, now exhumed in the N. Apennines. 13C and 18O composition of 125 calcite veins and 83 host rocks have been analyzed from 4 transects of the 500 m-thick fault zone. Stable isotopes have been coupled with structural analysis. Field and microstructural observations suggest that the evolution of structures in the deeper portion of the outcropping megathust, correspondent to 5 km depth, was genetically linked to episodic high Pf, resulting in cyclic fluid-induced brittle deformation interspersed with aseismic ductile creep. δ18O and δ13C values vary from samples of host rock and veins. For the hangingwall rocks the δ18O values show a great variability for each transect if they relate to the previous accretionary prism, from -8.23 to -2.16‰ PDB. If they relate to the slope sediments the isotopic values within each transect are more uniform as well as shown for the footwall samples. The accretionary prism-related rocks are also characterized by a high variability of δ13C, from -1.28 to 2.29‰ PDB, as well as the slope sediment related rocks, from -1.82 to 0.92‰ PDB. Systematic variations of isotopic values are instead observed in the vein samples. In general they show a depletion in δ18O and a shift from mostly positive to negative δ13C values with respect to both the hangingwall and footwall. For the shear zone veins O- and C-isotopic composition tend to be more uniform irrespective of the host rock values. Different localities, though, have different linear trends of δ13C and δ18O values. In general the isotopic data are consistent with a fault architecture where the deformation results concentrated on an array of discrete faults.
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