Polyphase deformation in the SE Cantabrian Arc recorded by calcite twins in veins.

Marina Armann1, Alexander Weh1, Stefan Krumm1& Martin Burkhard2

1Institut für Geologie, FA Universität Erlangen-Nürnberg, Germany

2Institut de Géologie, Université de Neuchâtel, Switzerland

marina.armann@geo.stud.uni-erlangen.de

Calcite e-twin-lamellae are commonly used to determine the magnitude and orientation of paleostresses. A series of cross-cutting calcite veins has been studied from weakly metamorphosed carboniferous limestones in the SE Cantabrian Zone (Spain).

The study area comprises the inner, SE part of the Cantabrian Arc. Deformation took place under shallow crustal conditions, with low grade metamorphism and a locally developed cleavage. Thin-skinned tectonics with several levels of décollements and wide ranges of different thrust systems have produced complex outcrop patterns.

The area was affected by at least three phases of variscan deformation and a later period of alpine deformation. The onset of deformation during Namurian times was accompanied by S-N compression with the formation of E-W trending fold axes and thrusts. Subsequent E directed deformation during Westphalian led to a complex arrangement of folds with the development of polyphase structures (Ramsay interferrence pattern 1 and 2) and strike-slip movements. During late Stephanian, thrusting complicated the polyphase structures with S facing folding, duplexes and thrusts. Finally, brittle alpine deformation completed the present outcrop pattern.

The metamorphic grade of the study area was determined from illite crystallinity data of 62 samples-indicating anchizonal conditions in 80% of the samples and another 20% of epizonal values. Depending on the metamorphic grade, three different types of twins (type I to type III) could be observed. Type I (microtwins) and type II (thick twins) are common in the anchizonal influenced study area. Type III (curved and twinned twins) occur in epizonal areas, mainly in the vicinity of major thrust zones where bending and recrystallization of twins is visible.

The twinning strain method of Groshong (1972) was applied to 21 samples of different vein generations in order to characterise paleo-"stresses". Observations show that the oldest variscan deformation (Westphalian) was strongly overprinted by the two younger Stephanian and early Permian deformations. Twinning strains could only be reconstructed for the two younger variscan deformation phases. Stephanian compression was oriented E-W in a thrusting regime, whereas the early Permian deformation was due to thrusting, a N-S directed compression. Alpine deformation did not leave any significant imprint in the form of calcite twinning.

 

References:

Groshong, R.H. (1972). Strain calculated from twinning in calcite. Bull. geol. Soc. Am. 83, 2025-2048.