Structures, microfabrics and textures of the Cordilleran-type Rechnitz metamorphic core complex, Eastern Alps

Shuyun Cao; Franz Neubauer; Manfred Bernroider; Junlai Liu; Johann Genser

doi:10.1016/j.tecto.2013.06.025

Structures, microfabrics and textures of the Cordilleran-type Rechnitz metamorphic core complex, Eastern Alps

Tectonophysics. 2013 Nov 26:608:1201-1225. doi: 10.1016/j.tecto.2013.06.025.

Authors

Shuyun Cao¹, Franz Neubauer¹, Manfred Bernroider¹, Junlai Liu², Johann Genser¹

Affiliations

¹ Dept. Geography and Geology, University of Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austria.
² State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Xueyuan Rd. 29, Haidian, Beijing 100083, China.

Abstract

Rechnitz window group represents a Cordilleran-style metamorphic core complex, which is almost entirely located within nearly contemporaneous Neogene sediments at the transition zone between the Eastern Alps and the Neogene Pannonian basin. Two tectonic units are distinguished within the Rechnitz metamorphic core complex (RMCC): (1) a lower unit mainly composed of Mesozoic metasediments, and (2) an upper unit mainly composed of ophiolite remnants. Both units are metamorphosed within greenschist facies conditions during earliest Miocene followed by exhumation and cooling. The internal structure of the RMCC is characterized by the following succession of structure-forming events: (1) blueschist relics of Paleocene/Eocene age formed as a result of subduction (D₁), (2) ductile nappe stacking (D₂) of an ophiolite nappe over a distant passive margin succession (ca. E-W to WNW-ESE oriented stretching lineation), (3) greenschist facies-grade metamorphism annealing dominant in the lower unit, and (4) ductile low-angle normal faulting (D₃) (with mainly NE-SW oriented stretching lineation), and (5) ca. E to NE-vergent folding (D₄). The microfabrics are related to mostly ductile nappe stacking to ductile low-angle normal faulting. Paleopiezometry in conjunction with P-T estimates yield high strain rates of 10^{- 11} to 10^{- 13} s^{- 1}, depending on the temperature (400-350 °C) and choice of piezometer and flow law calibration. Progressive microstructures and texture analysis indicate an overprint of the high-temperature fabrics (D₂) by the low-temperature deformation (D₃). Phengitic mica from the Paleocene/Eocene high-pressure metamorphism remained stable during D₂ ductile deformation as well as preserved within late stages of final sub-greenschist facies shearing. Chlorite geothermometry yields two temperature groups, 376-328 °C, and 306-132 °C. Chlorite is seemingly accessible to late-stage resetting. The RMCC underwent an earlier large-scale coaxial deformation accommodated by a late non-coaxial shear with ductile low-angle normal faulting, resulting in subvertical thinning in the extensional deformation regime. The RMCC was rapidly exhumed during ca. 23-18 Ma.

Keywords: Ductile deformation; EBSD analysis; Microstructure; Nappe stacking; Rechnitz window; Texture.