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Titel |
A Cordilleran-type metamorphic core complex: Rechnitz window, Eastern Alps |
VerfasserIn |
Shuyun Cao, Franz Neubauer, Manfred Bernroider, Junlai Liu, Johann Genser |
Konferenz |
EGU General Assembly 2013
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 15 (2013) |
Datensatznummer |
250075475
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Zusammenfassung |
The Rechnitz window group represents a Cordilleran-style metamorphic core complex, which
is almost entirely located within nearly contemporaneous Neogene sediments of the
Pannonian basin at the transition zone between the Eastern Alps and the Neogene
Pannonian basin. The western boundary of the South Burgenland High is a high-angle
normal fault dipping to the west, whereas the eastern boundary is rather a low-angle
normal fault operative during exhumation of the Penninic units exposed within
the Rechnitz window. Two tectonic units separated by a ductile thrust fault can be
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 formed as a result of subduction
(D1), (2) ductile nappe stacking (D2) greenschist facies-grade metamorphism 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 respectively to ductile low-angle
normal faulting. Palaeopiezometry of recrystallized quartz and calcite in conjunction
with P-T estimates yield high strain rates of 10-11 to 10-13 s-1, depending on the
temperature (400–350 ºC) and choice of piezometer and flow law calibration. Progressive
microstructures, strain analysis of the recrystallized quartz and calcite, texture analysis and
thermobarometric calculations indicate an overprint of the greenschist facies grade fabrics
(D2) by the sub-greenschist facies grain plane strain deformation (D3), resulting in
subvertical thinning in an extensional deformation regime. Phengitic mica from the
Eocene high-pressure metamorphism, which remained stable during D2 ductile
shearing, is still well preserved within late stages of final sub-greenschist facies
shearing. The interesting point is now that the rocks of both Penninic units underwent a
previous high-pressure metamorphism and subsequent lower greenschist facies
conditions. Chlorite geothermometry yields two main temperature groups, 376–328
°C, and 306–132 °C. The lower temperature group is interpreted to result from
late-stage hydrothermal overprint, which affected older microfabrics. Chlorite is
seemingly accessible to late-stage resetting. The spatial distribution of specific
microfabrics indicates that the RMCC underwent an earlier large-scale coaxial pure shear
deformation accommodated by a late non-coaxial shear (simple shear) with ductile
low-angle normal faulting during the main stage of exhumation between ca. 23–18 Ma. |
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