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| Titel |
Revisiting the Variscan transpressional tectonics in the Southwestern Iberian suture |
| VerfasserIn |
Irene Pérez Cáceres, David Martínez Poyatos, Jose Fernando Simancas, Antonio Azor |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250088849
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| Publikation (Nr.) |
 EGU/EGU2014-3016.pdf |
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| Zusammenfassung |
| The boundary between the Ossa-Morena Zone (OMZ) and the South Portuguese Zone (SPZ)
in southwest Iberia is a Variscan collisional suture with transpressive left-lateral
kinematics, contrasting with the dextral component that characterizes most of the
Variscan convergence in other regions of the Orogen. Recent work including new
structural and radiometric data has improved our knowledge on the geometry and
timing of deformations affecting the OMZ/SPZ suture, which can be summarized as
follows:
Closure of the Rheic Ocean in Late Devonian time is attested by high-pressure
and ophiolitic thin allochthonous units emplaced on the southern border of the
OMZ. The kinematic interpretation of early stretching lineations and tectonic
fabrics indicate that these units were emplaced in a tectonic regime of oblique
left-lateral convergence.
Transient transtension in Early Carboniferous time gave way to a narrow
aisle of newly-formed oceanic-like crust just over the foregoing Rheic Ocean
suture, accompanied by mafic magmatism intruded/extruded at both continental
sides. Radiometric dating has yielded the same age of around 340 Ma for the
oceanic-like mafic protholiths and their granulite/amphibolite facies tectonic
fabric, thus indicating the very ephemeral life of the oceanic-like strip.
Oblique convergence was resumed immediately after transtension, first causing
northward obduction of the oceanic-like unit and north-verging folding in
metasedimentary units of the southern border of the suture. Later on,
a south-vergent regional fold was developed synchronous with left-lateral
granulite-amphibolite facies shearing. Finally, shear deformation gave way to
a low pitch stretching lineation, thrusting the OMZ over SPZ, concentrated
on the southern limb of this regional fold and constituting a complex ductile
2-3 km-thick shear band evolving from amphibolite to greenschist facies,
developing: (i) high-temperature greenschists at the southern border of the
mafic oceanic-like unit, which propagated southwards progressively cooling to
low-temperature greenschists; (ii) disruption of layers that produced small fishes
of weakly deformed rocks, previously interpreted as sedimentary mélanges.
Radiometric dating of acid volcanic rocks included in the low-grade shear zone
has yielded an age of 337 Ma, putting an older limit to shearing. At late
Variscan time, a brittle left-lateral shear band partially obliterated the previous
syn-metamorphic shear zone.
Oblique convergence propagated southwards across the SPZ in Late
Carboniferous time, though lateral displacements decreased rapidly in favor of
shortening. This deformation has been roughly modelled as a transpressional
band characterized by 40% shortening and γ=1 shearing. An age of 330 Ma
yielded by a deformed granite at the northern part of the SPZ probably indicates
the arrival of deformation at that point, while stratigraphic data suggest that
deformation reached SW Portugal at around 310 Ma.
To conclude, the transpressive OMZ/SPZ boundary shows strain partitioning, with left-lateral
displacements concentrated in ductile to brittle shear zones affecting the suture units,
and moderately oblique shortening affecting a broad zone of the SPZ foreland. |
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