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| Titel |
Evolution of stress and strain during 3D folding: application to orthogonal fracture systems in folded turbidites, SW Portugal |
| VerfasserIn |
J. E. Reber, S. M. Schmalholz, S. M. Lechmann |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250020623
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| Zusammenfassung |
| We present field data and numerical modeling results which show the evolution of stress and
strain patterns during 3D folding resulting in an orthogonal fracture system. The
field area is located near Almograve, SW Portugal. The area is part of the Mira
Formation which itself is part of the South Portuguese Zone (SPZ). The structural
development of the SPZ is characterized by southwest vergent folding and thrust
displacement. The metamorphism in the SPZ increases from diagenetic conditions in the
southwest to greenschist-facies conditions to the northeast. The Mira Formation is
composed of turbiditic layers of Carboniferous age with low sandstone to shale
ratio. The data was gathered at three outcrops which show structures similar to
chocolate tablet structures in the folded sandstone layers. Chocolate tablet structures are
generated under simultaneous extension in two directions and show two fracture
systems of the same age which are perpendicular to each other. However, the Mira
Formation is located in a convergent area. Also, the outcrops near Almograve show two
fracture systems of different age. The fractures orthogonal to the fold axis and the
bedding are crosscut by fractures parallel to the fold axis and orthogonal to the
bedding.
Our hypothesis for the evolution of the observed fracture systems is as follows;
the older fractures which are now orthogonal to the fold axis and to the bedding
plane were generated during compression while the layers were still approximately
horizontal. They are parallel to σ1(i.e. mode 1 fractures). The second and younger
fracture family was generated in a phase where there is local extension in the fold
limbs. These fractures are orthogonal to the far-field σ1, parallel to the fold axis and
perpendicular to the bedding. The shortening direction is constant during the entire folding
process.
We test our hypothesis with numerical modeling. We use 2D and 3D finite element
codes with a mixed formulation for incompressible flow and a viscous rheology.
The stress and strain tensor components are calculated at each numerical nodal
point. The stress and strain fields are visualized through ellipses and ellipsoids
which are calculated using the eigenvalues of the respective tensors. The shortest
main axis represents the direction of the smallest stress σ3 and the longest main
axis represents the direction of the largest stress σ1. To generate two orthogonal
fracture systems in the fold limbs we expect a relatively rapid change of the stress
field in the fold limbs during folding. With a relatively slow change of the stress
field we would expect to see more than two fracture systems with a wide range
of fracture orientation which we did not observe in the field. The preliminary 2D
results show, as expected, a sudden flip of the main axes of the stress ellipse which
corresponds to a change from limb-parallel compression to extension. For the 3D
model we expect similar results and we will investigate the impact of different
deformation boundary conditions on the evolution of the 3D stress and strain fields. |
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