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| Titel |
Structures in an anhydrite layer embedded in halite matrix: Results from thermomechanical experiments under bulk plain strain |
| VerfasserIn |
M. Mertineit, G. Zulauf, M. Peinl, F. Zanella, O. Bornemann |
| 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 |
250023440
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| Zusammenfassung |
| Anhydrite layers from Gorleben salt dome, embedded in a halite matrix from Asse salt dome,
both northern Germany, were deformed under bulk plain strain using a thermomechenical
apparatus (Zulauf et al., 2007, 2009). The initial layer thickness Hi ranges from 0.85 to 2.5
mm. Further deformation conditions were as follows: T =345Ë C, Ïmax=4.59 MPa,
ezmax=-40%, Ä=2*10-7s-1.
During the deformation process, load cells record the stress along Y and Z. The displaced
material could escape in X.
The deformed samples were scanned using a computer tomograph at the
Universitätsklinikum Frankfurt/Main. The CT data allow the generation of 3D-modells using
the software Smoooth.
We deformed six samples with the layer (S) perpendicular to the X-axis and four samples
with the layer perpendicular to the Z-axis. Depending on the orientation of the layer (S-¥X
or S-¥Z), the expected structures should be folds or boudins, respectively, the geometry of
which should strongly depend on Hi.
In cases were the layer was orientated parallel to the shortening axis (S-¥X), the
anhydrite layer shows Mohr-Coulomb fractures. The fracture walls were thrust on top of each
other. The space between hanging and foot wall is filled with salt. The angle between the
fractures and the YZ-plain ranges from 10Ë to 25Ë , rarely up to 70Ë , dependent on the finite
strain. In thin layers (Hi=0.85 and 1 mm) rarely non-cylindrical folds developed. In both
cases (S-¥X and S-¥Z) the layer thickness did not significantly change during
deformation.
In cases were the layer was orientated perpendicular to the shortening axis (S-¥Z)
boudins developed by extensional fracture. The number of boudins and their size depend
strongly on the initial layer thickness Hi. With increasing layer thickness Hi the width of
boudins Wa increases linearly.
Wa = -0.2 + 1.4 * Hi (1)
This relation between Hi and Wa is further compatible with equation (16.4) of Price and
Cosgrove (1990) which also considers rheological parameters.
Moreover experiments carried out under bulk constrictional strain (Zulauf et al.,
2007, 2009) show a similar dependency of the initial layer thickness and boudin
width.
For microstructual investigations of the halite matrix, thin sections (XZ- and YZ-sections)
were prepared and etched following the method of Urai et al. (1987). First microfabric data
show that halite behaves viscous whereas anhydrite deforms by fracturing or rare folding
under the chosen deformation conditions. Halite deforms by climb-controlled dislocation
creep with strain hardening (Carter et al., 1993). Anhydrite, on the other hand,
was deformed in the brittle-plastic regime, characterized by twinning, kinking and
fracturing.
The subgrain size of halite has been used to estimate the differential stress (Schléder &
Urai, 2005, 2007), that was compared with the stress recorded by the load cells. The subgrain
size of deformed halite varies between 0.04 and 0.07mm, resulting in differential stresses
between 3.3 +1.5/-0.8 MPa (S-¥X) and 4.2 +3.0/-1.2 MPa (S-¥Z), although the
conditions for piezometry are not completely fulfilled (e.g. lack of steady state
during deformation in some samples). These stress values in the matrix fit with the
stress values recorded during deformation. Close to rigid anhydrite the subgrain size
decreases to values of 0.02 - 0.03 mm, reflecting peak stress up to 6.7 +3.7/-0.7
MPa.
We do not know the reasons why folding of the anhydrite layer is largely lacking,
although the viscosity contrast between halite and anhydrite should be appropriate for
folding. Possible reasons are the lack in confining pressure or mechanical anisotropies in the
undeformed anhydrite. Further investigations will focus on the texture of halite and on
microfabrics of the anhydrite.
References
Carter, N.L., Horseman, S.T., Russel, J.E. & Handin, J (1993): Rheology of rocksalt, J.
Struct. Geol., Vol. 15, No. 9/10, p. 1257-1271
Price, N.J.; Cosgrove, J.W. (1990): Analysis of Geological Structures, by Neville J. Price
and John W. Cosgrove, pp. 516., Cambridge, UK: Cambridge University Press, August
1990
Schléder, Z. & Urai, J. L. (2005): Microstructual evolution of deformation-modified
primary halite from the Middle Triassic Röt Formation at Hengelo, The Netherlands, Int. J.
Earth Sci., 94, p. 941-955
Schléder, Z. & Urai, J. L. (2007): Deformation and recrystallization mechanisms in
mylonitic shear zones in naturally deformed extrusive Eocene-Oligocene rocksalt
from Eyvanekey plateau and Garmsar hills (central Iran), J. Struct. Geol., 29, p.
241-255
Urai, J. L., Spiers, C. J., Peach, C. J., Franssen, R. C. M. W. & Liezenberg, J. L. (1987):
Deformation mechanisms operating in naturally deformed halite rocks as deducted from
microstructural investigations, Geol. Mijnbouw, 66, p. 165-176
Zulauf, G., Zulauf, J. & Bornemann, O. (2007): Deformation of a halite-anhydrite
sequence under bulk constriction: Preliminary results from thermomechanical experiments,
in: Wallner, M, Lux, K.-H., Minkley, W. & Reginal Hardy jr., H. (Eds.). The mechanical
behavior of salt-Understanding of THMC processes in salt, Taylor & Francies, London:
63-68
Zulauf, G., Zulauf, J., Bornemann, O., Kihm, N., Peinl, M., Zanella, F. (2009):
Experimental deformation of a single-layer anhydrite in halite matrix under bulk constriction:
1. Geometric and kinematic aspects, J. Struct. Geol. (submitted) |
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