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| Titel |
Aspects of the internal kinematics and dynamics of salt diapirs: Results from thermomechanical experiments |
| VerfasserIn |
G. Zulauf, J. Zulauf, M. Peinl, N. Kihm, 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 |
250025713
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| Zusammenfassung |
| The internal parts of salt diapirs are characterized by constrictional deformation
supporting steeply plunging prolate fabrics and related linear (L>S) fabrics (Talbot and
Jackson 1987). The youngest folds recognized in stems of salt diapirs are known from
German Zechstein salt as curtain folds (Kulissen- or Vorhangfalten, Hartwig 1925)
because the steeply inclined bedding planes define steeply plunging cylindrical
folds. The grain-shape lineation tends to parallel the hinge lines of curtain folds. In
cases of rheological stratification (e.g. stiff anhydrite or shale layers embedded
in a weaker halite matrix), the curtain folds should be associated with boudins,
the latter resulting from vertical extension parallel to the steep axes of the curtain
folds.
A new deformation apparatus has been used to model the internal kinematics of
rheologically stratified salt diapirs. Composite natural samples consisting of a single layer of
Gorleben anhydrite, embedded in matrix of Asse halite (both from Zechstein formation of
northern Germany), were constrictionally deformed at temperature, T = 345Ë C, strain rate, Ä
= 10-7 s-1, maximum viscosity, η = 2 x 1013 Pa s, and maximum finite strain, eX =
122%.
Viscous flow of Asse halite under the conditions listed above was accommodated by
dislocation creep, which can be approximated by the equation obtained experimentally by
Carter et al. (1993) for low stresses. Dislocation creep was related to formation of subgrains
which are forming a striking chessboard pattern in sections cut parallel to the major stretching
axis, X. The subgrain size, D, has been used to estimate the differential stress, Ï, using the
equation obtained by Schléder and Urai (2005) after combining the calibrations published by
Carter et al. (1993) and Franssen (1993). The piezometrically derived stress values are
between 2 and 6 MPa. Although the prerequisites for piezometry are not fully met in the
present case of Asse halite (e.g. steady-state deformation is not given in each run), the derived
stresses are quite similar to the actual stresses recorded by the load cells of the
machine.
At advanced state of constriction (eX > 90%) a strong increase in strain hardening of
halite led to a transient tension fracture that healed up and was shortened by folding during
the final phase of viscous deformation. Tiny prismatic anhydrite inclusions disseminated
inside the halite matrix were reoriented during constriction resulting in a linear grain-shape
fabric.
3D-images of the anhydrite layer, based on computer tomography, revealed rare kink
folds with axes subparallel to X, and boudins which result from brittle tension fracture. With
increasing layer thickness, Hi, the width of boudins, Wa, increases linearly and can be
described by
Wa = -0.3 + 1.3 * Hi (1).
The normalized width of boudins (Wd = Wa/Hi) is almost constant at 1.5 ±1.0. These
geometrical parameters can be used to reveal fracture boudinage under bulk constriction. The
oblique orientation of most of the boudins, with respect to the principal strain axes, results
from folding o the boudins by a second generation of folds, the latter with axes
subperpendicular to the layer. Similar structures have been produced using plasticine as rock
analogue (Zulauf and Zulauf, 2005).
The necks between the anhydrite boudins are different in shape and composition.
Some necks are entirely filled with viscous halite. Others show open space that is
coated with black organic matter (as shown by fluorescence microscopy) and/or with
halite, both resulting from precipitation from a fluid. Fluorescence microscopy
has also revealed organic matter inside fluid inclusions which are resting on grain
boundaries of initial (only naturally deformed) Asse halite. The shape of these fluid
inclusions varies significantly from isolated bubbles to finger like tubes (see also Urai
et al., 1987), all of which show a central part that is dark under the fluorescence
microscope (probably NaCl brine) and an outer bright rim consisting of organic matter. In
some cases the tubes are fusing into dark fluid films which are decorating the grain
boundary.
Grain boundary fluid inclusions are still present in experimentally deformed samples.
However, these fluid inclusions are stretched and are more irregularly distributed along the
grain boundaries compared to those of the initial samples. Organic matter is still
present in the outer rims of the inclusions as is shown by fluorescence microscopy. Of
particular interest are the interfaces of viscous halite and rigid anhydrite which
were acting as rheological boundaries, along which halite was strongly sheared. In
these high-strain domains the grain boundary fluid inclusions were also strongly
stretched resulting in accumulation and trapping of fluid phases at these sites. This
observation explains why the open space in the neck domains is coated with organic
matter. After the latter was expelled from deformed and fused grain boundary fluid
inclusions it migrated into the open neck space where it was precipitated. First
investigations using RAMAN spectroscopy have confirmed that the composition of the
organic matter of fluid inclusions and black coatings of open necks is the same. We
argue that the release of fluids from grain boundaries has significantly controlled
the strain hardening which is a characteristic feature at advanced states of finite
strain.
The new data presented above might have implications for selecting rock salt of the Asse
type as host rock for a radioactive waste repository. Further investigations will
focus on the texture (crystallographic preferred orientation) of deformed halite and
on the composition of the fluid inclusions inside both undeformed and deformed
samples.
References:
Carter, N.L. et al., 1993. J. Struct. Geol. 15, 1257-1271.
Franssen, R.C.M.W., 1993. PhD thesis, Rijksuniversiteit Utrecht.
Hartwig, G., 1925. Jahresberichte Niedersächsischer Geologischer Verein 17,
1-74.
Schléder, Z. and Urai, J.L., 2005. Int. J. Earth Sciences, 94, 941-955.
Talbot, C.J., Jackson, M.P.A., 1987. AAPG Bull. 71, 1086-1093.
Urai, J.L. et al., 1987. Geologie en Mijnbouw, 66, 165-176.
Zulauf, J., Zulauf, G., 2005. J. Struct. Geol. 27, 1061-1068. |
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