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| Titel |
Weakening processes in thrust faults: example of the Monte Perdido thrust fault (South Pyrenean orogenic prism, Spain). |
| VerfasserIn |
Brice Lacroix, Henri Leclere, François Souquière, Martine Buatier, Pierre Labaume |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250054436
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| Zusammenfassung |
| In thrust and fault-belts, the development of low angle thrust faults composes a real
mechanical paradox. Although fluid pressure is an important parameter controlling fault
reactivation, the presence of fluid could be responsible for fluid-rock interaction and trigger
the apparition of weaker hydrous minerals.
In this study, we aim to decipher the mechanical processes accompanying the shear
surfaces reactivation in thrust faults. The current investigations focus on the Monte Perdido
thrust fault (south Pyrenean orogenic prism). The fault zone consists of an interval of
intensely deformed clays-bearing rocks bounded by major shear surfaces. Structural analysis
in the fault zone evidence the presence of shear-surfaces veins developed at a low angle with
principal stress. Petrographic, SEM and TEM observations coupled to geochemical
and thermometric analysis in the fault zone highlighted two principal stages of
deformation:
The first stage corresponds to the development of calcite shear veins by the
reactivation of ancient weakness structures. Stable isotope analysis on calcite cement
from shear veins and their host sediments revealed that fault involved in a closed
hydrological system facilitating an increase of fluid pressure. The spatial relationships
existing between shear veins and cleavage seams attest that pressure solution is the
major deformation process. Applying the 2D reactivation model (Sibson, 1985) we
determined that shear reactivation was possible under suprahydrostatic fluid pressure
(Pf > Ïă3).
The second stage corresponds to shear reactivation involving clay minerals. Detrital
chlorite and illite were affected by dissolution recrystallization processes. Chlorite
precipitated along shear surface whereas illite recrystallized along cleavage surfaces
perpendicular to Ïă1.
These contrasted results allow to propose a model of fault reactivation. The incremental
reactivation of the previous shear surfaces was facilitated by a suprahydrostatic fluid pressure.
Furthermore, some evidences of hydrostatic fluid pressure during calcite cement precipitation
support that incremental shear surface reactivation follows a fluid pressure cycle. Pressure
solution processes triggered a calcite departure from host sediments to veins increasing the
relative clay minerals content. As a result, the host rock frictional coefficient decreased from
0,65 to 0,25.
Consequently, during the last stage of deformation, shear surfaces could be reactivate
without a high fluid pressure contribution. The presence of high illite and chlorite content in
host sediment and along shear surfaces acted as a fault weakening, facilitating stable creeping
deformation. |
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