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| Titel |
Grain-scale Response and Aggregate Deformation Processes in Experimentally Deformed Carbonate and Shale Gouges |
| VerfasserIn |
Joseph Clancy White, Lori A. Kennedy, Jennifer C. Haywood, Dan R. Faulkner |
| 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 |
250054084
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| Zusammenfassung |
| In limestone-on-shale thrust faults, such as those that are common in foreland-thrust regimes
world wide, carbonates typically comprise the hanging wall, and shale comprises the
footwall. Generally, a cataclasite is developed in both the carbonate and shale materials, and
there is a zone of mixing in which the shale cataclasite contains clasts of carbonate
cataclasite. Both the hangingwall and footwall cataclasites can be foliated and the degree of
strain partitioning between the cataclasites is unknown.
Triaxial frictional sliding experiments were conducted on 20 mm diameter by 50 mm
length cores containing a 1 mm thick, gouge layer along a 30Ë angle sawcut. Porous Berea
sandstone (Ï -17%) and impermeable Badshot dolomite were variously paired to give
contrasting forcing block properties. Gouge material was created from quartz-bearing
phyllosilicate-rich shale (31% quartz, 39% muscovite, 18% clinochlore, 11% feldspar)
combined in various volumetric proportions with reagent grade calcite powder (80% calcite,
20% dolomite) with an average grain size of ~5 μm. The mechanical tests demonstrated the
following relationships: (1) temperature is more important than pore fluid pressure in
decreasing the strength of carbonate gouge; (2) at room temperature carbonate
gouge is significantly stronger than any shale-carbonate composite gouge; (3) at
150Ë C, pure carbonate gouge is weaker than 75% and 50% shale composites; (4)
gouges containing shale continue to strain harden throughout the duration of the
experiments; and (5) it appears that pore fluid pressure is necessary for stick-slip (seismic)
behaviour. A general observation was that gouges were stronger than the Berea
sandstone (i.e. damage zone weaker than core zone) that led to attrition of the gouge
wall rock and its incorporation into the gouge. Measured ‘frictional’ strength is
in this case a combination of displacement in the gouge and fracture in the wall
rock.
In order to further elucidate the processes controlling the latter behaviour, specifically the
effect of pore fluid pressure, detailed microstructural study (optical microscopy, scanning and
transmission electron microscopy) of calcite and shale gouge deformed between
dolomite-dolomite and sandstone-dolomite blocks has been undertaken. At a first-order level,
the forcing blocks impose distinct boundary conditions, with the porous sandstone enabling
an irregular slip contact, whereas dolomite blocks have discrete contacts, typically with strain
localized here with intense grain-size reduction. The evolution of compositional and
mechanical layering within the gouges (Riedel shears and/or grain variations) controls the
hardening, softening and localization, with commensurate variations in grain-scale
deformation processes. Of particular interest is the inferred onset of intracrystalline
deformation in the calcite gouges, and what influence, if any, this rheological response has on
the aseismic-seismic transition during stick-slip. These observations from controlled
experiments provide a key database for comparison with naturally deformed fault gouge. |
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