5th Swiss Geoscience Meeting, Geneva 2007
Transition of cataclastic deformation to potential
diffusion creep in granitoid fault rocks
Onneken Julia*, Stünitz Holger*, Heilbronner Renée*
*Geologisch-Paläontologisches Institut, Bernoullistrasse 32, CH-4056 Basel
(julia.onneken@unibas.ch)
Fault rocks show a characteristic grain size distribution (GSD), which can be
described by its D-value, that is the slope in the log(frequency )-log(grain
radius)-plot. Natural and experimental fault rocks consist of gouge (or
cataclasites) and cracked grains (for which the original geometrical relationship
of the fragments can be reconstructed). Two D-values are found for all
distributions: small grain sizes (20nm – 2µm) have a low D-value (D<≈ 1.0) and
for larger grains (2µm – 20µm) the D-value is higher (D>≈ 2.0) (Keulen et al.
2007). Previous experiments have shown that the D>-values and the amount of
gouge increase with slip.
Reconnaissance experiments with a slow creep rate after rupturing exhibit a
temperature dependence, which suggest that this deformation is probably not
friction-controlled. The likely deformation mechanism is diffusion creep
facilitated by the small sizes of fragments. Extremely small grains (< 0.1 µm)
disappear and grains are more rounded after creep.
The reconnaissance experiments have been carried out on cracked specimens.
In order to use a reproducible sample geometry, triaxial simple shear
experiments are carried out on wet gouge of Versasca Gneiss between forcing
blocks in a Griggs' solid medium deformation apparatus. The cores are cut at
45° into two forcing blocks and 100 mg Versasca Gneiss powder (grain sizes
<200µm) with 0.2 Wt% H20 added, is placed between them. The samples are
initially deformed at room temperature or 300°C, 500 MPa confining pressure,
and partly 10-4 -10-5 s-1 strain rate in order to compact the gouge.
The experiments, which we have performed so far, give information on the
compaction behaviour of the cracked powder under confining pressure and 10%
strain. We want to attain a characteristic fault rock with a D-value of > 2 with as
little slip as possible. For the study of the creep behaviour, it is critical that the
samples have the characteristic porosity of natural gouge samples (1%). The
compaction should minimize the porosity. After then initial compaction, slow
creep experiments are carried out. A series of experiments at different
temperatures and stresses (corresponding to different creep rates) will be
performed to test for the transition between cataclastic deformation to potential
diffusion creep.
REFERENCES
Keulen, N., Heilbronner, R., Stünitz, H., Boullier, A.-M. & Ito, H. 2007: Grain
size distributions of fault rocks: A comparison between experimentally and
naturally deformed granitoids. Journal of Structural Geology, in press.
Heilbronner, R. & Keulen, N. 2006: Grain size and grain shape analysis of fault
rocks. Tectonophysics, 427, 199-216.