The rheology of fine-grained psammites deforming in slate belts under low grade metamorphic conditions
Ilse Kenis1, Janos L. Urai2, Manuel Sintubin1 & Wouter van der Zee 2, 3
1 Structural Geology & Tectonics Group, K.U.Leuven, Redingenstraat 16, 3000 Leuven, Belgium
2
Geologie-Endogene Dynamik, RWTH Aachen, 52056 Aachen, Germany3
now at: GeoMechanics International, Mainz, Germanyilse.kenis@geo.kuleuven.ac.be
In the High-Ardenne Slate Belt (Belgium), layer-perpendicular quartz veins in psammite layers acted as mechanical boundaries due to their difference in strength with respect to the psammites (veins > psammites > pelites; Urai et al., 2001; Kenis et al., 2002). During layer-parallel shortening in the early stages of the Variscan orogeny, this behaviour resulted in the development of double-sided mullions (Kenis et al., 2002). Based on a simplified analytical model, Urai et al. (2001) suggested that the shape of the mullions is a strong function of the stress exponent n of the power law () and thus a potential gauge for paleorheology. Since this analytical model is only a first approximation, a geomechanical model using the ABAQUS package was developed. In this model, a volume-constant steady state power law creep rheology is assumed for the three materials vein quartz, psammite and pelite. A parameter sensitivity analysis using this numerical mullion model confirmed the hypothesis that the stress exponent is an important determining parameter for the shape of the mullions. Moreover the analysis demonstrated that, in addition to the stress exponent, the initial aspect-ratio of the psammite segments between the veins and the difference in strength between the psammites and the vein quartz are important parameters. Because of the differences in initial shape of the psammite segments between veins each mullion is analysed in a separate case study. These case studies have been performed with the aim to constrain the flow law for the psammites. The vein quartz in the model is deformed following the power law creep equation as defined for wet quartz by Hirth et al. (2001). Moreover, the model is constrained by paleopiezometry in the dynamically recrystallized vein quartz, which is deformed but to strains less than those in the psammite. Using an iterative parameter fitting scheme, it is found that in all case studies the solution converges to a single set of parameters which define the complete flow law for the psammites, at low grade metamorphic conditions (T = 400 ºC, strain rate around 10-15 s-1, and a water-rich fluid phase at pressures close to lithostatic). This solutions show that an approximately ten-fold contrast in strength between the psammites and vein quartz, together with a stress exponent (n =1 +/- 0.2) for the psammites is required to produce the observed shape of the double-sided mullions in agreement with the differential stress and finite strain required in the vein quartz. These results are in good agreement with the microstructural work in the psammites. The latter shows a fine grained matrix rich in phyllosilicates, with clear evidence for deformation by solution transfer processes which is usually interpreted to indicate Newtonian (n » 1) rheology.
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Hirth, G., Teyssier, C., Dunlap, J., 2001. An evaluation of quartzite flow laws based on comparison between experimentally and naturally deformed rocks. Int. J. Earth Science, 90, 77-87.