Tracking Flow Geometry In Uniaxial Compression Of Short Cylinders Of Solnhofen Limestone Using The Microstructure In Calcite

Sergio Llana-Fúnez and Ernie Rutter

Rock Deformation Lab, Department of Earth Sciences, University of Manchester

Oxford Road, M13 9PL Manchester

sfunez@fs1.ge.man.ac.uk

There are a limited number of deformation geometries possible in standard rock deformation equipment. Traditionally, the aim has been to achieve a simple, constant and predictable flow geometry that matches homogeneously deformed rock bodies. In this study we have deformed short rock cylinders in axial compression, to try to produce a complex, three-dimensional ductile strain history (non-plane strain), in order to study the effect on grain shape and crystallographic fabric evolution (SPO and CPO) for their potential use in natural cases were deformation is heterogeneously distributed.

The experiments were run under constant confining pressure (~200 MPa), temperature (600 °C) and displacement rate (bulk ~5.183·10-6 m/s) using an externally heated Nimonic rig (named for the material from which the pressure vessel is fabricated), which uses water as the confining medium. Under the experiment conditions and at the grain size of the starting material (Solnhofen limestone), intracrystalline plasticity is the predominant deformation mechanism in the rock specimen. Solnhofen limestone has been used because of its initial small grain size (ca. 4 µm), that provides large number of grains to be deformed, and because the presence of impurities in the grain boundaries prevents grain growth at the conditions of the experiment (600 °C) and therefore the 3-dimensional grain shape reliably reflects local finite strain.

In this specimen geometry, and with the condition of no slip along the contacts, the deformation is dominated by the effects of flow between converging parallel plates. Thus in addition to a coaxial shortening component (i.e. flattening), there is a strong component of shear parallel to the forcing blocks as the middle part of the specimen is extruded sideways (remaining the bases of the specimen fixed) which increases with radial distance from the centre (i.e. general non-coaxial flow). The material is also forced to extend in the circumferential direction, by an amount that increases with the amount of radial extrusion accumulated. For the material eventually extruded beyond the confines of the parallel plates, the flow is dominated by the effect of circumferential extension (i.e. constrictional flow). Thus this experimental configuration is characterized by (a) zones of different deformation style and intensity, and (b) material being forced to pass from one strain path to another.

From our microstructural observations in experiments in calcite and from what is recorded in the CPO, we see a clear relation between maximum stretching and shortening and the texture patterns in inverse pole figures (IPF) with respect to the structural framework where both of these directions are expected to be. In addition to this, where material passes from one flow regime to another, it seems that CPO reflects mainly the geometry of the final increments of strain.

Acknowledgements

This research was supported through an European Community Marie Curie Fellowship, contract No. HPMF-CT-2000-00778. R. Holloway and E. Mariani are thanked for their valuable help during the experiments.