THE PROBLEM OF DERIVING BULK PROPERTIES OF HETEROGEOUSLY DEFORMING MATERIALS

When comparing rock deformation experiments and field data, experimentalists and field geologists are faced with the problems of extrapolation (spatial and temporal) and integration (from simple systems to complex ones).

The basis for extrapolation is usually a flow law, and it is assumed that a given deformation mechanism achieved in the laboratory is the same as that which occurs in nature. It is also assumed that the relatively simple systems which are investigated in the lab are representative of the more complex systems which are encountered in nature.

However, it is well known that the behaviour of a given composite or polyphase material cannot be estimated by somehow "averaging" the behaviour of its constituents. Also, localization and heterogeneity of deformation introduce additional complications.

This contribution presents an exemplary analysis of the texture development of experimentally and naturally deformed monophase and polyphase quartz bearing rocks. The aim of this exercise is to understand the development of the CPO and microstructure of the components in terms of the bulk behaviour. Once we understand this relation we may be able to reverse the procedure and integrate (and extrapolate) the bulk behaviour from the behaviour of its parts.

Using optical orientation imaging (CIP, http://www.unibas.ch/earth/micro), the bulk textures of experimentally sheared Black Hills Quartzite (BHQ) and the bulk quartz textures of naturally deformed Arolla Gneisses (Western Alps) are determined. Partial CPOs of the deforming old grains and of the recrystallizing new grains are calculated. Orientation tracking is used to analyse the development of selected texture components (typical of different slip systems).

The bulk CPO of the naturally deformed Arolla Gneiss and the bulk CPO of the BHQ are approximately identical and develop in an analogous fashion. However, in the gneiss, the development of the CPO appears to be delayed, requiring larger strains for a given fabric strength than the pure material.

After summarizing the analysis of the microstructure and texture components and re-integration of the bulk quartz behaviour of the BHQ and Arolla Gneiss, the original question will be addressed again: what is the definition of the "bulk rock" and what is the meaning of an average, extrapolated (geological) strain rate.