Natural Microgauges

Abstract

Microstructures have been used in the past to determine a sequence of deformation structures and shear sense. Many microstructures, however, seem to contain quantitative information as well. Obviously, the chemistry of minerals in rocks has long been used to obtain data on metamorphic conditions and isotope ratios have been used to determine the age of minerals and rocks, but such data can often only be obtained from powders or solutions, destroying the microstructure in the rock. This chapter presents a collection of microstructures which have been described in the literature as providing quantitative data from their geometry alone, observed by non-destructive methods. We have named this type of structures “microgauges”. There are presently microgauges for strain, vorticity, stress orientation, differential stress, pressure, strain rate, temperature and rheology. The subject is in full development and new tools are added at a regular pace.

Some geologists may wonder if this type of geometric microgauges cannot simply be replaced by geochemical tools. Clearly, some types like those for stress orientation, differential stress and vorticity cannot, but even gauges for pressure and temperature are useful in rocks where the chemistry cannot give a clear answer, such as quartzites, or as a method to check results of geochemical techniques.