Conglomerates used for geological strain analysis commonly show significant competence contrasts and measurable strain variations among the component clasts. Previous studies have employed these differences in strain, together with inclusion-matrix theory, to quantify the effective viscosity ratios among different rock types. We present a case study of a conglomerate that provides a new look at these approaches, and considers the rock as a multiphase mixture.

The Cesson Conglomerate from northern France is a tightly packed polymict conglomerate that can be simplified into four component rock groups: volcanics (mostly rhyolites), granites (granite-gneisses to microdiorites), psammites, and semipelites (semipsammite to pelite). We use Rf-f analysis of clasts from field photographs and rock specimens (in XZ and YZ sections) to determine the average strain ratios for each of the four rock groups. This reveals that the semipelite group strained the most, and granite the least. The mean strain from Rf-f analysis of all measurable clasts in a conglomerate will not generally provide a true measure of the whole-rock strain. We prefer Fry analysis for measuring the bulk strain, taking the mean value from analyses of field photographs and specimens. A simple theoretical model is then used to calculate the viscosity ratio of a particular rock type relative to the whole rock, in terms of their different strains. We determine rock/bulk viscosity ratios ranging from 0.8 for semipelite to 4.5 for granite. Measured relative to semipelite, the whole conglomerate has a viscosity ratio of 1.3, the volcanic or psammite clasts ~2 , and granites ~6.

These results broadly agree with those from other conglomerates: that the viscosity contrasts among common rock types span only about one order of magnitude. Such consistency from different conglomerates and settings leads us to conclude that a wide variety of rocks are approximately Newtonian in ductile geological deformation.