HETEROGENEOUS DEFORMATION AND DEFORMATION PROCESSES IN THE SEINE RIVER CONGLOMERATE

The Archean Seine River polymict conglomerate was examined in thin section in order to investigate partitioning of flow and assess the influence of fluids in the deformation of lithologically heterogeneous rocks. Within a single thin section, there is evidence for several different processes of deformation. Locally high concentrations of phyllosilicates and siderite at the boundaries of clasts provide evidence for solution transfer. Undulose extinction and recrystallization in quartz grains are evidence for dislocation-controlled creep. Fractures (not propagating into the matrix) cutting the most rigid clasts indicate brittle deformation. Quartz and calcite filled veins indicate the presence of fluids in these fractures. In some cases, the matrix flowed into fractures at clast boundaries, indicating that flow and fracture took place concurrently. Cross-cutting relationships of veins indicate that there were at least two generations of fluids present during ductile deformation.

Although the pressure, temperature, and bulk kinematics are constant, there are multiple reasons why deformation might be accommodated by different processes in any one sample, even within grains of the same mineral. Several important parameters that control deformation are not constant on the clast and crystal scales in a polymict conglomerate. Competence contrast (viscosity or elastic modulus) depends on the composition of neighboring grains. Also, stress varies in the matrix around an ellipsoidal clast, while being constant within that clast. The variations of stresses adjacent to rigid clasts result in significantly higher strain in a direction perpendicular to foliation and lower strain in a direction parallel to foliation. Local stress variations control solution and precipitation of minerals. The relative importance of dislocation-controlled creep and solution transfer also depends on local changes in stress, and on grain size, which is extremely heterogeneous in these rocks. The range of deformation processes operating simultaneously in a variety of minerals and clasts may eventually allow bulk conditions of stress and strain rate to be established.