Dyanna Czeck, Indiana University of Pennsylvania
In many deformed rocks, we can see evidence that competency contrasts have dramatic effects on the three-dimensional orientation of structural fabrics and the three-dimensional pattern of the strain. Here, a field example of a deformed polymictic conglomerate, the Seine Conglomerate, is shown in order to study the competency contrast effects on fabrics in detail. In a first-order description, we may describe the "deflection" of structural fabrics about more competent units embedded in less competent units, both at the scale of clast and mineral grain. However, by looking at the boundary zones between competency domains in detail and by considering the variation in strain, we can gain a more detailed glimpse of competency contrasts as fundamental agents in the initiation of structural fabrics.
In the Seine conglomerates, boundaries between "competent" and "less competent" domains can be observed at a range of scales (from approximately 0.1 mm-10 cm). In this rock, competency domains are primarily clasts or grains. Competency contrasts seem to play an important role in both the three-dimensional geometrical character of the foliations and the initiation of foliation formation. Foliations nucleate about more competent regions. Competency contrasts influence the networking geometries of small scale shear zones and foliations. The strain at which the competent domain and the corresponding major foliations become subparallel to the maximum finite strain is dependent on the scale of the competency contrast.