the strained shape. Mukhophadhyay (1973) argued that quartz grains in the Ardennes are good markers because they are sepa- rated enough not to interact with each other. This is intuitively reasonable. Markers used here, however, all show some interaction.

Impingement of grains upon each other produces elonga- tion of the grains or one of the grains. Elongation is always in the plane of stylolitization which corresponds with the direc- tion of secondary mica growth (ie. the cleavage plane). Grain impingement causes deviation from the strain path of an ellipse by imposing additional grain shape changes. These grain inter- action effects would be small if there were no grain boundary sliding, or viscosity contrasts, because bounding areas would deform with the clasts but would always retain their configur- ation. Two touching clasts would remain just touching through- out the deformation. Within the studied suite of rocks viscosi- ties differ between the clasts, and there is probably grain boundary sliding. Grain interactions do occur and, along with the initially irregular grain shapes, cause final strain marker distributions to have features which are not predicted by geometric deformation.

Viscosity contrasts

Clasts of all compositions were measured and included together in most of the strain determinations.

For discussion, let us assume a perfectly random distribu- tion of heterogeneous clasts. Clasts that are more viscous than the matrix will display a component of rigid body rotation when strained (Gay, 1968a, p. 219). Gay (1968a) pointed out that clasts, with viscosities much greater than the matrix, will exhibit little shape change, but will undergo rotation such that their long axis will coincide with that of the strain ellipse. Clasts, which are nearly equivalent to the matrix in viscosity, will deform geometrically and not rotate rigidly, thereby approxi- mating more closely the deformation predicted by geometric strain analyses.

Clasts with moderate to low viscosity ratio will approximate more closely the deformation predicted by geometric analysis of the strain. Their ellipticities will generally be greater than for the high viscosity ratio clasts. On the polar plot the distri- bution in a strained state would be directionally uniform, but display a spread in magnitude of final ellipse ratios. This would have the effect of inducing bimodality in the final ellipse dis- tribution on the polar plot, if the strain ellipse were taken to be the central part of the distribution. A characteristic of this type of induced biomodality is that the strain ellipse axes will parallel the two directional modes of the unstrained distribu- tion. Gay (1968a) assumed that the boundaries between matrix and clast are fixed (ie. there is no creep or slippage). In the pres- ence of pressure solution phenomenon this assumption would not hold.

A consequence of this assumption being invalid is that the rigid body rotation component of competent clasts need not occur. Gay (1968a) pointed out that clast interference has a ten- dency to lessen the control of viscosity contrast and therefore presumably decrease the effects of rotation.

Figure 61 illustrates bimodality of a distribution produced by viscosity contrast between clast and matrix. The distribu- tion tends to be bimodal where only quartz clasts are involved and even more accentuated where less viscous clasts are also present.

Figure 61. These are polar plots of the natural log of the axial ratio and orientation of the long axis of the final ellipses from sample 1. Plot A contains data from quartz grains and plot B combines these with data from chert grains. The difference in dispersion of the pattern from the origin is because the chert shows more deformation than the quartz. These differences are due to viscosity contrasts between the grains, and between the grains and the matrix.

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