EXPLORING THE RELATIONSHIP BETWEEN FINITE STRAIN AND CLAST ORIENTATION DISTRIBUTION: IMPLICATIONS FOR RIGID GRAIN VORTICITY ANALYSIS

Since the mid-1980s several analytical techniques have been developed for estimating flow vorticities of naturally deformed rocks. These techniques have been applied to samples from diverse tectonic settings including collisional, extensional, and strike-slip regimes. Clast-based vorticity estimation techniques utilize orientations of grains assumed to have behaved as isolated rigid particles suspended in a flowing viscous matrix. This method is arguably the most commonly used due to the relative ease of application of the technique and abundance of suitable material. A major assumption of the rigid grain technique is that a sufficient amount of strain has accumulated for grains above a critical aspect ratio to have rotated into their stable positions. Because finite strain magnitude is often difficult to extract from naturally deformed mylonites this assumption is rarely quantitatively verified.

We constructed a simple numerical model to explore the effect of variable finite strains on the development of the orientation distribution of a dense, homogeneous initial population of rigid clasts embedded in a viscous medium for several distinct general shear flows. Our initial results indicate that high finite strains (Rs >> 10) are required for high aspect ratio grains to rotate into orientations approaching their stable sink position—even for flows with a significant pure shear component. Additionally, we found that for flows with a high simple shear component, grains with aspect ratios below the critical value rotate into orientations sub-parallel to the flow plane and remain near this position until very large strains accumulate. Although grains below the critical aspect ratio will continually rotate with progressive deformation, rotation occurs so slowly in these orientations that the clasts are effectively stable at geologically realistic finite strains. Thus, it appears that the clast-based method produces poor results at low strains for any flow, and may become unreliable at very high strains for simple shear-dominated flows.