DOES THE RHEOLOGY OF NATURALLY DEFORMED ROCKS DEPEND ON FINITE STRAIN?

Deformed polymictic conglomerates provide an opportunity to directly compare the amount and style of deformation of the different rock types that comprise the clasts and matrix. During a given deformation, competent clast types resist deformation and record relatively little finite strain in comparison to incompetent clast types that deform more and easily and record more strain. By conducting an examination of relative clast strength at different outcrops that record progressively higher amounts of bulk finite strain, it is possible to test whether clast strength relationships stay the same throughout protracted deformation. We present results of such analyses from two locations: (1) the lithic lapilli tuffs and volcaniclastic sediments of the Triassic-Jurassic Koip Sequence from the Sierra Nevada of California, which was affected by the transpressional Cretaceous Gem Lake shear zone in the Sierra Nevada of California, and (2) the Seine metaconglomerate from northwestern Ontario, which was affected by an Archean transpressional deformation at approximately 2.7 Ga.

Within each field area, we conducted three-dimensional strain analysis at numerous outcrops recording progressively higher bulk finite strain. The finite strain recorded by the different clast types relative to that of the matrix demonstrates the competence contrast among the rock types. However, the observed clast strength relationships are not constant; at outcrops recording higher bulk strain, clast strength relationships are different from those observed at lower bulk strain. This indicates that the strengths of some clast types change as bulk finite strain increases. These results can be explained two ways: (1) clast rheology is strain dependent, or (2) clast and/or matrix rheology is described by a flow law with a power law exponent n ≠ 1.

Further analysis will focus on using microstructural characteristics of these rocks to distinguish between these two possible causes of clast strength evolution. The presence or absence of particular microstructures may allow us to place absolute constraints on the rheologies of different clast types. Quantitative studies of the rheological evolution of rocks have important implications for geodynamic modeling and our understanding of deformation of complex natural systems at all scales.