ESTIMATION OF FINITE STRAIN IN CLASTIC SEDIMENTARY ROCKS ASSUMING STRAIN PARTITIONING BETWEEN CLASTS AND MATRIX

The estimation of finite strain in sedimentary rocks is often based on the behavior of elliptical markers that are assumed to behave in a passive manner during deformation. The theory of passive deformation of elliptical objects (Ramsay 1967) underpins most if not all current methods of strain analysis including the R_f/φ (Dunnet 1969), centre to centre (Fry 1979) and nearest neighbor (Ramsay 1967, Mulchrone 2003) techniques. This assumption of passive behavior is frequently invalid when dealing with deformation of clastic low grade sedimentary rocks where marked strain partitioning between clasts and matrix is common. Mulchrone & Walsh (2006) have recently described a solution for the deformation of non-rigid viscous elliptical inclusions in a matrix of differing viscosity for a general 2D deformation, an approach that allows for slip between clast and matrix. This study will critically compare strain estimates in deformed clastic sedimentary rocks using this new approach with existing methodologies that assume passive behavior. Examples are drawn from the Devonian (Glashabeg Formation) of southwest Ireland, Cretaceous sandstones (Campanian Two Medicine Formation) from the Rocky Mountain Front of western Montana and deformed Miocene cobble conglomerates (Sixmile Creek Formation) of southwestern Montana. All of these lithologies show little evidence of penetrative intra-clast deformation. Preliminary results indicate that strain estimates based on traditional methods are very sensitive to the geometric mean of initial clast aspect ratios (R_i). The Mulchrone & Walsh (2006) methodology is not as sensitive to R_i values. This study clearly illustrates the need to take account of possible penetrative strain underestimates based on clast analysis using traditional methodologies.