Strain analysis and rheology contrasts in polymictic conglomerates: An example from the Seine metaconglomerates, Superior Province, Canada

Abstract

Deformed conglomerates typically display contrasting strains between different lithological clast populations and matrix, which can be exploited to provide information about effective viscosity contrasts. We analyzed three different clast populations (granitoid, mafic volcanic, and felsic volcanic) across a strain gradient in the Seine metaconglomerates of Ontario, Canada. We calculated finite strain using the R f/ φ method and bootstrap statistics. Granitoid clasts exhibited the least amount of strain, while the mafic and felsic clasts had similar, greater strains. We assumed that the bulk finite strain was recorded by the mafic clasts, given their large proportion of the rock and their similar composition to the matrix. We calculated effective viscosity contrasts using the finite strain information, assuming Newtonian viscous rheology. The two volcanic clast populations have similar effective viscosities, regardless of strain magnitude, with ratios of felsic/mafic ranging from 0.27 to 2.12 and average ratio of 1.19. The granitoid clasts are significantly more competent than the mafic clasts with effective viscosity ratios ranging from 2.35 to 12.39 and average ratio of 5.61. The results are consistent with a qualitative competence hierarchy of granitoid > felsic ≥ mafic, although the quantitative effective viscosity ratios change with strain magnitude. Consequently, the effective viscosity contrast between clast types is strain dependent and possibly deformation path dependent.