QUANTIFYING THE EFFECTS OF A WEAK PHASE ON THE RHEOLOGY OF NATURALLY DEFORMED, POLYPHASE ROCKS OF THE MOUNT ISA INLIER, QUEENSLAND, AUSTRALIA

We investigated the effect of weak phase (mica) abundance and connectivity on the rheology of naturally deformed, polyphase rocks of the Llewellyn Creek formation, Mt. Isa Inlier, Australia. A series of late stage veins were emplaced in turbidites (consisting of both quartzofeldspathic and phyllosilicate domains) prior to deformation. The turbidites and veins were subsequently deformed, with the veins consistently at a high angle to bedding in the quartzofeldspathic domains and at a low angle to bedding in the phyllosilicate domains. We interpret this vein geometry to reflect variations in strain, with low strain in the quartzofeldspathic domains and high strain in the phyllosilicate domains. Hence, the modal mineralogy exerts a primary control on strain partitioning.

We have identified a threshold mica percentage and connectivity at which the rheology of the rocks changes from a non-linear, high-viscosity rheology in the quartzofeldspathic domain to a linear, low-viscosity rheology in the phyllosilicate domain. The threshold occurs at the transition between the quartzofeldspathic and phyllosilicate domains where mica percentages change from 25 to 45% and Linear Mica Connectivities increase from 0.4 to 0.8. The threshold correlates with a change in dominant deformation mechanisms from dislocation creep accommodated flow in the quartzofeldspathic domain to diffusion assisted grain boundary sliding in the phyllosilicate domain. Additionally, the degree of connectivity of mica plays a very large role in strain localization. We observe isolated micas become mechanically more interconnected with increasing mica percentage, developing bedding-parallel foliation bands and shear bands at high strains. Increased mica connectivity occurs locally by intracrystalline cataclasis of quartz and feldspar grains that are adjacent to mica. Quartz paleopiezometry and flow law estimates put the strain rate at 10^-12 to 10^-13 s^-1 for the quartzofeldspathic domain and 10^-11 to 10^-12 s^-1 for the phyllosilicate domain.