NATURALLY DEFORMED POLYPHASE ROCKS PROVIDE QUANTITATIVE RHEOLOGICAL INFORMATION

The rheological behavior of polyphase rocks influences deformation at all crustal levels. However, the majority of both experimental and field-based analyses of rock rheology focus on monomineralic rocks. We exploit a combination of strain markers and natural variations in mineral content to quantify the rheology of deformed and metamorphosed turbidite sequences from the Eastern Fold Belt in the Mt Isa Inlier, Queensland, Australia. X-ray diffraction analyses of cores taken from a representative turbidite sequence show that from base to top of the sequence, micas increase from 25 to 50% while quartz and feldspar decrease from 75 to 40%. At the outcrop scale, the continuous deflection of initially bedding-perpendicular quartz-rich veins links variations in shear strain to these lithological changes. Specifically, larger strains (greater deflections) are recorded by the stratigraphically higher, mica dominated domain relative to the stratigraphically lower quartz- and feldspar-dominated domain. Electron backscatter diffraction shows measurable crystallographic preferred orientations in quartz in the quartzofeldspathic domain, which are absent in the mica dominated domain. Back-scattered electron images record evidence of solution-precipitation creep and grain-boundary sliding where mica dominates. Paleopiezometry indicates that the stress recorded by quartz increases slightly upwards in the section, from ~15 MPa to ~16 MPa. Correspondingly, the strain rate increases from ~8 x 10-13 s-1 in the quartzofeldspathic domain to ~1 x 10-12 s-1 in the mica-rich domain. Where relatively little mica is present, quartz was the dominant weak phase and deformation took place mainly via intracrystalline processes such as dislocation creep. Where mica content is high, deformation occurred via intercrystalline processes such as grain boundary sliding. These results suggest that deformation behavior is dependent on the identity, relative amounts, and spatial distribution of weak phases in polymineralic rocks.