Discontinuous ultramylonite zones cut Proterozoic granulite facies gneisses in MacRobertson Land, east Antarctica, and preserve evidence of ductile non-coaxial flow and reverse sense of shear. Cross-cutting relationships indicate that ultramylonite deformation involved overthrusting to the east, but progressively rotated to involve overthrusting to the north; rotation of the principal compressive stress axes is inferred. Extensive pseudotachylite developed during ultramylonitization, the history of individual ultramylonite zones having involved a single episode of pseudotachylite generation. Neoblastic sillimanite indicates ultramylonitization occurred at >520" C. On the basis of inferred recrystallized granulite facies mineral assemblages ultramylonitization occurred at >TOO" C, and s7.3 f 0.5 kbar, at aHz0 = 0.3 and low am. Comparison of these values with those suggested by metamorphic assemblages in rocks unaffected by mylonitization indicates that the R a p e r Complex experienced a late increase in pressure of 1-2 kbar during ultramylonitization. The P -T-a,,,, conditions of the ultramylonite zones are inferred to have been close to the solidus for minimum melting, pseudotachylite generation having involved a limited pressure drop during brittle fracturing at high strain rates. Most of the pseudotachylite veins are undeformed; the mechanism(s) of fracturing and melting must have caused strain hardening in rocks surrounding the ultramylonite, further strain having been mostly accommodated by a new or subsidiary shear zone. Renewed stress at reduced strain rates, or renewed stress in zones in which the proportion of pseudotachylite was significantly higher, could have led to the rare occurrences of deformed pseudotachylite. The preservation of fine-grained pseudotachylite is dependent on it remaining dry.Abbreviations used in tables and figures are after Kretz (1983), but with the following additions: ann = annite; east = eastonite; fcrd = iron cordierite; fs = ferrosilite; mgts = Mg-tschermak's molecule; naph = sodic phlogopite.
Fold axes and lineations across the southern margin of the Kisseynew gneiss belt were progressively reoriented from northwesterly to northeasterly during an Early Proterozoic, southwest-directed, compressive deformation cycle. Partitioning of deformation resulted in a concentration of non-coaxial strain along the boundary between the Kisseynew gneiss belt and Flin Flon–Snow Lake belt. The structural evolution was complicated by streamlining of the movement vector around the comparatively rigid Flin Flon–Snow Lake belt. The relationship of metamorphic assemblages to kinematic fabrics indicates that deformation was coeval with a prograde–retrograde metamorphic cycle. Progressive deformation occurred during a period of 30 Ma, which is consistent with Phanerozoic orogenic time scales.
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