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Comment on “Evidence and timing of crustal extension versus shortening in the early tectonothermal evolution of a Proterozoic continental rift sequence at Broken Hill, Australia” by G. M. Gibson, M. Peljo, and T. Chamberlain

[1] Gibson et al. [2004] presented structural and metamorphic histories of three key areas of the Willyama Supergroup, then argued for a high-grade metamorphism and accompanying extensional schistosity commencing circa 1700 Ma. In this discussion the basis for their structural and metamorphic history is challenged, for the following reasons: [2] 1. Their presented data and interpretation of that data are at gross variance with work presented here. [3] 2. Their data and interpretation are at gross variance with that of other structural geologists, several of whose pertinent studies have been ignored. [4] 3. Their reliance on discredited geochronology weakens their interpretation of metamorphic history. [5] 4. The timing of their proposed extensional deformation is at variance with published stratigraphic and geochronological evidence. [6] 5. There is no basis for the postulated high-pressure metamorphism. [7] 6. Major internal inconsistencies and misleading statements within the paper greatly weaken their argument. [8] Gibson et al.’s [2004] article is an outcome of the multidisciplinary Broken Hill Exploration Initiative (BHEI), but the paper was neither discussed with, nor seen by, other key BHEI contributors before publication. It is one of a number of papers and abstracts involving G.M. Gibson and/ or A.P. Nutman, proposing a geological history involving high-grade deformation extending from about 1700 Ma to at least 1670 Ma. Closely linked publications are Gibson and Nutman [2004], the ensuing discussion by Conor et al. [2005], and reply by Gibson and Nutman [2005]. Some discussion of those publications is unavoidable here. 1. Commentary on Structural Interpretations of Key Areas