Polyphase deformation and its relation to metamorphic crystallisation in rocks at Wilmot Pass, central Fiordland

Abstract

Abstract Three major fold generations are recognised in central Fiordland. First generation folds (F1) are uncommon, isoclinal, and developed during regional middle to upper amphibolite facies metamorphism. Minerals formed during this event include kyanite, garnet, biotite, hornblende, staurolite, and possibly sillimanite. First generation folds are strongly overprinted by north-south-trending second generation structures (F2). Macroscopic F2 folds are seldom observed, but are inferred to be both recumbent and isoclinal. Extensive metamorphic reconstitution accompanied, and may also have immediately preceded, second generation folding and is most pronounced within the environs of the Mt George intrusive belt. A thermal aureole is recognised with sillimanite and sillimanite-K feldspar zone assemblages. Rocks within the aureole are strongly foliated and texturally indistinguishable from country rocks. The aureole is inferred to have developed whilst the country rocks were still undergoing active deformation. The third phase of folding (F3) produced broad, open, macroscopic structures with steeply dipping axial surfaces, parallel to which major faults have often developed. Two major F3 structures are recognised; the Lyvia Synform and Mica Burn Antiform. These folds are asymmetric and essentially of post-metamorphic origin; they were accompanied by minor recrystallisation under greenschist-lower amphibolite facies conditions. Fold axes plunge gently to the NNE or SSW Two periods of faulting are recognised: an early period of thrust faulting which followed, or occurred late during, second generation folding, and a subsequent period of high angle, predominantly reverse faulting. The latter accompanied or postdates F3 and includes the Wall Mountains, Wilmot and Gladstone Faults as well as the Spey-Mica Burn fault system; these faults largely trend NNE or NNW. Thrust faulting was directed from west to east and includes 2 important structures: the Mount Troup Thrust and Mount Watson Thrust.