The tectonic evolution of the Rocky Cape geanticline in northwest Tasmania

This thesis is a cross-sectional study of the Rocky Cape Geanticline, one of the major tectonic elements of Tasmania. The Rocky Cape Geanticline consists of deformed, unfossiliferous, and dominantly unmetamorphosed sedimentary rock of presumed Proterozoic age. The internal structure is due to the Penguin Orogeny of late Proterozoic age. Two distinct lithological assemblages are recognised, corresponding to different basins of deposition. These basins have a northeast southwest trend, parallel to the axis of the Rocky Cape Geanticline, and flank the older Precambrian nucleus Which lies to the east. To the west of the axis of the geanticline is an unstable shelf sequence about 20,000 feet thick, consisting of Shale, Oiltstone, orthoquartzite and minor dolomite. This comprises the Rocky Cape Group; a revised definition is given for the Rocky Cape Group, and new names are given to the Constituent formations. First record of sedimentation indicates a starved euxinic environment. This was followed by a stable, Shallowwater, frees-circulating environment in which unusually large thicknesses of cross-bedded orthoquartzite accumulated. Palaeocurrent directions from cross-bedding have diametrically opposed, bimodal or polymodal patterns, which are interpreted as the expression of transverse dispersal currents. These may reflect a gently oscillating palaeoslope. Further to the west the Rocky (Cape Group is overlain, with a low-angle unconformity, by the Smithton Dolomite which has a basal conglomerate. East of the axis of the geanticline, a flysch- type sequence at least 15,000 feet thick accumulated. This is called the Burnie Formation, and is considered to be younger than the Rocky Cape Group. It consists of slate, quartz wacke and minor pillow lava. Axially directed turbidity currents were an important agent of sedimentation, although, large-scale festoon cross-bedding Indicates that traction movement by bottom currents was significant. These currents are considered to be Independent of the turbidity currents. A variety of sedimentary deformation structures occur. Most are post-depositional, and can be further classified into those due either to-down-Slope mass movement, (block slide, slump sheet, and fluxoturbidite); or to small vertical, horizontal, or toroidal movements, (load cast, psendonddule, diapiric contortion, ball-andpillow structure and convolute lamination). Liquefaction is considered to be an important process In the development of the latter category. The imposed tectonic cleavage is Commonly-controlled by the pre-existing sedimentary deformation, but crosscutting relationships also occur, and assist in the distinction between sedimentary and tectonic Structures. Major tectonic Movements during the Penguin Orogeny were to the southeast, so that the Burnie Formation was squashed between the older basement to the east and the Rocky Cape Group to the west. Albite dolerite (Cooee Dolerite) in the form of dykes, sills and sheets were intruded during the early stages of folding. The two main sedimentary assemblages are now separated by the Keith Metamorphics, a belt about five miles wide, composed of pelitic schist, basic schist and amphibolite. The western boundary is gradational across, strike, and displays cataclastic textures. The Keith Metamorphics was derived in part from the surrounding sediments and partly from basic igneous rock, believed to be equivalent to the Cooee Dolerite. This belt is interpreted as a high-angle Shear zone. Correlation with similar schists along the axis of the Rocky Cape Geanticline indicates that this is a fundamental structure, and it is here termed the Arthur Lineament. The structure in the Rocky Cape Group is simple, consisting of a series of folds trending parallel to the major axis. The folds are tight and asymmetrical near the shear zone, and become more open and symmetrical to the west. This folding affects the Smithton Dolomite. Detailed structural analysis of the Burnie Formation reveals a complex picture of progressive . deformation involving five phases, all on approximately the same axis. The first and third generations were the major events in the structural evolution. The major structure consists of two overturned synclines separated by a flat belt and an anticlinal hinge. The overall vergence is to the southeast, toward the older Precambrian nucleus. Mesoscopic folds and associated Structures of one phase tend to form in zones which are spatially segregated from zones of other generations. Thus, the first generation folds are Mostly confined to the overturned limbs of the major synclines, and the third generation folds are confined to the intervening fiat belt. Smaller scale segregation is demonstrated by the occurrence of the later phases in zonee of planar bedding between clusters of.first generation folds. The area of overlap of these zones is generally small, So that examples of refolded structures are not abundant, and a false impression is given of rapid changes in tectonic style of the one fold generation. The segregation of fold phases is most strongly expressed where the bedding remained kinematically active, a feature which suggests that there is a mechanical limitation to Superposed folding. In the first generation mesoscopic folds, the geometry of the arenite beds approximately fits that Of flattened concentric folds. At Sulphur Creek, the mesoscopic folds have a marked asymmetry, shown by common-limb thinning of the fold couplet, a "half-fan" cleavage, and offset carinate structures. These features may be explained by an obliquity between the initial axial plane and the plane of flattening. The flattening stage of fold development was followed by a disharmonic stage involving variable lateral shortening, and the formation of "slip-off" structures, oblique shear joints and boudinage. Cleavage formed continuously throughout the sequence of development of the P1 minor structures, although the mairpariod of formation was during the flattening stage. The change in behaviour of the arenite beds during folding from initially competent, to incompetent, and back to competent, may be due to the role of internal pore water during deformation. The evolution of the Rocky Cape Geanticline fits into a pattern of pxoterozoic Lower Palaeozoic orogenesis, in which the axis of sedimentation progressively shifted toward the older nucleus, and the tempo of - sedimentation and structural deformation increased to a climax in the Upper Cambrian.