Considerations on emplacement of Jurassic Dolerites

The emplacement of the Jurassic dolerites of Tasmania is considered in terms of the tectonic structure of Tasmania at the time of intrusion. At this time Tasmania consisted of a Permo-Triassic sedimentary cover draped in gentle downwarps over an uneven basement of folded Precambrian-Palaeozoic rocks, with uplifted highs in the west, northeast, and east. Characteristics required for the recognition of dolerite feeders in Tasmania, particularly cones, are discussed and the structures are examined in the light of these. In form, the cones are generally asymmetrical structures. In many cases there is insufficient evidence present to determine whether they represent rootless conical sheets emplaced under a flotational mechanism, or whether they represent conical feeders. Possible modes of emplacement are considered. Some of the cones appear to be secondary structures developed in the sedimentary cover at about the base of the Triassic sequence, possibly from cupola-like structures on underlying sill sheets. The intruding dolerite was controlled by, and also caused, extensive Jurassic faulting. The recognisable tectonic pattern is shown. The faulting in places suggests the development of horstlike and graben-like structures in the downwarps between the main pre-existing highs. Some of the major Tertiary faults of the State were originally lines of Jurassic movements, and there is a suggestion that the Tertiary Midlands, Cressy, Tamar, and possibly the Derwent and Oyster Bay trough structures were inherited, in part, from Jurassic structures. It is considered east-north-easterly and north-easterly tensional forces were important in producing the fault pattern and that it was controlled to a certain extent by the margins of the main pre-existing highs and structural trends in the basement. The greatest fracturing took place in the central trough of the main downwarp between the highs of western and eastern Tasmania. Dolerite magma probably rose here from feeder• dykes and its emplacement was controlled by compensation surfaces, giving rise to transgressive sills, meeting in numerous large dykes, with considerable rafting of sedimentary blocks. Away from the main downwarp, sill sheets were more extensively developed and in the broad scheme their emplacement apparently followed compensation paths. Cone structures were emplaced, apparently mostly on the margins of the downwarps and on small intervening highs. On the upper flanks of the main highs, feeders were mainly plug and dyke-like plugs. Where basement highs exceeded heights of about 4,000 feet above the basement level in the downwarps, dolerite feeders were lacking. At the time of the dolerite intrusions Tasmania was probably situated below the Antarctic Circle, just east of the Ross Sea, and comparisons are made with the similar Ferrar dolerites of Antarctica. Differences in the emplacement of the dolerite appear to be due to structural differences between the two regions at the time of emplacement. Only limited comparisons can be made between Jurassic trends associated with the dolerites of the two regions, but maxima in the trends of Tasmanian dykes were apparently similarly orientated to those of Antarctica