The structural and metamorphic history of the Wilmot and Franklin ranges, South-west Tasmania.
Creators: name: Boulter, Clive Anthony
Publisher: name: University Of Tasmania
Year: 2023

description: Tidally dominated, shallow-shelf sea conditions are indicated for the deposition of much of the late Precambrian sequence of the Frankland and Wilmot Ranges, Southwest Tasmania. Predominantly pelitic units may reflect tidal-flat or deltaic situations, and detritus in marine quartz sand horizons was derived from an environment of prevailing aeolian conditions perhaps by transgression. Overprinting proves five cleavage-forming deformation events followed by at least two conjugate kink sets and a regional 'faultdrag' rotation of all structural surfaces. The major geometrical features are the result of D 1 and D4 whilst D2 and D3 locally produce macroscopic folds; all D 1 to D5 events are essentially coaxial. Major D4 folds are generally tight upright structures with wavelengths and amplitudes of about four kilometres and they rotate all earlier events. Gravitational gliding was responsible for the emplacement of F 1 structures which were subsequently, in the main, only slightly modified by further overriding during the same event. D 1 folds root to the west and southwest and though F2 show the same direction of overthrusting it is uncertain whether D1/D 2 is a continuous event. D1/D 2 show orthotectonic characteristics whilst D4 is of a paratectonic style. D5 and the later conjugate kink bands are minor in scale. The regional rotation of deformations before and including those of D5 and the kinks is considered to have been caused by pre-Ordovician transcurrent movement on the Lake Edgar Fault. Quartz arenite, in D 1' first deformed by plastic deformation in hydrolytically weakened diagenetic quartz overgrowth but soon stress difference, in the grain-supported arenite, was taken up by intragranular plasticity of the detrital grains. Structural grains become apparent in outcrop at less than 10% shortening and good cleavages require less than 20% shortening. Penetrative fabrics developed in pelitic rocks in D l . All phases are extremely heterogeneous, with unstrained zones surviving to the present day. Post-D 1 cleavages usually involve microfolding of earlier fabrics, pressure dissolution and/or intracrystalline plasticity of quartz and mica. The relative importance of each mechanism is dependent on the pre-existing fabric and mica content of the rock. Investigations of strain in quartz arenite revealed a need to measure sedimentary fabrics from non-orogenic areas to provide a sound basis for work in deformed material where initial marker ratios and orientations were variable. Methods chosen must also allow for an independent check on the validity of two-dimensional strain ratios. The geometry of deformed cross-bedding shows that flexural slip was important in the formation of major F 1 structures which were modified by an average 25% flattening. As sedimentary structures are commonly modified or mimicked by deformation, care in interpretation is emphasised. Soft sedimentary, pre-tectonic clastic dykes are often planar and sub-parallel to cleavages where both structures are at an angle to bedding. Ready convergence of sedimentary and tectonic elements is thus demonstrated and the use of the approximate parallelism of dykes and cleavage to support tectonic dewatering is considered unsound.