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ItemDeformation and the thermobaric history of the eastern coast of Williams IslandMarks, Bianca ( 1997)Williams Island is located off the southern coast of the Eyre Peninsula of South Australia where the Palaeoproterozoic rocks of the Lincoln Batholith intrude a portion of an Archaean basement complex. The structures of the eastern coast of Williams Island are controlled by the rheological contrast between the mafic dykes and the felsic granite gneisses that comprise the batholith. Planes of rheological weakness exist at the dyke margins along which strain is localised. The plane of failure and the kinematics along it depends upon the orientation of the dyke with respect to the stress field. Displacements at cross-cutting dyke margins indicates the occurrence of three significant deformation events, D 1, D2 and D3. By comparison, the D1 is localised to a region of outcropping Jussieu Dykes, the D2 is pervasive and the D3 is confined to the discrete Northern and Southern Shear Zones. Associated with the latter two deformations is an increase in temperature and strain rate which controls the relative strength of the metabasic and the granite gneiss rocks. Brittle extensional structures, such as boudinage, form when the mafic dykes behave in a more competent manner relative to the host, whereas ductile extensional features, like pinch and swell, infer a greater homogeneity between the rock types. The rheological contrast is inverted with a preferential increase in strain resulting in granite boudinage. The D2 fabrics arc predominantly defined by a granulite two-pyroxene assemblage and the structural elements of D3 are characterised by minerals associated with amphibolisation. Average pressure calculations of representative assemblages give 7 ± 1 kbar for M2/D2 and 12 ± 2 kbar for M3/D3, which suggests crustal thickening over D2 - D3 time. Exhumation of the crustal block therefore occurred after peak D3.
ItemStructural and metamorphic constraints on Kimban Orogenesis from southern Eyre Peninsula, South AustraliaBerman, David ( 1997)Archaean to Mesoproterozoic lithologies at Whalers Way on southern Eyre Peninsula present a unique cross section of episodic deformation and magmatism within the Gawler Craton. D2 and D3 at Whalers Way were coaxial phases of progressive fold-thrust deformation. Progressive D2 deformation resulted in the formation of east dipping shear zones along which nappes with NW directed vergence were stacked coeval with the Kimban metamorphic peak. Domains of upright and recumbent D3 structure can be identified based on the orientation of S3. Domains of recumbent D3 structure retain transitions from open folds not associated with an axial planar foliation through to recumbent antiforms bound by shallow west dipping shear zones. S3 within domains of recumbent structure is non-pervasive and well developed only within D3 shear zones. In contrast a pervasive S3 developed within domains of upright D3 structure. Systematic variation in the pitch of L3 imply vertical strain was partitioned into domains of upright structure. Tectonic foliations have been used to establish linkages between the structural and metamorphic histories of Whalers Way. Assemblages that define S2 (M2) equilibrated at 6-7 kbar and 800-820°C (M2) whereas S3 (M3) assemblages equilibrated at 3-5 kbar and 700°C (M3). M2 and M3 peak temperatures were in excess of those which could result solely from thermal relaxation of an overthickened crust. Transient Kimban heat input is consistent with rapid post-D3 cooling implied by K-Ar geochronology. D2 and D3 were the earlier and latter phases of a single episode of transpressional deformation in which the change in kinematics from NW to NE directed transport was associated with a 90° swing in the orientation of the subhorizontal principal compressive stresses. The consistency of kinematic data and the temporal sequence of magmatism relative to deformation across the Eyre Peninsula suggest syn-D3 decompression at Whalers Way reflects regional scale processes. Because the terrain is inferred to have cooled rapidly, the presence of near isothermal decompression textures implies exhumation must also have been rapid and suggests significantly non-plane strain flow within steeply oriented crustal scale shear zones is an efficient mechanism for exhumation of the mid to lower crust.