Integrated structural geology and geophysical analysis of crustal-scale shear zones in the Gawler Craton, South Australia

Local-scale structural geological observations have been integrated with multi-scale potential field data analysis allowing assessment of shear zone kinematics and crustal evolution within the Archean Mesoproterozoic Gawler Craton, southern Australia. This methodology has been crucial to the successful reconciliation of kinematic and relative overprinting chronological data over a regional scale across the poorly outcropping, complexly deformed terranes that constitute the craton. A number of case studies are presented, which demonstrate the power of this methodology and collectively form the framework for a regional model that describes the tectonic evolution of the craton over the time period ca. 2800 to 1450 Ma. The resulting tectonic model demonstrates links between Australia and Laurentia. Analysis of shear zones in the western Gawler Craton indicates a polyphase shearing history dominated by a crustal-scale array of predominantly west-dipping shear zones. Initiation ofSZ, was coincident with crustal extension, high heat flow and deposition on thinned areas ofArchean crust with a dense lower crustal component. SZ1-R2 dextral transpressional movements reactivated SZ, during the Kimban Orogeny inverting marginal basins and vertically offsetting dense basement. SZ, R2 activity is constrained until ca. 1680 Ma. SZ3 structures record a major phase of crustal-scale sinistral strike-slip movement between ca. 1630 and 1540 Ma prior to their reactivation at ca. 1450 Ma (SZ3 R4) with west-side-up movements internal to the Archean continent and dextral strike-slip at the boundary of the Paleoproterozoic basins to the north. Within the Tallacootra ShearZone, D2 mylonitic fabrics record ca. 1730-1690 Ma shortening and correlate with SZ,. D3 sinistral shears initiated Riedel to regional-scale SZ2 strike-slip (SZ2). Extension during the Hiltaba Event led to the emplacement of aplite dykes into the shear zone. D4 dextral transpression during the Coorabie Orogeny reactivated the Tallacootra Shear Zone (SZ, R4) exhuming lower crust within a positive flower structure. Exposures at Yarlbrinda Hill comprise the margin of a highly deformed St Peter Suite pluton emplaced within the SW Gawler Craton at ca. 1611 Ma. D, records ductile thrusting, which overprints N-S trending dextral Yarlbrinda Shear Zone fabrics within ca. 1690-1622 Ma St Peter Suite and Tunkillia Suite granites. D2 deformation is recorded by brittle reactivation of first generation mylonitic fabrics, cataclasis, alteration and pseudotachylite development. D3 caused folding and tear faulting of the D, 2 shear zone. Extension and rotation led to Hiltaba Suite emplacement and extensional faulting during D4 and D3. Deformation of ca. 1622 Ma St Peter Suite granite is consistent with movement on the Yarlbrinda Shear Zone and requires the St Peter Suite to have been emplaced on the overriding plate during subduction. D, to D, deformation correlates with the Wartakan Orogeny. D1 structures within the Yarlbrinda Shear Zone are related to the emplacement of ca. 1690-1670 Ma Tunkillia Suite granitoids. D, is characterised by dextral transpression between ca. 1625-1611 Ma. D3 xv is recorded as a phase of partitioned W-side-up shearing/thrusting during emplacement of late St Peter Suite granites. During D4 the Nuyts Domain underwent counterclockwise rotation possibly coincident with the initial dextral movement on the Yerda Shear Zone and the latest phase of movement along the Yarlbrinda Shear Zone. This rotation requires a phase ofextension during the early phases of the Hiltaba Event. NE-trending sinistral and dip-slip shear zones record a phase of extension that overprints the Yarlbrinda Shear Zone contemporaneous with Hiltaba Suite magmatism. D6 reactivation of the Yerda Shear Zone and its dextral splays occurred during N-S oriented shortening. D2 to D5 deformations occurred during the Wartakan Orogeny, which is constrained to ca. 1630-1580 Ma and characterized by localised flat subduction, the evolution of which influenced the distribution and volume of arc-related magmatism. D6 is correlated with a phase of~N-S shortening. The Yerda Shear Zone forms a composite of several shear zones that form a major E-W-trending crustal boundary within the craton. The latest deformation recorded within outcropping Hiltaba Suite granite is characterized by uniaxially oblate finite strain. E-W-trending mylonites are partitioned from WNWtrending dextral C’ fabrics that developed under N-S shortening during the Kararan Orogeny. Pre1575 Ma kinematics show that the Yerda Shear Zone initiated as a major NE-directed thrust during the Wartakan Orogeny, which imposed an oroclinal geometry to the Gawler Craton. During subsequent NW-SE extension, normal faulting and sinistral strike-slip at ca. 1580 Ma led to localization of ca. 1575 Ma granite within the shear zone during dextral transtension in a back-arc setting. Detailed analysis of the Kalinjala ShearZone suggeststhat this structure is a primary suture, subsequently reactivated and reworked as a crustal-scale dextral transcurrent fault. Early events record the ca. 1850 Ma suturing between the proto-Gawler Craton and the North Australian Craton contrasting with previous tectonic models that suggest the Gawler Craton was amalgamated with the North Australian Craton during the ca. 1730-1690 Ma Kimban Orogeny.