Strain localisation and transcurrent reactivation in the granulite facies Kalinjala Shear Zone at Port Neill, South Australia

Abstract

The central portion of the Kalinjala Shear Zone, in the Port Neill region, South Australia, is recognised as a northeast-trending anomalously magnetic corridor coinciding with a sub-vertical transpressional dextral shear zone. This shear zone is a primary lithospheric structure, separating the nucleus of the Archean Gawler Craton from a suite of Paleoproterozoic granites (ca 1850 Ma) and earlier paragneiss. At Port Neill, there is an apparent 17 km dextral offset of a pre-existing Bouguer gravity anomaly by the Kalinjala Shear Zone. This is attributed to Kimban-aged (ca 1740–1690 Ma) deformation that is accompanied by peak granulite facies metamorphic conditions reaching pressures of ∼1.0 GPa and temperatures of ∼800 °C. This is consistent with the presence of leucosomes, within the sheared fabric, in the axial surface of mylonitic folds and in mafic gneisses. Strain has been localised within and adjacent to mafic dykes, predominantly hosted in the granitic rocks, with the highest strained areas having the greatest evidence of leucosome extraction. The local orientation of stretching lineations in this shear zone do not necessarily correlate with the transport direction but may reflect along- and across-strike variations in finite shear strain into alternating zones dominated by either constrictional or a superimposed flattening strain. Overprinting the dominant dextral movement are sinistral shears, with horizontal strike-slip movements, which post-date the peak metamorphic assemblages and are generally confined to sites containing late K-feldspar-rich pegmatites. The major movement recorded in the Kalinjala Shear Zone was both horizontal and vertical, occurring in response to regional exhumation of the Cornian-aged (ca 1850 Ma) sequences in the east, during the Kimban Orogeny, and was basically controlled by the nature of lithospheric plate convergence during accretion of a continental backarc onto the Gawler Craton.

KEY POINTS

1. Dextral transpression with late-sinistral along-strike extension and east-up exhumation is associated with 17 km of horizontal offset along the central section of the Kalinjala Shear Zone.

2. Deformation is dominated by three major Kimban-aged (ca 1730–1690 Ma) events, which overprint older ensialic backarc metasedimentary rocks, orthogneiss and granites of the Donington Suite.

3. Granulite facies metamorphic conditions control the nature of deformation and strain distribution within the shear zone.

4. The Kimban events record the collision of the Gawler Craton nucleus with the southern margin of the North Australian Craton.

Keywords:

• Kalinjala Shear Zone
• mylonites
• Gawler Craton
• Kimban Orogeny
• Paleoproterozoic
• transpression
• Donington Suite
• Tournefort Metadolerites
• granulite facies metamorphism

Acknowledgements

We wish to acknowledge the First Nations people of the Barngala and their language in which the word ‘kal:injala refers to the general locality of Port Lincoln. Potential-field data were sourced from the Geological Survey of South Australia and Geoscience Australia. Mark Fanning, John Parker, Mike Schwarz and members of the South Australian Geological Survey are thanked for discussions during various stages of our fieldwork. Jeff Vassallo and Hans Hoek contributed to this work during the initial fieldwork and during discussions as part of Jeff’s PhD thesis. In addition, Brett Marmo, Sharif Oussa, Libby Fontaine-Geary, James Wilkie and Chevaun Gellie contributed as part of their Honours mapping projects in the Port Neill to Tumby Bay areas. Anthony Reid and Caroline Tiddy are thanked for valuable comments resulting in improvements to the paper.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Detailed mapping of this portion of the southeastern Eyre Peninsula was initially undertaken by CW with support of ARC Large Grant A39533031 and by JS and PB as part of an ARC Linkage funded project LP04544301 in collaboration with the Geological Survey of South Australia, Department for Mining and Energy.