Southern Eyre Peninsula Research Articles

Tsunami or storm? A high-level coastal boulder field on the southern tip of Eyre Peninsula, South Australia

A high-level coastal boulder field at Whalers Way, southern Eyre Peninsula, South Australia, occurs at elevations of 20–30 m above present sea-level on a gently seaward-sloping, karst-weathered calcrete-capped structural bench, formed on the Upper Pleistocene Bridgewater Formation and underlying Carnot Gneisses (Archean–Paleoproterozoic Sleaford Complex). More than 1000 ex situ boulders, cobbles and pebbles of gneiss and mafic igneous rocks, as well as fragments of calcrete, occur within the boulder field and cover an area >6000 m2. All the crystalline rock clasts are derived locally from bedrock outcrops along the adjacent coastline, where metasedimentary granulite-facies of the Carnot Gneisses crop out along shore platforms and their backing cliffs, close to present sea-level. The boulder field is younger than Marine Isotope Sub-Stage 5a (MIS 5a; ca 80 ka) based on amino acid racemisation ‘whole-rock’ analyses of calcarenite of the Bridgewater Formation, which forms the calcrete bench on which the boulder field rests. The boulder field wraps around the western extremity of cliff-top dunes dated at 18–17 ka by optically stimulated luminescence, implying that the boulder deposit post-dates the formation of the cliff-top dunes. Given that the clasts in the boulder field show minimal weathering rinds, sea-level during the Last Glacial Maximum was some 125 m lower than present and that between 80 and 7 ka ago, sea-level never attained present levels within the region, emplacement of the boulder field by a high-energy wave event in the Holocene highstand, following the 7000 years BP culmination of post-Glacial sea-level rise, is suggested. Storm waves, rather than a tsunami, most likely explain the emplacement of the boulder field.

Shear zones in the southern Eyre Peninsula, South Australia: Quartz c-axis fabrics in granulite facies mylonitic orthogneisses and relationship to mafic dykes

The south eastern portion of the Eyre Peninsula, South Australia, is dominated by a suite of granites and orthogneisses deformed in steep-dipping shear zones during the Kimban Orogeny (1730–1690 Ma). The largest is the transpressional Kalinjala Shear Zone with shortening strains of ≥90% and shear strains of γ = ≥20, with other near-parallel secondary shear zones displaying similar kinematics. These ductile shear zones are dominated by mylonitic orthogneisses, with shearing commonly nucleating on, or adjacent to, precursor mafic dykes. The mafic dykes are highly sheared, folded and boudinaged with a newly formed matrix assemblage (orthopyroxene + clinopyroxene + garnet + magnetite + quartz + K-feldspar) that indicates the peak P/T metamorphic conditions of 0.7–1.0 GPa and 750–850 °C. Partial melting, developed as leucosomes, is associated with this metamorphism. Microstructural relationships between quartz crystallographic preferred orientation (CPO) and shape preferred orientations (SPO) analyses of these polymineralic rocks are best interpreted in terms of a granulite facies isothermal temperature regime of ~750 °C. The quartz c-axis fabrics, obtained using an optical fabric analyser, reveal a range of CPOs from weak girdles normal to the shear plane, to maxima parallel to the Y-strain axis. There is an overall symmetric orthorhombic pattern, with c-axis fabrics oriented with respect to the XY plane of flattening. There is a minor component of non-coaxial deformation in some shear zones. These fabrics indicate the increased activation of predominantly the prism-<a > slip system in quartz within the shear zones, which is characteristic for these high-temperature and high-strain deformation conditions.

Tectonic history related to the southern section of the Kalinjala Shear Zone, Eyre Peninsula, South Australia: And correlations with Terre Adélie Craton, Antarctica

The southern edge of the Eyre Peninsula, experienced a separation from proto-Antarctica in the Cretaceous and has been projected to extend into Terre Adélie Craton, Antarctica. The Archaean and Palaeoproterozoic rocks of the southern Eyre Peninsula, have been modified by the Kimban Orogeny (~1730–1690 Ma) with the development of the regionally extensive Kalinjala Shear Zone, which has been correlated with the Mertz Shear Zone in Antarctica. In this contribution, new structural observations associated with the granulite facies Kalinjala Shear Zone, together with other shear zones, all showing similar kinematics of dextral top-to-the-north transport will be described. The Kimban Orogeny overprints and reworks a major lithospheric boundary between the Gawler Craton and a sequence of preceding events related to the ~ 1850 Ma Cornian Orogeny. Shear zones were initiated during the early stage of the Kimban Orogeny (KD1) in a dextral simple-shear regime, transitioning into a pure-shear (KD2) dominated regime. With shortenings of > 90% in the Kalinjala Shear Zone localized into regions containing pre-existing Tourneford Metdolerite Dykes emplaced at ~ 1810 Ma into the granites of the Donington Suite (1850–1843 Ma) and a basement of 2000–1859 Ma paragneisses. Geometrical relationships reveal the operation of oblique extrusion during the Kimban Orogeny, where a coaxial component acted sub-horizontal and orthogonal to the trend of the Kalinjala Shear Zone. Regions of low-strain and a ‘pinched-in’ and inverted sequence of Mesoproterozoic metasedimentary cover, the Hutchison Supergroup, are bounded by major km-scale shear zones with steep west-dipping tectonic contacts. Post-Kimban subvertical east–west-trending joints and faults record a rifting event which can be related to a Gondwana breakup. With post-rift Neogene fault activity recording additional tectonic episodes where pre-existing basement structures exert significant control on their orientation and distribution. Palaeogeographical reconstructions between the Eyre Peninsula and the Terre Adélie Craton, based on matching basement geology and structural style, are compared and reconciled with this new information.

Defining the Eyre Conductivity Anomaly with the Tumby Bay MT transect

We present results of 36 broadband magnetotelluric (MT) stations along a 70 km ENE-WSW profile across the southern Eyre Peninsula, South Australia. The transect crosses the Eyre Peninsula Anomaly (...

Identifying lithospheric boundaries using magnetotellurics and Nd isotope geochemistry: An example from the Gawler Craton, Australia

Major lithospheric boundaries play a fundamental role in localizing crustal deformation and metal enrichment in the crust. Their identification in poorly exposed terranes remains challenging and typically relies on a combination of sparse outcrop and drill hole data extrapolated using regional gravity and magnetic datasets. The high complexity in potential field data as well as the limited depth sensitivity to mostly upper to mid-crustal levels increases the ambiguity in identifying coherent lithospheric domains as well as the trans-lithospheric boundaries that cross into the subcontinental lithospheric mantle. Here we present a different approach using a combination of deep-probing magnetotelluric and seismic reflection methods, integrated with Nd isotope and inherited zircon ages of crustally-derived felsic igneous rocks, to locate and characterize a concealed Archean-Proterozoic lithospheric boundary within the Gawler Craton, South Australia. The boundary extends for approximately ∼600 km in the eastern part of the craton, from the supergiant iron oxide-copper-gold-uranium (IOCG-U) Olympic Dam deposit in the north to outcrops of the Kalinjala Shear Zone on southern Eyre Peninsula in the south. Using our approach we show a significant reduction in the uncertainty of the location of this boundary between these two end-points. While a range of locations are permissible based on potential field data across northern Eyre Peninsula, our integrated approach points towards a precise location east of previous interpretations, beneath the upper crustal Roopena Fault.

Analysis of Saline Intrusion into a Coastal Aquifer: A Case History of Legacy Issues and Challenges to Water Security

Coastal aquifers are subject to seawater intrusion. Therefore, managing freshwater aquifers in coastal areas remain challenging. At present, determining safe yields from the coastal aquifers to prevent seawater intrusion is primarily based on the use of numerical simulation-optimization models or by the use of analytical models based on the Ghyben-Herzberg principle. This study examines the cause and effects of seawater intrusion into a coastal aquifer, Lincoln Basin in southern Eyre Peninsula, South Australia and shows that application of simple techniques would have prevented seawater intrusion. Three freshwater lenses, Lincoln A, B, and C of the Lincoln Basin, located about 13 km southwest of Port Lincoln township, have been developed as a town water supply source in 1960. The capacity of the basin has been assessed by three long-term pumping tests. Based on pump tests results, three areas were developed to supply 2×106 m3 per year distributed across three lenses as lens A : four wells to supply 0.84×106 m3, lens B: four wells to supply 0.5×106 m3 and lens C: four wells to supply 0.66 ×106 m3. Neither recharge to the freshwater lenses nor a water balance had been assessed, and a precautionary approach to groundwater extraction was not followed. The apparent driver for managing the basin was demand for the township. In this study, we assessed the recharge using two methods; water-table fluctuation (WTF) and the conventional chloride mass balance (CMB) method. Total recharge to the freshwater lenses is estimated at 1.6×106 m3 per year which is less than the average annual groundwater extraction from the basin during the 1961-1977 periods (average 2.14×106 m3). As a result mining of the groundwater storage has occurred in the basin leading to saline intrusion, upconing and lateral flow of brackish water into wellfield areas. The total volume extracted from the basin was 35×106 m3, which exceeded the average recharge over the 15 year period, 24×106 m3. Using analytical methods, the seawater/freshwater interface movement from its original position was estimated to be 35 m in lens A, 337 m in lens B and 188 m in lens C. For each pumping well at maximum discharge rate, the transient interface location directly underneath the well was calculated. This results in interface rises under pumping wells in lens A of 3.8 m, lens B of 0.5 m, and in lens C about 0.7 m. According to the risk-based groundwater allocation method, maximum extraction would have been as a proportion of 25% of the annual recharge. Thus, maximum annual abstraction limits for lens A, B and C would have been 210×103 m3, 72×103 m3 and 130×103 m3, totaling 412×103 m3.

An ecological climate change classification for South Australia

ABSTRACTTemperatures in South Australia have risen 0.5–1.5°C post-1950, a trend expected to drive ecological change. However, climate sensitivity, landscape resilience and management priorities vary among regions. We classified the vulnerability of South Australian plant assemblages to climate change in a framework that combines climate sensitivity and resilience. Generalised Dissimilarity Models (GDMs) of plant species composition were trained with species occurrence records from field plots. Resilience was represented by the proportional remnancy of native vegetation surrounding each location. Landscapes were classified using bivariate quantiles. Mean annual rainfall, summer maximum temperatures and spatial structure were important predictors of species turnover. GDMs (explaining 37–68% of deviance) were projected onto future climate scenarios to calculate sensitivity metrics. The Wheat Belt, mallee and Flinders Ranges were the most sensitive, representing a climatic transition characterised by rapid change in species assemblages. Southern Eyre Peninsula and western Kangaroo Island were classified as Resilient; the Mount Lofty Ranges, southern Flinders Ranges and eastern Kangaroo Island as Resistant; northern Eyre Peninsula and northern Flinders Ranges as Sensitive; and the Wheat Belt as Susceptible. The classification assists decisions on management priority and provenances or species selection for restoration and assisted migration in conjunction with other biodiversity measures.

Further evidence for two metamorphic events in the Mawson Continent

AbstractIn this study,in situand erratic samples from George V Coast (East Antarctica) and southern Eyre Peninsula (Australia) have been used to characterize the microstructural, pressure–temperature and geochronological record of upper amphibolite and granulite facies polymetamorphism in the Mawson Continent to provide insight into the spatial distribution of reworking and the subice geology of the Mawson Continent. Monazite U-Pb data shows thatin situsamples from the George V Coast record exclusively 2450–2400 Ma ages, whereas most erratic samples from glacial moraines at Cape Denison and the Red Banks Charnockite record only 1720–1690 Ma ages, consistent with known ages of the Sleaford and Kimban events, respectively. Phase equilibria forward modelling reveals considerable overlap of the thermal character of these two events. Samples with unimodal 1720–1690 Ma Kimban ages reflect either formation after the Sleaford event or complete metamorphic overprinting. Rocks recording only 2450–2400 Ma ages were unaffected by the younger Kimban event, perhaps as a result of unreactive rock compositions inherited from the Sleaford event. Our results suggest the subice geology of the Mawson Continent is a pre-Sleaford-aged terrane with a cover sequence reworked during the Kimban event.

Identifying Centres of Plant Biodiversity in South Australia.

We aimed to identify regional centres of plant biodiversity in South Australia, a sub-continental land area of 983,482 km2, by mapping a suite of metrics. Broad-brush conservation issues associated with the centres were mapped, specifically climate sensitivity, exposure to habitat fragmentation, introduced species and altered fire regimes. We compiled 727,417 plant species records from plot-based field surveys and herbarium records and mapped the following: species richness (all species; South Australian endemics; conservation-dependent species; introduced species); georeferenced weighted endemism, phylogenetic diversity, georeferenced phylogenetic endemism; and measures of beta diversity at local and state-wide scales. Associated conservation issues mapped were: climate sensitivity measured via ordination and non-linear modelling; habitat fragmentation represented by the proportion of remnant vegetation within a moving window; fire prone landscapes assessed using fire history records; invasive species assessed through diversity metrics, species distribution and literature. Compared to plots, herbarium data had higher spatial and taxonomic coverage but records were more biased towards major transport corridors. Beta diversity was influenced by sampling intensity and scale of comparison. We identified six centres of high plant biodiversity for South Australia: Western Kangaroo Island; Southern Mount Lofty Ranges; Anangu Pitjantjatjara Yankunytjatjara lands; Southern Flinders Ranges; Southern Eyre Peninsula; Lower South East. Species composition in the arid-mediterranean ecotone was the most climate sensitive. Fragmentation mapping highlighted the dichotomy between extensive land-use and high remnancy in the north and intensive land-use and low remnancy in the south. Invasive species were most species rich in agricultural areas close to population centres. Fire mapping revealed large variation in frequency across the state. Biodiversity scores were not always congruent between metrics or datasets, notably for categorical endemism to South Australia versus georeferenced weighted endemism, justifying diverse approaches and cautious interpretation. The study could be extended to high resolution assessments of biodiversity centres and cost:benefit analysis for interventions.

Investigation on the groundwater resources of the South Eyre Peninsula, South Australia, determined from laterally constrained inversion of tempest data

Groundwater in the Eyre Peninsula of South Australia is scarce with potable resources limited to the western coastal margin and the southern tip of the peninsula. Consequently an understanding of their extent has become increasingly important particularly with demand being close to current extraction limits. In September 2006, about 1000 line km of TEMPEST AEM data were acquired over the Southern Eyre Peninsula, in order to assist in the definition of freshwater lens systems and in particular aquifer bounds associated with them as part of a resource definition project. Following their acquisition, the TEMPEST data set was analysed for data quality and then transformed into conductivity depth images (CDI) using EMFLOW and subsequently using a smooth model inversion (see Fitzpatrick and Munday, 2007). In an effort to better define to better define the geometry of specific bounding surfaces of hydrogeological relevance the TEMPEST data were inverted through the application of the laterally constrained inversion (LCI) technique. This paper describes the initial results from the first application of the LCI to data from a fixed wing AEM system.

Distribution and status of the Osprey (Pandion haliaetus) in South Australia

The breeding population of Ospreys (Pandion haliaetus) in South Australia was estimated at ∼52 pairs in 2005. These were sparsely distributed (one pair/30.1 km of coastline) from the eastern Bunda Cliffs in the west of the state, to Kangaroo Island in the east, with most of the population on western and southern Eyre Peninsula in the Great Australian Bight region. Most nests were constructed on broken sections of coastal cliff and near-shore rock-stacks, with a small number on man-made structures and one active nest in a tree. The population is geographically isolated, with the westernmost territory in South Australia ∼1000 km from the nearest known breeding sites to the west, in Western Australia, the nearest population to the east is in north-eastern New South Wales, >1500 km distant. There is evidence for an overall population decline in South Australia as only historical breeding records were found for upper Spencer Gulf and for the Murray River indicating a substantial reduction in range. In addition, some nesting sites on Eyre Peninsula and Kangaroo Island are considered vulnerable to disturbance from unmanaged coastal recreation and the encroachment of residential and tourism development.

Tectonic evolution of the southern Gawler Craton,South Australia, from electromagnetic sounding

Long-period natural-source electromagnetic data have been recorded using portable three-component magnetometers at 39 sites in 1998 and 2002 across the southern Eyre Peninsula, South Australia that forms part of the Gawler Craton. Site spacing was of order 5 km, but reduced to 1 km or less near known geological boundaries, with a total survey length of approximately 50 km. A profile trending east – west was inverted for a 2D electrical resistivity model to a depth of 20 km across the southern Eyre Peninsula. The main features from the models are: (i) on the eastern side of the Gawler Craton, the Donington Suite granitoids to the east of the Kalinjala Shear Zone are resistive (>1000 Ωm); (ii) the boundary between the Donington Suite granitoids and the Archaean Sleaford Complex, which has much lower resistivity of 10 – 100 Ωm, is almost vertical in the top 10 km and dips slightly westwards; and (iii) two very low resistivity (<1 Ωm) arcuate zones in the top 3 km of Hutchison Group sediments correlate with banded iron-formations, and are probably related to biogenic-origin graphite deposits concentrated in fold hinges. Such features suggest an extensional regime during the time period 2.00 – 1.85 Ga. We suggest that the resistivity boundary between the Donington Suite and the Archaean Sleaford Complex represents a growth fault, typical for rift systems that evolve into a half-graben structure. In the graben basin, low-resistivity shallow-marine Hutchison Group sediments were deposited. Folding of the sediments during the Kimban Orogeny between 1.74 and 1.70 Ga has led to migration of graphite to the fold hinges resulting in linear zones of very low resistivity that correlate with banded iron-formation magnetic anomalies.

Metamorphic evolution and reworking of the Sleaford Complex metapelites in the southern Eyre Peninsula, South Australia

The partial melting processes of Al‐rich and cordierite‐absent garnet‐orthopyroxene‐bearing leucosome assemblages in the reworked Archaean Sleaford Complex in southern Eyre Peninsula, South Australia are examined. The Kimban Orogeny produced a pervasive development of garnet‐K‐feldspar‐dominated leucosome assemblages. The textural relationships suggest that the peak assemblage garnet‐orthopyroxene‐K‐feldspar involved a melt phase and formed through the divariant partial melting reaction garnet + biotite = orthopyroxene + K‐feldspar + liquid in the KFMASH system, during a prograde KD1 heating event. A cordierite‐bearing metapelite also preserves the peak KD1 assemblage of garnet‐cordierite‐orthopyroxene‐K‐feldspar, and the textural relationships suggest that peak assemblages formed through the crossing of the univariant partial melting reaction garnet + biotite = orthopyroxene + cordierite + K‐feldspar + liquid in the KFMASH system, during the KD1 prograde heating stage. It is suggested that peak metamorphic P‐T conditions reached 850–900°C and 0.9–1.0 GPa, and that the preservation of peak assemblages means that some melt was lost near or during the metamorphic peak. The pervasive interstitial K‐feldspar grains nucleated synchronously with decompression, and the second generation of small euhedral garnets are inferred to have formed through decompression‐generated partial melting during a KD2 high‐strain shear event. The P‐T conditions of 700–750°C and 0.5–0.6 GPa were attained during the KD2 high‐grade shear event. The inferred clockwise P‐T path from KD1 to KD2 is compatible with a collisional tectonic model during the Kimban Orogeny (ca 1730–1700 Ma), whereas syn‐KD2 decompression‐cooling was a result of rapid tectonic exhumation of the terrain.

Structural repetition of the Hutchison Group metasediments, Eyre Peninsula, South Australia

Progressive deformation of the Palaeoproterozoic Hutchison Group metasediments, eastern Eyre Peninsula, South Australia, accompanied the development of a ∼200 km‐long suture, the Kalinjala Shear Zone. High‐strain structures that developed during the Kimban Orogeny preserved in this supracrustal sequence at Sleaford Bay, southern Eyre Peninsula are: (i) KF1 sheath folds; and (ii) KD2 tight folds and reverse shears. Basement Archaean Sleaford Complex gneisses are interpreted to have been thrust over their cover rocks. Interleaving of individual Hutchison Group units by imbricate shears and folds are suggested to have caused an estimated strike‐normal shortening of up to 50%. However, this calculation is of less significance than the >15:1 strike‐parallel elongation of the package during KD1. Identical structural elements reported by earlier workers are also preserved in the stratigraphic type areas of the Hutchison Group, central Eyre Peninsula. Importantly, the stratigraphic Upper and Lower Middleback Jaspilites, in the Middleback Range, are reinterpreted as one unit structurally repeated by a kilometre‐scale KF1 sheath fold. Imbricate KD2 shear zones are inferred to have inverted original disconformities throughout the package. In light of these observations, we suggest that the currently constructed stratigraphic succession of the Hutchison Group should be regionally re‐examined, with a special focus in those areas less affected by the imbricate structures.

Geology and Characterization of Two Hydrothermal Nontronites from Weathered Metamorphic Rocks at the Uley Graphite Mine, South Australia

AbstractMining operations during the early 1990s at Uley Graphite Mine near Port Lincoln on southern Eyre Peninsula, South Australia, uncovered abundant nontronite veins in deeply weathered granulite facies schist, gneiss, and amphibolite of Palaeoproterozoic age. Two types of nontronite are present: a bright yellowish-green clay (NAu-1) distributed as veinlets and diffuse alteration zones within kaolinized schist and gneiss, and a massive to earthy, dark-brown clay (NAu-2) infilling fracture networks mainly in amphibolite or basic granulite. The nontronites are the product of low-temperature hydrothermal alteration of primary minerals, biotite, and amphibole. The principal chemical difference between NAu-1 and NAu-2 is a higher alumina content in NAu-1, which was either inherited during hydrothermal alteration of biotite in the host rock or acquired through recrystallization of nontronite during subsequent weathering and associated kaolinization. Sufficient bulk samples of both NAu-1 and NAu-2 were collected to supplement reference nontronite of the Source Clay Repository of The Clay Minerals Society. The clay fraction of the bulk samples is typically &gt;85%. NAu-1 contains minor kaolin and quartz which are easily removed to give a high purity nontronite of composition M+1.05[Si6.98Al1.02][Al0.29Fe3.68Mg0.04]O20(OH)4, similar to that of nontronite from Garfield, Washington. NAu-2 contains fewer total impurities but the presence of trace amounts of submicron carbonate and iron oxyhydroxide requires additional chemical treatment to produce a nontronite of purity comparable to NAu-1. Composition of NAu-2 was calculated as M+0.72[Si7.55Al0.45][Fe3.83Mg0.05]O20(OH)4, although infrared data indicate that at least some Fe is in tetrahedral coordination.

Electromagnetic investigation of the Eyre Peninsula conductivity anomaly

Seafloor and land magnetotelluric (MT) data were collected in the SWAGGIE (Southern Waters of Australia Geoelectric and Geomagnetic Induction Experiment) project in April-May 1998, from 30 seafloor and 23 land sites. The principal objective of the experiment was to delineate the strike and depth of a zone of high electrical conductivity, known as the Eyre Peninsula Anomaly (EPA) in South Australia. Three linear arrays of marine magnetotelluric instruments were deployed across the continental shelf and slope to locate the offshore extension of the EPA on the continental margin, and to image the continental-oceanic lithosphere-asthenosphere transition. A land array of magnetometers was deployed at the same time to better resolve the EPA in the southern Eyre Peninsula. Robust remote-reference processing of time-series magnetic and electric data gives good MT and geomagnetic depth sounding responses in the bandwidth of 101 to 105 s, corresponding to skin-depths of mid-crustal (10 km) to mantle transition (400 km) range. Initial processing of marine and land data clearly indicates that the EPA is continuous to the edge of the continental shelf, with a conductance greater than 15 000 S confined to a narrow, near-vertical zone. At sites distant from the EPA, one-dimensional MT inversions fit the data well and provide a background conductivity structure for two and three-dimensional forward and inverse modelling of the EPA.

Palaeoproterozoic Kimban mobile belt, Eyre Peninsula: Timing and significance of felsic and mafic magmatism and deformation

Within the Kimban mobile belt on the eastern margin of the Gawler Craton, two separate tectono‐thermal and up to four separate magmatic events can be recognised. The first (inferred) magmatic event involved underplating of the mafic, lower crustal protolith of the Donington Granitoid Suite at 2100–2000 Ma. Emplacement, around 1850 Ma, of the Donington and Colbert Granitoid Suites and the mafic, syn‐plutonic Jussieu dykes, together forming the Lincoln Batholith, comprise the second magmatic event. Donington Suite emplacement was accompanied by high‐temperature meta‐morphic conditions, indicated by syn‐intrusive migmatitic shearing in host rock of the Lincoln Batholith and by the generation of voluminous felsic selvages around the Jussieu dykes. This first tectonother‐mal event in the Lincoln Batholith is associated with D1 in Hutchison Group metasediments. The mafic Tournefort dykes, emplaced during the third magmatic event, are structurally and compositionally distinct from the Jussieu dykes. Their time of emplacement is constrained by the 1850 Ma Donington Suite rocks which they post‐date, and by ca 1720 Ma peak metamorphic assemblages contained within them. The main deformation within the Lincoln Batholith occurred during a second tectono‐thermal event, broadly associated with D2 and D3 in the Hutchison Group. This event occurred after Tournefort dyke emplacement, perhaps in the order of 100 million years after the first tectonothermal event. The Kalinjala Shear Zone was active during this second tectonothermal event, juxtaposing the Lincoln Batholith to the east with the Hutchison Group to the west. Since there is no evidence to date that indicates that the first tectonothermal event was associated with orogenic activity or that it was an integral part of the orogeny associated with the second tectonothermal event, the term ‘Kimban Orogeny’ should be restricted to the latter. The fourth magmatic event is represented by emplacement of the Moody Granitoid Suite into the Hutchison Group, pre‐ to syn‐D3. Within the Kimban mobile belt on southern Eyre Peninsula, only the Moody Suite represents true syn‐orogenic magmatism. The term ‘Lincoln Complex’, which is synonymous with a common genesis for intrusives of the second, third, and fourth magmatic events, is inappropriate and should be abandoned. Whereas the Donington Suite is chemically distinct from plate‐margin type calc‐alkaline magmatism, the Lincoln Batholith has some physical aspects in common with plate‐margin batholiths, notably the occurrence of mafic enclaves and the emplacement of mafic syn‐plutonic dykes, indicating the coexistence of mafic and felsic magma. Until the underlying mechanisms that control the chemical evolution and physical architecture of magmatic systems are understood, it is dangerous to use any of these characteristics to infer a tectonic setting by comparison with modern examples.

Electrical conductivity anomalies and their relationship with the tectonics of South Australia

Summary. Geomagnetic variation studies have been conducted in the Gawler Craton and Adelaide Geosyncline of South Australia. The magnetometer stations extend from the coast up to the southern edge of the 1970 array of Gough, McElhinny & Lilley. The coast effect is the dominant feature of the data but use is made of the hypothetical event technique to identify two zones of telluric current concentration. Both of them appear to be associated with linear zones of enhanced electrical conductivity within the crust. The Southern Eyre Peninsula anomaly lies within the Gawler Craton and may identify a major fracture or shear in the upper crust, The conductivity anomaly within the Adelaide Geosyncline appears to be the continuation of the Flinders anomaly discovered by the 1970 array study. It correlates well with the arcuate fold pattern of the Southern Flinders Zone of the

A crustal conductor on Eyre Peninsula, South Australia

Short period variations in the geomagnetic field components were recorded in a closely spaced network of sites on southern Eyre Peninsula, South Australia. They reveal the existence of a linear zone of very good electrical conductivity within the late Archaean–early Proterozoic Gawler Craton. There is strong enhancement of the horizontal component variations transverse to the zone and an abrupt phase change in the vertical component, particularly for periods near 2×103s. These features are consistent with a line or narrow thin sheet of current flowing in a conductor at a depth ⩽10 km; this almost certainly requires a major fracture or shear zone within the crystalline basement, for which there is supporting geophysical evidence. Superficial Cenozoic deposits blanket much of the area and consequently the basement geology is not known in detail, although there are structures parallel to the conductor 20 km to the east. Identification of a major linear crustal feature in such an area demonstrates the usefulness of geomagnetic variation studies for understanding deep geology.

Discordant K‐Ar dates from Proterozoic metasedimentary rocks in southwestern Eyre Peninsula, South Australia: An example of excess40Ar in slates?

Abstract K‐Ar total rock age determinations have been made on a sequence of metasedimentary rocks resting on a 2350 m.y.‐old basement in southern Eyre Peninsula, South Australia. The metasediments have an Rb‐Sr age of 1785 m.y., but K‐Ar isochrons suggest that relatively high temperatures persisted for a further 250 million years before the rocks became systems closed to K and Ar diffusion. A significant amount of 40Ar was trapped in the metasediments at the time of closure of the K‐Ar system, 1550 million years ago.