The timing and mechanisms of crustal growth, the role (if any) of modern-style plate tectonics and potential secular changes, during the Archaean are poorly understood. To provide constraints on these questions, we present isotopic and geochemical data for the well exposed, classic, Paleoarchean to Mesoarchean Pilbara Craton (>3.5 to <2.8 Ga), and the large, but poorly outcropping, largely Neoarchean Yilgarn Craton (>3.0 to 2.6 Ga), both in Western Australia. Both are dominated by typical Archaean granite-greenstone geology. Regional Sm-Nd data from felsic magmatism indicates both cratons are comprised of large proto-cratonic cores with relatively uniform Nd TDM model ages - c. 3.5-3.6 Ga for the eastern Pilbara, c. 3.1-3.3 Ga for the western Yilgarn. Distinct isotopic breaks separate these proto cratons from marginal terranes with both significantly younger, but also more domainally-variable, TDM model ages. The cratonic nucleii are characterised by episodic felsic magmatism spanning 650 Ma (from >3.47 Ga to 2.85 Ga) for the Pilbara Craton and 350 Ma years (3.0 Ga to 2.63 Ga) for the Yilgarn Craton. In both, this magmatism was dominated by transitional TTG-type compositions, and shows secular variations to more potassic, siliceous compositions, consistent with an increasing component of crustal reworking. Definitive arc-related magmatism, e.g., boninites, calc-alkaline andesites, sanukitoids, are largely absent. The surrounding marginal terranes are characterised by isotopically younger domains that broadly correspond to geological domains. Importantly, these domains are either characterised by primitive isotopic signatures (i.e., Nd TDM ages close to crystallisation ages), and/or contain evidence for arc-related magmatism, i.e., boninites, sanukitoids (Pilbara), calc-alkaline andesites (Yilgarn). The Pilbara cratonic nucleus is best interpreted to have formed as a result of vertical crustal growth in an episodic plume-environment. The Yilgarn cratonic nucleii possibly formed in a similar manner, though the evidence is not as clear. Subsequent marginal arc-related magmatism affected both cratons and the marginal terranes in both are best interpreted as representing lateral crustal growth and terrane accretion, not dissimilar to modern day plate tectonics. Best indications are that such accretion commenced at least by 3.2 Ga.

Archaean TTGs, as originally defined comprise intermediate to felsic rocks characterised by high Na2O/K2O, low to moderate LILE contents and no potassium-enrichment with increasing differentiation. The majority of such rocks also carry a high-pressure signature, identified by elevated Sr (i.e., are Sr-undepleted) and Eu, fractionated REEs, with low HREE (Y- and HREE-depleted), and high Sr/Y. These latter characteristics are also often used to define TTGs and are commonly thought to reflect an origin via slab melting (with or without mantle-wedge interaction), largely as a necessary corollary of a warmer Archaean mantle. Our work, however, shows that within many Archaean terranes there exists a subclass of granites (which we call transitional TTGs), that when compared to `true? TTGs have higher LILE contents, show strong enrichment in LILEs (e.g., K2O) with increasing differentiation, and tend towards more siliceous compositions (68-77% SiO2), but still possess a similar high-pressure signature. Such transitional TTGs are contemporaneous with, or postdate true TTGs but where present also grade to more mafic compositions that overlap with true TTGs. These Transitional TTGs dominate some cratons, the best example being the Yilgarn Craton where true TTGs form only a small percentage of total granites. Sm-Nd isotopic and inherited zircon data indicate that the petrogenesis of most transitional TTGs requires the involvement of pre-existing crust. What is not clear, given the difficulty in reconstructing tectonic environments in the Archaean, is whether this crustal component represents input via the subduction process (e.g., subducted sediments), represents a response to thicker pre-existing crust (AFC processes), or whether these rocks form from pure crustal melts in thickened Archaean crust. Comparison of both TTG-types with ?apparent? modern-day TTG analogues ? adakites ? shows somewhat similar groupings. The major adakite group (group 1, mostly 58-68% SiO2), is characterised by a narrow range in La/Sm (5.5-3.0), and Sm/Yb (1.5-3.5), moderate Sr (400-700 ppm) and low to moderate LILE. This group overlaps significantly with true TTGs although the latter tend to have higher La/Sm (up to 9) and Sm/Yb (up to 10+). The second adakite group has higher Sm/Yb (6-10), La/Sm (6-9) and Sr (<500 to 1500 ppm), and typically higher LILE contents, and appears to be confined to continental arc regimes and is most similar to transitional TTGs. Notably both TTG-types extend to significantly more silica and LILE-rich compositions than seen in the majority of adakites. A distinctive, significantly more mafic (50-60+% SiO2) adakite group with high to very high Sr (up to 1500-2500 ppm), and high Sm/Yb (to 10-12), appears distinct from all Archaean TTGs. Group 1 adakites are most consistent with melting of a MORB source leaving an amphibole-bearing (i.e., non-eclogitic) residue, while group 2 adakites are interpreted to represent either melting at higher pressure or greater degrees of slab melting (leaving an eclogitic residue), with their higher LILE contents reflecting either subducted sediment input or processes related to the presence of continental crust (e.g., AFC). In contrast, the range in La/Sm and Sm/Yb in true TTGs suggests greater involvement of garnet during melting in Archaean times (garnet amphibolite to eclogite residue) relative to modern day (group 1) adakites, perhaps reflecting a greater depth of generation or drier melting. Importantly, although pure crustal melting can not be ruled out, analogies with group 2 adakites shows that transitional TTGs can be produced as slab melts particularly in continental arc environments ? both models maybe applicable and may have varied in relative importance through time. Further, the presence of transitional TTGs in Archaean cratons can be used to infer significant pre-existing continental crust. (THIS ABSTRACT MODIFIED FROM ORIGINAL).

Modern adakite forms in subduction zones with unusually high geotherms that result in high-pressure melting of subducted basaltic crust. Close geochemical similarities between adakite and Archaean TTG, which forms the major component of Archaean crust, have supported a view that Archaean TTG is likewise subduction related. However, recent studies have shown that conditions of adakite formation are not unique to a subducting slab and can be attained in basaltic lower crust in both subduction and non-subduction environments. Non-subduction environments may be equally relevant to the genesis of Archaean TTG. Heat flow calculations suggest that Archaean subduction (if it occurred), must have been at a low angle ? the same style of subduction that produces most modern adakites. However, if Archaean TTG was derived from a subducting slab, then like most modern adakites, it should show 1) evidence for interaction with a mantle wedge, and 2) an association with diagnostic subduction influenced magmas such as high-Mg andesite (sanukitoid), Nb-enriched basalts and boninites. Whereas this does appear to be the case for some Late Archaean terrains (e.g. Superior Province), such is not unequivocally the case for older terrains. This suggests either that Early Archaean TTG is not subduction related, or that the style of Early Archaean subduction was significantly different from modern subduction, including low-angle or flat subduction. The volume of preserved felsic crust in the Early Archaean Pilbara Craton, Western Australia, requires a complimentary volume of dense mafic material (residual after basalt melting) equaling a combined thickness of ~170 km, which is difficult to conceptualize through solely magmatic processes (e.g. underplating, mantle plume). Subduction provides a means whereby large volumes of mafic crust can be progressively cycled through a melting zone, but if applicable to the Early Archaean, features such as typically low Mg#, Cr, and Ni indicate that TTG melts of that crust must have avoided interaction with the mantle. We suggest that Early Archaean slabs were pushed or thrust beneath overriding basaltic crust, without the development of a mantle wedge. Early Archaean TTGs are melts of the underthrusted slab and their petrogenesis combines components of the subduction model for modern adakite and models for lower crustal sodic melts (e.g. Cordillera Blanca ? Peru; Ningzhen ? China).

As part of its program to define the extents of the Australian Legal Continental Shelf on the Kerguelen Plateau AGSO acquired over 5500 km of new seismic data including the first regional datasets over Elan Bank and in the Labuan Basin. This report presents results of a geological framework study carried out to underpin the Australias Law of the Sea claim on the Kerguelen Plateau. It provides an up to-date analysis of the stratigraphy, structure, geological evolution and petroleum prospectivity of the Kerguelen Plateau region taking into account recent ODP drilling, geological sampling, seismic reflection and refraction data, as well as potential field data.

4 reproducible student activities suggested answers Suitable for primary levels Year 6 and secondary level Years 7-8

Elan Bank, a large western protrusion of the Kerguelen Plateau, is a microcontinent that originally lay between India and Antarctica in Gondwana. The acticle analyses seismic stratigraphy and crustal structure of the Elan Bank and discusses tectonic history of this feature. The paper contributes to understanding of dispersal and accretion of continental fragments in association with both plate tectonics and hotspot activity has likely been a significant process for much of Earth's history.

Introduction to a thematic issue of AJES on the Tasmanides. Most papers were originally presented at the 15th Australian geological Convention in Sydney, 3-7 july 2000.

The Whundo Group, in the Pilbara Craton of northwestern Australia, is exceptional amongst Mesoarchaean, or older, volcanic sequences in that it preserves geochemical characteristics that are extremely difficult to interpret in any way other than reflecting modern-style subduction processes, most likely at an intra-oceanic arc. The group includes boninites, interlayered tholeiitic and calc-alkaline volcanics, Nb-enriched basalts, adakites, and shows evidence for flux melting of a mantle wedge. The geochemical data are also consistent with geological relationships that infer an exotic terrane with no felsic basement. These data provide clear evidence for modern-style subduction processes at 3.12 Ga.

Passive margins worldwide record the combined effects of faulting and magmatism that achieve stretching within the crust, but their relative contributions are difficult to evaluate. The ~1000 km-long Great Australian Bight margin appears to be magma starved, yet its moderate breadth and deepwater setting indicates unusually large amounts of heating during rifting, raising questions concerning the significance and timing of magmatism, the along-strike variability of strain, and the factors causing the localization of rifting along these margins. We re-evaluate existing onshore and offshore gravity, magnetic, seismic reflection, and well data from the Australian margin to investigate the evolution of the margin from rift initiation to breakup and the onset of seafloor spreading. Our results indicate that the southern Australian margin evolved through at least two phases of extension prior to breakup, and that the stretching is spatially and temporally distinct. Rift systems along the Australian margin were established between 165-140 Ma. The rift basins are generally simple, widely-distributed half-graben structures. Strain localized within the centre of this broad rift zone, to a short-lived, possibly magmatic, narrow rift system between 92-83 Ma, immediately prior to the onset of seafloor spreading between Australia and Antarctica. Structural and stratigraphic relations indicate that seafloor spreading initiated at ~83 Ma within the centre of the Great Australian Bight, and then propagated both to the east and west. To the east, or northern sector of the Otway Basin continental rifting continued until approximately 65 Ma. Euler deconvolution and analytical signal results calibrated by seismic reflection data indicate that magmatism commenced immediately before or during Stage-2 rifting (92-83 Ma). Significant crustal thinning accompanied the two rift phases, magmatism during the late stages of rifting may explain the deepwater setting of the Bight Basin.

Existing models for the evolution and break-up of weakly magmatic passive margins have been derived largely from observations of the North Atlantic margins. It is therefore unknown whether the observations from the Iberian margins are unique and site specific. Studies of Australo-Antarctic margins provide a broader spectrum for the development of conceptual models of the formation of weakly magmatic passive margins. Weakly magmatic passive margins record large amounts of extensional strain prior to breakup, but the role of magma intrusion and the along-strike variability of strain remain poorly understood. We re-evaluate existing onshore and offshore gravity, magnetic, seismic reflection, and well data from the Australo-Antarctic margins to probe the evolution of along-axis segmentation during progressive stages of rifting through to breakup and the onset of seafloor spreading. Our integrated plate reconstruction and geodynamic model for the break-up of the Australian-Antarctic conjugate margin provides a regional framework for analyses of continental breakup in weakly magmatic rift systems. The southern Australian margin evolved through at least two distinct phases of extension, but the timing of the phases was diachronous along the length of the rift: 1) strain concentrated along large offset border faults now located beneath the continental shelf break; 2) localisation of strain to a narrow zone of incipient break-up. Minor extrusive magmatism occurred only during the second rifting stage and within ~10 My prior to the first well-defined seafloor spreading anomaly.