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.

This set is composed of a selection of geoscience booklets, paper models and an image set - Climate Change booklet - Time and Life Booklet - Volcanoes booklet - Earthquakes booklet - Australian Earthquakes image set - Plate Tectonics booklet - Plate tectonics 3D paper model set Suitable for secondary Year levels 7-12

Wide-angle seismic data from ocean bottom seismographs, together with gravity and deep marine reflection profiling data along the Vulcan transect in northern Australia, define the crustal-scale features between the Precambrian Australian craton and the Timor Trough. The transect provides an outline of crustal and upper mantle architecture across the major boundary between the Australian and SE Asian plates when linked with earlier deep marine seismic profiling. Near the Australian coast, relatively unaltered Precambrian Kimberley Basin rocks are inferred to extend to the edge of a shallow-water shelf area (Yampi Shelf) with a crustal thickness of 35 km. The crust then thins to 26 km under the outer shelf near the Timor Trough. Over the same distance Palaeozoic/Mesozoic basin sequences are interpreted to thicken to 12-13 km, inferring an attenuation of Precambrian basement rocks from 35 to 13-14 km across the margin (ß=2.6). On the outer shelf, the Vulcan Sub-Basin is a trans-tensional rift within Permo-Triassic platform areas (Ashmore Platform, Londonderry High). Within the lower crust under major bounding faults at the sub-basin/platform margins, there are elevated P-wave velocities to 7 km/s, suggesting emplacement of intrusive, more mafic rocks at depth during basin-forming processes. At mid-crustal levels, near the top of the inferred attenuated Precambrian crustal rocks, there are strong near-vertical-incidence reflections at about 13 km depth that are interpreted to be a detachment or further evidence of intrusive rocks. Additionally, seismic energy reflected at wide angles from within the upper mantle at 38-45 km depth indicates that compositional boundaries/heterogeneities continue at depth.

Lying off the south-west coast of Australia some 2,500 m below sea level is the broad, rectangular Naturaliste Plateau, which forms a relatively flat ledge half way between sea level and the abyssal plain. How the plateau came to be in this position, and whether its origin is continental or oceanic, are two questions relevant to the evolution of the south-east Indian Ocean.

Geophysical data were acquired by Australia and Japan from 1994-2002 on the deep-water continental margin offshore from Queen Mary Land, East Antarctica in the general locality of Bruce Rise. This paper presents a regional interpretation of these data and outlines the tectonic history.

Models for the crustal evolution of the Yilgarn Craton have changed in the last 25 years from generally autochthonous greenstone development on sialic crust (Gee et al. 1981, Groves & Batt 1984) to alloch-thonous models that highlight the importance of accretionary tectonics (Myers 1995). Recent models highlight the importance of mantle plumes and long-lived convergent margins for both Au and Ni (Barley et al. 1998). The role of sialic crust in the development of the abundant mineral systems in the Yilgarn, and Archaean cratons in general, however, remains problematic. Felsic rocks from across the Yilgarn Craton are used as crustal probes, with their geochronology, zircon inheritance and Nd isotopic character used to constrain the age and extent of basement terranes. The studies reveal a collage of crustal fragments and implicate both autochthonous and allochthonous crustal development, with increasing importance of accretionary tectonics, particularly after 2.8 Ga. The crustal evolution places significant constraints on the development of metallogenic associations.

The origins of high heat production (HHP) granites - with high concentrations of the heat-producing elements Th, U and K (HPE) - is controversial, particularly large areas of such rocks. To constrain possible controls on HHP granites, we have investigated temporal changes in Th, U and K contents of Paleoarchean to Mesozoic granites in Australia, and how these relate to peri-ods of HHP magmatism. Australian HHP granites range in age from Mesoarchean to Triassic, but are most abundant in the Neoarchean, the Paleoproterozoic - early Mesoproterozoic, and the Carboniferous. HHP magmatism ranges from relatively short lived (<30 Ma) geographically-restricted events in the Neoarchean and Carboniferous, to geographically widespread, (possibly unrelated) repetitive events over an extended time period (ca. 1800 to 1500 Ma) for the Proterozoic.

This record outlines models for the tectonic evolution of Australian Proterozoic terranes, and the mineral systems that are likely to have operated in particular regions at particular times.

Felsic units of the Hiltaba Association Granites and the comagmatic Gawler Range Volcanics (together the GRHVP) can be divided into four supersuites: the I-type Malbooma and Jenners Supersuites; and the A-type Roxby and Venus Supersuites. All units are aged between ~1595 - 1575 Ma. Major and trace element modelling of granites of both the strongly fractionated and evolved Malbooma and moderately fractionated and evolved Jenners Supersuites suggests derivation by crystal fractionation from granodiorite compositions. Neodymium isotopes of the granites and volcanics indicate a more primitive Nd input than available from the known Archaean and Palaeoproterozoic crust alone. However, these felsic rocks are thought to be derived by partial melting of granodiorite compositions, rather than being the result of extensive fractionation from basalts. Mafic rocks of the GRHVP have variable isotopic and chemical signatures. The Lady Jane Diorite at Tarcoola has Nd ~0.2, and a composition incompatible with OIB derivation, but compatible with partial melting of a crustally-contaminated MORB. Some of the alkaline mafic/ultramafic rocks have Nd values as high as +4 [1]. Maximum zircon saturation temperatures of ~800C and ~900C for the I- and A-type supersuites respectively, are significantly lower than those of ~1000C measured by other geothermometers [2] for the Yardea Dacite (Roxby Supersuite), but show that the A-type supersuites were higher temperature than the I-type supersuites. The distribution of high temperature A-type granites shows some correlation with areas of coeval iron oxide copper-gold mineralisation. The coincidence of very high temperature granites with crustal Nd signatures, and mafic rocks with primitive to weakly evolved signatures indicates an extensional environment with very elevated geotherm and mantle upwelling. However, the felsic rocks are dissimilar to those associated with mantle plumes, and therefore a back arc distal to a continental-oceanic subduction zone setting is suggested, perhaps analagous to the present Altiplano Puna region of the Central Andes.

Basement architecture off western Tasmania is a legacy of late Neoproterozoic-Cambrian subduction-related processes, ocean basin closure and multiple accretionary events, culminating in formation of the Delamerian-Ross and western Lachlan orogens. Structures associated with these fold belts were subsequently reactivated during late Mesozoic-Cenozoic Gondwana breakup and the separation of Australia from Antarctica, strongly influencing the pattern and geometry of offshore rifting, including formation of an ocean-continent transform boundary off western Tasmania. Seismic reflection profiles combined with recently acquired high resolution aeromagnetic data permit this boundary and its associated reactivated basement structures (Avoca-Sorell fault system) to be mapped in greater detail than has hitherto been possible and point to a transform margin dominated by steeply outward-dipping structures and deep sedimentary basins similar to other transform margin ocean-continent boundaries, including the highly prospective Côte d'Ivore-Ghana region off the west African coast. Basement highs and rotated pre-rift crustal blocks adjacent to the west Tasmanian transform margin incorporate significant volumes of granite as well as a lower crust of probable Mesoproterozoic age that is locally juxtaposed against lower Paleozoic sequences intruded and/or floored by basaltic and ultramafic material. Aeromagnetic anomalies sourced from these basement rocks change orientation from NW- to NE-trending across the Avoca-Sorell fault system and can be traced laterally into regions of known onshore basement geology, highlighting both the tectonic significance of this structure and its origins during lower Paleozoic deformation accompanying the Delamerian-Ross orogeny.