On plexus representation of dissimilarities

Correspondence analysis has found widespread application in analysing vegetation gradients. However, it is not clear how it is robust to situations where structures other than a simple gradient exist. The introduction of instrumental variables in canonical correspondence analysis does not avoid these difficulties. In this paper I propose to examine some simple methods based on the notion of the plexus (sensu McIntosh) where graphs or networks are used to display some of the structure of the data so that an informed choice of models is possible. I showthat two different classes of plexus model are available. These classes are distinguished by the use in one case of a global Euclidean model to obtain well-separated pair decomposition (WSPD) of a set of points which implicitly involves all dissimilarities, while in the other a Riemannian view is taken and emphasis is placed locally, i.e., on small dissimilarities. I showan example of each of these classes applied to vegetation data

High-T granulites and polymetamorphism in the southern Arunta Region, central Australia: Evidence for a 1.64 Ga accretional event

Mapping and sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon geochronology in the newly defined Warumpi Province of the southern Arunta Region, central Australia has identified a succession of granulite facies metasedimentary rocks (the Yaya Metamorphic Complex) that has undergone a polymetamorphic Proterozoic evolution. Pelitic metasedimentary rocks of the Yaya Metamorphic Complex have a maximum deposition age of 1661+/-10 Ma, and underwent granulite facies metamorphism synchronous with intrusion of granite, charnockite and gabbro. In quartz-poor cordierite-rich rock types, peak metamorphic P-T conditions of >800 deg. C and 9-10 kbar were followed by near-isothermal decompression, with the breakdown of orthopyroxene, sillimanite and garnet to sapphirine and cordierite. Metamorphic zircon rims from a cordierite-rich granulite have an age of 1638+/-8 Ma, and two migmatitic metapelite units have similar metamorphic ages of 1641+/-14 and 1639+/-20 Ma. Amphibolite facies felsic migmatite of the Madderns Yard Metamorphic Complex has a protolith age of 1688+/-16 Ma, with evidence for metamorphism at 1640 Ma. Further localised migmatisation at amphibolite facies conditions took place at 1571+/-5 and 1149+/-3 Ma. Our evidence suggests that a major orogenic event, named here the Liebig Orogeny, affected the southern Arunta Region at 1640-1635 Ma. Zircon evidence suggests that there was less than 20 million years between the timing of deposition and deep crustal metamorphism of the Yaya Metamorphic Complex, and high-T decompression textures suggest rapid exhumation, consistent with a crustal thickening event. The distal effects of the Liebig Orogeny in the North Australian Craton include intraplate tectonism, volcanism and a hairpin bend in the apparent polar wander path, suggesting that it is an event of regional significance. We propose that the Liebig Orogeny reflects the accretion of the Warumpi Province onto the North Australian Craton at 1640 Ma

The Musgrave Province - NT's most underexplored terrane

Most mineral exploration within the Palaeo- to Mesoproterozoic of the Northern Territory has been focused on the Palaeoproterozoic basement terranes of the North Australian Craton and on Northern Australian platform cover rocks such as the McArthur Basin. In comparison, the more juvenile Proterozoic crust that lies to the south of the North Australian Craton, including the Warumpi Province (southwestern Arunta) and Musgrave Province3, has received little attention from explorers. The Musgrave Province within the NT is one of the most underexplored Proterozoic terranes on the Australian continent, with an average of 1 drillhole for every 210 km2. The geological framework of the Musgrave Province was a focus of NTGS studies in the 1990s (see List of selected NTGS publications in the Musgrave Province) and a summary of the geology of the Musgrave Province has recently been published (Edgoose et al 2004). Recent flying of the 2001 Eromanga and 2004 Simpson airborne surveys have completed high-resolution airborne magnetic coverage of the Musgrave Province at exploreable depths. NTGS is continuing its investigations in the Musgrave Province through collaborative research programs with the University of Adelaide.Ian R Scrimgeour, Christine J Edgoose, Dorothy F Close and Ben P Wad

Thrust tectonic styles of the intracratonic Petermann and Alice Springs Orogenies, Central Australia

T. Flottmann, M. Hand, D. Close, C. Edgoose, and I. Scrimgeou