ALBERT

Beschreibung: The Molucca Sea Collision Zone in eastern Indonesia is the site of an orthogonal collision between two active subduction systems. Both the Halmahera subduction zone, to the east, and the Sangihe subduction zone, to the west, have subducted oceanic lithosphere of the Molucca Sea Plate, which has now been completely consumed. Both volcanic arcs were active since the Neogene and provide a means of probing the element fluxes through the two systems. The geochemistry of Neogene and Quaternary lavas from each volcanic arc is compared to constrain changes in the mass fluxes through the systems and the processes controlling these fluxes at different times during their history. Both arcs show increased evidence for sediment recycling as the collision progressed, but for contrasting reasons. In Halmahera this may represent an increased sediment flux through the arc front, while in Sangihe it may simply reflect a greater opportunity for melting of sediment-fluxed portions of the mantle wedge. In both cases the change in arc geochemistry can be related to the evolving architecture of the particular subduction zone. The Halmahera lavas also record a temporal change in the chemistry of the mantle component that resulted from induced convection above the falling Molucca Sea Plate drawing compositionally distinct peridotite into the mantle wege.

Beschreibung: Recycling of oceanic plate back into the Earth's interior at subduction zones is one of the key processes in Earth evolution. Volcanic arcs, which form above subduction zones, are the most visible manifestations of plate tectonics, the convection mechanism by which the Earth loses excess heat They are probably also the main location where new continental crust is formed, the so-called subduction factoiy' About 400f modern subduction zones on Earth are intra-oceanic. These subduction systems are generally simpler than those at continental margins as they commonly have a shorter history of subduction and their magmas are not contaminated by ancient sialic crust. They are therefore the optimum locations for studies of mantle processes and magmatic addition to the crust in subduction zones. This volume contains a collection of papers that exploit the relative simplicity of intra-oceanic subduction systems to provide insights into the tectonic, magmatic and hydrothermal processes associated with subduction.

Beschreibung: In the Huckitta region of the eastern Arunta Inlier, central Australia, two terrains with distinct metamorphic histories are separated by a zone of sinistral strike-slip mylonitic deformation and reworking, the Entire Point Shear Zone (EPSZ). To the south of the EPSZ, in the Harts Range Group, Ordovician (c. 470 Ma) intraplate granulite facies metamorphism (c. 800{degrees}C, 8-10 kbar) was followed by decompression to c. 7 kbar. In contrast, the Kanandra Granulite, to the north of the EPSZ, is characterized by Palaeoproterozoic high-grade metamorphism at 770-850{degrees}C and 5-7 kbar, followed by inferred near-isobaric cooling. Juxta-position of these terrains along the EPSZ occurred at upper amphibolite facies conditions (700{degrees}C, 7 kbar), and resulted in extensive reworking of the Kanandra Granulite. Monazite growth within EPSZ mylonites is dated at 445 {+/-} 5 Ma, whilst a garnet amphibolite gives a Sm-Nd isochron age of 434 {+/-} 6. The timing of this deformation is broadly coincident with the inferred onset of south-vergent compressional deformation in the Harts Range region to the south. This suggests that juxtaposition of the Ordovician granulite terrain with the surrounding Proterozoic terrains occurred during intraplate sinistral transpression in the late Ordovician. Further reworking of the Kanandra Granulite occurred at mid-amphibolite to greenschist facies conditions, during north-vergent mylonitic deformation that exhumed the Ordovician high-grade terrain during the 400-300 Ma Alice Springs Orogeny. Although this zone of Palaeozoic reworking is 〈5 km wide, it forms the northern margin of Palaeozoic high-grade intraplate deformation and represents a major tectonic boundary in central Australia.

Beschreibung: This paper is an overview of drift exploration methods for lode Au deposits in areas of thin and thick cover of glacial sediments within the Abitibi Greenstone Belt of central Canada. It summarizes a large volume of data produced by government regional surveys and case studies as well as that from industry-led gold exploration programs. Regional till surveys can be used as targeting mechanisms for further Au exploration. Anomalies are defined by a series of samples with elevated Au concentrations that lie along significant bedrock structures, occurring in clusters or as isolated samples in areas of low sample density. Thresholds between background and anomalous Au grain abundances or Au concentrations are variable and depend on location within the Abitibi Greenstone Belt. Case studies around known deposits provide examples of geochemical and mineralogical signatures of Au deposits that can be expected in till down-ice. These serve as sources of information on appropriate sampling methods and size fractions to analyse, and on ice flow patterns, local glacial stratigraphy and suitable till units for sampling. Two methods for measuring the Au content of till are commonly used: (1) a count of visible Au grains and (2) geochemical elemental analysis. Close to source, till contains thousands to hundreds of thousands ppb Au and several hundred Au grains. The Au grains vary from coarse sand to silt sizes and have pristine shapes. The presence of high Au concentrations in till indicates that the ore zones subcrop and that glacial processes have produced Au dispersal trains down-ice.

Beschreibung: Previous stable oxygen isotopic data from surface-dwelling foraminifera indicate that Eocene tropical sea surface temperatures (SSTs) were significantly lower than at present. Here we show that stable isotopic analyses ({delta}18O, {delta}13C) of the late mid-Eocene mixed-layer dweller Morozovella spinulosa are consistent with mid-Eocene mid-latitude SSTs close to, or slightly lower than modern temperatures at Blake Nose, western North Atlantic. In contrast, isotopic analyses of the benthic foraminifer, Nuttalides truempyi reveal a gradual fall in mean bottom-water temperatures from 8 to 7 {degrees}C over c. 500 ka years. These deep intermediate-water temperatures are significantly higher than modern ones and are similar to intermediate- and bottom-water temperatures recorded from earlier in Palaeogene and late Cretaceous time. Large shifts are seen in the {delta}18O and {delta}13C values of the planktonic foraminifers, of up to 1{per thousand} and 2.6{per thousand}, respectively, that probably reflect temperature and nutrient fluctuations controlled by regional changes in upwelling intensity and runoff. The surface to benthos {delta}18O gradient decreases from 3{per thousand} PDB to a minimum of c. 0.5{per thousand} PDB over 400 ka, which could relate to the intensity of upwelling. Spectral analysis reveals precessional forcing in the foraminiferal {delta}18O records, which shows the direct influence of low-latitude insolation on surface-water stratification. Monsoonal wind systems may have forced the upwelling cycles and/or freshwater input. The benthic foraminifer {delta}18O record also contains the obliquity cycle, in addition to the precessional cycles, indicating the inheritance of mid- and high-latitude forcing to subtropical deep waters.

Beschreibung: As a result of its relative buoyancy, continental crust is rarely subducted, meaning that successive episodes of continental deformation impart a complex geological character that is not found in younger oceanic lithosphere. This character is largely the result of two related processes: (1) reactivation, involving rejuvenation of discrete structures; and (2) reworking, involving the repeated metamorphism, deformation and magmatism of a previously tectonized crustal or lithospheric volume. Characterizing the style, distribution and timing of reactivation and reworking in different continental settings should therefore provide a crucial data set with which to evaluate the spatial patterns, temporal evolution and dynamic controls of tectonic rejuvenation of the continents and continental lithosphere. This volume presents a combination of review and research papers, which highlight some of the issues and problems associated with the characterization and modelling of continental reactivation and reworking.

Beschreibung: In contrast to oceanic lithosphere, the continents are manifestly composed of the products of tectonic processes whose cumulative duration spans much of the Earths history. Most continents contain Archaean nuclei that are enclosed by Proterozoic and Phanerozoic tectonic domains. The evolution of post-Archaean continental volumes has included additions of new continental material, but it has also involved repeated modification of parts of the existing continental lithosphere during periods of tectonic rejuvenation. This generally involves processes such as the formation of new structural fabrics, the over-printing of metamorphic assemblages and the generation and emplacement of magmas. Such behaviour can occur repeatedly throughout the geological record because the quartzofeldspathic continental crust cannot be subducted due to its relative buoyancy and weakness compared with its oceanic counterpart and the underlying lithospheric mantle. Thus, the character of the continents is significantly influenced by the way in which the existing lithosphere responds to new tectonothermal events that follow geologically significant cessations of activity for millions to hundreds of millions of years (Sutton & Watson 1986). Existing continental lithosphere may be modified during its incorporation into new collisional systems, for example the involvement of the Hercynian basement' in the Alpine collision. However, the most dramatic manifestations of continental tectonic rejuvenation occur during intraplate orogeny, where a coherent pre-existing lithospheric volume undergoes large-scale failure. Notable modern examples of intraplate orogeny are the Cenozoic Tien Shan and the Mongolian Alti in north Asia, which are forming in response to the Himalayan collision (e.g. Hendrix et al. 1992; Dickson ... This 250-word extract was created in the absence of an abstract.

Beschreibung: Fault zones control the location, architecture and evolution of a broad range of geological features, act as conduits for the focused migration of economically important fluids and, as most seismicity is associated with active faults, they also constitute one of the most important global geological hazards. In general, the repeated localization of displacements along faults and shear zones, often over very long time scales, strongly suggests that they are weak relative to their surrounding wall rocks. Geophysical observations from plate boundary faults such as the San Andreas fault additionally suggest that this fault zone is weak in an absolute sense, although this remains a controversial issue. Our understanding of fault-zone structure and mechanical behaviour derive from three main sources of information: (1) studies of natural fault zones and their deformation products (fault rocks); (2) seismological and neotectonic studies of currently active natural fault systems; (3) laboratory-based deformation experiments using rocks or rock-analogue materials. These provide us with a basic understanding of brittle faulting in the upper crust of the Earth where the stress state is limited by the frictional strength of networks of faults under the prevailing fluid-pressure conditions. Under the long-term loading conditions typical of geological fault zones, poorly understood phenomena such as subcritical crack growth in fracture process zones are likely to be of major importance in controlling both fault growth and strength. Grain-size reduction in highly strained fault rocks produced in the plastic-viscous and deeper parts of frictional regime can lead to changes in deformation mechanisms and relative weakening that can account for the localization of deformation and repeated reactivation of crustal faults. Our understanding the interactions between deformation mechanisms, metamorphic processes and the flow of chemically active fluids is a key area for future study. An improved understanding of how fault- or shear-zone linkages, strength and microstructure evolve over large changes in finite strain will ultimately lead to the development of geologically more realistic numerical models of lithosphere deformation that incorporate displacements concentrated into narrow, weaker fault zones.

Beschreibung: The formation of clay minerals within active fault zones, which results from the infiltration of aqueous fluids, often leads to important changes in mechanical behaviour. These hydrous phyllosilicates can (1) enhance anisotropy and reduce shear strength, (2) modify porosity and permeability, (3) store or release significant volumes of water, and (4) increase fluid pressures during shearing. The varying interplay between faulting, fluid migration, and hydrous clay mineral transformations along the central Alpine Fault of New Zealand is suggested to constitute an important weakening mechanism within the upper section of this crustal discontinuity. Well-developed zones of cataclasite and compacted clay gouge show successive stages of hydrothermal alteration, driven by the cyclic, coseismic influx of meteoric fluids into exhumed amphibolite-facies rocks that are relatively Mg rich. Three modes of deformation and alteration are recognized within the mylonite-derived clay gouge, which occurred during various stages of the fault's exhumation history. Following initial strain-hardening and frictional melting during anhydrous cataclastic breakdown of the mylonite fabric, reaction weakening began with formation of Mg-chlorite at sub-greenschist conditions (〈320 {degrees}C) and continued at lower temperatures (〈120 {degrees}C) by growth of swelling clays in the matrix. The low permeability and low strength of clay-rich shears are suitable for generating high pore-fluid pressures during faulting. Despite the apparent weakening of the c. 6 km upper segment of the Alpine Fault, the upper crust beneath the Southern Alps is known to be actively releasing elastic strain, with small (〈M 5) earthquakes occurring to 12 km depth. We predict that larger events will nucleate at c. 6-12 km along an anhydrous, strain-hardened portion of the fault.

Beschreibung: Gravitational instability of the continental lithospheric mantle is often associated with orogenic activity. Recent theoretical and experimental developments in the understanding of the convective instability of a dense layer, with non-Newtonian viscosity (representing lithosphere) above a less dense fluid layer (representing asthenosphere) are reviewed. These developments offer an explanation for why the continental lithospheric mantle might be generally mechanically stable in spite of a thermally induced density stratification, which one might expect to be unstable. Gravitational stability of this system depends on the initial amplitude of a disturbance to the stratified system, a disturbance that is most likely to be provided by localized lithospheric thickening associated with plate convergence. If the constitutive law that describes the deformation of dry olivine is applicable to the subcontinental mantle, the perturbation required to produce instability could be created by localized horizontal shortening of the order of 10%. If the wet olivine flow law is applicable, the required amount of shortening may be on the order of only 1%, in each case provided that it occurs in a time short compared with the thermal diffusion timescale of the lithosphere. The long-term stabilization of continental lithosphere may thus be associated with dehydration. Under circumstances of localized lithospheric convergence, the buoyancy of the continental crust plays an important role in determining the form of downwelling. If the crust is strong compared to mantle lithosphere, instability generally takes the form of localized down-welling beneath the centre of the convergent zone. If the crust is weak compared to mantle lithosphere, downwelling commences on the margins of the convergent zone as the buoyant crustal layer resists thickening. The initial instability may then trigger rapid extension of the lithospheric mantle beneath the convergent orogen. The extension is driven by asymmetric cold downwellings that move away from the centre of the convergent zone in a way that bears some resemblance to a delaminating slab or a retreating subduction zone. With these results in mind, some of the geological and geophysical evidence for lithospheric instability in modern orogens of Southern California, the South Island of New Zealand, the Mediterranean, and Central Asia are reviewed. Seismological evidence from Southern California and New Zealand suggest that these young orogens provide examples of lithospheric instability, in which downwelling occurs beneath the centre of the convergent zone where the crustal thickening is maximum. In contrast, the Alboran Sea and Tyrrhenian Sea basins show that extension has followed convergence as downwelling has retreated away from the convergent zone, and lithospheric mantle beneath the centre of convergence apparently has been replaced by asthenosphere. The Tien Shan and Tibetan Plateau provide large modernday examples of continental convergence. In each case there is strong evidence that the mantle lithosphere has undergone some form of instability that has led to at least part of it being replaced by hot asthenosphere. Images provided by teleseismic tomography of variations of seismic wave speeds beneath these orogens suggest that mantle lithosphere has been locally renewed following gravitational instability triggered by orogenic convergence.