The stable isotope signature of kilometre-scale fracturedominated metamorphic fluid pathways, Mary Kathleen, Australia

Abstract Large calcite veins and pods in the Proterozoic Corella Formation of the Mount Isa Inlier provide evidence for kilometre-scale fluid transport during amphibolite facies metamorphism. These 10- to 100-m-scale podiform veins and their surrounding alteration zones have similar oxygen and carbon isotopic ratios throughout the 200 × 10-km Mary Kathleen Fold Belt, despite the isotopic heterogeneity of the surrounding wallrocks. The fluids that formed the pods and veins were not in isotopic equilibrium with the immediately adjacent rocks. The pods have δ¹³C_calcite values of –2 to –7% and δ¹⁸O_calcite values of 10.5 to 12.5%. Away from the pods, metadolerite wallrocks have δ¹⁸O_whole-rock values of 3.5 to 7%. and unaltered banded calc-silicate and marble wallrocks have δ¹³C_calcite of –1.6 to –0.6%, and δ¹⁸O_calcite of 18 to 21%. In the alteration zones adjacent to the pods, the δ¹⁸O values of both metadolerite and calc-silicate rocks approach those of the pods. Large calcite pods hosted entirely in calc-silicates show little difference in isotopic composition from pods hosted entirely in metadolerite. Thus, 100- to 500-m-scale isotopic exchange with the surrounding metadolerites and calc-silicates does not explain the observation that the δ¹⁸O values of the pods are intermediate between these two rock types. Pods hosted in felsic metavolcanics and metasiltstones are also isotopically indistinguishable from those hosted in the dominant metadolerites and calc-silicates. These data suggest the veins are the product of infiltration of isotopically homogeneous fluids that were not derived from within the Corella Formation at the presently exposed crustal level, although some of the spread in the data may be due to a relatively small contribution from devolatilization reactions in the calc-silicates, or thermal fluctuations attending deformation and metamorphism. The overall L-shaped trend of the data on plots of δ¹³C vs. δ¹⁸O is most consistent with mixing of large volumes of externally derived fluids with small volumes of locally derived fluid produced by devolatilization of calc-silicate rocks. Localization of the vein systems in dilatant sites around metadolerite/calc-silicate boundaries indicates a strong structural control on fluid flow, and the stable isotope data suggest fluid migration must have occurred at scales greater than at least 1 km. The ultimate source for the external fluid is uncertain, but is probably fluid released from crystallizing melts derived from the lower crust or upper mantle. Intrusion of magmas below the exposed crustal level would also explain the high geothermal gradient calculated for the regional metamorphism.     

The Lower Permian Wasp Head Formation,Sydney Basin: high‐latitude,shallow marine sedimentation following the late Asselian to early Sakmarian glacial event in eastern Australia

The Lower Permian Wasp Head Formation (early to middle Sakmarian) is a ～95 m thick unit that was deposited during the transition to a non‐glacial period following the late Asselian to early Sakmarian glacial event in eastern Australia. This shallow marine, sandstone‐dominated unit can be subdivided into six facies associations. (i) The marine sediment gravity flow facies association consists of breccias and conglomerates deposited in upper shoreface water depths. (ii) Upper shoreface deposits consist of cross‐stratified, conglomeratic sandstones with an impoverished expression of the Skolithos Ichnofacies. (iii) Middle shoreface deposits consist of hummocky cross‐stratified sandstones with a trace fossil assemblage that represents the Skolithos Ichnofacies. (iv) Lower shoreface deposits are similar to middle shoreface deposits, but contain more pervasive bioturbation and a distal expression of the Skolithos Ichnofacies to a proximal expression of the Cruziana Ichnofacies. (v) Delta‐influenced, lower shoreface‐offshore transition deposits are distinguished by sparsely bioturbated carbonaceous mudstone drapes within a variety of shoreface and offshore deposits. Trace fossil assemblages represent distal expressions of the Skolithos Ichnofacies to stressed, proximal expressions of the Cruziana Ichnofacies. Impoverished trace fossil assemblages record variable and episodic environmental stresses possibly caused by fluctuations in sedimentation rates, substrate consistencies, salinity, oxygen levels, turbidity and other physio‐chemical stresses characteristic of deltaic conditions. (vi) The offshore transition‐offshore facies association consists of mudstone and admixed sandstone and mudstone with pervasive bioturbation and an archetypal to distal expression of the Cruziana Ichnofacies. The lowermost ～50 m of the formation consists of a single deepening upward cycle formed as the basin transitioned from glacioisostatic rebound following the Asselian to early Sakmarian glacial to a regime dominated by regional extensional subsidence without significant glacial influence. The upper ～45 m of the formation can be subdivided into three shallowing upward cycles (parasequences) that formed in the aftermath of rapid, possibly glacioeustatic, rises in relative sea‐level or due to autocyclic progradation patterns. The shift to a parasequence‐dominated architecture and progressive decrease in ice‐rafted debris upwards through the succession records the release from glacioisostatic rebound and amelioration of climate that accompanied the transition to broadly non‐glacial conditions.     

Out of the Rat Race

After the social policy reforms of 1991 important changes in internal migration in New Zealand occurred. These resulted in changing housing costs and falling incomes which led to the movement of low income households, particularly beneficiaries and state house tenants, from urban to rural areas and small towns. A survey of 404 people who moved to the Southern Wairarapa towns of Carterton, Martinborough and Featherston from New Zealand urban areas since 1985 shows that a proportional increase in beneficiary movement occurred after 1991. Housing costs, relative to income, are important factors in the decision to move for a range of migrants. This pattern of migration has important regional development ramifications.     

SUGGESTIONS FOR A CONSISTENT TERMINOLOGY FOR SEISMIC ANISOTROPY1

Seismic anisotropy is an unfamiliar concept to many geophysicists and the use of misleading and ambiguous terminology has made it more difficult to understand. I suggest here a consistent terminology in which simple expressions have specific meanings similar to their colloquial meanings. It is hoped that use of such language will help to make the increasing number of papers reporting seismic anisotropy more readily comprehensible to the non-specialist. This not a manual of anisotropy, and it is not intended for theoreticians. It is a list of terms which may make anisotropy a little easier to understand for those more familiar with wave propagation in isotropic solids.     

The vertebrate fauna of the type Cromerian

The rich vertebrate fauna of the Upper Freshwater Bed of West Runton, Norfolk. England (pollen assemblage zone Cr II) is described and related to vegetational conditions based on analysis of fossil pollen and macroscopic plant remains. The general picture is of fishes associated with aquatic plants in a slow-flowing river, amphibious vertebrates living in water-side habitats, and a terrestrial fauna generally reflecting a regional mixed oak forest cover, but also including grassland voles associated with local herb-dominated communities on the river floodplain. The fossil assemblage appears to have accumulated largely from the remains of terrestrial vertebrates washed in as the river eroded its banks. A reassessment of the fossil voles indicates that only five species occur, although ten were previously described. The sparse fauna from the overlying marine gravel (zone Cr III) is also discussed. On the basis of vertebrate faunas and other evidence, sites in England and Germany arc correlated with the type Cromerian.     

Trace fossils from proglacial lake sediments

Trace fossils have been discovered in Middle Pleistocene proglacial lake sediments from St. Albans, Hertfordshire, England. They indicate that this environment was capable of supporting a variety of benthonic animals. The fossils are described and tentatively assigned to several invertebrate groups.     

The lacustrine sediment record of Oberwinkler Maar (Eifel, Germany): Chironomid and macro-remain-based inferences of environmental changes during Oxygen Isotope Stage 3

The lacustrine record of Oberwinkler Maar (Eifel, Germany) is the northernmost continuous record documenting the Weichselian Pleniglacial in central Europe – a period characterized by multiple abrupt climate oscillations known as the Dansgaard/Oeschger cycles. Here, the results of a high-resolution study of chironomid remains are presented, with a focus on the earlier part of Oxygen Isotope Stage (OIS) 3 (60–50 kyr BP) covering four stadial/interstadial cycles. During the stadials, the chironomid fauna of the former lake was dominated by many cold-stenothermic chironomid taxa, indicating a cold, oligotrophic lake. The concentrations of chironomid remains were lower during the interstadials, and featured a higher number of warm-indicating taxa. This could have been the result of a higher summer temperature at the study site, but also of bottom-water anoxia, an increase in trophic state or a combination of all these factors. During the stadial intervals, a taxon that is restricted to (sub-)arctic environments is present in our record, suggesting a change in the temperature regime rather than in-lake processes as the driving mechanism for the changes in the chironomid record. Although, consistently, there was a response of the lake ecosystem to climate changes, the amplitude of this response was not constant. This study provides unique and independent evidence of the impact of Dansgaard/Oeschger climate variability on the European continent during the earlier part of OIS-3.     

The effect of heterogeneous stress and strain on metamorphic fluid flow,Mary Kathleen,Australia, and a model for large-scale fluid circulation *

In the Proterozoic Mary Kathleen Fold Belt, northern Australia, infiltration of large volumes of externally derived fluid occurred synchronously with regional amphibolite-facies metamorphism and deformation. This paper develops a model of structurally controlled fluid migration by comparing the distribution of fossil fluid pathways with the inferred stress and strain patterns during the deformation. Intense fluid flow was localized within strong, relatively brittle meta-intrusive bodies, and in discrete, veined, brecciated and altered zones around their margins. In metasediments folded in a ductile manner outside these areas, fluid infiltration was negligible. The direct correlation between structural styles and the magnitude of veining and metasomatism suggests control of permeability enhancement, and hence fluid flow, by deformation. Finite difference modelling of a strong body in a weaker matrix has been used to evaluate the variation of stresses during the deformation, from which it is clear that stress and strain heterogeneities have systematically influenced the development and maintenance of metamorphic fluid pathways. Particular regions in which mean stress may be significantly lower than the average lithostatic pressures include the ‘strain shadow’zones adjacent to the strong bodies, other dilatant zones around the bodies, and the bodies themselves. This geometry is favourable not only for localized brittle deformation under amphilobite facies conditions, but also for focused fluid flow in the low mean stress regions, as evidenced by the abundance of veins. Fluid access through these metamorphic aquifers occurred during tensile failure episodes, with particularly large dilations and decimetre-scale veining in areas of strain incompatibility. It appears likely that fluid circulated many times through the Fold Belt, with flow concentrated in the metamorphic aquifers. A model is developed that explains both the structurally focused fluid flow and the postulated multi-pass recirculation by dilatancy pumping, the ‘pump engines’comprising the low mean stress zones.