bluuebunny/crossref_metadata_2025 huggingface dataset:
New data on the age of the uppermost ORS and the lowermost post-ORS strata in Dickson Land (Spitsbergen) and implications for the age of the Svalbardian deformation
Authors: Piepjohn Karsten , Brinkmann Lars , Grewing Anke , Kerp Hans
Published: April 2008

In NW Spitsbergen, the infill of a large Old Red Sandstone (ORS) basin was affected by the Svalbardian deformation shortly after the sedimentation of the uppermost ORS units. In the Billefjorden area, along the eastern margin of the basin, folded and thrustfaulted Devonian deposits are unconformably overlain by undeformed Carboniferous clastic sediments and platform carbonate deposits. To re-examine the age of the Svalbardian deformation, samples for palynological investigations were taken from the youngest deformed ORS strata and the oldest post-Svalbardian sediments. The results of palynological investigations show that the folded and thrust-faulted uppermost ORS unit, the Plantekløfta member of the Mimerdalen Formation (Andrée Land Group), is Late Famennian in age. The lowermost undeformed and unconformably overlying post-ORS unit, the Triungen member of the Hørbyebreen Formation (Billefjorden Group), is ?Late Tournaisian to Viséan but not Famennian in age. Thus, the compressional west-directed folding and thrusting of the Svalbardian deformation took place after Late Famennian and before Late Tournaisian time.
Climatic modelling in the Cretaceous using the distribution of planktonic Foraminiferida
Authors: Hart Malcolm B.
Published: April 2008

During the Cretaceous, planktonic Foraminiferida underwent a major diversification, colonizing a wider range of water depths and geographical regions. Orville Bandy, over 30 years ago, was the first to recognize the potential of using this regional distribution to reconstruct different palaeoceanographic regimes. Using new data from the South Atlantic Ocean, Antarctic Ocean and the Indian Ocean it is possible to show the poleward migration of warm-water taxa during the Cretaceous and compare it with data already available from the Northern Hemisphere. Instead of the present-day nine latitudinal zones based on planktonic taxa it is only possible to identify five with any degree of reliability. These are from north to south, the Boreal, Transition, Tropical (= Tethyan), Transition and Austral. In some of the developing oceans during the Cretaceous (e.g. the Eastern Indian Ocean, South Atlantic Ocean) there are local biogeoprovinces with quite distinctive local assemblages and morphotypes. While these do not reflect on the climatic zonation they provide an interesting insight into the development of the plankton. The foraminiferal distributions are compared with climatic maps produced by the assessment of other data sources.
Environmental changes in pre-evaporitic Late Miocene time in the Lorca Basin (SE Spain): diatom results
Authors: Jurkschat Th. , Fenner J. , Fischer R. , Michalzik D.
Published: April 2008

During the Late Miocene the Lorca Basin, which was an integrated part of the Betic Strait, underwent desiccation. The present investigation was completed to determine the response of the diatom flora to environmental changes before this event. Near the city of Lorca (Province of Murcia, SE Spain) the outcrop area around the Serrata ridge was studied and a stratigraphic section of 125m has been measured and sampled (115 samples). The Serrata ridge is capped by a 25m thick gypsum bed (Gypsum Member of the Serrata Formation). The profile below these gypsum layers comprises marl, sandstone, gypsum and partially laminated diatomite (Varied Member of the Serrata Formation). The frequency of radiolaria, siliceous sponge spicules, silicoflagellates, ebridians, diatoms and diatom species has been determined quantitatively. For the lower 87.5m of the section, which contains sufficiently well preserved diatoms, an age of latest Miocene (6.6-5.3 Ma; Magnetic Epoch 6 and 5) can be assigned by the presence of the diatom species
Asterolampra acutiloba
and
Nitzschia reinholdii
, applying the stratigraphic ranges of the species used in the North and Equatorial Pacific and in the Atlantic. In total, 116 diatom species and species-groups have been identified. However, the number of species per sample varies only between 13 and 36 species (a minimum of 300 valves counted). This low diversity, together with a dominance of one species-group,
Thalassionema nitzschioides
(up to 80%), which tolerates considerable ecological changes (e.g. in salinity) indicate that environmental stress must have prevailed during the sedimentation of the Varied Member of the Serrata Formation. Abundance fluctuations of this species-group, as well as changes in the composition of the remaining diatom assemblage and decreases in abundances of the siliceous microfossil groups, reflect repeated periods of interruption of the still marine influenced deposits before the deposition of thick gypsum layers. Several repetitive changes are indicated, where each cycle is characterized by an increase in meroplanktonic species (e.g.
Paralia sulcata, Actinoptychus senarius
) and by a contemporary decrease in the holoplanktonic species towards the top, suggesting progressive environmental stress and a shallowing of the basin. Five of these cycles are clearly distinguished characterizing the initial stages of the evaporitic desiccation event in the Lorca Basin. Benthic diatoms are present only in low abundances (2–4%) and show no correlation with these fluctuations.
Cooling evidence from Pleistocene shelf assemblages in SE Sicily
Authors: Di Geronimo I. , Di Geronimo R. , La Perna R. , Rosso A. , Sanfilippo R.
Published: April 2008

A short Lower Pleistocene section, cropping out in SE Sicily, is studied. The sequence mainly consists of richly fossiliferous sandy layers with
Arctica islandica
, deposited in mid-shelf environments. Molluscs, bryozoans, serpuloideans and calcareous algae are investigated, to identify species with palaeoclimatological implications. Apart from some well-known North Atlantic molluscs, other species are identified as palaeoclimatological tools. Cooling evidence is provided by such palaeoclimatological indicators and by the increasing shell size in some species.
Palaeoceanography and numerical modelling: the Mediterranean Sea at times of sapropel formation
Authors: Rohling E. J. , De Rijk S. , Myers P. G. , Haines K.
Published: April 2008

Palaeo-circulation concepts based on (micro-)palaeontological records are compared with results of a general circulation model (GCM), in a review of the eastern Mediterranean conditions during sapropel S1 deposition (7–8ka
bp
). We discuss two conceptual models, based on proxy data, which integrate bottom water anoxia and high productivity during sapropel formation. One infers a weakened version of the present-day anti-estuarine circulation, whereas the other invokes a reversal of the circulation to an estuarine type. Although both seem reasonable, in view of the available palaeoceanographic proxy data, they imply completely different states of the Mediterranean fresh-water budget. Numerical modelling has been based on three available reconstructions of changes in the west-east salinity gradient compared with present-day conditions (small decrease, intermediate decrease and reversing). The three scenarios give surprisingly similar results. The interface between the surface and intermediate water has shoaled significantly compared with the present, which fuels a deep chlorophyll maximum. All three simulations show stagnant deep waters, which could cause bottom water anoxia. Reversal of circulation is not observed, not even in the reconstruction of most extreme salinity changes. The combined approach of palaeoceanographic and numerical modelling reviewed in this paper shows that modelling provides an indispensable feedback on the viability of the various proxy-based conceptual models.
The contribution of Quaternary vertebrates to palaeoenvironmental and palaeoclimatological reconstructions in Sicily
Authors: Bonfiglio Laura , Marra Antonella C. , Masini Federico
Published: April 2008

In Sicily few studies have been devoted to the climatic-environmental changes of the Pleistocene and Holocene period. Most of the studies on Quaternary vertebrates in Sicily have been focused on the evolutionary-taxonomic aspects of the fauna. Sicily experienced at least four vertebrate dispersal events during Quaternary time, which are of different provenence (African and/or European) and have been controlled by filtering barriers of different intensities. The marked endemism and the extremely low diversity of the fossil assemblages of early and early-mid-Pleistocene time do not allow detailed interpretations. By way at contrast, younger assemblages are more diverse and, although they display some endemic characters, are similar to those of southern peninsular Italy. The late mid-Pleistocene and early Late Pleistocene assemblages (
Elephas mnaidriensis
faunal complex) are characterized by the occurrence of a red deer (
Cervus elaphus siciliae
), a dwarf fallow deer-like endemic megalocerine (
Megaceroides carburangelensis
), auroch (
Bos primigenius siciliae
), bison (
Bison priscus siciliae
), elephant (
Elephas mnaidriensis
), hippopotamus (
Hippopotamus pentlandi
), boar (
Sus scrofa
), brown bear (
Ursus
cf.
arctos
) and three large social carnivores (
Panthera leo, Crocuta crocuta
and
Canis lupus
). Most of these taxa, except for the megalocerine, are characterized by slightly reduced body size compared with the same taxa from mainland Europe. These assemblages are indicative of a climate with temperate, Mediterranean affinity and of landscapes in which forested areas were associated with more open environments. The relatively low abundance of the red deer and the dominance of the megalocerine in several assemblages suggests that a Mediterranean-type forest locally dominated some of the climatic phases. The assemblages of the youngest Late Pleistocene period on Sicily are characterized by a dramatic drop in diversity, with the disappearance of elephant, hippopotamus, bison, the endemic megalocerine, and the largest predators. This would indicate an environmental crisis probably linked to the drier climatic oscillations of late Pleniglacial time, as is suggested also by the spread of the ground vole, which is the dominant small mammal in several assemblages. The Late Glacial period is characterized by the spread of equids (horse and wild ass), which are indicators of open landscapes and of xerophytic steppe-like cover. The beginning of the Holocene period is characterized by the expansion of forested areas and by a more humid climate, as suggested by the abundance of red deer, and by the dispersal of the common dormouse (
Glis glis
) and water vole (
Arvicola
sp.).
Groundwater protection in a Celtic region: the Irish example
Authors: Misstear Bruce D. , Daly Donal
Published: April 2008

One of the key environmental objectives of the proposed EU Water Framework Directive is that Member States must prevent the deterioration of groundwater quality. A national groundwater protection scheme for Ireland has been published recently. This scheme shows certain broad similarities to the groundwater protection policy for England and Wales, incorporating the concepts of groundwater vulnerability, source protection zones and responses to potentially polluting activities. However, the Irish scheme is different in several important respects, reflecting the different hydrogeological conditions and pollution concerns in Ireland. Some of these hydrogeological conditions and pollution concerns are common to the other Celtic regions. A major feature of the Irish scheme is the importance given to subsoil permeability in defining groundwater vulnerability. At present, the subsoil permeability is classified in qualitative terms as high, moderate or low. For the protection scheme to be defensible, it is essential to adopt a systematic and consistent approach for assigning subsoil units to these permeability categories. In mapping groundwater vulnerability, it is also useful to take account of secondary indicators such as groundwater recharge potential, natural and artificial drainage density and vegetation characteristics. Another important issue in Ireland is the protection of groundwater in karst areas, since these areas are especially vulnerable to contamination.
The Triassic Sherwood Sandstone aquifer in Northern Ireland: constraint of a groundwater flow model for resource management
Authors: McNeill G. W. , Cronin A. A. , Yang Y. , Elliot T. , Kalin R. M.
Published: April 2008

The most important aquifer in Northern Ireland is the Triassic Sherwood Sandstone formation that lies under the urban area of Belfast and the areas to the east (Newtownards) and south (Lagan Valley) of the city. The management of this groundwater resource is important in providing a sustainable supply for both public and industrial users. A groundwater flow model that collated relevant meteorological, hydrological and hydrogeological information for the Lagan Valley and Newtownards areas was developed. This conceptual model was coupled with geochemical and isotopic data (using inverse modelling) to constrain the groundwater flow parameters. The results of
14
C-dating suggest some of the groundwater in the aquifer may be up to 4000 years old, and that structural controls play a major role in both the flow rate and the spatial distribution of groundwater within the Sherwood Sandstone aquifer.
Modelling the potential impact of climate change on a shallow coastal aquifer in northern Scotland
Authors: Malcolm R. , Soulsby C.
Published: April 2008

The potential impact of climate change on a freshwater wetland in a coastal dune system at St. Fergus, north-east Scotland, was assessed using hydrogeological modelling techniques. A range of climate change scenarios were input into a numerical model (MODFLOW) that had been calibrated for the site using contemporary field data. The combination of parameter and hydrogeological uncertainty in models, together with uncertainty over likely site-specific climate change in the future, dictates that such modelling studies are often inconclusive. Nevertheless, a range of scenarios was forecast which indicates that future average recharge and flooding of the wetland is likely to fall within the range of interannual variation observed at the site, regardless of whether recharge increases or decreases. It is therefore likely that any resulting ecological impacts will be limited and reflect subtle hydrological changes that will affect different species in different ways. The utility of such a modelling approach is in providing environmental managers with a range of likely scenarios that can be incorporated into future management plans and policies to maximize flexibility of response.
Mid- to Late Eocene organic-walled dinoflagellate cysts from ODP Leg 171B, offshore Florida
Authors: Van Mourik Caroline A. , Brinkhuis Henk , Williams Graham L.
Published: April 2008

The well-calibrated mid- to late Eocene sediment record of ODP Leg 171B (Site 1053A, Blake Nose) allows a detailed stratigraphic and palaeoenvironmental analysis of the dinoflagellate cyst (dinocyst) content. The recovered assemblages are a mixture of inner neritic, outer neritic and oceanic species. The autochthonous dinoflagellates, principally those of the
Impagidinium
group, indicate an oceanic milieu, with possibly some shallowing of water depth towards the top of the section. This trend is also indicated by a corresponding increase of inner neritic dinocysts. The close agreement in the abundance peaks of inner neritic dinocysts and terrestrial palynomorphs indicats that both are allochthonous. This is confirmed by the much higher number of neritic species found in JOIDES Holes 1 and 2, on the continental shelf of eastern Florida, immediately to the west of the Blake Nose. Lower-latitude species found in Hole 1053A, but not occurring at higher latitudes during late Eocene time, are
Diphyes colligerum
and
Thalassiphora delicata
. The presence of these, and other lower-latitude species, confirms that warmer-water conditions persisted during mid- to early late Eocene time in the vicinity of Site 1053. Eighteen new taxa are described, two of them formally:
Charlesdowniea proserpina
sp. nov. and
Oligosphaeridium anapetum
sp. nov.
Long-term thermal consequences of tectonic activity at Mount Isa, Australia: Implications for polyphase tectonism in the Proterozoic
Authors: McLaren S. , Sandiford M.
Published: April 2008

Mount Isa is a Palaeo-Mesoproterozoic terrane in Northern Australia characterized by >300 Ma of episodic tectonic activity prior to effective cratonization. This tectonic activity has resulted in dramatic changes in the heat production distribution in the crust and must have been accompanied by long-term changes in thermal regimes. Primary differentiation of crust initially enriched in heat producing elements has been achieved by felsic magmatism over much of the 300 Ma history, often associated with extensional deformation. The flux of heat producing elements from lower to mid-upper crustal levels associated with this magmatism was sufficient to cause long-term lower crustal cooling of at least 200°C. The accumulation of the radiogenic intrusives (which comprise
c.
230f surface outcrop and have heat production rates averaging 5.2 µWm
−3
) in the mid-upper crust resulted in a highly stratified heat production distribution. One consequence of this distribution is that small changes in the depth to this heat production, through processes such as deformation, erosion and the deposition of sediments, lead to significant changes in deep crustal temperatures (up to 100°C) and consequently lithospheric strength. These considerations suggest that the long-term evolution of the Mount Isa region partly reflects the progressive concentration of heat-producing elements in the upper crust leading to a long-term increase in lithospheric strength, and eventually to effective cratonization. The long-term cooling and strengthening trend was locally countered by the role of subsidence during basin formation which, through burial of heat producing elements in the existing crust and the accumulation of more heat production in insulating sediments, helped to localize subsequent contractional deformation.
The response of mineral chronometers to metamorphism and deformation in orogenic belts
Authors: Parrish Randall R.
Published: April 2008

Mineral chronometers, especially accessory minerals using the U-Pb decay system, can reveal important information regarding the environmental conditions and duration of metamorphic-deformation events during the re-working of older rocks. Minerals such as zircon can newly grow during amphibolite facies or granulite facies events, providing direct ages of metamorphism. Pre-existing minerals like monazite, allanite, and titanite can preserve a component of their original age in spite of upper amphibolite facies re-working and very thorough recrystallization of the rock fabric during mylonite development. The degree of Pb loss can be used to deduce, at least semi-quantitatively, the temperature and duration of the subsequent event. In well-studied examples, the relative retentivity of Pb is highly predictable, and this helps place strong constraints on relative closure temperatures, even when laboratory experimental data are lacking or inconclusive. A number of examples are presented from a wide variety of geological environments to illustrate the response of U-Pb isotope systematics within accessory minerals to superimposed deformation, metamorphism and/or mineral growth.
The effects of early Cambrian metamorphism in western Dronning Maud Land, East Antarctica: a carbon and oxygen isotope study of fluid-rock interaction in the Sverdrupfjella Group
Authors: Johnstone Warren P. , Harris Chris
Published: April 2008

The Sverdrupfjella Group in western Dronning Maud Land is a 1200 to 900 Ma orogenic belt that experienced a thermal overprint in the early Cambrian. Evidence for distinct episodes of fluid-rock interaction is found in calc-silicate rocks, veins and retrograde mineral assemblages. A sequence of altered but undeformed basalts exhibit extreme
18
O depletion (
δ
18
O as low as −1.8%) apparently due to interaction with meteoric water during regional (possibly early Cambrian) metamorphism. In the central Kirwanveggen, metasomatic calc-silicates, and retrograde mineral assemblages are associated with late high-strain zones of probable Cambrian age. The former have δ
13
C values which overlap those of massive metacarbonate units 150 km to the NE, and imply regional scale movement of fluids. The latter record
18
O depletion relative to unretrogressed equivalent rocks and suggest interaction with externally derived fluids.
The glacial transport and physical partitioning of mercury and gold in till: implications for mineral exploration with examples from central British Columbia, Canada
Authors: Plouffe A.
Published: April 2008

Mercury glacial dispersal was measured in the clay-sized fraction (< 0.002 mm) and heavy mineral concentrate (0.063–0.250 mm, specific gravity > 3.3 g/cm
3
) of till in a region of bedrock cinnabar occurrences, in central British Columbia, Canada. Most of the Hg in till occurs as sand-sized cinnabar (HgS) grains. A longer dispersal train was measured with the heavy mineral concentrates because Hg concentrations in heavy minerals yielded a higher ratio between anomalous and background concentrations when compared to the clay-sized material. It is proposed that geochemical or mineralogical analyses on a specific grain size fraction or density fraction of till, where the desired metal resides, result in a higher contrast between anomalous and background concentrations. Such a great contrast translates into a longer detectable dispersal train and hence, a larger target for mineral exploration. Therefore, in drift exploration programs, it is crucial to identify the mode of occurrence of a sought commodity in till; this can be achieved in part with a simple partitioning study whereby metal concentrations are measured in specific grain size fractions of till. Physical partitioning results for Au in the study area indicate that close to the bedrock source, large metal concentrations in some cases are present in the sand- (0.063–2 mm) and granule-sized (2–4 mm) fractions. Therefore, the significance of a regional Au anomaly, commonly defined in the silt plus clay-sized fraction of till could be evaluated by further determining the Au content of coarser size fractions (sand and granule).
The nature and tectonic significance of fault-zone weakening: an introduction
Authors: Rutter E. H. , Holdsworth R. E. , Knipe R. J.
Published: April 2008

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.
Implications of earthquake focal mechanisms for the frictional strength of the San Andreas fault system
Authors: Townend John , Zoback Mark D.
Published: April 2008

Analysis of stress orientation data from earthquake focal plane mechanisms adjacent to the San Andreas fault in the San Francisco Bay area and throughout southern California indicates that the San Andreas fault has low frictional strength. In both regions, available stress orientation data indicate low levels of shear stress on planes parallel to the San Andreas fault. In the San Francisco Bay area, focal plane mechanisms from within 5 km of the San Andreas and Calaveras fault zones indicate a direction of maximum horizontal compression nearly orthogonal to both subvertical, right-lateral strike-slip faults, a result consistent with those obtained previously from studies of aftershocks of the 1989 Loma Prieta earthquake. In southern California, the direction of maximum horizontal stress near the San Andreas fault is nearly everywhere at a high angle to it, similarly indicating that the fault has low frictional strength. Thus, along these two major sections of the San Andreas fault (which produced great earthquakes in southern California in 1857 and central and northern California in 1906), the frictional strength of the fault is much lower than expected for virtually any common rock type if near-hydrostatic pore pressure exists at depth, and so low as to produce no discernible shear-heating anomaly.

Our findings in southern California are in marked contrast to recent suggestions by Hardebeck & Hauksson that stress orientations rotate systematically within
c.
25 km of the fault, which prompted a high frictional strength model of the San Andreas fault. As we utilize the same stress data and inversion technique as Hardebeck & Hauksson, we interpret the difference in our findings as being related to the way in which we group focal plane mechanisms to find the best-fitting stress tensor. We suggest that the Hardebeck & Hauksson gridding scheme may not be consistent with the requisite
a priori
assumption of stress homogeneity for each set of earthquakes.

Finally, we find no evidence of regional stress changes associated with the occurrence of the 1992 M7.4 Landers earthquake, again in apparent contradiction with the findings of Hardebeck & Hauksson.
Non-volcanic rifted margins, continental break-up and the onset of sea-floor spreading: some outstanding questions
Authors: Boillot G. , Froitzheim N.
Published: April 2008

During the last 20 years, regional studies on the West Iberia margin and on the former margins of the Tethys have considerably advanced the understanding of processes related to continental break-up and the onset of sea-floor spreading. However, some questions remain outstanding. To tentatively answer these, a coherent interpretation of available data is proposed, based on the detachment fault concept applied to the continental as well as the oceanic lithosphere, and on the hypothesis of a multi-staged rifting process. The interpretation addresses the nature of the lower crust beneath non-volcanic passive margins, the origin of ophicalcites, the probable time gap between syn- or post-rift crystallization of gabbros and extrusion of basalts on the sea floor, and the significance of dipping reflectors within oceanic lithosphere adjacent to non-volcanic passive margins. The interpretation also considers the symmetry v. asymmetry of continental rifting and break-up, the location of the ocean-continent boundary, and the possible association of magnetic quiet zones with ultramafic sea floor (serpentinized peridotite) bordering non-volcanic passive margins.
Subsidence, deformation, thermal and mechanical evolution of the Mesozoic South Alpine rifted margin: an analogue for Atlantic-type margins
Authors: Bertotti Giovanni
Published: April 2008

The geological record of the currently exposed South Alpine transect of the Mesozoic passive continental margin provides information on the evolution of Atlantic-type margins. Before the onset of rifting, in Mid-Triassic to Carnian time, strong subsidence affected the central parts of the Southern Alps, in the Lombardian basin. No major fault is documented for this time span. Thermal conditions were strongly perturbed. Continental rifting began in Norian times, and until the beginning of the Liassic period was characterized by overall subsidence of the Lombardian basin with rates up to 200 mm ka
−1
. The strong subsidence is a result of continuing extension and of generalized crustal cooling. Subsidence rates were still important in Liassic times, although lower than in Late Triassic time. At the end of the Liassic period, the site of extension shifted towards the west, where crustal break-up eventually took place in Mid-Jurassic times. Previously poorly documented features such as the very strong subsidence in the initial rifting stages, the changing geometry and mechanics of normal faults are here associated with the thermal interactions between pre-existing thermal anomalies and rifting.
The Steinmann Trinity revisited: mantle exhumation and magmatism along an ocean-continent transition: the Platta nappe, eastern Switzerland
Authors: Desmurs Laurent , Manatschal Gianreto , Bernoulli Daniel
Published: April 2008

The close association of serpentinites, basalts and radiolarites, later known as the Steinmann Trinity, was clearly described by Steinmann from the south Pennine Arosa zone and its southern prolongation, the Platta nappe of the eastern Swiss Alps. This classical ‘ophiolite’ is distinctly different from typical fast-spreading ridge associations and can be compared with the transitional crust occurring along non-volcanic passive continental margins in present-day oceans. It includes serpentinized peridotites that we interpret as subcontinental mantle rocks, which were exhumed along low-angle detachment faults and locally overlain by extensional allochthons of continental crust, minor gabbroic intrusions, tholeiitic pillow lavas and flows and a succession of oceanic sediments. The serpentinized peridotites record deformation at falling temperatures during extension leading to final exposure of the mantle rocks at the sea floor and their inclusion in tectono-sedimentary breccias (ophicalcites). Field relationships, and mineral-chemical and radiometric data show that the gabbros intruded already serpentinized mantle rocks at shallow depth 161 Ma ago. They are Mg gabbros, Fe gabbros and Fe-Ti gabbros, cut by dioritic pegmatoid veins and albitite dykes, which originated by differentiation from the same parental magma. All gabbros show the same metamorphic evolution, i.e. intrusion at relatively low pressure, oceanic hydration at elevated temperature and a subsequent static oceanic alteration. The pillow lavas stratigraphically overlie the exhumed mantle rocks and the tectono-sedimentary breccias related to the exhumation of both mantle rocks and gabbros. However, both gabbros and pillow basalts are characterized by εNd values typical for an asthenospheric mid-ocean ridge-type source of the melts. They may be the products of a steady process that combined extensional deformation with magma generation and emplacement. They appear to document the onset of sea-floor spreading across an exhumed subcontinental mantle during the earliest phases of a slow-spreading ridge.
Evolution of the Aveiro Cretaceous aquifer (NW Portugal) during the Late Pleistocene and present day: evidence from chemical and isotopic data
Authors: Condesso De Melo M. T. , Carreira Paquete P. M. M. , Marques Da Silva M. A.
Published: April 2008

A combined hydrogeological, geochemical and isotopic investigation was performed in the Aveiro Cretaceous coastal aquifer in order to investigate the rock-water interaction and the evolution of palaeowaters. The results indicate an aquifer with well-defined freshwater (<50 mg l
−1
Cl
−
throughout most of the aquifer as far as the coastline and with very slow chemical kinetics for the water-rock interactions. The low mineralization may be explained by the mainly siliciclastic composition of the aquifer sediments, while calcite dissolution and cation exchange were also considered to be involved in the groundwater chemical evolution. This interpretation is in agreement with lithology and facies changes, and was validated by the PHREEQC-2 modelling results. Radiocarbon ages indicate a smooth gradient across the aquifer, implying continuous flow during the late Pleistocene and Holocene. Noble gas ratios indicate that the mean annual air temperatures were lower by 5–6°C at the last glacial maximum (LGM). In contrast to most areas with continental palaeowaters, the environmental isotopes indicate enrichment (0.8–1.0%. in
18
O), interpreted as reflecting the composition of the oceans at the time of the LGM at this maritime site, as well as the constancy of the air mass circulations over the period to the present day. An outlet for the aquifer is inferred offshore to account for the observed geochemical data, with the possibility that freshwater could still also be found offshore.
Before the hills in order stood: the beginning of the geology of time in England
Authors: Fuller John G. C. M.
Published: April 2008

That order should govern the nature of the world is an idea not confined to England, though the history of science in this country demonstrates again and again that a conception of Divine Order lay at its heart. To people of earlier days, want of order implied confusion, displacement, derangement, time out of joint, even the presence of malevolent power — ‘when the planets in evil mixture to disorder wander’. The divine scheme revealed by scripture was a frame and support for Earth science. It told an indisputable story of an ordered beginning, a diluvial reordering, and a future end in dissolution. It was a story backed by secular law, and no thinking person could have been unaware of it. Yet ‘when’ and ‘how’ were legitimate questions, answered in detail by hexaemeron writers. It is an educational curiosity in England that a particular Biblical chronology drawn up in 1650 accompanied scriptures printed for use in schools until 1885 — a matter of consequence to the history of all geological thought in this country.
Jean-André de Luc and nature’s chronology
Authors: Rudwick Martin J. S.
Published: April 2008

Jean-André de Luc (or Deluc) (1727–1817), who first proposed the term ‘geology’ almost in its modern sense, was one of the most prominent geologists of his time. His ‘theory of the Earth’, published in several versions between 1778 and 1809, divided geohistory in binary manner into two distinct phases. The fossiliferous strata had been formed during a prehuman ‘ancient history’ of immense but unquantifiable duration. Then the present continents had emerged above sea-level in a sudden physical ‘revolution’, at the start of the Earth’s ‘modern history’ of human occupation. De Luc argued that the rates of ‘actual causes’ or observable processes (erosion, deposition, volcanic activity, etc.) provided ‘natural chronometers’ that proved that the ‘modern’ world was only a few millennia in age; and he identified the natural revolution at its start as none other than the Flood recorded in Genesis. So ‘nature’s chronology’ could be constructed from natural evidence, to match the well-established historical science of chronology based on textual evidence from ancient cultures. De Luc’s natural chronology was restricted to the recent past, but it provided a template for later geologists to develop a geochronology extending into the depths of geohistory. The historical importance of de Luc’s work has only been obscured by the myth of intrinsic conflict between science and religion.
Current issues and new developments in deformation mechanisms, rheology and tectonics
Authors: De Meer S. , Drury M. R. , De Bresser J. H. P. , Pennock G. M.
Published: April 2008

We present a selective overview of current issues and outstanding problems in the field of deformation mechanisms, rheology and tectonics. A large part of present-day research activities can be grouped into four broad themes. First, the effect of fluids on deformation is the subject of many field and laboratory studies. Fundamental aspects of grain boundary structure and the diffusive properties of fluid-filled grain contacts are currently being investigated, applying modern techniques of light photomicrography, electrical conductivity measurement and Fourier Transform Infrared (FTIR) microanalysis. Second, the interpretation of microstructures and textures is a topic of continuous attention. An improved understanding of the evolution of recrystallization microstructures, boundary misorientations and crystallographic preferred orientations has resulted from the systematic application of new, quantitative analysis and modelling techniques. Third, investigation of the rheology of crust and mantle minerals remains an essential scientific goal. There is a focus on improving the accuracy of flow laws, in order to extrapolate these to nature. Aspects of strain and phase changes are now being taken into account. Fourth, crust and lithosphere tectonics form a subject of research focused on large-scale problems, where the use of analogue models has been particularly successful. However, there still exists a major lack of understanding regarding the microphysical basis of crust- and lithosphere-scale localization of deformation.
In situ
experimental study of roughness development at a stressed solid/fluid interface
Authors: Brok Bas Den , Morel Jacques , Zahid Mohsine
Published: April 2008

Theory and experiments have demonstrated that the initially flat surface of an elastically strained solid is morphologically unstable. The elastic strain energy of a rough, corrugated surface is lower than that of a flat one. Hence, stress forces the surface into a rough structure, but the associated increase in surface energy counteracts this roughening. In this way an equilibrium surface roughness consisting of μm-scale grooves and ridges can develop if the solid is transported, e.g. by diffusion through an aqueous solution, from sites of high stress to sites of low stress. We report
in situ
experimental observations of the surface of elastically strained potassium (K-) alum single crystals held in K-alum solution. The observations confirm earlier reports of the development of stress-induced μm-scale grooves on the surface of this material. The
in situ
observations show, however, that the stress-induced surface morphology is not a static, but a dynamic structure. The grooves are mobile, and may for example propagate or increase or decrease in length. They may move upwards, downwards, or remain where they are. Others rotate and undulate. It is suggested that if stress is high enough, grain boundaries in (wet) rocks could posses a similar structure of channels, continuously changing position and orientation, in line with the so-called ‘dynamically stable’ island-channel grain boundary structure that is essential to several pressure solution models.
Quantitative texture analysis of glaucophanite deformed under eclogite facies conditions (Sesia-Lanzo Zone, Western Alps): comparison between X-ray and neutron diffraction analysis
Authors: Zucali M. , Chateigner D. , Dugnani M. , Lutterotti L. , Ouladdiaf B.
Published: April 2008

X-ray and neutron diffraction techniques have been applied to quantitative texture analysis of a glaucophanite from the Sesia-Lanzo Zone (Western Italian Alps), naturally deformed under eclogite facies conditions. The comparison has been carried out in order to reveal the limits and problems of texture analysis related to strongly deformed polymineralic. Different methods of measuring and computing the orientation distribution function from diffraction data have been tested, in particular X-rays, direct peak integration, and neutron diffraction using Rietveld-texture analysis. Due to grain-size problems and heterogeneity of individual amphibole minerals, neutron radiation is shown to be the best probe for characterizing the whole rock: being more penetrative than conventional X-rays, a larger volume of the mineral aggregate is sampled, giving better statistics. However, results obtained by summing the corresponding individual spectra of at least three X-ray diffraction experiments on parallel slabs of the same specimen also give statistically valid, semiquantitative results that reproduce the overall textures. The quantitative texture analysis shows the strong texture of the two generations of amphiboles (AmpI and AmpII), which are mainly characterized by [001]*-directions at an angle of about 10° to the mineral lineation and by (
hk0
) planes describing girdles around the lineation. The texture is comparable to those described in the literature for amphibole deformed under different temperature and pressure conditions, and the pronounced asymmetry of the [001]* directions with respect to the mineral lineation is consistent with a non-coaxial component that occurs during the deformation.
Non-linear feedback loops in the rheology of cooling-crystallizing felsic magma and heating-melting felsic rock
Authors: Burg Jean-Pierre , Vigneresse Jean-Louis
Published: April 2008

At least six major parameters control the rheology of partially molten systems: melt content, rate of melt production, reaction to strain of the solid component, reaction to strain of the molten component, temperature and chemical composition of the source rocks. We examine their interactions to understand the rheology of partly molten rocks and partly crystallized magmas. In particular, this paper focuses on the rheology in the transitional domains between two pairs of thresholds that bracket a transitional regime between solid state and fluid behaviour during melting and crystallization, respectively. We review related information and point out non-linear effects that develop during heating of melting rocks and cooling of crystallizing magmas. Owing to the non-linear interactions, positive or negative feedback loops accelerate or damp the system. Melt in migmatite experiences shear-softening which, along with strain partitioning, facilitates melt segregation. Conversely, the increasing number of rigid crystals during cooling increases the suspension viscosity (shear hardening), which soon inhibits magma movement. These effects reinforce the asymmetry between solid-to-melt and melt-to-solid transitions. They severely contradict the concept of one rheological critical melt percentage valid for both melting and crystallization transitions. Fabric acquisition competes with nucleation and crystal growth, thus leading to hysteresis of the stress-strain rate curves. Implications for field observations include horizontal magma segregation, magma extraction and successive magma intrusions.
Do calcite rocks obey the power-law creep equation?
Authors: Renner Jörg , Evans Brian
Published: April 2008

The power-law creep equation,
ε̇
∞

σ
n

exp(−
Q/RT
), is commonly used to relate strain rate,
ε̇
, stress,
σ
, and temperature,
T
, for thermally activated dislocation creep in rocks. When triaxial deformation experiments on marble and limestone samples are performed at temperatures of 400–1050°C, to strains <0.2, and with strain rates between 10
−3
and 10
−7
s
−1
, the variations in strength among different rocks at nominally identical conditions are much larger than the experimental uncertainty. During dislocation creep, the strengths of various limestones and marbles decrease with increasing grain size, similar to the Hall-Petch effect in metals. The stress sensitivity of strain rate,
n
′ =
∂
ln
ε̇
/
∂
ln
σ
, and the temperature sensitivity of strain rate,
Q
′ = −
R
∂
ln
ε̇
/
∂
(1/
T
), differ greatly for the various calcite aggregates. There is a systematic dependence of
n
′ and
Q
′ on stress, grain size, and perhaps, temperature, and there is no interval in stress where
n
′ is constant. Thus, the steady-state power-law equation is an inadequate description of dislocation creep in calcite rocks. To improve the constitutive law, it may be necessary to include at least one additional state variable that scales with grain size.
The influence of dynamic recrystallization on the grain size distribution and rheological behaviour of Carrara marble deformed in axial compression
Authors: Ter Heege J. H. , De Bresser J. H. P. , Spiers C. J.
Published: April 2008

Strain localization and associated rheological weakening are often attributed to grain size reduction resulting from dynamic recrystallization. Most studies investigating rheological changes due to dynamic recrystallization regard recrystallized grain size as a single value that is uniquely related to stress during steady-state deformation. However, rock materials invariably exhibit a grain size distribution with distribution parameters that may be altered by dynamic recrystallization during transient deformation, affecting the rheological behaviour. This study aims to investigate the effect of deformation conditions on the evolution of grain size distribution and rheological behaviour during dynamic recrystallization in the approach to steady state. To study this, we have deformed Carrara marble to natural strains of 0.15–0.90 in axial compression at temperatures of 700–990°C, stresses of 15–65 MPa, strain rates of 3.0 × 10
−6
–4.9 × 10
−4
s
−1
and a confining pressure of 150 or 300 MPa, and analysed the grain size distribution of each sample. The results show that during dynamic recrystallization, grain size distributions evolve by a competition between grain growth and grain size reduction. The relative importance of grain-size-sensitive creep increases as the average grain size is reduced with strain. Minor weakening is observed, which is probably insufficient to cause strain localization in nature.
GPR survey of a Holocene aeolian/fluvial/lacustrine succession, Lauder Sandhills, Manitoba, Canada
Authors: Havholm K. G. , Bergstrom N. D. , Jol H. M. , Running G. L.
Published: April 2008

A stream cut-bank at Flintstone Hill, Lauder Sandhills, Glacial Lake Hind Basin, southwestern Manitoba, exposes an extensive Holocene sedimentary sequence. Ground penetrating radar (GPR) is used to determine lateral continuity of four major stratigraphic units (wetland complex, Mid-Holocene aeolian dune, sandsheet/fluvial, Late Holocene aeolian dune) and to image primary sedimentary features within the dune strata. Depth of GPR signal penetration is only about 5 m. Signal attenuation is attributed to the presence of primarily pedogenic silts/clays and carbonate- and iron sesquioxide-rich horizons within the sandier units. Nevertheless, the stratigraphic units are shown by GPR, and confirmed by core data, to be laterally continuous within the survey area. Finer-scale dune strata are not well displayed in the aeolian units. Signal attenuation above the Mid-Holocene unit and relatively high iron sesquioxide concentrations within the unit limit imaging of well-preserved dune foreset strata. In the Late Holocene unit, depositional strata are poorly preserved and GPR primarily images soil A-horizons formed on surfaces of temporary stabilization within the dune sediment.