bluuebunny/crossref_metadata_2025 huggingface dataset:
The Skiddaw Group (English Lake District) reviewed: early Palaeozoic sedimentation and tectonism at the northern margin of Avalonia
Authors: Stone P. , Cooper A. H. , Evans J. A.
Published: April 2008

The Skiddaw Group is a 5 km thick sequence of Tremadoc-Llanvirn turbiditic mudstone and sandstone, including a major olistostrome, which occupies the northern part of the Lake District Lower Palaeozoic Inlier. Sporadic outcrop and borehole records indicate that similar strata extend beneath other parts of northern England. To the west, the Manx Group of the Isle of Man is a regional correlative. The Skiddaw Group was deposited on the Avalonian margin of the Iapetus Ocean, with constituent sediment derived largely from an earlier, possibly Precambrian, continental margin volcanic arc. Nd isotope data confirm the absence of juvenile detritus. Olistostrome emplacement in the late Arenig preceded subduction-related uplift of the deep-marine Skiddaw Group to form the subaerial basement to the mainly Caradoc, Borrowdale and Eycott volcanic groups. The scale of the unconformity beneath the volcanic rocks requires considerable pre-volcanic disruption and erosion of the Skiddaw Group prior to structural disturbance by volcanotectonic faulting. Volcanism ended in the late Caradoc when thermal reequilibration, coupled with possible further extension, allowed marine transgression through the early Silurian. Ultimately, convergence of Avalonia with Laurentia initiated thrust imbrication of the Skiddaw Group as the Southern Uplands thrust belt extended across the sutured Iapetus Ocean. Thrust-related hydration caused widespread resetting of Rb-Sr isotope systems during the 430–420 Ma interval. A penetrative slaty cleavage with a broadly Caledonian trend was imposed during the Early Devonian, Acadian Orogeny and cuts an earlier, bedding-parallel (compaction) fabric. Later phases of Acadian compression probably involved reactivation of thrusts within the Skiddaw Group with associated strain partitioning resulting in domainal crenulation cleavage. Granite intrusion at
c.
400 Ma coincided with the final cleavage episode.
A comparison of the Ribband Group (southeastern Ireland) to the Manx Group (Isle of Man) and Skiddaw Group (northwestern England)
Authors: McConnell B. J. , Morris J. H. , Kennan P. S.
Published: April 2008

The lithostratigraphy of the Ribband Group of southeastern Ireland is revised through comparison to the equivalent Avalonian margin sequences of the Manx(Isle of Man) and Skiddaw (northern England) Groups. Four tracts are recognized in the Ribband Group, within which fossil age control and the ‘coticule package’ marker horizon are used to constrain lithofacies comparisons. Volcanic arc rocks in the Ribband and Manx Groups contrast with the passive margin provenance of the Skiddaw Group.
On the uplift and denudation of the Weald
Authors: Jones David K. C.
Published: April 2008

The evolution of the Weald has been the subject of continuing controversy over nearly two centuries of investigation. This paper reviews current knowledge of the area’s development during the Tertiary against this background and produces an evolutionary geomorphology focused on a cross-section reconstruction of the Weald stretching from the south coast to the Thames. Estimations of the original thickness of Chalk (400–460 m) and other Mesozoic strata indicate a complex fold structure which, if ever fully developed, would have had a crestal elevation of 1400 m. The additional accumulation of an estimated 60–220 m of Palaeogene sediments reveals gross denudation of up to 1350 m in 66 Ma or 20.4 m Ma
−1
, a low rate on relatively erodible strata indicating periods of relative morphostasis in the Tertiary. Four models of evolution are identified — ‘Neogene flexuring and erosion’, ‘pulsed tectonism focusing on Palaeogene denudation’, the classic Wooldridge and Linton interpretation, and the model detailed in this paper — and applied to the cross-sectional reconstruction to produce spatial and temporal patterns of gross denudation and denudation rates. The analysis reveals that all models are possible theoretically and that denudation rates lie in the lower half of the range attributed to ‘normal denudation’. The model detailed in this paper has a proposed pattern of evolution for the Central Weald that involves early Palaeogene uplift, warping and denudation, followed by slower denudation in the later Palaeogene, stimulated erosion in the Neogene due to tectonic activity, culminating with severe denudation and relief generation in the Quaternary due to uplift of up to 260 m. The Chalk downlands, by contrast, are considered to have developed from the Pleistocene exhumation and dissection of areas that evolved more slowly in the Tertiary and were dominated by low relief landsurfaces at no great elevation above contemporary summits that experienced lengthy periods of morphostasis.
What do experiments tell us about the relative contributions of crust and mantle to the origin of granitic magmas?
Authors: Patiño Douce Alberto E.
Published: April 2008

The origin of different kinds of granitic rocks is examined within the framework of experimental studies of melting of metamorphic rocks, and of reaction between basaltic magmas and metamorphic rocks. Among the types of granitic rocks considered in this chapter, only peraluminous leucogranites represent pure crustal melts. They form by dehydration-melting of muscovite-rich metasediments, most likely during the fast adiabatic decompression that results from tectonic collapse of thickened intracontinental orogenic belts. All other granitic rocks discussed here represent hybrid magmas, formed by reaction of basaltic melts with metamorphic rocks of supracrustal origin. These hybrid rocks include Cordilleran granites, formed at or near convergent continental margins, strongly peraluminous ‘S-type’ granites, alumina-deficient ‘A-type’ granites, and rhyolites associated with continental flood basalts. The differences among these types of granites reflect differences both in their source materials and in the pressures at which mantle-crust interactions take place. In turn, these variables are correlated with the tectonic settings in which the magmas form. Hybrid mafic cumulates are also produced by mantle-crust interactions, simultaneously with the granitic melts. These cumulates range from orthopyroxene + plagioclase-rich assemblages at low pressure to clinopyroxene + garnet-rich assemblages at high pressure, and are known to be important constituents of the lower continental crust. With the exception of peraluminous leucogranites, generation of granitic magmas is almost always associated in space and time with growth, rather than just recycling, of the continental crust.
Permo-Triassic and Jurassic extension in the northern North Sea: results from tectonostratigraphic forward modelling
Authors: Odinsen Tore , Reemst Paul , Beek Peter Van Der , Faleide Jan Inge , Gabrielsen Roy H.
Published: April 2008

We have undertaken 2D forward modelling across the northern North Sea, based on reprocessed, interpreted and depth-converted deep reflection seismic lines NSDP84-1 and −2 (15 s twt) and refraction data. Two separate stretching phases, Permo-Triassic and Jurassic, are recognized. The cumulative stretching is consistent with the observed crustal structure and the overall basin configuration, as reproduced by forward modelling. Good agreement between observed and modelled top basement level, and crustal thickness below the platform areas are particularly emphasized. Crustal-scale modelling indicates that crustal thickness varied across the northern North Sea at the onset of the Permo-Triassic rifting, from
c
. 35 km in the platform areas to less than 30 km in the interior of the basin. This may be ascribed to Devonian(-Carboniferous?) crustal stretching. Thinning of the crust has progressively been narrowed, from post-Caledonian extensional collapse, to less regional Permo-Triassic basins, and finally development of the Viking Graben area in the Jurassic-early Cretaceous time. Most of the Permo-Triassic stretching occurred between the Øygarden Fault Zone to the east and the Shetland Platform (southern transect) and the Hutton Fault alignment to the west. The width of the Permo-Triassic basin was
c
. 120–125 km, with calculated
β
mean
between 1.38 and 1.40. Permo-Triassic
β
mean
estimates across the present Horda Platform vary between 1.33 and 1.39. The Jurassic
β
mean
estimates for the same area vary between 1.08 and 1.13. Across the Viking Graben, Permo-Triassic
β
mean
varies between 1.28 (southern transect) and 1.41 (northern transect). This is lower than estimates for the Jurassic
β
mean
, which amounts to 1.53 and 1.42. Permo-Triassic and Jurassic
β
mean
estimates across the East Shetland Basin are 1.29 and 1.11, respectively. Lithospheric thermal evolution reflects the general differences between Permo-Triassic and Jurassic stretching, with a much wider thermal perturbation during the former and a focusing and lateral migration towards the east of the peak thermal elevation during the latter. There are still uncertainties related to the degree of (de)coupling between the upper crust and upper mantle during the Permo-Triassic and the Jurassic rift phases. These uncertainties are related to the interplay between age, strain rate, crustal rheology, crustal thickness and long-lived zones of weaknesses.
Detachments and low-angle faults in the northern North Sea rift system
Authors: Fossen Haakon , Odinsen Tore , Færseth Roald B. , Gabrielsen Roy H.
Published: April 2008

Several master faults in the North Sea basin tend to flatten to give low dips at depth, and in this sense form detachments in the rift system. Such low angle faults are identified in the western flank of the Viking Graben (Tampen Spur area), where they occur as both intra- and supra-basement detachments. Interference between detachments and steeper faults results in ramp-flat-ramp geometries. In the eastern part of the Gullfaks fault block, a supra-basement detachment is probably associated with anomalously high late Jurassic extension in the Gullfaks Field area. The low-angle Gullfaks detachment also helps explain the presence of sets of parallel east-dipping faults (domino systems), a common feature in the collapsed hanging wall to low-angle detachments. Similar detachments probably exist beneath the Gullfaks Sør block and SE of the Visund fault block. All of these are interpreted as late Jurassic collapse structures directly related to active late Jurassic extensional tectonics. Strong indications of intra-basement detachments are also found in the Tampen Spur area. These detachments are formed by major normal faults that flatten in the basement, as seen beneath the Visund fault block. This geometry may to some extent be related to fault rotation during repeated phases of extension in the Palaeozoic-Early Mesozoic period. However, abrupt flattening of some of the faults in the basement indicates that the master faults follow some of the many pre-existing mechanically weak zones in the basement, primarily low-angle Devonian extensional shear zones or Caledonian thrusts.
Syn-rift evolution and resulting play models in the Snorre-H area, northern North Sea
Authors: Nøttvedt Arvid , Berge Anker M. , Dawers Nancye H. , Færseth Roald B. , Häger Kjell O. , Mangerud Gunn , Puigdefabregas Cai
Published: April 2008

An upper Jurassic, wedge-shaped syn-rift succession, comprising the Heather and Draupne Formations, is present in the hangingwall trough of the Snorre Fault Block. The succession is bounded to the west by the Statfjord East Fault, whereas it onlaps the snorre Fault Block to the east. It consists of a two-fold coarsening-upward sequence from shale to sandstone of shallow marine/shoreline origin.

Active fault block rotation and subsidence in the Snorre-H area commenced in the Mid-Bathonian and lasted through the Ryazanian. The Heather Formation was deposited during the early rift stage (Mid-Bathonian-Early Oxfordian; 3° cumulative tilt), whereas the Draupne Formation (Late Oxfordian-Ryazanian; 9° cumulative tilt) accumulated during the main and late rift stages. The lower part of the Heather Formation was likely deposited across a submerged tilted fault block terrain, with a predominant extra-basinal sediment supply. Deposition of the upper Heather Formation, however, was governed by gradually emerging footwall islands, albeit yet without significant local erosion. As a result of increased fault/block rotation during deposition of the Draupne Formation Shale, Sequences I–II (late Early Oxfordian-early Mid-Volgian) footwall islands became firmly established, providing a predominant local sediment source to the Snorre-H sub-basin. Clay and silt were supplied from erosion of the Heather Formation on the Snorre-H hangingwall, with subordinate input of sand from the Statfjord East footwall. Subsequent deposition of the Draupne Formation, Sequences III–V (Late Mid-Volgian-Ryazanian), was governed by significant relief on the footwall islands, causing deep erosion into the Brent Group on the Snorre-H hangingwall dip-slope and leading to progradation of the Upper Draupne Sandstone shoreline complex across the Snorre-H area. Deposition of the Draupne Formation and temporal shoreline position were likely partly controlled by northwards fault-tip propagation of the Statfjord East Fault.

Various syn-rift play models and depositional reservoir facies are present within the Snorre-H hangingwall basin. They include dip-slope shallow marine/shoreline sands, basin floor gravity transported sands and likely footwall talus sands enveloped in organic rich shales of the Draupne Formation. The distribution of reservoir facies is intimately linked to exposure and erosion of the middle Jurassic Brent Group below the syn-rift unconformity.
Crustal stress and tectonics in Norwegian regions determined from earthquake focal mechanisms
Authors: Lindholm Conrad D. , Bungum Hilmar , Hicks Erik , Villagran Mario
Published: April 2008

A database of 109 earthquake focal mechanisms for Scandinavia and Svalbard has been compiled, comprising 29 new solutions. Stress directions extracted from these earthquake focal mechanisms have been calculated for southeastern and southwestern Norway, the Norwegian Sea and the Barents-Finnmark region. The principal horizontal stress is
c.
N45°W, and indicates a clockwise rotation as one moves from south to north, in accordance with stress trajectories that can be expected from ridge push. The data indicate that in southern Norway normal faulting regimes are more dominant in onshore regions, whereas thrust faulting dominates the offshore seismic activity. Offshore mid-Norway a preference for normal faulting is also found, whereas the shallow earthquakes in Finnmark, northern Norway, are all reverse. Focal depth distributions were studied in the northern North Sea region, revealing substantial differences with depths in the range 5–30 km.
The interplay of faulting and folding during the evolution of the Zagros deformation belt
Authors: Sattarzadeh Y. , Cosgrove J. W. , Vita-Finzi C.
Published: April 2008

In this short paper satellite images, aerial photographs and seismic sections are used to show that pure buckle folds, pure forced folds and folds intermediate between the two have all formed, and are still forming, in association with the compression tectonics currently occurring in the Zagros deformation belt which is situated along the northeastern margin of the Arabian plate. The type of folding and its distribution can be linked directly to the distribution of ancient basement faults and to the rheological profile of the cover sequence.
Tectonically enhanced forced regressions: examples from growth folds in extensional and compressional settings, the Miocene of the Suez rift and the Eocene of the Pyrenees
Authors: Gawthorpe Robert L. , Hall Matt , Sharp Ian , Dreyer Tom
Published: April 2008

This paper examines the stratal geometries and facies stacking patterns associated with forced regressions around fault-propagation folds in extensional and compressional settings. Case studies are documented from: (i) the Miocene of the Suez rift and (ii) the Eocene of the Ainsa piggyback basin, Pyrenees. Despite the different tectonic settings, the stratal geometries and facies stacking patterns are remarkably similar. Distinctive sharp-based shoreface sandstones, formed as a result of forced regression, were deposited around growth anticlines. The forced regressive shoreface sandstones ‘shale-out’ rapidly basinward away from the growth anticlines and sit abruptly within offshore mudstones of highstand (HST) and transgressive (TST) systems tracts along the flanks of the growth anticlines.

As fold amplification proceeded, older sandbodies were rotated to dip more steeply, and there is commonly a 2–5° angular difference between successive forced regressive sandbodies. This progressive tilting, coupled with marine erosion during relative sea-level fall has completely removed HST and TST deposits near anticline crets, and led to vertical amalgamation of individual forced regressive sandbodies. The resulting stratal geometries clearly result from the tectonic enhancement of forced regression.
Suprasubduction Zone Origin of the Pozanti-Karsanti Ophiolite (Southern Turkey) Deduced from Whole-rock and Mineral Chemistry of the Gabbroic Cumulates
Authors: Parlak Osman , HÖck Volker , Delaloye Michel
Published: April 2008

The Pozanti-Karsanti Ophiolite Complex is situated in the eastern Tauride Belt and represents a remnant of the Mesozoic Neotethyan Ocean. It consists of three distinct nappes: (1) an ophiolitic mélange; (2) a metamorphic sole; and (3) ophiolitic rocks. The oceanic lithosphere section of the Pozanti-Karsanti Ophiolite comprises mantle tectonites, ultramafic-mafic cumulates, isotropic gabbros, sheeted dykes and basaltic volcanic rocks. These units are cut by isolated microgabbro-diabase dykes at all structural levels. New results are presented on the whole-rock and mineral chemistry of the gabbroic cumulates. Well-layered, low-Ti gabbroic cumulates, showing adcumulate to mesocumulate textures, are represented exclusively by gabbronorites. The mineral chemistry of gabbronorites from the Pozanti-Karsanti Ophiolite indicates that these cumulate rocks have been produced by the low-pressure crystal fractionation of basaltic liquid. Magnesium numbers (Mg-numbers) of clinopyroxene, orthopyroxene and amphibole range from 89 to 73, 80–66 and 80–72, respectively. Plagioclase compositions range from An
94
to An
84
. The coexistence of calcic plagioclase, magnesian clinopyroxene and orthopyroxene indicates that the cumulate gabbronorites from the Pozanti-Karsanti Ophiolite were formed in an arc environment. The covariation of Al
2
O
3
and Mg-numbers of both clinopyroxene and orthopyroxene show features typical of low-pressure igneous intrusions such as the Skaergaard and Tonsina Complexes, but differ from the high-pressure ultramafic cumulates found in the same arc. The cumulate gabbronorites probably represent shallower levels in the arc which were subsequently juxtaposed against deeper level ultramafic cumulates either during accretion or later faulting.
Neogene and Quaternary Volcanism of Southeastern Turkey
Authors: Arger Jan , Mitchell John , Westaway Rob W. C.
Published: April 2008

Potassium-argon dating indicates two episodes of basaltic magmatism in south eastern Turkey at
c.
19–15 and
c.
2.3–0.6 Ma. Each produced olivine-titanaugite basalts, whose chemical compositions are difficult to classify using any conventional model in both the Anatolian continental fragment and the Arabian Platform. It is proposed here that both episodes of volcanism, and the associated crustal thickening and surface uplift, result from heating of the mantle lithosphere by crustal thickening caused by inflow of plastic lower crust from adjoining regions. Thus, although this study region has remained in a plate boundary zone for tens of millions of years, its volcanism has no direct relationship to local plate motions. It is suggested instead that both episodes of volcanism are the result of loading effects caused by glacial to interglacial sea-level variations, which will cause net flow of lower crust from beneath the offshore shelf to beneath the land: the moderate glaciations of Antarctica which began in the Early-Middle Miocene, and the more intense lowland glaciations of the northern hemisphere which began around
c.
2.5 Ma.
Micromorphological analyses of microfabrics and microstructures indicative of deformation processes in glacial sediments
Authors: Menzies John
Published: April 2008

Various forms of sediment deformation can be detected within glacial sediments at the microscopic scale. Analyses of these forms leads to a preliminary classification of microfabrics and microstructures of brittle, ductile and polyphase modes of deformation in glacial sediments. With the development of a taxonomy of different microfabrics and microstructures these processes, once differentiated, permit insights into glacial sediments to a scale and level of detail hitherto unknown. Examples are presented that illustrate some of these different forms of deformation often within a single glacial sediment sample. This research suggests many of the past ideas with regard to details of glacial depositional processes, especially in terms of diamicton deposition and subsequent classifications, need to be re-evaluated.
Fluctuations in the carbonate production of Phanerozoic reefs
Authors: Kiessling Wolfgang , Flügel Erik , Golonka Jan
Published: April 2008

A comprehensive database on Phanerozoic reefs is used to evaluate the carbonate production of reefs through time. Net, gross and export carbonate productions of 2760 Phanerozoic reefs are calculated and the cumulative production for 32 time slices is evaluated. The total amount of carbonate produced in the reef ecosystem in a given time slice is a function of global reef abundance, average reef size and the relative amount of carbonate exported from the reefs. Carbonate production of reefs is usually low, but characterized by prominent peaks in the mid-Silurian Givetian-Frasnian, the Late Triassic, the Late Jurassic, the mid-Cretaceous and the Neogene.

The determinants of reefal carbonate production are correlated with a variety of intrinsic and extrinsic parameters such as palaeogeographic setting, dominant biota, reef type, bioerosion, petrographic composition, eustatic sea level, oceanic crust production rates, atmospheric CO
2
concentrations, and global nutrient level. The calculated carbonate production, however, is rarely correlated with particular Earth system parameters. This implies that either the controls on reefal carbonate production are too complex to allow reliable predictions, or biotic factors represent more important controls than physico-chemical parameters. The constructed curve of Phanerozoic reefal carbonate export production is also poorly correlated with proposed curves of global shallow-water carbonate production suggesting that reefs rarely contributed in a quantitatively significant way to the global carbonate budget.
The Late Palaeozoic relations between Gondwana and Laurussia
Authors: McKerrow W. S. , Mac Niocaill C. , Ahlberg P. E. , Clayton G. , Cleal C. J. , Eagar R. M. C.
Published: April 2008

Reconstructions based on biogeography, palaeomagnetism and facies distributions indicate that, in later Palaeozoic time, there were no wide oceans separating the major continents. During the Silurian and Early Devonian time, many oceans became narrower so that only the less mobile animals and plants remained district. There were several continental collisions: the Tornquist Sea (between Baltica and Avalonia) closed in Late Ordovician time, the Iapetus Ocean (between Laurentia and the newly merged continents of Baltica and Avalonia) closed in Silurian time, and the Rheic Ocean (between Avalonia and Gondwana and the separate parts of the Armorican Terrane Assemblage) closed (at least partially) towards the end of Early Devonian time. Each of these closures was reflected by migrations of non-marine plants and animals as well as by contemporary deformation. New maps, based on palaeomagnetic and faunal data, indicate that Gondwana was close to Laurussia during the Devonian and Carboniferous periods, with fragments of Bohemia and other parts of the Armorican Terrane Assemblage interspersed between. It follows that, after Early Devonian time, the Variscan oceans of central Europe can never have been very wide. The tectonic evolution of Europe during Devonian and Carboniferous time was thus more comparable with the present-day Mediterranean Sea than with the Pacific Ocean.
Review of Nd isotopic data and xenocrystic and detrital zircon ages from the pre-Variscan basement in the eastern Bohemian Massif: speculations on palinspastic reconstructions
Authors: Hegner Ernst , Kröner Alfred
Published: April 2008

Nd crustal residence ages combined with xenocrystic and detrital zircon ages of Neoproterozoic and early Palaeozoic granitoid gneisses and metasediments from the NE Bohemian Massif (West Sudetes and Erzgebirge) suggest an origin from a basement with significant Grenvillian and Svecofennian-Birimian-Amazonian age components. Large contributions from Archaean crust appear to be missing. Nd model ages of 1.4–1.7 Ga as well as Meso- to Palaeoproterozoic zircon xenocrysts ages for the Lugian Domain (Jizerské, Krkonoše, and Orlica-Sniezník Mountains) are interpreted as evidence for a predominantly Palaeoproterozoic basement that underwent rejuvenation during Meso- and Neoproterozoic-Ordovician magmatic events. Our interpretation of the Nd model ages may be relevant for the western part of the Variscan Belt, where a similar range of Nd model ages characterizes large crustal segments. Samples from the Silesian Domain (Desná Dome Staré Město Belt, Velké Vrbno Unit) of the Brunia Microcontinent, in thrust contact with the Lugian Domain, yielded Nd crustal residence ages of 1.1–1.3 Ga, which coincide with the oldest ages of xenocrystic zircons. Brunia is interpreted as a predominantly juvenile Grenvillian-age crustal segment, possibly constituting a former arc batholith outboard from the more ensialic setting of the Lugian Domain. Nd crustal residence ages for our 77 whole-rock samples, including 41 new data, as well as 185 xenocrystic and detrital zircon ages from the northern and northeastern Bohemian Massif agree with those of Neoproterozoic rocks from northern South America and support an origin from a northwest Gondwana palaeogeographical location.
Review of geochemical variation in Lower Palaeozoic metabasites from the NE Bohemian Massif: intracratonic rifting and plume-ridge interaction
Authors: Floyd P. A. , Winchester J. A. , Seston R. , Kryza R. , Crowley Q. G.
Published: April 2008

During early Palaeozoic time the Cadomian basement of the northern margin of Gondwana underwent extensive rifting with the formation of various crustal blocks that eventually became separated by seaways. Attenuation of the continental lithosphere was accompanied by the emplacement of anatectic granites and extensive mafic-dominated bimodal magmatism, often featuring basalts with an ocean crust chemistry. Intrusive metabasites in deep crustal segments (associated with granitic orthogneisses) or extrusive submarine lavas at higher levels (associated with pelagic and carbonate basinal sediments) show a wide range of chemical characteristics dominated by variably enriched tholeiites. Most crustal blocks show the presence of three main chemical groups of metabasites: Low-Titholeiitic metabasalts, Main Series tholeiitic metabasalts and alkalic metabasalt series. They differ in the degree of incompatible element enrichment (depleted to highly enriched normalized patterns), in selected large ion lithophile (LIL) to high field strength element (HFSE) ratios, and abundances of HFSE and their ratios. Both the metatholeiite groups are characterized by a common enrichment of light REE-Th-Nb-Ta. High Th values (or Th/Ta ratios) and associated low ε
Nd
values (especially in the Low-Ti tholeiitic metabasalts) reflect sediment contamination in the mantle source rather than at crustal levels, although this latter feature cannot be ruled out entirely. The range of chemical variation exhibited is a consequence of the melting of (a) a lithospheric source contaminated by a sediment component (which generated the Low-Ti tholeiites), and (b) a high-level asthenospheric mid-ocean ridge basalt (MORB)-type source that mixed with a plume component (which generated the range of enriched Main Series tholeiites and the alkali basalts). It is considered that a plume played an important role in the generation of both early granites and the enriched MORB-type compositions in the metabasites. Its significance for the initial fragmentation of Gondwana is unknown, but its presence may have facilitated deep continental crust melting and the fracturing into small crustal blocks. The early-mid-Jurassic plume-instigated break-up of the southern Gondwana supercontinent is considered to be a possible tectonic and chemical analogue for Early Palaeozoic Sudetic rifting and its magmatic products.
Passive margin detachment during arc-continent collision (Central European Variscides)
Authors: Oncken O. , Plesch A. , Weber J. , Ricken W. , Schrader S.
Published: April 2008

In the Rhenish Massif and Ardennes, the Rheno-Hercynian fold and thrust belt of the Mid-European Variscides exposes a telescoped complete Late Palaeozoic passive margin, which was detached from the lower crust during Carboniferous collision with a continental arc (Mid-German Crystalline High on the leading edge of Armorica). Geometric analysis and isostatically corrected balanced cross-sections show that the basal detachment propagated from the oceanic realm into the passive margin by repeated ductile footwall failure during lithospheric flexure of the weak lower plate (effective elastic lithospheric thickness <4 km) under the load of the advancing upper plate. Early collision was associated with offscraping of the uppermost slope sediments in frontal accretion mode. Subsequent detachment of the remaining passive margin cover was initiated after subduction of the ocean-continent transition to about 6 kbar depth. Rocks and fabrics from the detachment show that, at this pressure, they crossed the brittle-plastic transition at the fossil 300–400 °C isotherm. Newly failed segments of the basal detachment propagated along this isotherm and branched off towards the foreland by progressively downstepping from the passive margin sediments into the basement of the downflexed lower-plate crust. This evolution coincided with cyclic changes in accretion mode of the lower-plate upper crust to the advancing orogenic wedge (repeated changes from basal to frontal accretion) as well as with a related stepwise propagation of a narrow foreland basin. Propagation of the detachment segments and the related imbricate fans in the lower plate, moreover, was controlled by the geometry of the basin structure by localizing branch lines and ramps along earlier growth faults.
Syn-convergent high-temperature metamorphism and magmatism in the Variscides: a discussion of potential heat sources
Authors: Henk A. , von Blanckenburg F. , Finger F. , Schaltegger U. , Zulauf G.
Published: April 2008

A period of pervasive high-temperature metamorphism and igneous activity from 340 to 325 Ma is a well-established characteristic of the Variscan Orogen of Central Europe. During this stage, the internal zone of the orogen was virtually soaked by granitic to granodioritic magmas. Petrological data point to temperatures of 600–850 °C at upper- to mid-crustal levels. These elevated temperatures occurred during the final convergence stage and may be comparable with similar processes inferred from geophysical evidence for the present-day Tibetan Plateau, in both regional extent and significance for the orogen’s evolution. We review various geodynamic scenarios that may have provided the heat for melting and metamorphism, and compare model predictions with field data from the Variscides. All lines of evidence point to a geodynamic scenario that led to thickening of the continental crust with increased internal radiogenic heating, but without simultaneous thickening of the mantle lithosphere. Possible mechanisms include convective removal of the thermal boundary layer, delamination of part of the lithospheric mantle, and subduction of the mantle lithosphere of the downgoing plate. However, with the present stage of knowledge it is virtually impossible to single out one of these three mechanisms, as their geological consequences are so similar.
The Svalbardian-Ellesmerian deformation of the Old Red Sandstone and the pre-Devonian basement in NW Spitsbergen (Svalbard)
Authors: Piepjohn Karsten
Published: April 2008

In NW Spitsbergen, the Late Silurian to Late Devonian infill of an Old Red Sandstone (ORS) basin was affected by west-vergent folding and west-directed thrusting during the Early Carboniferous (Tournaisian) Svalbardian deformation. The brittle, predominantly compressional structures of the Svalbardian Fold-and-Thrust Belt are concentrated along at least five narrow, more or less north-south-trending zones. Three zones are exposed in the Devonian infill of the ORS basin. The involvement of the post-Caledonian ?Late Silurian to Earliest Devonian Viggobreen weathering zone and deposits Early Devonian in two thrust zones within the basement of the western basin margin indicates that the Svalbardian deformation also affected the basement areas along the west coast of NW Spitsbergen. Structures of the Svalbardian Fold-and-Thrust Belt are exposed within an area at least 100 km wide between the Billefjorden Fault Zone in the east and the west coast of NW Spitsbergen. Therefore, the Svalbardian deformation represents a much more important fold belt than previously recognized. On the basis of the timing, the large extent and the orientation of the fold-and-thrust zones, the Svalbardian Fold-and-Thrust Belt appears to represent the eastern continuation of the Ellesmerian Fold Belt in North Greenland and the Canadian Arctic Archipelago.
Groundwater resources and vulnerability in the Cretaceous Chalk of Northern Ireland
Authors: Barnes S.
Published: April 2008

A confined Cretaceous Chalk aquifer underlies approximately one quarter of Northern Ireland, yet little is known about its groundwater resource potential. This issue has been addressed on the catchment scale by analysing spring discharge and hydrochemical fluctuations.

The Chalk springs are recharged by allogenic leakage and surface runoff from overlying Tertiary basalts. Sources connected to river-sinks show greater variation in flow and quality reflecting a much shorter residence time than those predominantly derived from the diffuse recharge. Discharge from the confined region becomes proportionally significant during prolonged dry spells, but is typically a minor component compared with groundwater circulation volumes in the unconfined region.

Spring flood recessions are rapid (recession coefficients up to 0.125 per day) and suggest that the Chalk has a high hydraulic conductivity and a low storage capacity. These characteristics together, with the essentially impermeable matrix, are consistent with an aquifer dominated by a dispersed fracture network.

Conceptual aquifer classification suggests that the outcrop region is a highly sensitive karst aquifer. The subcrop areas can only be exploited via boreholes and are likely to be less productive, although the water quality has been shown to be more stable and less vulnerable to contamination.
K-T boundary spherules from Blake Nose (ODP Leg 171B) as a record of the Chicxulub ejecta deposits
Authors: Martínez-Ruiz F. , Ortega-Huertas M. , Palomo-Delgado I. , Smit J.
Published: April 2008

The Cretaceous-Tertiary (K-T) boundary interval recovered by the ODP Leg 171 at Site 1049 (Blake Nose, NW Atlantic) contains a thick (9–17 cm) spherule bed marking the boundary. The spherules are mainly perfect spheres with a lesser proportion of oval spherules. They usually range from 100 to 1000 μm. This bed represent the diagenetically altered impact ejecta from Chicxulub and further supports this structure as the site of the K-T impact. Mineralogical and geochemical investigations indicate that impact-generated glass was altered to smectite. Transmission electron microscopy observations revealed in some spherules that smectite is forming from a Si-rich or Ca-rich material, which could suggest a precursor similar to Haitian glasses. The variable thickness and the presence of some Cretaceous planktonic foraminifera and clasts of Cretaceous chalk suggest reworking of the ejecta material. However, the spherule bed confirms that a large volume of the Chicxulub ejecta material reached the Blake Nose Plateau.
Pervasive Pan-African reactivation of the Grenvillian crust and large igneous intrusions in central Dronning Maud Land, East Antarctica
Authors: Paech H.-J.
Published: April 2008

The geological history of central Dronning Maud Land (cDML) is described on the basis of age data, recent observations, and published data. The ages of both the protoliths of the metamorphic rocks of cDML and their primary metamorphism are Grenvillian (1200 to 1000 Ma). Following a long hiatus for which geochronological data are lacking, the Grenvillian crust of cDML was intruded by abundant Pan-African partly charnockitic granitoids and anorthosites of the Dronning Maud Land igneous province. These intrusions occurred predominantly in two Pan-African igneous episodes. The early igneous episode (about 600 Ma), in which the Grubergebirge anorthosite intruded, was followed by Pan-African high-grade metamorphism, intense ductile deformation and then medium-grade retrogression during later Pan-African events (570 to 520 Ma). The Grenvillian structures and metamorphic signature were pervasively overprinted or completely eradicated. The Pan-African metamorphism and tectonism were completed prior to the late Pan-African igneous episode (500 Ma), in which predominantly post-kinematic granitoids, as well as a small anorthosite, intruded. Thus, the present tectonic structure of the crust in cDML was formed during Pan-African events by overprinting of Grenvillian basement and intrusion of granitoids/syenites and anorthosites.
Regional till geochemical surveys in the Canadian Cordillera: sample media, methods and anomaly evaluation
Authors: Levson Victor M.
Published: April 2008

Basal tills have become a widely used regional geochemical sampling medium in recent years in the Canadian Cordillera. They reflect the primary composition of the source bedrock and contrast with B-horizon soil that can be developed on a variety of glacial and non-glacial surficial sediment types. Detailed sedimentological data are critical to collect and they are used to differentiate basal tills from other visually similar sediments including englacial and supraglacial tills, colluvial debris flow deposits, and very poorly sorted, glaciofluvial or glaciolacustrine sediments (e.g. diamictons or gravelly muds). The variable transport and depositional processes that form these different sediments make interpretation of geochemical data difficult. Deep (usually >0.75 m) C-horizon sampling of basal till minimizes the complicating effects of pedogenesis, weathering, surface washing and gravity remobilization of the tills. The latter processes, particularly pronounced in the wet, steep terrain, typical of much of the Canadian Cordillera, lead to depleted concentrations of heavy minerals (notably Au) and hydromorphic dispersion of mobile elements in the near surface sediments. Also, elements that are preferentially concentrated in the fine fraction can be selectively removed by surface waters.

Offset sampling lines, oriented perpendicular to the dominant ice-flow direction, are most effective for detecting regional geochemical anomalies which are typically narrow and elongated parallel to ice-flow. Erratics trains and till anomalies are usually a few to several kilometres long and up to one or more kilometres wide. For some metals such as Au, anomalies are generally larger and more readily detected in till than in B-horizon soil. Surface till anomalies reflect up-ice sources and not the immediately underlying bedrock; down-ice displacements of > 500m often occur in areas of thick till. Basal till anomalies usually can be traced to source along linear transport paths reflecting topographically controlled valley-glacier flow in mountainous areas and unidirectional ice-sheet flow in many plateau areas, chiefly representative of the last glacial event. Interpretations of till geochemical data are enhanced with a clear understanding of the surficial and bedrock geology, Quaternary stratigraphy, ice-flow history and down-ice dispersal characteristics around known mineral deposits.
The application of heavy indicator mineralogy in mineral exploration with emphasis on base metal indicators in glaciated metamorphic and plutonic terrains
Authors: Averill Stuart A.
Published: April 2008

Indicator mineralogy is used to explore for a wide variety of mineral commodities. The method utilizes minerals which are sufficiently heavy to be readily concentrated in the laboratory, often colourful and possess other useful physical and chemical properties. The minerals also must be source specific. Some indicator minerals are true resistate minerals. The others, although less resistant, are stable in oxidized glacial drift and many non-glacial sediments. A few, such as gold grains, are silt sized but most are coarse grained. Grain size has a major impact on indicator mineral dispersal patterns in glacial drift.

The coarse-grained indicator minerals are of two main types: (1) kimberlite indicator minerals (KIMs); and (2) metamorphosed or magmatic massive sulphide indicator minerals (MMSIMs). KIMs are enriched in Mg and Cr and most MMSIMs are enriched in Mg, Mn, Al or Cr. These indicator elements cannot be diagnosed geochemically in anomalous heavy mineral concentrates because the concentrates contain other, more plentiful non-indicator minerals containing the same elements in the same chemical form. Chalcopyrite is also a very useful MMSIM but the number of surviving grains in a dispersal train is too low for detection by selective geochemical analysis.

MMSIMs are derived from three main types of base metal deposits and their associated alteration or reaction zones: (1) volcanosedimentary massive sulphides (encompassing volcanogenic, Sedex and Mississippi Valley subtypes) in medium to high grade regional metamorphic terrains; (2) skarn and greisen deposits; and (3) magmatic Ni-Cu sulphides. The variety of MMSIMs associated with Ni-Cu deposits is astonishing, apparently reflecting mineral hybridization related to assimilation of sulphurous sedimentary rocks by ultramafic magmas. Cr-diopside is one of the best indicators of fertile Ni-Cu environments although not necessarily of the actual Ni-Cu deposits.

Heavy indicator mineralogy is much more sensitive than heavy mineral geochemical analysis and offers many exploration benefits in regional exploration programs including: (1) sampling efficiencies; (2) enlargement of both the bedrock target and dispersal train; (3) coverage of a wider range of mineral commodities; (4) undiminished sensitivity in areas of overabundant non-indicator heavy minerals; (5) visual evidence of points of origin of dispersal trains; and (6) indications of the economic potential of the source mineralization. It is most effective as a reconnaissance exploration tool and is particularly well suited for testing gneissic volcanosedimentary and plutonic terranes where base and precious metal deposits are highly modified and difficult to recognize by other prospecting methods.
Lake sediment geochemical methods in the Canadian Shield, Cordillera and Appalachia
Authors: Cook Stephen J. , McConnell John W.
Published: April 2008

Lake sediment geochemistry has been used in Canada since the 1970s for mineral exploration and resource evaluation in glaciated regions of Appalachia, the Canadian Shield and the western Cordillera which are of low to moderate relief. Geochemical signatures of bedrock and mineralization within a lake’s catchment basin are commonly reflected in the chemical constituents of the organic-rich Holocene sediment which has been transported from source by a combination of mechanical and hydromorphic processes. Lake sediment geochemical surveys have been used successfully to discover base metal, Au, Mo, W, Sb, Sn, U, and REE mineralization. The scale of such surveys determines the size and density of lake sediment sampling. In regional surveys, large lithological targets such as greenstone belts or chemically distinctive intrusives can be identified by low density (
c.
1 per 10–15 km
2
) sampling of relatively larger lakes. Smaller targets such as mineralized areas require higher density (
c.
1 per 4–5 km
2
) sampling using a greater number of smaller lakes. Regional surveys are typically helicopter-borne, and employ a tubular grab sampler which permits rapid reconnaissance-scale coverage of large areas. Centre-basin profundal lake sediment, or gyttja, is an ideal sample medium because of its homogeneity and abundance of fine-grained organic matter, which complexes with many trace elements. Multi-element analytical techniques in common use include inductively coupled plasma emission spectrometry (ICP-ES), ICP mass spectrometry (ICP-MS) and instrumental neutron activation analysis (INAA). Reliability of data is assessed by analysis of known standards, site duplicates and sample splits. PC statistical software packages facilitate the interpretation of geochemical data, and desktop GIS packages aid in further interpretation and generation of multi-layer maps.
Regional and local-scale gold grain and till geochemical signatures of lode Au deposits in the western Abitibi Greenstone Belt, central Canada
Authors: McClenaghan M. Beth
Published: April 2008

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.