bluuebunny/crossref_metadata_2025 huggingface dataset:
Deep-sea pelagic sediments and palaeo-oceanography: a review of recent progress
Authors: Leggett J. K.
Published: April 2008

Understanding of present and past patterns of sedimentation, circulation, fertility, biogeography and chemistry in the oceans has advanced greatly in the last decade. In achieving this a diverse range of geological and geophysical techniques has been drawn on. Egress zones of active hydrothermal systems venting sulphide-laden hot water at spreading centres, long predicted on theoretical and other grounds, were discovered using deep-towed geophysical packages and have been fruitfully studied using submersibles. The chemistry of the venting solutions, and the apparent scale of the hydrothermal processes, have engendered a rethink of the oceanic fluxes of many elements. Continuing deep-sea drilling increasingly refines reconstructions of continental-drift events and the consequent changes in circulation patterns. The Deep-Sea Drilling Project planning has become more problem-specific, and on ‘palaeo-environment’ legs, sediment successions have been cored more continuously than was earlier the case. As a result, palaeo-oceanographers have been provided with a flood of data germane to the reconstruction of past behaviour of the CCD, to appreciation of past states of oxygenation of the oceans and of carbon preservation patterns (the ‘black-shale problem’), to documentation and interpretation of hiatus patterns, to calculation of global sediment budgets through time, and to calibration of the overall cooling of the Cenozoic hydrosphere. Isotopic determinations and organic geochemistry have become key tools in these studies. Global sea-level stands, clearly important as a regulator of conditions in the pelagic realm (though to an arguable extent) have been documented using seismic stratigraphy.

Attempts at synthesis of the complex spectrum of palaeo-oceanographic changes evident through time in the pelagic relam stress the rhythmicity with which the oceans have oscillated between broadly uniformitarian and non-uniformitarian conditions. Syntheses build towards identification of driving forces and responses.
Deformation processes in accretionary wedges—examples from the SW margin of the Southern Uplands, Scotland
Authors: Knipe R. J. , Needham D. T.
Published: April 2008

The deformation which accompanies the evolution of accretionary complexes at active margins is discussed using examples from the Southern Uplands of Scotland. The area was part of a northward-dipping subduction complex during Ordovician and Silurian times before the final closure of the Iapetus Ocean in the Early Devonian. The structures present in the SW margin of the Southern Uplands may be assigned to four different processes typical of accretionary-prism evolution. These are (a) downslope gravity-driven movement, (b) accretion-related folding and thrusting, (c) late accretion or collision processes, and (d) post-collision adjustments. The problems of recognizing and separating these processes in the Southern Uplands are discussed as well as the similarities which occur between overthrusting processes in foreland thrust-and-fold belts and the underthrusting processes taking place in accretionary complexes. In addition an attempt is made to assess the effects of the changes in deformation behaviour occurring during lithification which accompanies accretion. A model is presented here which relates the evolution of folds and thrusts to the changing material properties of sediments during accretion, and emphasizes the importance of the deformation behaviour of the hanging wall to accreting packages in these processes.
Geochemical characteristics of collision-zone magmatism
Authors: Harris Nigel B. W. , Pearce Julian A. , Tindle Andrew G.
Published: April 2008

This paper reports the results of a systematic geochemical study of intermediate and acid intrusive rocks from a number of continent-continent collision zones of Phanerozoic age. Four groups of intrusions can be recognized, each associated with a particular stage in the tectonic evolution of a collision zone.

Pre-collision calc-alkaline (volcanic-arc) intrusions
which are mostly derived from mantle
modified by a subduction component and which are characterized by selective enrichments in
LIL elements.

Syn-collision peraluminous intrusions (leucogranites)
which may be derived from the
hydrated bases of continental thrust sheets and which are characterized by high Rb/Zr and
Ta/Nb and low K/Rb ratios.

Late or post-collision calc-alkaline intrusions
which may be derived from a mantle source
but undergo extensive crustal contamination and can only be distinguished from volcanic-arc
intrusions by their higher ratios of Ta/Hf and Ta/Zr.

Post-collision alkaline intrusions
which may be derived from mantle lithosphere beneath
the collision zones and which carry high concentrations of both LIL and HFS elements.

The geochemical evolution of crustal melts within groups (ii) and (iii) can be viewed in terms of the dehydration reactions, volatile transfer and transient geothermal gradients that result from thrust tectonics in the zone of collision.
Geochemical and tectonic evolution of the Damara Belt, Namibia
Authors: Hawkesworth C. J. , Menzies M. A. , van Calsteren P.
Published: April 2008

The Damara is a high-grade, apparently intracratonic, Pan-African Orogenic Belt which has been the subject of unusually detailed geochemical and petrological studies. Basement inliers (2.0–1.0 Ga) underlie predominantly continental sediments, deposited in the period 1.0–0.7 Ga, and these are overlain unconformably by molasse-type deposits at least 580 Ma old. Basic volcanic rocks are rare, but the centre of the Damara Belt is characterized by large volumes of syn- and post-tectonic granitic rocks intruded between 750 and 450 Ma. Many were derived from the pre-existing crust as indicated by their initial
87
Sr/
86
Sr = 0.705–0.76, ɛ
Nd
= 0 to −17, δ
18
O = +7.1 to +15.2‰. Moreover, combined Nd- and Sr-isotope studies on sediments, granites and basement rocks suggest that at 500 Ma, in the centre of the belt, 2.0 Ga basement was sandwiched between rocks with younger model ages above and below. Metamorphic temperatures and pressures reached ∼620°C and 7–8 kb in the S where the Rb/Sr biotite ages are 500–480 Ma and ∼660°C and 3–4 kb in the centre of the belt where the biotite ages are 460–440 Ma. Prograde metamorphism (and partial melting) in the central part of the belt continued after the closure of the biotite ages along the margins and since the former is a zone of domal uplift structures, the crustal melting and diachronous metamorphism are attributed to differential uplift in an area of thickened continental crust. There is good evidence that the evolution of the Damara was intimately linked with that of the N-S trending belt now exposed in South America, although in detail the relationship between metamorphism and the inferred tectonic movements is still poorly understood. Nonetheless, since preliminary data indicate that the N-S belt may have formed along a destructive plate margin, it is speculated that for at least some of its history, the Damara evolved in an extensional intracontinental basin behind a zone of active subduction.
Collision and Cordilleran orogenesis: an Andean perspective
Authors: Dalziel I. W. D.
Published: April 2008

The Andean Cordillera is the result of interaction between oceanic lithosphere, forming the floor of the Pacific Ocean Basin, and S American lithosphere during the Mesozoic and Cenozoic. Break up of Gondwanaland was accompanied by widespread extensional tectonics in its S American segment during the Early to Mid-Mesozoic. The trend, dimensions and nature of the resulting basins were to a considerable extent controlled by the structure of the pre-Jurassic basement. Elongate basins formed along virtually the length (
c
. 7,500 km) of the Pacific margin from Northern Peru (5°S) to the Northern Scotia Ridge (56°S). These basins developed within, or immediately behind, a magmatic arc.

Mid-Cretaceous uplift and compression, contemporaneous with the global increase in seafloor spreading rates, took place along the length of this Pacific margin composite back-arc trough. Horizontal shortening accompanied the uplift, but only S of lat. 50°S did extensive penetrative fabrics develop in the basin called the Magellan Geosyncline by Aubouin (1965) because of its partially ophiolitic floor (the ‘Rocas Verdes’) and infilling of deep-marine volcaniclastic turbidites. This Mid-Cretaceous event may have been critical in initiating the process of Cordilleran orogenesis. Subsequently part or all of the Late Cretaceous batholiths were intruded into the uplifted back-arc trough as the locus of magmatic activity moved eastwards to its present location and dominantly E-verging fold-and-thrust belts developed along the eastern flank of the Cordillera.

Thus in the development of the present Andean Cordillera, S of 5°S, ‘collision’ was confined to the re-welding of a zone of failure between an ensialic magmatic arc and its parent continent. For the most part this involved little horizontal displacement and reflects, therefore, what might be described as ‘accordian tectonics’, a curtailed version of the Wilson cycle. Only in the very far S, in Tierra del Fuego and S Georgia, does crust appear to have been consumed in the arc-continent ‘collision’.
Arenig and Llanvirn graptolite biostratigraphy, Canadian Cordillera
Authors: Lenz A. C. , Jackson D. E.
Published: April 2008

Lower Ordovician graptolites of the Canadian Cordillera (southeastern British Columbia to northern Yukon) are strongly allied to those of the Victorian sequences of Australia and the nearly identical succession of New Zealand, consequently the Stage classification of Victoria is adopted with only minor modifications. Biostratigraphic zones recognized herein, and their Victorian correlatives, are
Tetragraptus approximatus
(upper Lancefieldian),
T. fruticosus
(Bendigonian),
Didymograptus protobifidus
(Chewtonian),
Isograptus victoriae lunatus
(very poorly known) and
I. v. maximodivergens
(lower and upper Castlemanian, respectively),
Oncograptus
possibly with lower and upper divisions (Yapeenian),
Paraglossograptus tentaculatus
divisible into a lower unit equivalent to Dariwillian 1 and 2, and an upper unit correlative with about Dariwillian 3 and characterized by the appearance of
Sinograptus
, and
Diplograptus decoratus
Dariwillian 4 and higher).

Zone by zone diversity studies of Pacific Faunal Province graptolites indicate maximum species’ diversity in the
fruticosus
and
tentaculatus
zones and lowest diversity about the
lunatus
Zone.
Biostratigraphy of the Ordovician
Nemagraptus gracilis
Zone
Authors: Finney Stanley C. , Bergström Stig M.
Published: April 2008

The middle Ordovician
Nemagraptus gracilis
Zone has long been used as a prominent, reliable correlation datum, having been recognized on every continent except Africa and Antarctica. However, there is some disagreement today on its definition and its boundaries. A re-examination of the original definition of the zone and evaluation of pertinent graptolite and conodont biostratigraphical data from key sections in Scotland, Wales, Scania, New York, Texas, Alabama and Australia indicate that the first appearance of
N. gracilis
is synchronous worldwide and can thus be used to define the base of the zone. Additional studies, however, are necessary before an internationally recognized upper boundary of the zone can be defined.

The Berrybush Burn section in Scotland is proposed as a general reference section of the
N. gracilis
Zone graptolite assemblage, with the Fågelsång and Albany sections in Scania and New York, respectively, as secondary reference sections. The Fågelsång, Sweden and Calera, Alabama sections are proposed as local reference sections for the lower boundary of the zone in, respectively, Europe and North America.
Cluster analysis of isograptid individuals from Bullengarook, Victoria, Australia
Authors: Schleiger N. W.
Published: April 2008

Bullengarook Slate Quarry is a locality high in the Ca3 Zone and less than 100 m below the Castlemaine-Yapeenian boundary. The diversity of isograptids at the quarry is high with juvenile
Pseudisograptus
occasionally occurring with the
Isograptus
species.
The Ordovician graptolite-bearing strata of Xiushui drainage basin, Jiangxi, southern China and their correlation
Authors: Yu Jianhua , Fang Yiting
Published: April 2008

The Xiushui drainage basin of Jiangxi is situated on the northern margin of the Jiangnan biogeographical province and adjoins the southern Yangtze province. The Ordovician System in this area is well developed, with a clear base and top, and is rich in fossils. It is one of the best Ordovician sections in China. The stratigraphy, faunal characteristics and correlation of the Ordovician graptolite-bearing strata have been studied. The lower and upper boundaries of the Ordovician System are also discussed.
Graptolite zones of upper Ordovician to middle Silurian age in a continuous section at Ziyang, Shaanxi, China
Authors: Fu Li-pu
Published: April 2008

Silurian strata are well developed in Qinling, China where there are many continuous sections (Fu 1980; Lin 1979; Mu 1980). The Bajiaokou section in Ziyang, Shaanxi is described, and is one in which all the international Llandovery graptolitic zones occur. It may be divided into more than twenty graptolite zones from the
Amplexograptus disjunctus yangtzensis
Zone to the
Cyrtograptus centrifugus
Zone.
Graptolite zones in the Ordovician and Silurian of the Gorny Altai
Authors: Obut A.M. , Sennikov N.V.
Published: April 2008

The Ordovician and Silurian graptolite zonal scale, based on studies of the stratigraphic distribution of graptolite assemblages in Great Britain, is compared with the problems of using data in other regions where often no stratotypes for zonal subdivisions are presently defined. Assuming such local stratotypes in future we can note that the total graptolite scale, being a sum of sections with biostratigraphic graptolite zones distinguished therein, serves as an instrument for exact determination of geological time; and perhaps this scale does not need the distinction of stratotypes of zonal subdivision having as it does in each case exclusively chronostratigraphic significance.

Zonal graptolite successions distinguished in different regions, and zonal subdivisions of the total graptolite reference scale, often have index species of the same name, and the same chronostratigraphic extent of zonal subdivisions. As a rule, stratotypes for provincial, regional and local graptolite zones are not distinguished, but some investigators offer characteristics of so-called standard sections where the whole successive set of zones with rich graptolite assemblages is given.

According to a recommendation of The Stratigraphic Code of the U.S.S.R. (Stratigraphicheskii Kodeks S.S.S.R., 1977) stratotypes of regional biostratigraphic zones ought to be distinguished. In those cases where the world set of stratotypes is recognized in the same section of graptolite zones for the territory; a local reference section. The standard section necessary both for intra-regional correlations and for inter-regional ones can include either the full set of graptolite zones of a system, series, stages or only a limited number of zones. The Ordovician and Silurian of the Gorny Altai is used as an example of the above principles.
Families of Retiolitida and their phylogenetic relations
Authors: Obut A. M. , Zaslavskaya N. M.
Published: April 2008

Isolated material of retiolitid graptolites from boreholes in the Russian and Siberian platforms is described and has enabled a reassessment of the classification and phylogeny of this large group of graptolites to be made. The Order Retiolitida (Mikhaylova 1970) is considered to consist of the following families: Archiretiolitidae (Bulman 1955); Pseudoretiolitidae (Obut & Zasalavskaya 1974); Retiolitidae (Lapworth 1873); and Plectograptidae (Bouček & Münch 1952). The new subfamilies Pseudoplegmatograptinae (Retiolitidae) and Gothograptinae (Plectograptidae) are proposed and the taxa
Agastograptus robustus
gen. and sp. nov. and
Pseudoplectograptus
gen. nov., both assigned to the Plectograptinae, are erected.

All material used is deposited in the Institute of Geology and Geophysics of the Siberian Branch of the Academy of Sciences of the U.S.S.R., Novosibirsk.
Mode of secretion of graptolite periderm, in normal and retiolite graptolites
Authors: Bates D. E. B. , Kirk N. H.
Published: April 2008

The construction of graptolite periderm is discussed with particular reference to the rival extra-thecal tissue (secretion) and pterobranch (mortaring) models, and it is concluded that only the former model fits the observed structures. Evidence from a number of retiolite graptoloids demonstrates that they also have a continuous periderm, and were constructed in a similar manner.
Assemblages of Silurian graptolites in various facies of the East Baltic region
Authors: Paškevičius J.
Published: April 2008

All the facies beginning with black clay to carbonate mud in a complete and continuous geological section with graptolites (from the
cyphus
Zone to the
lochkovensis
Zone) can be distinguished in the Lithuanian Silurian. The assemblages of graptolites were determined, and an attempt was made to analyse dependence of the graptolites on various facies. During this work it was ascertained that the most numerous general species and individuals in the assemblages are found at the junctions of clayey and carbonate facies. These assemblages must have been developed on the continental slope of the Silurian sea. The number of species in general slightly decreases in the direction of the past open sea (black-clay facies), to say nothing of the shelf zone (the carbonate facies), where graptolites are especially scarce. Here one can trace a certain analogy between the distribution of zooplankton of the present-day seas and that of the graptolites, which composed the macrozooplankton of the seas of the geological past.

Thus the graptolites being a pelagic group of the zooplankton of the past, lived, developed and were widespread in the deeper part (probably in mezopelagic) of the open sea, mostly beginning at the continental slope.
Modern pterobranchs: observations on their behaviour and tube building
Authors: Dilly P. N.
Published: April 2008

There is a considerable difference in the ultrastructure of fusellar tissue between fossil graptolites and modern pterobranchs. The phylogenetic significance of these differences is uncertain. It was thought that the ultrastructure of the earliest, and therefore probably the simplest, stage of formation of the pterobranch prosiculum might provide additional relevant clues. The larva escapes from the original dome-shaped seal by producing a hole, and immediately produces a series of fuselli around the aperture. The zooid uses the cephalic shield to construct the tube. The size of the original fuselli is very variable, but soon achieves adult size. The fuselli are very similar to those of adult erect tubes, and have oblique sutures. There is much extraneous material incorporated into the wall. It is proposed that the fibrils of the stroma of the coenecium somehow mature after they have been secreted. Histochemical tests suggest that the stroma contains a considerable amount of acid and sulphated mucopolysaccharides. Tests for chitin and keratin have been uniformly negative. Similar positive histochemical reactions have been achieved on the cephalic shield. Adult tube building and repair are reported for the first time.
Late Weichselian transgression, erosion and sedimentation at Gullfaks, northern North Sea
Authors: Carlsen Randi , Løken Tor , Roaldset Elen
Published: April 2008

A prograding sequence of near shore sediments is identified at the border of the northern North Sea plateau (water depth 135 m) and the western slope of the Norwegian Trench. In Late Weichselian time (probably about 15 000 years
bp
) the relative sea level was approximately 180 m lower than today. During this low sea-level stand, the older consolidated glaciomarine sediments in the slope area were exposed to coastal erosion, leading to a prograding unit of sand fining outwards to silt and clay. During rising sea-level the coastal erosion gradually reached a lodgement till deposited at the top of the plateau, resulting in coarser sediments such as stones and gravel, fining outwards to sand down the slope. The plateau area was transgressed shortly after 13 000 years
bp
.
North Atlantic sea-surface temperatures for the last 1.1 million years
Authors: Ruddiman W. F. , Shackleton N. J. , McIntyre A.
Published: April 2008

A 1.2 Ma time series of north-east Atlantic sea-surface temperature (SST) has been assembled from the spliced record of piston core K 708-7 (0.68-0 Ma) and hydraulic piston cores taken in nearby DSDP Hole 552A (1.2–0.68 Ma). A no-analogue fauna precludes making credible SST estimates in the interval 1.2–1.1 Ma, but the record above 1.1 Ma is suitable for SST estimates and related time-series analysis.

The amplitude of SST variation is considerably higher in the Brunhes than in the upper Matuyama. This is due both to colder glacial SST minima after 0.85 Ma, and to increasingly warm interglacial SST maxima from 0.7 to 0.4 Ma. The dominant periodicity in the SST signal is centred near 95 000 years; it increases in amplitude by a factor of four from the bottom of the record to the top, with the largest increase occurring between 0.7 and 0.4 Ma. This suggests a relatively gradual evolution of ‘100 000-year’ power over half a million years of the late Quaternary, rather than a single abrupt change at 0.9 Ma.

Significant, but progressively smaller, spectral peaks occur at periods of 54 000, 41 000, 31 000, 23 000 and 19 000 years. Both the 54 000-year and 31 000-year signals may be a response to insolation forcing by minor obliquity terms. Long-term δ
18
O records show small responses by the ice sheets at or near these two periods, indicating the probable initial response of the climate system to these insolation rhythms. In-phase relationships between δ
18
O and SST at these periods suggest that the ice sheets then impose these rhythms on the ocean with no lag. The mechanism by which such small insolation and ice-volume signals become enhanced in the SST record is not clear.
Meltwater influences and palaeocirculation changes in the North Atlantic during the last glacial termination
Authors: Williams Douglas F. , Fillon Richard N.
Published: April 2008

Major meltwater plumes during the last glacial termination 13 000 years
bp
are recorded in the δ
18
O anomalies of planktonic foraminifera from North Atlantic and Gulf of Mexico marine sediments. These distinct meltwater plumes represent the discharge of isotopically light freshwater from specific portions of the Northern Hemisphere ice sheets. Plume geometries show that northward incursions of warm, normal saline waters occurred during the last glacial termination, thereby delivering latent and sensible heat to high latitudes of the North Atlantic and enhancing the rate of ice sheet melting.
Gas Prospects in the Variscan Thrust Province of Southern England
Authors: Taylor J. C. M.
Published: April 2008

Recently published seismic and other investigations into the Late Carboniferous — Early Permian thrust and fold belts of the Appalachians, the Franco-German border, and to a limited degree so far of southern England, suggest that Variscan deformation is characterised by thin-skinned tectonics. This provides a rationale for recognising flat-lying thrust slices and mapping them in the subsurface. It also implies that the over-ridden rocks may not have been subjected to the excessive burial or the degree of disturbance once assumed. In the Variscan province of southern England the existence of potential source and reservoir rocks can be expected on regional grounds. Consideration of the likely burial and thermal history suggests there may be areas where gas if not liquid hydrocarbons from Palaeozoic source rocks could be preserved. Further geochemical, geophysical and petrophysical investigation seems warranted.
Thermal gradients in the continental crust
Authors: Chapman D. S.
Published: April 2008

A set of preferred geotherms for the continental crust is calculated using assumptions of steady state conductive heat transfer. The calculations use observed heat flow as the principal constraint, temperature and pressure dependent thermal conductivity functions, and a radiogenic heat generation profile that is consistent with a generalized petrological model of the lithosphere. Properties of these geotherms include: (1) a nearly constant gradient through the upper crust because the decrease in thermal conductivity at higher temperatures for felsic rocks counteracts the decreasing heat flow caused by crustal radioactivity; (2) divergence of the geotherms amounting to temperature differences of more than 500 K between rifts and shields at Moho depths, and (3) convergence of geotherms below 250 km in the asthenosphere as convective heat transfer dominates.

Emphasis is also placed on understanding the sensitivity of calculated temperatures to measureable properties. Computed geotherms are overall most sensitive to surface heat flow or reduced heat flow, and uncertainties in this quantity will ultimately limit the accuracy of temperature estimations for the deeper crust and lithosphere. Otherwise, the sensitivity to parameters such as heat production, thermal conductivity and its temperature dependence throughout the lithosphere and crustal thickness depends on depth and whether the region is characterized by high or low heat flow. Upper mantle heat production, pressure coefficients of thermal conductivity, and the position of the upper/lower crust boundary are relatively insensitive parameters for lower crustal temperature calculations.

Dynamic events accompanying crust forming processes lead to significant perturbations of the steady state static temperature field described above. Pressure-temperature pairs from granulite terrains plot near high temperature limits for those steady state geotherms and imply either a transient P-T-t path or a magmatic heat input from the mantle accompanying crustal thickening.
Geochronology and related isotope geochemistry of high-grade metamorphic rocks from the lower continental crust
Authors: Moorbath S. , Taylor P.N.
Published: April 2008

We discuss the distribution of Rb, Sr, Sm, Nd, U, Th and Pb in medium-to-high pressure granulites, which are considered representative of the material of the lower crust. The depletion of large-ion-lithophile, heat-producing elements in such granulites is emphasized, together with its consequencs for radiogenic isotope evolution. Geochronological studies of granulites show that sometimes high-grade metamorphism closely follows original crust formation, whilst in other cases a long time interval separates these major events. Isotopic criteria for the recognition of lower crustal involvement in magma genesis are also discussed.
Sedimentary carbonates as isotopic marker horizons at Lake Turkana, Kenya
Authors: Abell P. I. , McClory J. P.
Published: April 2008

The Plio-Pleistocene sediments of the Koobi Fora region of Lake Turkana contain a variety of carbonate horizons: molluscs, stromatolites, indurated calcitic sediments and concretions. The stable oxygen isotope ratios of these carbonates are largely unchanged since deposition, and constitute a record of environmental conditions at the time of deposition. Two factors influence these isotope ratios: (1) temperature, which produces more negative δ
18
O values with increasing temperature; and (2) evaporative concentration of the heavier oxygen isotope in the rainfall and body of the lake, which makes for more positive values of δ
18
O under arid conditions. The major change in climate which took place at about the time of deposition of the KBS Tuff is isotopically plain in most sections, but stratigraphic control is not sufficient to correlate other significant changes. Nevertheless, the general trends in environmental change during the deposition of any particular section of sediments can be clearly perceived provided there are enough carbonate horizons. Gastropods provide the least equivocal evidence for environmental conditions, and a survey of the entire continent of Africa for stable isotope ratios in contemporary gastropods has been conducted to provide a database for comparison of environmental conditions in the fossil gastropods.
Sedimentation by termites in the Malawi Rift Valley
Authors: Crossley R.
Published: April 2008

Homogeneous red clayey sand sheets, averaging 5 m thick, cover a discontinuous area totalling 8800 km
2
in the Karonga section of the Malawi Rift Valley floor. The sheets accumulated in a terrestrial environment, sedimentation was slow and gradual, and fossil termite mounds occur at the base of the sheet in one area. Topographic considerations rule out deposition by overland flow, and texture is incompatible with aeolian sedimentation. Modern termites (
Macrotermes falciger
) are transporting sediment from beneath the sand sheets onto the ground surface. The textures of fossil and modern
Macrotermes
mound deposits are very similar to the textures of the sand sheets. It is concluded that the sand sheets have accumulated through vertical transfer of sediment by
Macrotermes
. Scrutiny of the literature suggests that similar deposits may be widespread in African savanna and rainforest areas, both within the African Rift System and in epeirogenic basins. Equivalent sediments must occur in ancient tropical sequences since fossil termites are first recorded in the mid-Cretaceous.
The role of hydrocarbon source rocks in petroleum exploration
Authors: Brooks J. , Cornford C. , Archer R.
Published: April 2008

Petroleum is generated from organic-rich sediments (source rocks) containing organic matter originating from biological materials. During burial of sediments, the increase in temperature results in a series of geochemical reactions which leads from biopolymers to geopolymers, often collectively called kerogen, which are precursors of petroleum.

The amount, type and composition of petroleum generated is dependent upon the nature and geological history of the source rock. The most important parameters are the nature of the organic matter in the source rock and its maturity governed by its time/temperature history.

Petroleum exploration is now using modern understandings of hydrocarbon generation, migration and accumulation to help improve its success in predicting petroleum discoveries. The initial stages of an exploration programme can regionally evaluate the source rocks, together with the hydrocarbon potential of each source rock horizon and integrate this information with the geological development of the basin to enhance predictions of where and when hydrocarbon generation and accumulation have occurred.
The formation of organic-rich deposits in two deep-water marine environments
Authors: Morris R. J.
Published: April 2008

Organic-rich sediments have been collected from deep-water core sites in the Eastern Mediterranean (6500–9000 years BP) and the Guinea Basin (50,000–250,000 years BP estimated). The results of geochemical analyses suggest that similar processes may have been involved in their formation. The same processes are known to give rise to present day organic-rich oozes on the continental shelves off Peru and Namibia. The deep-water organic deposits are believed to have been a direct result of periods of greatly increased productivity in the overlying water column. Their formation is not thought to have involved the development of widespread stagnant conditions in the associated body of water, conditions which are often taken to be essential for the preservation of organic matter in deep water.
The magmatic evolution of the Scourian complex at Gruinard Bay
Authors: Rollinson H. R. , Fowler M. B.
Published: April 2008

Scourian (Archaean) gneisses at Gruinard Bay form a bimodal suite of trondhjemites and amphibolites in which the trondhjemites form about 75% of the complex. The gneisses form a large-scale agmatite complex in which amphibolite, ultramafic rocks and tonalitic gneiss are enclosed in trondhjemitic and associated granitic gneiss. All lithologies have been metamorphosed to hornblende-granulite facies and subsequently retrogressed. Amphibolites are the earliest recognizable component of the agmatite complex and have a varied geochemistry, which in part reflects the heterogeneous nature of their source. It is postulated that they are fragments of Archaean ocean floor. As such they are potential source rocks for the quartzofeldspathic gneisses, and calculations show that the tonalite and trondhjemite can be derived by 30% and 20–30% partial melting respectively, of a basaltic source, similar in chemistry to the amphibolites, leaving a residue of clinopyroxene, garnet and hornblende. Granitic gneisses associated with the trondhjemite require a slightly lower degree of partial melting (
c.
10%) and an eclogitic residue. Comparisons with experimentally determined phase equilibria indicate that the partial melting took place in the pressure range 10–20 kb, significantly shallower than the depth of partial melting in modern subduction zones.
The geochemistry of Lewisian marbles
Authors: Rock N. M. S. , Davis A. E. , Hutchison D. , Joseph M. , Smith T. K.
Published: April 2008

To rectify the dearth of published petrographical and analytical data for Lewisian marbles, 307 samples, from nearly all outcrops known, have been examined in thin section, and 114 samples analysed for 35 elements. Lewisian marbles can be divided texturally into: 1)
macrocrystalline
(generally white, pure to silicate-rich)
eg
from Glenelg; 2)
microcrystalline
(often coloured, ornamental, pure to silicate-rich),
eg
from Iona; 3)
microbrecciated
(grey, crushed, fine-grained, silicate-opaque-rich),
eg
from the Loch Maree Group (LMG). Lewisian marbles are geochemically typical of ancient carbonates, tending to have lower contents of trace elements (notably Sr) than younger (
eg
Scottish Dalradian) marbles. Whereas all analyzed marbles from the South Harris and Coll foreland outcrops and from the Glenelg and Kintail inliers are
dolostones
(%MgO 15–20, %SiO
2
5–15), those of the eastern inliers (Glen Strathfarrar, Shinness) are pure
limestones
(%MgO < 10, %SiO
2
< 10). Elsewhere, limestones and dolostones are intimately associated or interbanded (
eg
Scardroy inlier). Microbrecciated LMG marbles (Flowerdale belt of Gairloch; Furnace belt of Loch Maree) have notably higher chalcophile and siderophile element contents than macrocrystalline marbles in adjacent belts (Cloiche of Gairloch; Gleann Tulacha of Loch Maree). Aside from such local differences, however, limestones and dolostones of the inliers, LMG and remaining foreland reveal few non-trivial differences in composition. Moreover, Lewisian marbles collectively show strong Pearson and Spearman correlations between elements concentrated in accessory phases (
eg
Y-La-Ce, K-Rb-Al, Fe-Co-V). Given established secular variations in the chemistry of global carbonate rocks, these relationships between both marble groups and chemical variables are difficult to explain if some Lewisian marbles (
eg
S. Harris) represent late Scourian, and others (
eg
LMG) early Proterozoic sediments. They may provide a tentative hint that most Lewisian marbles were deposited contemporaneously.
The role of mid-crustal shear zones in the Early Proterozoic evolution of the Lewisian
Authors: Coward M. P. , Park R. G.
Published: April 2008

The major shear zones in the Lewisian complex are either steep NW-SE zones, with a generally dextral strike-slip component, or were subhorizontal before subsequent deformation. They appear to share the same NW-SE movement direction. The Outer Hebrides lie mostly within a major mid-crustal ‘flat’ which on the mainland, at a higher structural level, is seen only north of Loch Laxford and south of Loch Torridon. Inclined NW-SE-striking shear zones at Diabaig, Carnmore and Loch Laxford are interpreted as ramps by which this zone descends below the central region.

Movements on these major zones record a long period of probably intermittent activity from
c.
2600 Ma to
c.
1400 Ma. Early movements (Inverian) were widespread on the mainland, and indicate a generally overthrust regime with a small dextral component. A major change in kinematic regime occurred during dyke emplacement and in the early Laxfordian (D
1
–D
2
) where relative movements appear to have been dominantly strike-slip and extensional (transtensional). In the later Laxfordian (D
3
) major upright folds and steep dextral shear zones indicate a return to a dextral transpressional regime.

This sequence, together with evidence from Greenland, can be interpreted in the context of a reconstructed N. Atlantic, as the result of relative movements between two Archaean ‘plates’ to N and S of the combined Nagssugtoqidian-Lewisian belt.

The Inverian-Nag I structure indicates a dominantly N-S convergence. A change to a NW-SE convergence direction in the Early Laxfordian explains a dextral strike-slip regime in the Lewisian and convergence in W Greenland. A further change to NNW-SSE convergence in the later Laxfordian would explain the D
3
transpressional regime in Scotland.
Sedimentary models for extensional tilt-block/half-graben basins
Authors: Leeder M. R. , Gawthorpe R. L.
Published: April 2008

Extensional tectonism produces characteristic half-graben/tilt-block systems whose facies mosaics are influenced by tectonically induced slopes resulting from hanging wall downtilting and footwall uplift. The characteristic asymmetrical subsidence vectors that therefore develop across the graben also exert a fundamental control upon facies distributions. A number of predictive tectono-sedimentary facies models are presented in which these various influences are explored. Alluvial fans and cones react to tilting by becoming segmented, those in the hanging wall showing down-dip hanging wall off-lap and those sourced in the footwall showing progradation from the apex. Lake and coastal waters react instantly to tilting, causing transgression and seiche-induced erosion. Axial through-flowing river channels and delta lobes tend to migrate or avulse towards the axis of maximum subsidence but may be constrained by the toes of footwall-sourced fans. Peat accumulation or soil development are accentuated up the hanging wall dip slope away from the locus of maximum deposition. In coastal areas, fan deltas sourced in the footwall pass offshore into small submarine fans whilst axial fans issue from delta fronts where individual fan lobes may migrate under fault control. In carbonate provinces the footwall scarp may become a bypass margin whilst the hanging wall dip slope may undergo a ramp-to-rimmed shelf evolution with time.
Crustal extension in the Basin and Range Province, southwestern United States
Authors: Hamilton W.
Published: April 2008

Cenozoic extension of areally varying ages and amounts has on average doubled the width of the Basin and Range Province. Extensional structures that formed at all depths down to 20 km, and which range in age from Oligocene to Holocene, are widely exposed and are here interpreted in terms of a model of depth-varying deformation. The middle crust is extended by discontinuous ductile shear as internally underformed lenses slide apart along gently dipping zones of mylonite. The tops of these lenses are undulating detachment faults, the composite area of which increases with time as deep lenses slide out from underneath shallower ones. Brittle blocks of upper-crust bedrock above the detachments respond first by rotating between range-front faults, the same direction of rotation being maintained across a series of lenses, and then by pulling completely apart, while basinal strata fill the gaps and are dragged directly on detachment faults. Some faults rise gently from the main detachment zones and surface as range-front faults. Most tilted-block ranges are isolated atop detachments.

Detachment faults cut out crust. Beneath them are mid-crustal rocks of any age and type and above them are mostly upper-crustal rocks, including extensive syndeformational basin sediments rotated to steep or moderate dips. As attenuation proceeds and components rise, detachment faults evolve from ductile to brittle, develop splays, and are themselves broken by steep brittle structures related to new, deeper detachments. Parts of detachment faults remain active even after exposure at the surface, but slip on them is then limited to the down-dip direction. It is inferred from seismic reflection profiles and rock-mechanic considerations that the unexposed lower crust is extended by more pervasive ductile flattening.
Rupture characteristics of normal faults: an example from the Wasatch fault zone, Utah
Authors: Bruhn Ronald L. , Gibler Pamela R. , Parry William T.
Published: April 2008

The Salt Lake fault segment is one of several independent rupture segments in the Wasatch normal fault zone, Utah. The segment is about 35 km long and consists of several, approximately linear fault sections that intersect in geometrical barriers defined by bends and branching of the fault trace. The dips of these fault sections vary based on a preliminary model of the fault zone, with estimated dips ranging from 45 to 90°. The palaeo-stress tensor was characterized by sub-vertical maximum principal compressive stress, a minimum principal stress axis trending about 050° and
φ
≈ 0.2−0.5. The average slip direction across the fault segment was ≈240°. Three large, non-conservative barriers occur within the fault segment—at the northern and southern ends of the segment, and a branch point in the centre of the segment. Bifurcation of the rupture zone into two primary fault systems has presumably increased the fracture toughness of the northern part of the segment due to the overlap of secondary faulting adjacent to the primary fault planes. Two large historic earthquakes in the eastern Basin and Range and Rocky Mountain transition region have ruptured bifurcated fault zones similar in structure to the Salt Lake segment; the Hebgen Lake, Montana earthquake (
M
= 7.5) in 1959 and the Borah Peak, Idaho earthquake (
M
= 7.3) in 1983. The earthquake foci were located at depths of 12–18 km and the ruptures propagated towards the regions of fault-zone bifurcation in both cases. A characteristic earthquake (
M
≈ 7−7.5) may initiate at either the centre or the southern end of the Salt Lake segment. In the first case, the rupture would propagate bilaterally, but in the latter case the rupture would propagate unilaterally to the N. A unilateral propagation direction is preferred based on inferences drawn from the structural geology of the segment and direct comparison with the rupture traces of the Borah Peak and Hebgen Lake earthquakes.
Extensional tectonics and crustal structure: deep seismic reflection data from the northern North Sea Viking graben
Authors: Beach Alastair , Bird Tim , Gibbs Alan
Published: April 2008

A preliminary interpretation of a deep seismic reflection profile across the northern North Sea provides an insight into the crustal structure beneath the major extensional rift, the Viking graben. The Viking graben was initiated during the early Mesozoic and evolved during the Jurassic-Cretaceous into an asymmetrical half-graben, fault-bounded on its western side. Deep seismic reflections enable an interpretation of the position of the Moho to be made, and an estimate of the amount of crustal extension, within the plane of the section, of 50% has been calculated. The pre-Mesozoic crust has been thinned to a minimum of 14 km, but generally varies from 15–20 km beneath the rift. Interpretation of extensional faults within the Jurassic generally gives less than 50% extension, and much of the early Mesozoic faulting is probably not imaged. Calculation of Mesozoic subsidence in the graben leads to an estimated stretching factor of 3 or greater. A larger scale of linked tectonics involving major strike-slip movements along the length of the graben is invoked to explain the discrepancy between subsidence and crustal extension. Fault-related subsidence occurred in three distinct phases of finite duration—during the Triassic (40 My), the late Jurassic (30 My), and the early Tertiary (30 My). During the late stages of pervasive crustal thinning (late Jurassic-Early Cretaceous) a series of major fault and shear zones developed, dipping E and passing through the crust, offsetting the Moho. The change in tectonic style corresponds to a change from a dominantly extensional to a dominantly strike-slip mode of rift evolution. The seismic profile also images the Magnus basin related to the Great Glen fault system, and the Horda Platform near the coast of Norway.
Structures associated with extensional tectonics in the Suez rift
Authors: Chénet P. Y. , Colletta B. , Letouzey J. , Desforges G. , Ousset E. , Zaghloul E. A.
Published: April 2008

The opening of the Suez rift, which was initiated at the beginning of the Miocene (23 Ma), was the result of a regional extensional stress more or less perpendicular to the rift axis. A detailed study of selected areas within the onshore part of the rift (Abu Durba, Gebel Ekma and Wadi Dib) shows that the fault geometries visible in the Precambrian basement and the overlying sedimentary rocks are distinct; brittle deformation in one, ductile deformation in the other. In the basement (metamorphic and granitic rocks), faults display a rectilinear pattern. Two sets of faults are predominant; (i) N140-150E faults, parallel to the Gulf trend, with pure dip-slip displacement, (ii) cross-faults (transfer faults) trending N00 to N30E with strike-slip component. These faults delineate diamond-shaped tilted blocks. In the overlying sedimentary rocks (Cretaceous-Eocene sands, shales and limestones) faults are spoon-shaped. Spoon faults are generally superimposed over concave wedges of the basement blocks but are decoupled from basement fault sets. The decoupling is accommodated by ductile deformation and detachment in the Cretaceous marks.