bluuebunny/crossref_metadata_2025 huggingface dataset:
Mechanics of tectogenesis in plate collision zones
Authors: Murrell S. A. F.
Published: April 2008

Although continental parts of tectonic plates can be stretched, fractured and rifted, it appears that orogenic belts only form on continental margins or in previously stretched and thinned regions of continents and cratons. This is because continental crust and lithosphere is much weaker under the action of extensional forces than it is under contractional forces. The stretching and shortening of crust and lithosphere is discussed using McKenzie’s (1978) approach and the processes by which thinned or thickened lithosphere is restored to normal thickness are discussed in some detail. It is shown that the expressions for the initial and final changes of surface elevation are the same, apart from a difference in sign, for the cases of stretching and shortening when no material is added, or removed (by erosion). It is also shown that there is a critical value of the initial crustal thickness (∼ 17 km in McKenzie’s (1978) model) which must be exceeded if stretching is to cause crustal subsidence and shortening is to cause crustal elevation. Thinner crust, when shortened, will subside. This may be important in shortening processes at continental margins, where the thicker continental crust will rise and oceanward, the thinner crust will subside. The thermal and metamorphic changes associated with stretching (and thinning) and shortening (and thickening) are discussed. In the deeper parts of shortened and thickened crust prograde metamorphism will occur, possibly leading to anatexis. The strength and deformation mechanisms of crust and lithosphere are summarized. Finally, a collision tectonics model is presented based on the concept of shortening (with concomitant thickening) of the colliding passive and active margins, in which the important role of differential elevation changes across strike is emphasized. In the heart of the orogen will be a thickened crust consisting of thickened basement, a thick deformed sediment pile and nappes thrust from the margin. On a time scale of some tens of Ma, a thick metamorphosed and granitized lower crust will form after sufficient shortening has taken place.
A possible new pattern of cortical deposit in Tremadoc dendroid graptolites from chert nodules
Authors: Urbanek Adam , Mierzejewski Piotr
Published: April 2008

Rhabdosome fragments of dendroids from upper Tremadoc chert nodules, studied with TEM, show an unusual pattern of cortical tissue. In contrast to previously studied tissues it consists of an irregular meshwork of wavy, branching and interconnecting fibrils. Differentiated in size and thickness, the fibrous material includes thick, strongly branched fibres. Their presence is a characteristic feature of the cortex in the dendroids examined. The observed pattern of ultrastructure is tentatively described as ‘reticulate fabric’. Its nature remains obscure. It may represent a primary feature, thus being an unusual state of aggregation of fibrous material, but its secondary origin due to preservational degradation of normal dendroid cortex seems more plausible.
Stratigraphic significance of
Glyptograptus persculptus
group graptolites in central Nevada, U.S.A.
Authors: Berry William B.N.
Published: April 2008

Glyptograptids of the
G. persculptus
group,
Glyptograptus laciniosus
, and climacograptids of the
C. normalis
group (herein described as
C. miserabilis
) occur in the Hanson Creek Interval, or Formation, at Copenhagen Canyon, central Nevada. The graptolites occur in the basal part of an interbedded thin-bedded limestone and chert sequence overlying a quartzsand unit that is recognizable over a wide area in central Nevada and seems to reflect regional shallowing of the seas at the time of its deposition.
Dimorphograptus
sp. cf.
D. confertus
is present in the stratigraphically highest beds of the limestone-chert unit. Conodonts suggesting an early Silurian age are present in the graptolite-bearing sequence. Ashgill (late Ordovician) faunas have been found beneath the quartz-sand unit. Analysis of depositional environments of the Ashgill and early Silurian conodont-bearing beds suggests that deposition was continuous from the late Ordovician into the early Silurian in the area. The
G. persculptus
group glyptograptids display a significant range in variation. Some are relatively narrow and appear to be parallel-sided for much of their length, whereas others are relatively wide and taper noticeably in their proximal regions.
Top Ordovician and lowest Silurian of Dob’s Linn
Authors: Williams S. Henry
Published: April 2008

The Upper Hartfell Shale (including Anceps Bands A-E and the Extraordinarius Band) and the lowest Birkhill Shale have been extensively collected at Dob’s Linn revealing many forms previously unrecorded from Britain. The first two Anceps Bands (A and B) contain a distinct fauna from the following three (C-E) and two new subzones are suggested for the
D. anceps
Zone, an earlier
D. complexus
and a later
P. pacificus
Subzone, which should be more widely correlatable units. A major extinction occurred between Anceps Band E and the Extraordinarius Band (
C? extraordinarius
Zone) which contains only a few diplograptid species, as does the basal Birkhill shale (
G. persculptus
Zone). The
O? acuminatus
Zone commences at 1.6 m above the base of the Birkhill Shale with the first occurrence of
A. ascensus
while the monograptid
A. ceryx
appears at 1.9 m. The base of the
O? acuminatus
Zone is considered to be the most suitable horizon for the Ordovician-Silurian boundary.
The Silurian graptolite-bearing strata in China
Authors: Huo Shi-cheng , Shu De-gan
Published: April 2008

Details of the Silurian graptolite sequences of China, particularly those of the South China, Kunlun-Qinling and Yangzi provinces, are given and a graptolite zonal sequence proposed for the Silurian of China. The recognition of the upper and lower boundaries of the Silurian in China is discussed and a two-fold division of the Llandovery and a three-fold division of the Wenlock proposed on the basis of graptolite zones. The base of the Silurian is taken at the base of the
Glyptograptus persculptus
Zone; the Silurian/Devonian boundary at the base of the
Monograptus uniformis
Zone. It is shown that close correlations are possible between the proposed zonal scheme for China and those in general use in Eastern and Western Europe.
The monotypic ‘population’ accompanying the lectotype of
Saetograptus varians
(Wood 1900)
Authors: Palmer D. C.
Published: April 2008

The lectotype of
Saetograptus varians
(Wood) is accompanied by a monotypic assemblage of graptolites, which is shown to have current orientation and bimodal size distribution. The morphology of the two size classes is compared with that of the lectotype and the development of the nema analysed. This paper is the initial stage in a study of the semispinose Ludlow graptolite
S. varians
and is based on an examination of the lectotype and its accompanying fauna.
Late Mesozoic and Cenozoic sediment flux to the central North Atlantic Ocean
Authors: Thiede Jörn , Ehrmann Werner U.
Published: April 2008

A history of Mesozoic and Cenozoic palaeoenvironments of the North Atlantic Ocean has been developed based on a detailed analysis of the temporal and spatial distribution of major pelagic sediment facies, of hiatuses, of bulk sediment accumulation rates, and of concentrations and fluxes of the main deep-sea sediment components. The depositional history of the North Atlantic can be subdivided into three major phases: (a) Late Jurassic and Early Cretaceous phase: clastic terrigenous and biogenic pelagic sediment components accumulated rapidly under highly productive surface water masses over the entire ocean basin; (b) Late Cretaceous to Early Miocene phase: relatively little terrigenous and pelagic biogenic sediment reached the North Atlantic Ocean floor, intensive hiatus formation occurred at variable rates, and wide stretches of the deep-ocean floor were covered by slowly accumulating terrigenous muds; (c) Middle Miocene to Recent phase: accumulation rates of biogenic and terrigenous deep-sea sediment components increased dramatically up to Quaternary times, rates of hiatus formation and the intensity of deep-water circulation inferred from them seem to have decreased. However, accumulation rate patterns of calcareous pelagic sediment components suggest that large scale reworking and displacement of deep-sea sediments occurred at a variable rate over wide areas of the North Atlantic during this period.
Late Neogene submarine erosion events along the north-east Atlantic continental margin
Authors: Stein R. , Sarnthein M. , Suendermann J.
Published: April 2008

During the last 5 Ma, bottom currents have formed two major hiatuses along the continental margin off north-west Africa. In addition, slumping may have caused hiatuses at Site 397. The hiatuses extend from Gibraltar to the equator between 2800 and 4000 m water depth. At the hiatuses there has been up to 30 m erosion, or reduced sedimentation, at DSDP Sites 544B, 397, 141, and 366. Bio- and magneto-stratigraphy and oxygen-isotope curves relate the hiatuses to cooling events of the North Atlantic Deep Water at about 2.75 and 1 Ma.

A dynamic ocean model of deep-water circulation provides insights into how climate induced changes in the deep-water current regime.
Sediment waves in the eastern equatorial Atlantic: sediment record during Late Glacial and Interglacial times
Authors: Sarnthein M. , Mienert J.
Published: April 2008

Standing and migrating sediment waves along the West African continental rise (3000–4500 m water depth) are characterized by a regular spacing of wave length periods that are multiples of 280 m, and by long-term pelagic sediment composition. Interpretations of their origin must account for the following evidence: (1) The wave crests generally run parallel or oblique to the continental rise and also to prevailing (net) currents, as confirmed by SEA BEAM data; (2) Migrating waves along the West African continental margin are directed only upslope, i.e. towards the East. (3) The waves have been accreting for a long time, (>1 Ma). (4) Glacial to interglacial differences in the evolution of sediment accumulation rates on crests and troughs may indicate that the accreting regime was intermittent, and that the waves were more levelled prior to 7500 years
bp
due to increased deposition of siliciclastic matter in the troughs. (5) Absent patterns of grain-size sorting in the fine-grained muds contradict ‘conventional’ models of current-related sediment transport.

A superposition of large tidal components and slow thermohaline net currents (Lonsdale 1978), secondary current processes (Unsöld, pers. comm.), or a superposition of internal waves and thermohaline currents (Kolla
et al.
1980) are regarded as the most probable modes of origin.
Late Pliocene to Recent planktonic foraminifera from the North Atlantic (DSDP Site 552A): quantitative palaeotemperature analysis
Authors: Hooper P. W. P. , Funnell B. M.
Published: April 2008

Hydraulic piston cores from Hole 552A (56′02.56′N., 23′13.88′W.), recovered during Leg 81 of the Deep Sea Drilling Project, provide a near-perfect record of the glacial history of the North Atlantic. Oxygen isotope data gathered by Dr N.J. Shackleton were used to select 40 samples representing maxima of glacial and interglacial events, from the initiation of ice-rafting at 2.5 Ma to the present. For each sample the planktonic foraminiferal assemblage was analysed, enabling distinctive warm and cold water assemblages to be identified.

The relative abundance distributions of individual species were compared with the isotope data to assess temperature preferences, including those of two extinct species:
Globorotalia puncticulata
(Deshayes) and
Neogloboquadrina atlantica
(Berggren). A similar correlation for the coiling direction ratio in
Neogloboquadrina pachyderma
(Ehrenberg) gave a coefficient greater than 0.9, showing this to be an especially sensitive indicator of temperature at this latitude.

From these data a preliminary quantitative palaeotemperature analysis, using a simple transfer function, enabled a sea-surface temperature curve to be calculated for the Late Pliocene and Pleistocene to Recent, which correlates well with both the isotopic data and ecological data for extant species.
Diatom biostratigraphic and palaeoceanographic interpretations for the middle to high latitude North Atlantic Ocean
Authors: Baldauf Jack G.
Published: April 2008

Diatoms are useful for biostratigraphic correlation of Neogene sediment sections in the North Atlantic Ocean. The warm-temperate assemblage observed in cores from the middle and high latitude Atlantic enables partial application of the diatom zonations defined in the eastern equatorial Pacific. Sixteen of the equatorial Pacific diatom zones are recognized in cores from the middle latitudes of the North Atlantic.

The diatom assemblage implies that surface waters were warm during the early Miocene in the Rockall Plateau region. A decrease in the quality of diatom preservation, and the occurrence of numerous hiatuses at approximately the early-middle Miocene boundary, suggest a major change in oceanic circulation. An increase in deep water overflow from the Norwegian Sea may have been a contributing factor. Although middle Miocene sediments at both middle and high latitudes are generally enriched with silica, a possible restriction of siliceous sedimentation towards the high latitudes is observed during this time. The interval representing the early late Miocene and earliest Pliocene is generally devoid of diatoms.

Conditions favouring siliceous sedimentation continued at high latitudes and returned to middle latitudes of the North Atlantic during the late Pliocene, maybe in response to increased intensity of the Gulf Stream. The abundance and preservation of diatoms in high latitude Quaternary sediments is partially controlled by climatic fluctuations. The occurrence of the
Denticulopsis seminae
group suggest that cool, low saline waters may have occurred in the North Atlantic between 1.1 and about 0.70 Ma.
Pulsation tectonics as the control of North Atlantic palaeoceanography
Authors: Sheridan Robert E.
Published: April 2008

A new theory of pulsation tectonics involving the cyclic eruption of thermal plumes from the D″ layer in the lowermost mantle is proposed as a control of the first-order cycles of palaeocean changes. Plumes carry heat rapidly away from the core/mantle boundary, changing conditions in a way that results in the quiescence of the earth’s magnetic field. The rising plumes, after a phase lag, induce faster plate spreading and widespread intraplate volcanism. Subsequently, colder rock returning to the lower mantle re-establishes the D″ thermal boundary layer and sets up conditions for more turbulent core convection, which in turn allows frequent magnetic reversals. Meanwhile, lower mantle plumes cease erupting, and the plates spread more slowly. The cycles of fast and slow plate spreading and quiet and reversing magnetic field have a period of 60–100 Ma.

Changes in spreading rate on a global scale are probably a major cause of large scale (100–300 m) eustatic sea-level changes. Major global sea-level high-stands in middle to late Jurassic and in middle to late Cretaceous times coincide with fast spreading. Consequential changes in albedo, global temperature, humidity, forestation, atmospheric and oceanic CO
2
, oceanic acidity and oxygenation lead to variations in the preservation of black shales in the oceans, and in the level of the calcite compensation depth (CCD). In the North Atlantic, the climatic factors forced by rising sea-level caused increased black shale preservation and a shallowing of the CCD. With falling sea-levels, less black shale is preserved and the CCD is deeper.
Laboratory Studies into the Generation of Natural Gas from Coals
Authors: Higgs M. D.
Published: April 2008

The thermal histories of a Palaeozoic and a Tertiary coal have been investigated by laboratory simulation experiments in which a suite of samples, with natural maturities ranging from 0.4–3.0% vitrinite reflectance (VR), were heated under identical conditions of time and temperature. The cumulative hydrocarbon yield (measured as methane equivalents) was found to be 170 ml/g organic carbon (Org. C) for the Paleoazoic coal and 225 ml/g org. C for the Tertiary coal. Peak hydrocarbon generation was estimated to occur at approximately 1% reflectance for both coals. Carbon dioxide generation for both coals is rapid and can be considered to have ended by 1% reflectance. Nitrogen generation is negligible for both coals over the maturity range investigated.
Geology and Petrophysics of the Victor Field
Authors: Conway A. M.
Published: April 2008

The Victor Gas Field is located in the Southern North Sea Gas Province some 140 km east of the Lincolnshire coast. The Lower Permian Leman Sandstone Formation (Rotliegendes Group) forms the reservoir and consists entirely of sandstones with occasional silty shales in the basal section. It rests uncomformably on the Carboniferous, which was uplifted, gently folded and eroded during the Variscan Orogeny.
Electrical resistivity in continental lower crust
Authors: Haak V. , Hutton R.
Published: April 2008

The application of d.c. resistivity, magnetotelluric—both natural and artificial source—and geomagnetic variation techniques has provided lithospheric electrical resistivity models which generally display a relatively low resistivity region at lower crustal/upper mantle depths. In this paper, model parameters are summarized for the lower crust in tectonically inactive regions and discussed in more detail with respect to specific studies undertaken in Europe, N. America and Africa. Special attention is drawn to the resolution of the model parameters and to the fact that the depth to the low resistivity layers and the ratio of their thickness to their resistivity can normally be well determined.

The petrological interpretation is non-unique. Although electrical resistivity is strongly temperature dependent, the temperature within the lower crust in inactive regions is too low to give an
in situ
decrease in resistivity of the major rock phases. A more acceptable interpretation involves compositional variations—both lateral and vertical—within the crust associated with an increased amount of free fluids (e.g. water) or of free carbon (e.g. graphite). The correlation of the low resistivity structures in the lower crust with sedimentary layers involved in large-scale overthrusting or related tectonic processes is an attractive idea while the generation of low resistivity shear zones during metamorphism could also be an effective explanation of the observations. Some of the elongated anomalies of low resistivity which are discussed may correspond to Proterozoic plate boundaries and subduction zones.
The role of deformation, heat, and thermal processes in the formation of the lower continental crust
Authors: Murrell S. A. F.
Published: April 2008

Because as much as ninety per cent of the continental crust originated in Archaean times its structure has been modified by prolonged, though punctuated, thermal and tectonic processes. In spite of considerable heterogeneity arising from a complex tectonothermal history, continental lower crust is now revealing significant geophysical structures (low-velocity layers, high-conductivity layers, high seismic reflectivity). It is suggested that these structures are a consequence of tectono-thermal metamorphism which produces trapped layers of high pressure aqueous pore fluids. These pore fluids also result in deep seismic activity. A brief review of crustal tectonics and crustal growth based on concepts mainly due to Tarling (1978) leads to the postulate that the nature of the lower continental crust depends on the age of the last significant tectono-thermal event (Archaean, Proterozoic, and Phanerozoic structures differing significantly) due to the continuing reduction in the Earth’s internal energy and in the vigour of tectonic activity of the oceanic lithosphere (‘sea-floor spreading’) associated with mantle convection. The Phanerozoic has been the era of plate tectonics, marked by deformation at the margins of lithosphere plates or at sutures or by stretching and rifting within plates, where the deformation has caused crustal metamorphism. In the Proterozoic the continental crust was relatively much more stable than it was either in Archaean or Phanerozoic times, and it was transformed more by thermal effects of widespread igneous activity than by interactive plate margin tectonics. The Archaean was the period in which the major part of the continental crust was formed by igneous differentiation associated with mantle convection, and by sedimentary processes. The primal character of the processes at this time, together with the greater geothermal energy, distinguishes them from the similar processes of the Phanerozoic era. During a prograde metamorphic event dehydration of a particular crustal layer will tend to cause a build up of volume and pressure of entrapped pore water, and under such ‘undrained’ conditions tectonic stress and dilatancy-hardening will cause cataclastic flow distributed throughout the layer. Thick fluid-filled layers of this type could produce the geophysical characteristics observed in the lower crust. Where ‘drained’ conditions occur strain-softening may result under tectonic stress, leading to the formation of localized faults and fractures, which may be too thin to be geophysically detectable. The drainage of pore fluids from a metamorphically dehydrated layer due to tectonic fracturing (e.g. at an ocean-continent margin) could also cause the layer to show less contrast of geophysical properties, and make it undetectable. This might explain some of the apparent discontinuity of deep crustal structures.
Intraplate seismicity induced by stress concentration at crustal heterogeneities—the Hohenzollern Graben, a case history
Authors: Fuchs K.
Published: April 2008

The intraplate seismicity of the Hohenzollern Graben in southwest Germany is concentrated within a small region on the northern end of long shear lineaments extending from the Alps into their northern foreland. Strong shocks with magnitudes not more than 5.6 started in 1911 and repeat about every 30 years with a tendency to migrate northward and at the same time to become shallower. Recent seismic reflection and refraction work have suggested that the Hohenzollern Graben is located in the southeastern border region of an anomalous low velocity, probably high temperature body, near the Urach geothermal anomaly. It is also suggested that the top of the lower crust dips sharply from this body to the southwest. The hypocentres of the deepest earthquakes become shallower as the lineaments approach and traverse the Hohenzollern Graben in the direction towards the Urach body. From all these observations taken together, it appears that the release of seismic energy is concentrated into the area of the Hohenzollern Graben by an extensive lower crustal heterogeneity. The stress concentration in the brittle upper crust is induced by the thickening of the lower crust with its ductile behaviour. The viscosity of the lower crust in the Urach body has suffered further reduction by increased temperatures. Future research has to clarify the dominant cause of the stress concentration: is it the heterogeneity in material or the heterogeneity in thermal properties?
Fluid inclusions in rocks from the lower continental crust
Authors: Touret J.
Published: April 2008

In granulites, which constitute a major, if not exclusive rock type of the lower continental crust, fluid inclusions are abundant in many rock-forming minerals. Most contain a characteristic ‘carbonic’ fluid, high density CO
2
possibly mixed with variable quantities of N
2
and more rarely, CH
4
· NaCl brine inclusions have recently been observed in metasedimentary granulites from southern Norway. P-T estimates from synmetamorphic pure CO
2
inclusions agree with solid mineral thermobarometry in three selected examples of: (a) high-pressure granulites (Fura Complex, Tanzania), (b) intermediate-pressure granulites (southern Karnataka, India) and (c) low-pressure granulites (West Uusimaa Complex, Finland). These data support a model of fluid distribution of the lower crust based on the predominance of density-controlled, mantle-derived CO
2
-rich fluids coexisting immiscibly with NaCl brines. In southern Norway, most of the CO
2
was associated with or derived from carbonate-rich melts emplaced with deep seated, intermediate synmetamorphic intrusives.
Geochemical constraints on the origin of Archaean tonalitic-trondhjemitic rocks and implications for lower crustal composition
Authors: Rudnick R. L. , Taylor S. R.
Published: April 2008

The rare earth element (REE) concentrations of high-grade trondhjemitic gneisses from the Archean Kapuskasing Structural Zone, central Ontario, are similar to REE concentrations of Archaean tonalites and trondhjemites world-wide: REE patterns are steeply fractionated (La/Yb)
N
= 20–100), with variable positive Eu anomalies and fractionated HREE ((Gd/Yb)
N
= 2–4). Calculated models for the melting of a mafic source with a flat REE pattern and variable mineralogy show that amphibole alone is not capable of producing the large amount of REE fractionation observed in Archaean tonalites; garnet amphibolite (with >20% garnet), mafic garnet granulite or eclogite are the most likely sources. The presence of garnet constrains the mafic source to be within the lower crust (deeper than 25 km) or upper mantle. Such crustal or upper mantle melting depths are not geochemically distinguishable, yet have important implications for the composition of the Archaean lower crust.
Eclogite facies metamorphism in the lower continental crust
Authors: Carswell D. A. , Cuthbert S.J.
Published: April 2008

Occurrences of medium-temperature eclogite-facies assemblages in the Norwegian Caledonides, Western Alps and Central European Variscides are reviewed with reference to a generalized tectono-thermal model for their stabilization in continental plate collision zones involving A-type subduction. It is demonstrated that the respective pressure-temperature-time paths for lower continental crust and uppermost mantle during such orogenesis contrast with that for upper crust, and that mineralogical features, such as chemical zoning in garnets, may monitor the tectonic location and thermal evolution of different crustal levels in the resultant thrust-nappe stack. Phanerozoic orogenic belts of this type typically show evidence of the generation of early eclogite-facies assemblages subjected to partial greenschist- or amphibolite-facies overprint some 40–60 Ma later. The survival and ultimate surface exposure of the high-pressure assemblages requires that their exhumation is rapid (mean uplift rate around 1 mm/year) relative to the rate of thermal relaxation in the tectonically thickened continental crust.
Composition of the Canadian Precambrian shield and the continental crust of the earth
Authors: Shaw D. M. , Cramer J. J. , Higgins M. D. , Truscott M. G.
Published: April 2008

Following a summary of the pre-Archaean history of the earth’s crust, the present compositions of the upper and lower continental crust are appraised.

There is no sure evidence that the Archaean crust differed in bulk composition from the present day: estimates that it was richer in ferrides and poorer in incompatible elements are difficult to distinguish from regional variations.

Estimates of lower crustal composition vary considerably according to the manner in which they were determined. A new estimate is nevertheless presented.

The abundances of heat-producing elements in both the upper and lower crust are in accord with measured values of heat flow at the surface.
High grade metamorphism in the granulite belt of Finnish Lapland
Authors: Raith M. , Raase P.
Published: April 2008

The granulite complex of Finnish Lapland is located in the prominent collisional suture which separates the Archaean crustal blocks of the South Lapland and Inari cratons. The protoliths of the granulite complex represented a geosynclinal sequence of Proterozoic flysch-type sediments with intercalations of pyroclastic, effusive and intrusive rocks of tholeiitic to rhyolitic composition and calc-alkaline affinity. Deformation and progressive metamorphism set in about 2.2 b.y. ago by eastward subduction of the volcanosedimentary sequence and culminated in the granulite facies event during the collision of the Archaean crustal blocks. The granulite complex thereby was thrust westward into higher crustal levels onto the cooler basement unit of the South Lapland craton and was overthrust by the Inari craton. High-grade metamorphic equilibration of the rocks continued until about 1.9 b.y. during the isostatic uplift of the thickened crust.

The strongly sheared lower part of the granulite complex consists of sillimanite-garnet granulites, garnet-biotite gneisses, and hypersthene-plagioclase rocks. Its intensely migmatized middle and upper parts are dominated by medium to coarse-grained sillimanite-garnetbiotite ± cordierite metatexites and diatexites. An up-dated evaluation of P-T-X(CO
2
,H
2
O) conditions indicates that the post-kinematic assemblages in the granulite complex equilibrated at 830 to 760°C and 7.2 to 6.2 kbars, in the presence of fluids rich in carbon dioxide (X

CO
2

= 0.9 − 0.7). Increased activity of water in the middle and upper parts of the complex resulted in widespread anatexis and advanced re-equilibration of the rocks. The high heat flow in the granulite complex is explained by intense upward motion of hot fluids in the strongly sheared and foliated granulites. The rock series of the adjacent South Lapland and Inari cratons equilibrated at conditions of the upper amphibolite facies (720–750°C and high activities of water). The changes in the mineral assemblages and P-T-X(CO
2
,H
2
O) data across the units indicate that heat transfer and flux of CO
2
-rich fluids from the upthrusted hot granulite complex into the cooler basement unit was essentially confined to the proximity of the narrow thrust zone. The cooling path of the granulite complex passed near the Al
2
SiO
5
triple point (∼500°C/3.5 kbars) thus demonstrating the slow thermal relaxation of this rock unit.
Controls on the composition of modern sediments, Lake Turkana, Kenya
Authors: Yuretich R. F.
Published: April 2008

Source rock composition plays a primary role in controlling the sediment composition in modern Lake Turkana, but interactions between sediment and lake water both during and after deposition may serve to modify this distribution. The Omo river drainage basin, which comprises about 58% of the Lake Turkana catchment area, is underlain primarily by Tertiary mafic volcanics exposed to a humid, tropical climate. In contrast, the smaller Kerio-Turkwel River drainage basin (28% of the total lake catchment) is extremely arid, and bedrock exposures are dominated by Precambrian gneisses and schists. Sediments delivered by the Omo system reflect the intense weathering of the area; fine grain sizes dominate and consist of kaolinite, along with iron oxides and resistate minerals with high Al and Ti. The Kerio-Turkwel sediments are generally coarser (more silt and fine sand) and contain appreciable quartz, feldspar, smectite and illite. The chemistry and mineralogy of the bottom sediments near the river mouths reflect these differences, but compositional changes in deep water deposits may be caused by reactions with the alkaline and slightly saline (2500 ppm TDS) lake water. The sediments here are very fine grained (71 ± 9% clay) and, despite an apparently small influx of smectite into the lake, Mg-rich smectite is the principal clay. Hydrodynamic segregation of smectite from coarser materials could explain this trend, but geochemical calculations suggest that the reaction of weathered aluminosilicates with Mg
2+
, HCO
3
−
and SiO
2
in the lake water may also produce diagenetic smectite. Calcite is also precipitated inorganically, particularly in the more distal parts of the lake. Hydrogeochemical balances predict other mineral-water reactions to remove dissolved species, but direct evidence for these is yet to be documented.
Mire sediments in East Africa
Authors: Hamilton A. , Taylor D.
Published: April 2008

Mires (peat-forming systems) are very localized within East Africa, being concentrated in areas of higher rainfall and lower temperature. The main centre is the high ground of the wetter part of the Western Rift, in southwestern Uganda, Rwanda, Burundi and probably Zaire. Peat-forming species include trees, such as
Erica
and
Syzygium
, sedges and
Sphagnum
. Mire stratigraphy is very variable between sites, reflecting a diversity of environmental contexts. Many changes in the characteristics of mire sediments in particular profiles appear to be related to climatic events. Some recent changes in mire vegetation are the result of human influence.
Depositional context of Plio-Pleistocene hominid-bearing formations in the Middle Awash valley, southern Afar Rift, Ethiopia
Authors: Williams M. A. J. , Assefa Getaneh , Adamson D. A.
Published: April 2008

The Awash flows from the Ethiopian Highlands via the Ethiopian Rift into Lake Abhe in the Afar Rift. Over 8000 km
2
of Neogene fluvio-lacustrine deposits crop out in the Middle Awash valley, of which we have surveyed 300 km
2
E of the river between 10°25′N and 10°45′N. Pleistocene sediments adjacent to the Awash floodplain consist mostly of unfossiliferous brown clays with sand and gravel interbeds which contain vertebrate (including hominid) fossils and prehistoric stone artefacts (Oldowan and Acheulian). Palaeocurrent directions in these fossiliferous sands invariably parallel the ephemeral sandbed channels that flow W from the basalt escarpment to the Awash. We interpret the brown clays as palaeo-Awash floodplain deposits, in contrast to the fossiliferous gravel-sands, which are mainly of local provenance. Away from the river, alternating Pliocene clays and diatomites, with minor fossiliferous gravel-sands, are capped by a 3.8–4.0 Ma primary air-fall tuff. Facies changes are more frequent above this ‘Cindery Tuff’, indicating tectonic disruption of the early Pliocene lakes, reflected also in a progressive increase in fluvio-volcanic gravels from Pliocene to early Pleistocene.
Preliminary magnetostratigraphic record of Pleistocene deposits, Lake Natron Basin, Tanzania
Authors: Thouveny N. , Taieb M.
Published: April 2008

The declination of characteristic magnetization has been used to determine the polarity changes in two volcano-sedimentary sequences exposed in the Lake Natron basin. Radiometric dating and biostratigraphic data allow us to link the magnetostratigraphy to the standard geomagnetic polarity time scale. In the Lake Natron palaeomagnetic record, the Matuyama Epoch is well defined, possibly including the Olduvai Event (1.87–1.67 Ma). Other normal polarity zones have to be considered cautiously.
Introduction
Authors: Fleet A. J. , Brooks J.
Published: April 2008

Organic-rich marine sediments are the source of most of the world’s oil. The need for an understanding of actual and potential marine source rocks is therefore of vital economic importance, but also poses many fascinating ‘academic’ questions. This introduction attempts to outline and link the papers on actual and potential marine petroleum source rocks which make up this volume. These papers were originally presented at a Geological Society meeting in London in May 1983. The volume is divided into three main parts. The first deals with concepts and methods of study, the second with depositional processes and environments, and the last with the stratigraphic record. Four topics merit discussion in many of the papers: the roles of primary productivity and stratification, or restricted circulation, in controlling organic matter preservation; the relationship between sedimentation rate and the organic content of potential source rocks; the palaeo-oceanographic characteristics of past oceans in contrast to those of the Plio-Pleistocene ones; and the need for a multidisciplinary approach to the study of organic-rich
and
associated sediments.
The significance of carbonate ooids in petroleum source-rock studies
Authors: Ferguson J.
Published: April 2008

Study of carbonate ooids from areas of present day ooid production in the Arabian Gulf and the Bahama Banks has shown that they contain significant amounts of light hydrocarbon gases. It is concluded that these gases are generated within the ooids themselves by the bacterial degradation of the contained algal proto-kerogen, in an anaerobic environment. Continuation of the degradation of the contained organic matter by artificial maturation, using enhanced temperature and pressure, in the presence of natural sea-water, has shown that an evolutionary series of kerogen and soluble hydrocarbons exist, which can be related to material isolated from British Jurassic oolitic limestones.

Two principal conclusions arise from these studies. Firstly, that carbonate ooids might provide a model for evaluating the potential of carbonates generally, as source rocks. Secondly, because the reactants and products are frequently isolated by the emplacement of the first phase of calcite cement early during diagenesis, thus trapping and possibly ‘fossilizing’ them, detailed studies of suitable oolitic limestone might lead to better understanding of organic reactions at different stages in diagenesis.
Hydrothermal petroleum from diatomites in the Gulf of California
Authors: Simoneit B. R. T. , Kawka O. E.
Published: April 2008

The plate boundary zone within Guaymas Basin, Central Gulf of California, is a set of spreading axes offset by a central transform fault and covered by a thick sedimentary sequence. The high heat flow and concurrent hydrothermal circulation cause extensive thermal alteration and leaching of the organic and mineral matter covering the rift graben. The rapid vertical migration of the leach and subsequent quenching by colder seawater results in the precipitation of hydrothermal deposits rich in sulphides and organic pyrolysates at the sediment surface. Deposits from the northeastern half of the Basin’s Southern Trough have been sampled using the D.S.R.V.
Alvin
, to examine the organic condensate character and regional patterns, from which fractionation processes during hydrothermal fluid migration may be inferred.

With the characterization of the bitumen, three major patterns are resolved:

(1)

Hydrocarbons with
n
-alkane predominance and no carbon number preference are the most common pattern;

(2)

branched and/or unsaturated hydrocarbon predominance relative to
n
-alkanes mainly within the kerosene-diesel range;

(3)

biodegraded material, dominated by a naphthenic hump, with a minor contribution of resolved compounds. Samples with predominant
n
-alkanes are further defined by their molecular weight range into distinct subgroups. Waxy condensates versus kerosene-range
n
-alkanes are found at different localities and may reflect regional differences in hydrothermal fluid exit temperatures.

It is postulated that variable pyrolytic temperatures and selective fractionation of pyrolysates during migration, with differential condensation and precipitation under varied thermal regimes, induce the observed general patterns. In addition, post-depositional processes such as biodegradation and removal of low molecular weight components by water solubilization can affect the bitumen character.
Organic carbon and metal accumulation rates in Holocene and mid-Cretaceous sediments: palaeoceanographic significance
Authors: Bralower T. J. , Thierstein H. R.
Published: April 2008

The proportion of the organic carbon produced in oceanic surface waters that is buried in the underlying sediments is highly variable. The organic-carbon preservation factor, the proportion of preserved to produced organic carbon, is much higher in anoxic than in oxic deep waters. Predictability of organic-carbon preservation factors from sedimentary evidence would enable estimation of past primary production rates from measurable accumulation rates of organic carbon in ancient sediments.

Accumulation rate patterns of organic carbon, transition metals, and excess transition metals in Holocene and mid-Cretaceous sediments are evaluated as predictors of organic-carbon preservation factors. Holocene transition-metal accumulation rates are controlled dominantly by bulk accumulation rates. Euxinic deep water environments, characterized by high organic-carbon preservation factors, are more successfully identified by sedimentological than by geochemical criteria.

Sedimentological evidence from several mid-Cretaceous intervals implies widespread deep-water anoxia, high organic-carbon preservation factors, and correspondingly low primary productivity for large parts of the mid-Cretaceous oceans.
Metamorphic evolution of the mainland Lewisian complex
Authors: Sills J. D. , Rollinson H. R.
Published: April 2008

The equilibration conditions of early Scourian (Badcallian) granulite-facies metamorphism appear to be similar for the whole mainland Lewisian outcrop. Pyroxene and garnet assemblages equilibrated at about 7–8 kb and 750–800°C. Peak granulite facies conditions may have been slightly higher. An early high-pressure gt-cpx assemblage partially broke down to lower pressure opx + plag ± magt ± sp ± hb symplectites, indicating decompression after the peak of metamorphism. At a later stage, there was growth of a more Fe-rich garnet. Discordant trondhjemite and granite sheets, which were also metamorphosed to granulite facies, indicate very high magmatic temperatures of 890–1030°C from Fe-Ti oxides and 1000°C from hypersolvus feldspars.

The Scourian complex was extensively retrogressed during the Inverian as a result of the influx of hydrous fluids at about 600°C.
The boundary between Proterozoic and Archaean crustal blocks in central West Greenland and northern Labrador
Authors: Korstgård J. , Ryan B. , Wardle R.
Published: April 2008

The contact between the Early Proterozoic Nagssugtoqidian Mobile Belt and the Archaean craton in West Greenland, and that between the Churchill Province and the Archaean (Nain) craton in Northern Labrador represent two cross-sections of an Early Proterozoic orogenic boundary. The Archaean cratons in both regions display the peripheral effects of Proterozoic orogenic events. The best indicators of Proterozoic effects on the Archaean cratons are the Late Archaean-Early Proterozoic dykes and the Early Proterozoic supracrustal rocks which unconformably overlie the gneisses and dykes. In Greenland the boundary zone evolution is characterized by Late Archaean (Nag. 1) transcurrent shearing, Early Proterozoic Kangâmiut dyke intrusion and subsequent (Nag. 2) ductile overthrusting. Ductile overthrusts are characteristic of the Nagssugtoqidian-Archaean boundary and attest to substantial shortening of the crust in this region. In Labrador the boundary region has been divided into several distinct zones. Although facets of its evolution are analogous to the boundary in Greenland there are several fundamental differences, the most important of which is that transcurrent movement appears to have been dominant over ductile overthrusting. These differences are provisionally related to contrasting tectonic settings of the two areas during Early Proterozoic continental collision, the Greenland segment being the frontal collision zone whilst the Labrador segment documents slightly oblique transcurrent motion.
The regional tectonic setting and possible causes of Cenozoic extension in the North American Cordillera
Authors: Coney P. J.
Published: April 2008

Timing and tectonic setting of middle Cenozoic crustal extension in the North American Cordillera supports the concept that an overthickened crustal welt formed behind or astride the thrust belts as a result of compression during the Mesozoic to early Cenozoic. At the end of the Laramide Orogeny the gravitationally unstable welt collapsed by deep-seated crustal extension. The extension was aided by a lowering of crustal viscosity resulting from a complex pattern of volcanism and a reduction in intraplate compressive stress. As plate regimens evolved along the Pacific margin during the late Cenozoic, subduction progressively ceased as did compressive stress also. An evolving transform boundary and a massive Cordilleran-wide lithospheric uplift allowed a second phase of extension to develop across the already thinned and thermally weakened crust to form the Basin and Range Province, being active up to the present time.
Crustal extension along a rooted system of imbricate low-angle faults: Colorado River extensional corridor, California and Arizona
Authors: Howard Keith A. , John Barbara E.
Published: April 2008

The upper 10 to 15 km of crystalline crust in the 100-km-wide Colorado River extensional corridor of mid-Tertiary age underwent extension along an imbricate system of gently dipping normal faults. Detachment faults cut gently down-section eastward in the direction of tectonic transport from a headwall breakaway, best expressed in the Old Woman Mountains, California. Successively higher and more distal allochthons are displaced farther from the headwall, some as much as tens of kilometres. The basal fault(s) cut initially to depths of 10 to 15 km, the palaeothickness of a tilted allochthonous slab of basement rocks above the Chemehuevi-Whipple Mountains detachment fault(s). Hanging wall blocks tilt consistently toward the headwall as shown by dips of capping Tertiary strata and of originally horizontal Proterozoic diabase dykes. Block tilts and the degree of extension increase northeastward across much of the corridor. The faults are interpreted as rooting under the unbroken Hualapai Mountains and Colorado Plateau on the down-dip side of the corridor in Arizona. Slip on faults at all exposed levels of the crust was unidirectional, and totals an estimated 50 km. These data and inferences support the concept that the crust in California moved out from under Arizona along a rooted, normal-slip shear system. Brittle thinning above the sole faults affected the entire upper crust, and in places wholly removed it along the central part of the corridor. Upwarp exposed metamorphic core complexes in footwall domes.