bluuebunny/crossref_metadata_2025_split huggingface dataset:
Electrical resistivity in continental lower crust
Authors: Haak V. , Hutton R. 
Published: April 2008

Summary
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

Summary
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

Summary
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

Summary

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

Summary

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

Summary
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

Summary
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

Summary
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

Summary

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

Summary

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

Summary

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

Summary
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

Summary

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

Summary
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

Summary

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

Summary
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.