bluuebunny/crossref_metadata_2025_split huggingface dataset:
Organic-rich Cretaceous sediments from the North Atlantic
Authors: Summerhayes C. P. 
Published: April 2008

Summary
Organic-rich sediments are abundant in the early and middle Cretaceous of the deep North Atlantic, in the calcareous Blake-Bahama Formation (Valanginian-Barremian), in the siliceous Hatteras Formation (Aptian-Cenomanian), and, in some places, in the Plantagenet Formation (Turonian-Santonian).

The regional patterns of organic facies and organic enrichment reflect:

(1)
the rate of supply of marine versus terrestrial organic matter,

(2)
the rate of burial of organic matter,

(3)
the oxygenation of the bottom water, and

(4)
the oceanographic history of the North Atlantic basin.

Rates of supply of marine organic matter (most now present as amorphous matter) seem to have been similar in the eastern and western North Atlantic, though they became higher in the east in Cenomanian and younger times, when coastal upwelling intensified there. Rates of supply of terrestrial organic matter were higher in the west, and off Iberia and in Biscay (except in the Cenomanian-Turonian). Organic matter is most abundant where rates of sedimentation were highest, presumably because organic matter was rapidly removed from the zone of intensive recycling near the sediment water interface. Some of the organic rich deposits were introduced by downslope displacement from the oxygen minimum zone on the continental margins.

The evidence suggests that bottom waters were intermittently suboxic (0.5–0.2 ml/l O
2
) or anoxic (<0.2 ml/l O
2
), encouraging preservation of organic matter on the deep sea floor. A more or less permanently anoxic expanded oxygen minimum zone is also implied.

While the global Cretaceous ocean seems to have been generally poorly oxygenated, local events within the North Atlantic give it a somewhat different history of accumulation of organic matter from other ocean basins. Nevertheless, the global organic enrichment event at the Cenomanian-Turonian boundary is clearly visible in the North Atlantic.
The carbon isotope record of the Cenozoic: history of organic carbon burial and of oxygen in the ocean and atmosphere
Authors: Shackleton N. J. 
Published: April 2008

Summary

Carbon isotope measurements in deep-sea sediments may be used to obtain the history of the mean
13
C content of CO
2
dissolved in the ocean, of
13
C gradients within the ocean, and of the globally averaged
13
C content of the carbonate accumulated on the sea floor. The last has some interest in relation to energy reserves, because it provides a means of monitoring the changing rate of accumulation of organic matter on the globe.

Data are shown for the
13
C composition of Cenozoic marine carbonates, based on a suite of sites from DSDP (Deep Sea Drilling Project) Leg 74 in the South Atlantic. If the published estimates of global accumulation rates for marine carbonate are recalculated on the basis of recent timescales for the Cenozoic it is found that changes were rather subtle, contrary to the conclusion of Davies and Worsley (1981), so that a model of constant carbon input may be used to estimate changes in organic carbon accumulation from the
13
C data. A cumulation of changes in the global organic carbon reservoir is also obtained and is used to infer changes in the amount of oxygen in the atmosphere, which was 20% higher than today during most of the Cenozoic. However, the solubility of oxygen in the warm early Cenozoic oceans was about 20% lower than today’s value, so that the early Cenozoic ocean had a similar dissolved oxygen content to today’s ocean.
The Lewisian complex: a typical Precambrian high-grade terrain?
Authors: Park R. G. , Tarney J. 
Published: April 2008

Summary

The main structural, lithological and geochemical features of the Lewisian complex are summarized in relation to available geochronology and an attempt is made to integrate this information into an evolutionary model for the Lewisian from the Archaean through the Proterozoic. The phase of extensive crustal generation which formed the early Lewisian (Scourian) complex occurred at about 2900 Ma and was marked by the production of vast volumes of tonalite which, together with supracrustal components, were affected by strong horizontal thrusting and deformation. The deeper parts of the Lewisian crust were subject to (Badcallian) granulite facies metamorphism culminating at
c.
2700 Ma. Major NW-SE shear zones developed shortly after this and resulted in the segmentation of the Lewisian crust and juxtaposition of different crustal levels. This marked a period of quite extensive retrogression of the high-grade gneisses, which continued (with further local shearing) during and after the emplacement of the Scourie dyke suite at
c.
2400 Ma. Extension of the Lewisian crust occurred in the southern (Gairloch) area around 2000 Ma and extensive outpourings of mafic volcanics, with associated exhalative mineralization, took place accompanied by sedimentation in the developing basin to form the Loch Maree Group. Igneous activity related to the South Harris igneous complex may have occurred at this period on the Outer Hebrides. Laxfordian deformation and metamorphism began before 1900 Ma and resulted in major reactivation of the earlier shear zones between and within the juxtaposed crustal blocks. More pervasive deformation accompanied the closure of the Loch Maree volcano-sedimentary basin, with major overthrusting and isoclinal folding. Culmination of Laxfordian metamorphic activity occurred at
c.
1900 Ma with migmatization and granite injection at Laxford and on Harris, and locally in the southern area. Shear deformation, increasingly more brittle and localized, continued to affect the Lewisian complex until 1400 Ma and probably until 1000 Ma. Interestingly, the main shear zones established at the end of the Archaean crust-forming episode at 2600 Ma continued to be the focus of tectonic, metamorphic and magmatic activity over the following 1500 million years.
Geochemistry of the Scourian complex: petrogenesis and tectonic models
Authors: Tarney J. , Weaver B. L. 
Published: April 2008

Summary
The segmentation of the Lewisian complex which occurred at the end of the Archaean resulted in the juxtaposition of zones representing different crustal levels, including deep crustal granulites. Critical aspects of the geochemistry of Lewisian gneisses are considered in relation to their petrogenesis and tectonic models. Most of the Lewisian is made up of a bimodal suite of tonalitic-trondhjemitic gneisses enclosing mafic-ultramafic enclaves and layered complexes which are often associated with metasediments. These mafic components have a low-pressure petrogenesis and represent tectonic inclusions incorporated into the complex at the time of gneiss generation. They are more abundant in the granulites. The silicic gneisses have a higher pressure petrogenesis and their geochemistry is consistent with hydrous partial melting of amphibolites in a shallow-dipping subducting slab with hornblende as a residual phase.
Granulite metamorphism occurred or peaked some 200 Ma or so after the phase of gneiss generation at 2.9 Ga and terminated about 2.5 Ga. Concentrations of U, Th and Rb are at least an order of magnitude lower in the granulites than in the equivalent amphibolite-facies gneiss zones, and those of K and Pb about 50% lower. Concentrations of other elements are not significantly different. Currently popular models for the removal of these elements are not significantly different. Currently popular models for the removal of these elements from the deep crust through partial melting or transport in a carbonic fluid phase are examined, but each has problems in accounting for all the available data. An alternative model, considering the low contents of heat-producing elements as a primary feature of silicic magmas emplaced into the deep crust, is explored. This relies more on fluid processes removing heat-producing elements from the ocean crust during subduction, followed by hydrous melting of the amphibolite to produce water-saturated magmas which cannot rise to high crustal levels. In contrast to modern subduction zone magmas, which are largely derived from the mantle wedge that has been modified by addition of a fluid-transported, LIL element, ‘subduction component’, Archaean sodic tonalite-trondhjemite magmas result from hydrous melting of amphibolite which has lost that component. Granulite-facies metamorphism is an independent process that is commonly superimposed on deep crustal rocks which are already low in heat-producing elements, and during which further loss of these elements may occur, but granulite-metamorphism may also be superimposed on more normal crustal rocks without significant chemical changes.
Internal evolution of the major Precambrian shear belt at Torridon, NW Scotland
Authors: Wheeler J. , Windley B. F. , Davies F. B. 
Published: April 2008

Summary
The Torridon shear belt is a NW-striking region of heterogeneous reworking within the Lewisian complex. Mapping of the deformation fabric intensity reveals a geometry of low-strain, lozenge-shaped blocks surrounded by an anastomosing set of shear zones which form a linked system of flats and lateral ramps. These various orientations of shear zones share a common movement direction but show both sinistral-reverse and dextral-normal shear sense. This is true both in the Inverian, when the linked system was established, and in the Laxfordian, when shearing followed the established network but was localized along the ductile Scourie dykes. Possible explanations for the occurrence of opposed shear senses include local differential shear, and a modification of the Lister & Williams model of deformation. In the latter, shear zones generally show a sense of shear opposite to the overall displacement on the belt, and this underlines the need for a comprehensive analysis of the geometry of such shear belts.
The Inverian structure controlled emplacement of the Scourie dyke swarm, which in turn controlled the localization and orientation of the Laxfordian structures. Given the ductility of the dykes, this leads to a simple explanation for the coaxiality of Inverian and Laxfordian structures. A tentative model for the evolution of the Torridon shear belt is given, in which emphasis is placed on the importance of dykes as easy-slip horizons.
The structural history of the Canisp Shear Zone
Authors: Attfield P. 
Published: April 2008

Summary
The Canisp Shear Zone is a major structure in the central region of the mainland Lewisian. Evidence is presented to show that there have been two major phases of movement. In the first phase, during the Inverian, a broad belt of steeply-inclined foliation was formed bounded to the south by the north limb of the Lochinver antiform. The associated lineation plunges steeply to the SE, indicating a dip-slip movement with downthrow to the north, and a small dextral strike-slip component.
Quartz-epidiorite and ultrabasic dykes were then emplaced sub-parallel to the shear plane, their emplacement was controlled by pre-existing planes of weakness.
The second, Laxfordian, phase of movement reactivated the Inverian structure. Deformation was concentrated in narrow zones of high strain, with an associated strong, penetrative lineation plunging shallowly ESE. The shear sense is dominantly dextral strike-slip with a small downthrow to the north.
This change from dominantly dip-slip to dominantly strike-slip movement agrees with models for other areas of the Lewisian (Coward & Park, this vol.) and appears to be associated with a change in style towards more localized zones of deformation.
The Outer Hebrides Fault Zone: evidence for normal movements
Authors: White S. H. , Glasser J. 
Published: April 2008

Summary

The Outer Hebrides Fault Zone is a major structural feature within the Lewisian of the Outer Hebrides. It is well exposed in the Uists where it consists of high-strain strands of ultracataclasite, cataclasite and phyllonite in a crush melange. Microstructural studies of the fault rocks indicate that the sequence crush melange-cataclasite-phyllonite is typical of that for
c
-type mylonite development and reflects increasing retrogression. This, plus the anastomosing pattern of the high-strain zones, suggests that the fault rocks in the Uists mark the edge of a major fault zone. Fold and shear band asymmetries indicate a major phase of normal movement. It is not possible to assign definite ages to either, but it is clear that the formation of the crush melange preceded that of the high-strain strands, which appear to be immediately post-Caledonian.
The palaeomagnetic record in the Lewisian terrain
Authors: Piper J. D. A. 
Published: April 2008

Summary

Palaeomagnetic investigations of the Lewisian rocks identify a diverse remanence record which varies from zone to zone. The characteristic (A) remanence in the central mainland zone is a strong and stable NW downward-directed and magnetite-held component. This occurs sporadically elsewhere but is largely displaced in the northern mainland zone and in Lewis and Harris by lower blocking temperature/coercivity components with more variable directions; in addition hematite overprints occur sporadically in the central zone and in the Loch Torridon area of the southern mainland zone. Geological relationships and palaeomagnetic field tests suggest that all of these components were acquired during post-Laxfordian uplift and cooling of this terrain but a steep E downward-directed component is a feature of pre-Laxfordian belts in the south and may be considerably older. The collective data define migration of the geomagnetic field axis during uplift and cooling of the terrain and are linked in general terms to mineral ages in the range
c.
1720–1500 Ma; further rock magnetic and
39
Ar/
40
Ar investigations are in progress to characterize the areal significance of the components of remanence and to date them more precisely.
Mid-Archaean basic magmatism of southern West Greenland
Authors: Hall R. P. , Hughes D. J. , Friend C. R. L. 
Published: April 2008

Summary

Three groups of metamorphosed basic igneous rocks post-date the Amîtoq gneisses (
c.
3700 Ma) and pre-date the Nûk gneisses (
c.
2900 Ma) in the Archaean craton of southern West Greenland. These are the Malene metavolcanic amphibolites, the Ameralik dykes within the Amîtsoq gneisses and a series of large metagabbro-anorthosite complexes. The Malene rocks range from komatiitic to tholeiitic in chemical affinities and occur as extensive intermixed horizons throughout the craton. Metavolcanic amphibolites which are intercalated with the older Amîtsoq gneisses are chemically similar to those which are isolated from them. They probably all represent lavas erupted in an oceanic environment, some of which were subsequently tectonically intercalated with the older sialic crust. Significant geochemical differences between the Malene metavolcanic rocks and the Ameralik basic dykes, which intrude the Amîtsoq gneisses, suggest that these dykes could not have acted as feeders to the Malene lavas. The petrogenesis of the Malene rocks is considered in terms of normative behaviour and of the system CaO-MgO-Al
2
O
3
-SiO
2
(CMAS). Both demonstrate the importance of olivine and clinopyroxene in the evolution of the Malene magmas and suggest that they were derived from relatively garnet-depleted and clinopyroxene-enriched mantle compared to average modern mantle. Evidence for a complementary aluminous portion of the Archaean mantle is provided by the abundance of metagabbro-anorthosite rocks and associated aluminous tholeiitic metavolcanic amphibolites.
The Archaean high grade gneiss terrain in E Hebei Province, NE China: geological framework and conditions of metamorphism
Authors: Sills J. D. , Wang K. , Yan Y. , Windley B. F. 
Published: April 2008

SummaryThe Archaean high-grade gneiss terrain of eastern Hebei Province, N.E. China, is unusual in that it contains abundant banded iron-formations at amphibolite or granulite facies. The Archaean terrain is divisible into three: an area between Taipingzhai and Malanyu comprising granulite-facies tonalitic gneisses formed about 2.5 Ga (the Qianxi Gneisses); farther south is an amphibolite-granulite-facies terrain consisting of enclaves of iron-formations and metasediments (Shuichang supracrustal association) within orthogneisses (Qianan Gneisses) which are in general more potassic; in the extreme south of the Archaean outcrop the oldest rocks, the Caozhuang group (c.3.5 Ga in age) consist of a series of amphibolites, iron-formations and metasediments enclosed within granodioritic and granitic gneisses. The Qianan Gneisses comprise ‘charnockites’ ofc.2.7 Ga age, tonalitic to granodioritic gneisses and a variety of highly deformed granodioritic and granitic gneisses of uncertain age.P-Tconditions range from 7–8 kb andc.700°C in the north near Taipingzhai to 5–6.5 kb and 650–750°C near Shuichang and 4.5–6.0 kb and 600–650°C in the southernmost area. Thus deeper crustal levels are exposed in the north, but it is not known whether the transition is gradual or whether different crustal levels have been brought together by thrusting or by vertical movements along shear zones.
Active normal faulting and crustal extension
Authors: Jackson J. A. 
Published: April 2008

SummaryA world-wide review of fault-plane solutions and focal depths for large normal-faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6–15 km on faults dipping between 30 and 60°. In the few cases where levelling or seismic data are good enough, these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust. There is evidence that, in some cases, as a result of the enormous transitory increase in strain rate during large earthquakes, rupture continues into the normally ‘ductile’ lower crust on surfaces with substantially gentler dips. These low-angle surfaces may be analogous to some of the ‘detachments’ seen in metamorphic core complexes of the western USA, but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the ‘detachments’ and the blocks on either side. Such contrasts, however, are unlikely to introduce substantial curvature to an originally planar shear zone.If the observed spread in active normal-fault dips is caused by rotation of the faults and the blocks they bound during extension, then a maximum β value of 1.7 can be accommodated byseismicactivity on a single generation of normal faults. With continued extension, either a new generation of steeper faults, cutting the rotated first faults, is likely to form, or the deformation will continue aseismically on faults dipping at less than 30°.Large earthquakes have not been observed to nucleate on very low-angle (<20°) normal faults within the continental crust anywhere in the world. Such faults can of course move aseismically, but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active. Thus extensional models that require concentrated simple shear on large sub-horizontal faults within the brittle upper crust will also require a spatial separation between aseismic, very low-angle faulting and seismic high-angle faulting. Since seismogenic high-angle faults dominate the topography of extending regions, upper-crustal very-low-angle faulting, if it occurs on a large scale, presumably does so in areas that are relatively flat.
The tensile strength of the lithosphere and the localization of extension
Authors: Lynch H. David , Morgan Paul 
Published: April 2008

Summary
Continents appear to rift in preference to oceans. Furthermore, some areas of continents appear to be more susceptible to rifting than others. Experimental rock mechanics data are used to estimate lithospheric strength for lithospheres of different structure, thereby to investigate the possible causes of rift localization. Using optimum creep parameters for silicic, mafic and ultramafic rocks, we find that lithospheric strength is inversely related to both crustal thickness and heat flow. By virtue of its thinner crust, oceanic lithosphere is inherently stronger than continental lithosphere. We find that oceanic lithosphere older than about 10 Ma should be able to withstand the lithospheric forces exerted on it by gravity sliding and plate interactions. Rifting or ridge-jumps are therefore only likely to occur in very young oceanic lithosphere. Low heat flow continental shields should also be able to withstand likely lithospheric forces without significant deformation. As the heat flow increases, however, the lithosphere is weakened dramatically. A smaller amount of weakening is associated with crustal thickening. Thus, unless rifting is localized by a strongly heterogeneous stress field, it will be by anomalously weak lithosphere. This lithosphere is likely to have anomalously high heat flow and/or crustal thickness prior to extension.
Some important consequences of lithospheric extension
Authors: Keen C. E. 
Published: April 2008

Summary
This paper briefly summarizes two of the important by-products of lithospheric extension during rifting. These are partial melting and the segregation and migration of basaltic melt to crustal levels, and the dynamical consequences of viscous flow in the lower lithosphere and asthenosphere. In both cases, the rift-stage subsidence may be significantly different from that predicted by a simple stretching model and may allow relative uplift of the rift zone, as well as subsidence. The dynamical model also predicts significant uplift of the rift shoulders, doming over the entire rift zone, and asymmetrical continental breakup at rifted continental margins. The relative importance of these predictions depends on the thermal and mechanical properties of the lithosphere. The model predictions help explain some of the discrepancies between the observed behaviour of rifts and results obtained from the simple stretching models.