bluuebunny/crossref_metadata_2025_split huggingface dataset:
Mesozoic marine organic-rich shales
Authors: Hallam A. 
Published: April 2008

Summary

A survey of Mesozoic organic-rich shales in space and time indicates that they are widely distributed through much of the era but are concentrated especially at two levels: Upper Jurassic (Kimmeridgian-Volgian/Tithonian) and Middle Cretaceous (Aptian-Turonian). Stable carbon isotope studies prove to be a particularly useful tool in both palaeoenvironmental and diagenetic analysis. Anoxic events in the ocean may be characterized by high concentrations of δ
13
carbonate carbon. The carbon isotope values obtained may, however, reflect diagenesis in the sulphate reduction zone rather than conditions in the original sea water. Calcite concretions formed at successive stages in diagenesis may record a change from strongly negative to strongly positive values of δ
13
C, indicating a switch from the sulphate-reduction to the fermentation zone.

Analysis of benthic palaeoecology leads to a classification of marine mudrocks into barren laminite, shelly laminite, shelly shale and shelly mudstone, of which the last lacks any significant quantity of kerogen. By analogy with Recent sediments an attempt is made at a tentative estimate of the oxygen content at the sediment/water interface during the original deposition. Environments of deposition are considered in general terms and the respective merits of several facies models for the Mesozoic deposits are briefly discussed.
A review of petroleum source rocks in parts of the Middle East
Authors: Stoneley R. 
Published: April 2008

Summary
The oils in the Mesozoic and Tertiary reservoirs of the region surrounding the inner Persian/Arabian Gulf were derived from Jurassic and Cretaceous argillaceous and carbonate source rocks, which accumulated at various times in more or less restricted intrashelf basins and depressions, and were preserved under anoxic conditions. Maturation, migration and accumulation took place from the late Cretaceous onwards, at different times in different parts of the region; however, Upper Cretaceous and Tertiary sediments are, for the most part, still immature. This sourcing history is one of a number of favourable circumstances contributing to the richness of the region.
The extensional strength of the continental lithosphere: its dependence on geothermal gradient, and crustal composition and thickness
Authors: Kusznir N. J. , Park R. G. 
Published: April 2008

Summary
The response of lithosphere to an applied tectonic tensile force and the resulting stress distribution with depth has been investigated using a mathematical model incorporating the elastic, plastic and brittle behaviour of lithospheric material. Lithospheric strength is shown to be primarily controlled by lithospheric rheology and as a consequence is critically dependent on geothermal gradient and lithospheric composition. The rheologies of the upper crust, lower crust and mantle are assumed to be controlled by dislocation creep in quartz, plagioclase and olivine respectively.

The critical level of tensional tectonic force required to generate geologically significant strains has been calculated as a function of surface heat flow, and the predicted lithosphere strength compared with available levels of tensile tectonic force arising from subduction plate boundaries and isostatically compensated plateau uplift loads. The model predicts significant extensional deformation in regions with surface heat flow >65 mWm
−2
subjected to a tensile tectonic force of 3 × 10
12
N m
−1
and is in good agreement with observed examples of intraplate extension.

Lithosphere strength is critically controlled by the crustal thickness since the quartzofeldspathic rheology of the crust is weaker than the olivine rheology of the mantle. A decrease in crustal thickness thus increases the strength of the lithosphere. However, lithospheric extension also increases the geothermal gradient serving to weaken the lithosphere. The rate of extension is critical in determining which of these processes predominates. Fast strain rates (> 5 × 10
−15
sec
−1
) produce a weakening of the lithosphere (i.e. strain softening) while slower strain rates lead to strengthening of the lithosphere (strain hardening). Extensional style is consequently controlled by the lithospheric extension rate; fast extension producing, through strain softening, intense localized lithospheric extension with high (potentially infinite)
β
values, and slow extension, through strain hardening, giving broader regions of lithosphere extension with finite β values of the order of 1.5.

For intermediate geothermal gradients (
q
= 55−70 mWm
−2
) the model predicts a low stress-low strength region at the base of the crust due to the contrast between plagioclase and olivine rheology at the Moho. Other low-strength regions are predicted within the crust at major compositional (and rheological) boundaries. These low-strength zones are expected to control the location of detachment horizons by which crustal extension occurs particularly at slower strain rates. High geothermal gradients favour the shallower detachment horizons at the expense of the deeper horizons. The reverse is true for low geothermal gradients.
Lithospheric stretching, detached normal faulting and footwall uplift
Authors: Barr David 
Published: April 2008

Summary

Lithospheric stretching can successfully account for the overall evolution of many sedimentary basins, and detached normal faulting the detailed geometry of the upper crust. In an instantaneously stretched, isostatically compensated basin, the equations which describe these two processes can be combined to define a ‘notional depth to decollement’. Only at this level can the sole to the normal fault system maintain a constant depth below sea-level during extension. The notional depth to decollement depends primarily on the mean density of the basin fill and for a constant-density basin fill (e.g. sea water) coincides with the level of no vertical motion during stretching. In a sediment-filled basin, the notional depth to decollement will increase with the stretching factor
β
as early-deposited sediments are compacted and the mean density of the sediment column increases. In general, a physical sole fault will not lie at the notional depth to decollement, and must move vertically to maintain isostatic equilibrium: such movement precludes the use of balanced cross-section techniques to determine the physical depth to decollement.

In typical crustal situations, uplift of the sole fault will be more common than subsidence and will in turn cause uplift of any residual, unfaulted basement blocks which rest upon it. Footwall uplift can also be modelled using area-balance constraints, referred to the notional depth to decollement rather than to the physical sole fault. The amount of uplift depends primarily on the initial fault spacing. Three fields can be distinguished: one in which footwalls subside at an increasing rate as
β
increases, one in which they are uplifted above sea-level then subside below sea-level, and one in which they are uplifted then subside, but always remain above sea-level. Similar relationships exist in an uncompensated basin, where the depth to the physical sole fault replaces the notional depth to decollement.

Curves showing uplift and subsidence versus β have been constructed in dimensionless form (referred to depth to decollement) and for a model basin with an exponential sediment compaction relationship. They agree closely with uplift/subsidence histories inferred from seismic and well data for the North Sea, and with published descriptions of other areas (the Armorican margin, the Aegean Sea).
A physical model for Cenozoic extension of western North America
Authors: Sonder L. J. , England P. C. , Wernicke B. P. , Christiansen R. L. 
Published: April 2008

Summary
We investigate the possibility that the onset and development of Cenozoic extension in western North America was governed by the potential energy contrast within, and mechanical properties of, lithosphere that was previously thickened during the Sevier and Laramide Orogenies. The strength of the lithosphere can be defined by its vertically averaged properties; to a first approximation, this strength is too great for geologically significant extension to occur unless the Moho temperature exceeds about 700°C (±100°). This means that there may be a delay between the end of compression and the beginning of extension, the length of which depends on the pre-thickening thermal structure and the compressional strain. Delays of up to 100 My may occur for the lowest initial Moho temperatures investigated (<450°C), or extension may follow immediately on release of compression if the initial Moho temperature exceeds about 700°C. The total extensional strain that is achieved depends on the potential-energy contrast between the thickened lithosphere and its surroundings. Partial melting of peridotite to produce basaltic magma is possible after small degrees of extension, but depends strongly on details of the initial temperature condition in the lower part of the lithosphere.
The results of these calculations agree with observations of the Cenozoic extensional history of North America: late-Mesozoic/early-Tertiary compression in the Pacific Northwest was accompanied by extensive calc-alkaline magmatic activity and was followed almost immediately by extension; in the northern and southern Great Basin—which show respectively, little and no evidence of syn-compressional igneous activity—the gap between compression and extension was 20–40 Ma (N) to about 70 Ma (S).
Lithospheric extension on the Antarctic Peninsula during Cenozoic subduction
Authors: Garrett S. W. , Storey B. C. 
Published: April 2008

Summary
The magmatic arc of the Antarctic Peninsula displays a wide range of features related to extensional tectonics at a convergent margin. Cenozoic extensional features are concentrated in a linear belt on the western side of the peninsula. The West Coast Magnetic Anomaly (WCMA) indicates the presence of a composite batholith 90 km wide and 1 500 km long, part of which may have been emplaced during an initial splitting of the continental arc. Horst and graben structures subsequently developed in segments bounded by continental fracture zones reflecting those of the oceanic lithosphere. This suggests the influence of processes associated with the late stages of subduction. The arrival of segments of the ocean ridge at the trench may have resulted in the extension and uplift of the adjacent portion of the continental plate. The final set of these interactions began at about 7 Ma opposite the northern Antarctic Peninsula and coincided with the dramatic slowing of spreading at the surviving ocean ridge and the development of continental alkaline volcanism. Continued extension in response to the tendency of the subducted plate to continue sinking led to total failure of the continental lithosphere and the opening of Bransfield Strait, a marginal basin floored by oceanic crust.
Extensional tectonics and the fragmentation of Gondwanaland
Authors: Dalziel I. W. D. , Grunow A. M. , Storey B. C. , Garrett S. W. , Herrod L. D. B. , Pankhurst R. J. 
Published: April 2008

Summary
Evidence of widespread lithospheric extension, bimodal volcanism, and uplift and erosion, accompanying the fragmentation of Gondwanaland, is reviewed. One regime of extensional phenomena appears to be intimately associated with a Pacific-margin convergent-plate regime. A second appears to bear no direct geometrical relationship to the margin, but may reflect a thermal anomaly that resulted in the breakup of Gondwanaland. Possible causes of this anomaly are discussed. The two regimes overlap in space and time in the South Atlantic-Weddell Sea area. A causal relationship between the two is possible but remains non-proven.
Extensional structures on the western UK continental shelf: a review of evidence from deep seismic profiling
Authors: Cheadle M. J. , McGeary S. , Warner M. R. , Matthews D. H. 
Published: April 2008

Summary
BIRPS deep seismic reflection data collected on the western United Kingdom continental shelf show the existence of a wide variety of sedimentary basins, most of which originated during Palaeozoic-Mesozoic crustal extension. Symmetrical interior-fracture basins are numerous, but more complex basins are also common and show the importance of fault reactivation and the influence of pre-existing structures on basin development. All of the major basement-penetrating faults are interpreted as having been reactivated. They do not visibly cut through the entire crust and into the upper mantle. The crystalline crust thins dramatically beneath the basins indicating local crustal extensions of up to 60%, although regional extension is less than 30%.
The typical BIRPS profile shows a highly reflective lower crust sandwiched between an unreflective upper crust and upper mantle. This pattern of reflectivity appears to be characteristic of deep seismic data collected from within extended regions. The relatively small amount of extension which has affected this region suggests that the highly reflective lower crust is more likely to be due to lithological variation formed by mafic igneous intrusion and underplating during crustal extension than to-extensional ductile fabrics within the lower crust.
Fault patterns generated during extensional deformation of crystalline basement, NW Scotland
Authors: Laubach Stephen E. , Marshak Stephen 
Published: April 2008

Summary
The fracture array in the Lewisian basement of NW Scotland is dominated by two orthogonal sets. Parallel to and perhaps in part reactivating these fractures are two sets of faults, each with apparent normal offset. The NE-trending faults are longer and regionally more significant. The NW-trending faults commonly terminate against NE-trending faults and divide the hanging wall rocks of NE-trending faults into independent blocks. The propagation of principal NE-trending faults into the basement was accommodated discontinuously by relative differential displacement of these crustal blocks. The limited data suggest that incremental slip episodes were down-dip and coaxial on the NE-trending faults, but not on the NW-trending faults. The presence of non-coaxial low-angle slip lineations on NW-trending faults suggests either that master NE-trending faults rotated during progressive deformation, or that the dip of these faults decreases with depth. The dominance of NE-trending faults, and the kinematics of fault movements suggest that movement on the faults accommodated NW-SE extension.
Thin-skinned N-S extension within the convergent Himalayan region: gravitational collapse of a Miocene topographic front
Authors: Royden L. H. , Burchfiel B. C. 
Published: April 2008

Summary

Recent work by Burg
et al.
indicates the presence of E-W striking, gently N-dipping normal faults in the High Himalayas and southern Tibet, that formed during the post-collisional convergence of India and Tibet. These faults extend for at least 600 km along strike. We interpret them as probable late (?) Miocene extensional features with perhaps several tens of kilometres downward northerly displacement. A simple elastic model suggests that these normal faults may have formed during gravitational collapse of the Miocene topographic front between India and Tibet. In this interpretation, gravitational collapse occurred by southward motion, relative to India and Tibet, of a wedge of crustal rock bounded above by gently dipping normal faults and below by thrust faults that probably dip N. N-S extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere. Such faults may be common, but so far mainly unrecognized, features developed during convergence in many orogenic belts.
Hydrocarbon migration by hydraulic fracturing
Authors: Mandl G. , Harkness R. M. 
Published: April 2008

Abstract

The fracturing of cap rocks of very low permeability—in particular shales—by the intrusion of hydrocarbons under various
in situ
stresses has been considered. The conditions are discussed for the domination of the migration mode by the formation of either hydrocarbon dykes or sills, or a combination of both. Application of concepts of pro-elasticity and fracture mechanics to the process of dyke formation indicates the key roles of capillary pressure, premigration water pressure and
in situ
rock stresses, and provides conditions for maintaining open dykelets, for the change in width and spacing of growing fractures, and their closure, and explains differences in the behaviour of oil- and gas-filled dykelets.

In discussing the situations where the state of
in situ
stresses favours the development of bedding-parallel intrusion fractures, various mechanisms are suggested which may allow interconnecting dykelets to form and feed hydrocarbon sills at stratigraphically higher levels.
Faulting mechanisms in high-porosity sandstones; New Red Sandstone, Arran, Scotland
Authors: Underhill John R. , Woodcock Nigel H. 
Published: April 2008

Summary
Faults in the ‘New Red’ aeolian sandstones of Arran are unusual, firstly for occurring as closely-spaced (less than 1 m) often conjugate sets affecting large volumes of rock, and secondly for forming upstanding fault zones with numerous anastomosing strands of granulated rock, each preserving a small increment of slip. Anisotropy, such as bedding and cross-bed sets, has no discernible effect on fault behaviour. In contrast to the underlying Carboniferous rocks, large displacements are rarely concentrated on a single fault plane within the high-porosity sandstones. The proposed cause is slip-hardening of each fault after a very small displacement (less than 10 mm) causing the next slip increment to be taken up through undeformed rock rather than on the original plane.
The common factor in recent records of similar faults elsewhere is their occurrence in high-porosity sandstones. Because of the low grain-contact strength, these rocks are partly analogous to unconsolidated sediment. The high porosity promotes high grain-contact stresses which induce rapid cataclasis during initial slip. Grain fracture and spalling of iron oxide coatings and quartz overgrowths produce a seam with reduced grain size, poorer sorting, higher angularity and lower porosity than the unfaulted rock. These factors collectively strengthen the seam because the coefficient of friction is increased, even though cohesion is reduced. This results in a Mohr failure envelope that lies outside the envelope of the undeformed rock for most stress states. A transient pore pressure increase in the fault seam may be important during slip.
Rocks deformed by this slip-hardened faulting preserve a record of each increment of strain. If the displacement on each individual fault seam is the same, the geometry of the total fault systems is directly related to the bulk strain. Quadrimodal systems observed by us in Arran, and by others elsewhere, are probably a response to triaxial strain and show that bimodal ‘Andersonian’ fault systems are only special plane strain cases. If the bulk strain is irrotational, both the orientation and relative magnitude of the principal strains might be estimated.
Quaternary peralkaline silicic rocks and caldera volcanoes of Kenya
Authors: Macdonald R. 
Published: April 2008

Summary
The late-Quaternary trachytic caldera volcanoes of the Kenya rift provide an unrivalled opportunity for studying the mechanisms of evolution of large peralkaline volcanic complexes and assessing the fundamental problems of magma genesis and chemical differentiation. In a northern set of centres (the Barrier, Emuruangogolak, Silali and Paka) basalt is a major component and mugearites, while scarce, may also be present. Caldera collapse was possibly of Kilauean type. Geochemical variations and the relationships between eruptive rocks and suites of plutonic nodules are consistent with fractional crystallization as the dominant differentiation mechanism in these centres.
The southern basalt-absent centres (Menengai, Longonot and Suswa) show Krakatoa-style collapse. The pyroclastic sequences, especially syncaldera ash-flow deposits, indicate the ubiquity of striking compositional zonations within the magma chambers. Chemical variations in these centres have been ascribed to complex interplays of side-wall crystallization, magma mixing and liquid-state differentiation processes, especially involving volatile complexing. The ultimate origin of the trachytes at these centres is still debatable; crystal fractionation and partial melting of basalt are both viable mechanisms. The different development of the two types of centre may be related to the growth of a volatile-rich cap, which profoundly influences the types of differentiation mechanism.
Niger-Nigerian alkaline ring complexes: a classic example of African Phanerozoic anorogenic mid-plate magmatism
Authors: Bowden P. , Black R. , Martin R. F. , Ike E. C. , Kinnaird J. A. , Batchelor R. A. 
Published: April 2008

Summary
The Pan-African orogeny, regarded as a major event in the construction of Gondwanaland, played a dominant role in providing a variety of source materials for mineralization, melting or assimilation as components of the Benin-Nigerian shield. Although the parent magmas of the silica-oversaturated alkaline rocks which constitute the Niger-Nigerian anorogenic province were initiated in the asthenosphere, it was the exploitation and reactivation of the Pan-African shear zones and transcurrent faults during the fragmentation of Gondwanaland that controlled the locations of Phanerozoic intra-plate magmatism in W Africa.
The Niger-Nigerian anorogenic province represents one of those regions where progressive uplift has been accompanied by periodic sequential development of chains of volcanoes now exposed as ring complexes with Palaeozoic and Mesozoic ages. The magmatic lineage comparable with other alkaline provinces can be established from preserved volcanic sequences. Associated andesite compositions can be attributed to magma mixing.
In Niger, an important petrogenetic parameter is the occurrence of leucogabbros, anorthosites and monzoanorthosites as part of the Palaeozoic sub-volcanic association with syenites, peralkaline granites and biotite granites. The partially eroded volcanic cover of rhyolitic ignimbrites has provided the source for substantial uranium deposits. The southern centres of upper-Silurian-lower-Devonian age are mineralized with columbite and cassiterite.
The general geological and geochemical features of the Nigerian Triassic-Jurassic anorogenic centres are well known. While their magmatic derivation from mantle and crustal sources can be conclusively demonstrated, petrological and geochemical research has shown that many of the Nigerian anorogenic centres demonstrate substantial evidence for post-magmatic metasomatism linked to mineralization. There are certain parallels between alkali metasomatism in alkaline granite ring complexes and fenitization associated with carbonatites.