bluuebunny/crossref_metadata_2025 huggingface dataset:
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

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

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

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

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

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

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

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

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

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

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

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

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.
Turbidity-current and debris-flow pathways to the Cape Verde Basin: status of long-range side-scan sonar (GLORIA) surveys
Authors: Kidd Robert B. , Hunter Peter M. , Simm Robert W.
Published: April 2008

A regional GLORIA survey in the southern Cape Verde Basin identified highly variable patterns of backscattering intensity that were interpreted as resulting from seabed roughness and small-scale slope changes. It was inferred that their origin was linked to turbidity-current pathways across the continental rise. Farther north, a survey of the Saharan Continental Rise SE of Madeira identified even more spectacular changes in backscattering. Follow-up 3.5 kHz echo-sounder surveys together with core-sampling and bottom-camera studies allowed an assignment of acoustic facies to sediment type. In this area turbidity-current channels and deposits have been overrun by long-range debris flows resulting from the late-Quaternary Saharan sediment slide. Preliminary results of a further GLORIA survey that links the previously studied areas with a single swath along the continental rise are reported in this paper. The resulting sonographs show details of submarine-slide-debris-flow complexes on this margin that had previously been mapped from 3.5 kHz records. They extend across the lower continental rise to the plain areas. Others do not extend as far as the lower rise. Debris flows can no longer be thought of as unequivocal indicators of proximality in the geological record.
Organic carbon oxidation and preservation in NW Atlantic continental margin sediments
Authors: Heggie D. , Maris C. , Hudson A. , Dymond J. , Beach R. , Cullen J.
Published: April 2008

We present distributions of pore-water inorganic metabolites (nitrate, ammonia, manganese and iron), dissolved organic matter (DOM) including dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) and solid-phase organic carbon from the Hatteras Abyssal Plain, the Hatteras Continental Rise and Slope and the northern Bermuda Rise areas of the eastern United States seaboard. Organic carbon oxidation rates in the sediments were calculated from models of the pore-water inorganic metabolites and the solid-phase organic carbon distributions. The pore-water model results were consistently higher than rates determined from the sediment organic carbon distribution. This result may reflect an inappropriate use of bulk sediment mixing coefficients in simple solid-phase models to estimate rates of interfacial diagenetic processes. The organic carbon oxidation rates in the sediments were combined with the organic carbon burial rates to assess the particulate organic flux into the sediments, and these results were compared with the particulate organic rain caught in sediment traps. Trap fluxes were consistently higher than model-estimated fluxes. We cannot make an unequivocal interpretation of the organic carbon mass balance, but we suggest that rates of benthic metabolism estimated from inorganic metabolite and sediment organic carbon distributions underestimate rates of organic carbon cycling because some fraction of the particulate organic carbon rain caught in traps is remineralized as DOM. Pore-water enrichments of DOM, DOC and DON were a factor of 5–10 higher than bottom-water concentrations, suggesting a flux of DOM to bottom waters. DOM fluxes to bottom waters contribute to abyssal metabolism in the benthic boundary layer and not to sediment metabolism.
Early Proterozoic Metavolcanic Suites of the Northernmost Part of the Baltic Shield
Authors: Pharaoh T. C. , Warren A. , Walsh N. J.
Published: April 2008

Metavolcanic rocks of Proterozoic age occur in three major supracrustal belts in the northernmost part of the Baltic Shield, in Norway and the USSR.

In the west, the
Kautokeino Belt
, and its continuation into the tectonic windows of Finnmark, contains thick sequences of tholeiitic metabasalt the age of which is poorly constrained at present. In the Repparfjord Window, these basalts are strongly enriched in Fe, Ti, LIL and LREE and are chemically similar to modern continental flood basalts. Stratigraphic equivalents in the Alta area and Kautokeino Belt are less enriched in incompatible elements, and more like MORB in composition, although a within-plate setting is indicated by the field evidence. These differences are believed to reflect heterogeneity of the mantle source region, with a locally ‘enriched’ source in the Finnmark windows.

The
Karasjok Belt
of central Finnmark forms the northward continuation of the Kittilä Belt of northern Finland. Amphibolitic rocks derived by metamorphism of tholeiitic basalts dominate the tectonostratigraphy of the greenstone belt. These are associated with metamorphosed komatiitic rocks which have recently yielded a Sm-Nd age of 2.1 Ga. The Karasjok Belt is interpreted as an early Proterozoic supracrustal sequence thrust westwards as an allochthonous sheet during uplift of the Lapland Granulite Complex in the Svecokarelian Orogeny at about 1.9 Ga.

The
Pechenga Complex
of the USSR continues westward into Finnmark, and overlies the Archaean gneisses of the Kola Complex. The Kola Superdeep Borehole penetrated a metamorphosed supracrustal sequence 7 km thick, dominated by basalts and andesites of calcalkaline composition. The MORB-normalized geochemical patterns of these lavas indicate that they were erupted in a mature continental volcanic arc.

The geochemical data support geotectonic models invoking rifting of pre-existing continental crust in the early Proterozoic. Komatiitic liquids were locally able to reach the earth’s surface, but probably only in the rift zones suffering greatest extension. Calcalkaline arc magmatic suites along the western edge of the Kola Craton reflect subduction of‘oceanic’ lithosphere, although no trace of this is preserved along the suture now marked by the Karasjok Belt.
Flow and flow mechanisms in crystalline rock
Authors: Black J. H.
Published: April 2008

Recent work on the hydrogeology of crystalline rocks has evolved rapidly from basic data gathering to specific experiments for evaluating details of flow mechanisms. Single-borehole data from around the world are similar and independent of detailed geology and test techniques. It appears that most faults in crystalline rocks have an effective hydraulic conductivity of around 1 × 10
−7
ms
−1
with values for intact rock about five orders of magnitude lower. The porosity of unweathered crystalline rock measured in the laboratory usually varies between 0.1% and 1% with weathered samples attaining up to 15%. When large data sets are considered it is possible to correlate depth and hydraulic conductivity. The interpretation of hydrogeological tests has evolved from basic porous media through fissures bounded by impermeable rock and fissures bounded by permeable rock to networks and channels. Whereas most hydraulic testing is adequately interpreted using the fissured porous medium concept, tracer tests require highly complex fissure network models with channelling. There is growing evidence that groundwater flow is concentrated in a limited network of channels. This seems to apply on both the laboratory and the regional scale.
Tectonic control of desert sediments in rift basins ancient and modern
Authors: Frostick L. E. , Reid I.
Published: April 2008

Desert sediments are frequently associated with continental rifts or aulacogens in the stratigraphic record. In this circumstance, rift structure and the timing and magnitude of tectonic events exert an overriding control on the distribution and character of alluvial sediments. Recent advances in the understanding of rift structure make a reassessment of ideas on the relationship between faulting, drainage patterns and alluvial facies distribution timely. Existing models of symmetrical graben with large alluvial fans shed from the footwalls of planar, normal boundary faults do not accord with observations in recent rifts. For example, half-graben dominate the structure throughout the African rifts. Boundary faults are mostly listric and the associated back-tilting of fault-blocks, combined with the doming that frequently accompanies rifting, causes back-shedding of rivers away from the basins. Alluvial fan development at fault scarps is, therefore, limited. The maximum observed fan radius in Lake Turkana is, for example, only 8 km, or 10% of the half-graben width at that point. The supply of sediment to rift basins can be dominated by axial rivers (
eg
Sugata R. flowing into L. Logipi in Kenya, the Awash R. of the Afar in Ethiopia, and the Rhine R. flowing through the Rhine rift), but there are a surprising number of basins that do not have major axial inputs (
eg
L. Tanganyika and L. Malawi in East Africa and L. Baikal in Siberia). In these cases, the importance of smaller, but more numerous rivers draining across the roll-over becomes obvious. These rivers are subjected to rapid changes in base level and longitudinal gradient: movement of a boundary fault alters the geometry of the lake basin and base-level falls as the lake adjusts. This is accompanied by the aggradation of coarse alluvial deposits across the roll-over. Clast size gradients in these deposits are well defined for the region between the basin margin and the main depocentre, with typical measured values of −3 mm km
−1
. During the long, intervening periods of tectonic quiescence these and other deposits are reworked and redistributed downslope and offshore. Deltas build out into the lake, but microstratigraphy reveals that they are not of the classic Gilbert type. Short- and long-term fluctuations in lake level cause large-scale migration of the shoreline so that lake muds and alluvial sands are interdigitated over many kilometres in sequences which vary from a few metres to tens of metres in thickness. A synthesis of detailed work carried out on the sedimentology and geomorphology of a modern rift of the common half-graben type provides a new model of sedimentation that will prove useful in interpreting ancient deposits with hydrocarbon potential.
The diagenesis of the Great Estuarine Group, Middle Jurassic, Inner Hebrides, Scotland
Authors: Hudson J. D. , Andrews J. E.
Published: April 2008

The Bathonian Great Estuarine Group consists of sandstones, silty shales and mainly shelly limestones, deposited in micro-tidal brackish lagoons in a warm, seasonal climate. The rocks show, in general, a lack of intense early diagenesis or deep-burial diagenesis, and are thus suitable for the study of shallow-burial diagenetic processes. Early diagenetic changes can be directly linked to depositional environment. They were only important on the lagoon margins, where cyanobacteria flourished in schizohaline ‘algal marsh’ settings and dolomite formed in response to evaporation of low-salinity lagoonal waters. During burial to a few hundred metres, and after initial compaction, cementation by ferroan calcite occurred. This was pervasive in limestones and formed large concretions in sandstones. Quartz and feldspar grains in sandstones were marginally corroded by the porewaters that precipitated calcite, and some feldspars had previously or concurrently suffered partial solution. Aragonitic mollusc shells were replaced by calcite in permeable rocks but remained unaltered in shales. Widespread volcanism in the Palaeocene buried the area beneath basaltic lavas and dykes were intruded. These events produced little metamorphic or burial-diagenetic change, except in close proximity to minor intrusive contacts or in the vicinity of plutons. The evolution of porewaters can be monitored in the carbon and oxygen isotopic composition of diagenetic calcites. After initial burial, they were dominantly of meteoric derivation. Vitrinite reflectance measurements confirm the mild thermal history of most of the sediments, and show the local effects of heating by igneous intrusions.
A new look at the causes and consequences of the Icelandic hot-spot
Authors: Bott M. H. P.
Published: April 2008

New support for the hypothesis of a lower mantle plume rising near Iceland comes from recent developments in the theory of whole-mantle convection and is inferred from the large heat input required to support the major topographic swell which affects much of the N Atlantic and Greenland. The associated geoid and gravity highs are interpreted as due partly to the swell and its deep compensation and partly to the pressure anomaly caused by the plume. Rapid lateral migration of partially fused material from the plume in the asthenosphere prior to continental break-up caused the early Tertiary volcanism, with subsequent igneous activity concentrated at the ridge crest to form the Icelandic transverse ridge. Progressive expansion of the upper mantle thermal anomaly in response to pressure, in association with the complexities of plate evolution in this region, may account for the early uplift of S Greenland and later uplift of the Blosseville Kyst region. The low density upper mantle would also be expected to give rise to an anomalously high ridge-push force.
Early Tertiary volcanic rocks in Well 163/6-1A, Rockall Trough
Authors: Morton A. C. , Dixon J. E. , Fitton J. G. , Macintyre R. M. , Smythe D. K. , Taylor P. N.
Published: April 2008

Well 163/6-1A, located in the northern part of the Rockall Trough, proved the presence of a thick sequence of extrusive igneous rocks below Upper Palaeocene sediments. K-Ar age dating suggests that the lavas were extruded at approximately 55 Ma. The lava sequence comprises three distinct lithologies. The upper part of the pile consists of olivine tholeiites that show alkalic and picritic tendencies and have a distinct within-plate composition. These are underlain by another group of olivine tholeiites that are much closer in composition to normal mid-ocean ridge basalt (N-type MORB). The Sr and Pb isotopic compositions of the basalts suggests possible contamination by continental crustal material.

The basalts are underlain by a sequence of cordierite-phyric dacites of remarkably homogeneous composition. Their highly aluminous nature, high Ni and Cr contents and their Sr and Pb isotopic compositions indicate that they are not differentiates of a basaltic parent magma, and are considered to have originated by melting of argillaceous and, possibly, arenaceous rocks at depth. Organic-rich black shale lithologies may have been involved.
Biological marker compounds as indicators of the depositional environments of petroleum source rocks
Authors: Volkman J. K.
Published: April 2008

Recent studies concerned with the use of biological marker compounds for assessing the source of organic matter in sediments and crude oils are reviewed and critically assessed. Compounds which are indicative of inputs from bacteria and phytoplankton are reviewed as well as some compounds for which origins have not been determined. The use of ratios of C
27
and C
29
steranes for assessing the depositional environment of petroleum source rocks is shown to be particularly open to misinterpretation. Some inferences based on pristane/phytane ratios also need to be reassessed. Areas of research where more studies would be valuable are highlighted.
Palaeolimnology and organic geochemistry of the Middle Devonian in the Orcadian Basin
Authors: Duncan A. D. , Hamilton R. F. M.
Published: April 2008

An interdisciplinary study of the Middle Devonian lacustrine laminites from the Orcadian Basin of NE Scotland has enabled the construction of a predictive source-rock model which we have applied to the Moray Firth area.

Parameters characterizing areal source-rock extent, primary organic input, and the depths, salinity levels and redox potential of the depositional water column are discussed, together with the quantity and maturity of the preserved sedimentary organic matter.

On the basis of this model, we suggest that a Middle Devonian component is present in the Beatrice crude (an atypical North Sea oil) in the Moray Firth Basin. This proposal is largely supported by molecular geochemical evidence.
Styles of soft-sediment deformation on a Namurian (Carboniferous) delta slope, Western Irish Namurian Basin, Ireland
Authors: Martinsen O. J.
Published: April 2008

Soft-sediment deformation features from a delta-slope succession exposed in the Western Irish Namurian Basin, Ireland, are described and related to three main styles of deformation: (i) slumping, (ii) sliding and (iii) water-escape structures. While (i) and (ii) were formed by near-surface gravitational gliding of sediment, (iii) was formed by mainly vertical motion of sediment and escaping pore fluids. Deformation structures formed by slumping include slump folds and large- and small-scale faults, internal shear surfaces, deformation of early calcareous concretions and boudinage. The slumps were initially dominated by plastic deformation, whereas brittle deformation dominated the late stages. Deformation structures formed by sliding are mainly macroscopic faults, but microfaults and slide folds also formed locally. The slides were at all stages dominated by brittle deformation. Water-escape structures include mud and sand dykes and volcanoes, collapse depressions, mud diapirs and small-scale loading. These structures can be either related to or independent of slumping and sliding. While slumps are not restricted to any particular part of the delta slope, the occurrence of slides is, to a large degree, dependent upon grain size. Mud slides only occur on the muddy upper delta slope, while sand slides are only found on the lower delta slope. Water-escape structures tend to be more common on the lower delta slope.
Controls on internal structure and architecture of sandstone bodies within Upper Carboniferous fluvial-dominated deltas, County Clare, western Ireland
Authors: Pulham A. J.
Published: April 2008

Deposition of Upper Carboniferous (Namurian R1-R2) deltas in the Clare Basin, western Ireland, was influenced by a combination of processes: fluvial, basinal (notably waves), synsedimentary deformation, and rapid subsidence rates within the receiving basin. Fluvial processes predominate; consequently the major deltaic sandstone bodies are fluvial channels and fluvial-dominated mouth bars. The internal structure of single channel sandstones reflects high energy tractive processes and aggradation of sands in relatively straight channel reaches. The structure of mouth-bar sandstone bodies reflects a variety of frictional and buoyant river-mouth processes. Depositional processes within the Clare Basin are often overprinted by syndepositional deformation. Delta-front sequences exhibit abundant evidence of synsedimentary faulting, slumping, clay diapirism and gravity sliding. Deformed packets of sediment are up to 60 m thick and over half of all delta-front sequences are disturbed. These processes led to a modification of mouth-bar structure and geometry. Some fault-bounded delta-front sandstone bodies fill fault-induced depocentres. Large, multistorey and laterally composite channel sandstones in the S of the basin may reflect major deformation across the delta plain, related to rapid subsidence and compaction rates. In contrast, the architecture and structure of delta-plain sequences in the N of the basin are consistent with a more stable fluvial-dominated setting.
Facies and development of the Middle Jurassic Brent Delta near the northern limit of its progradation, UK North Sea
Authors: Brown S. , Richards P. C.
Published: April 2008

The lithofacies patterns in the Middle Jurassic Brent Group, the principal hydrocarbon reservoir in the UK northern North Sea, are examined in seven wells. The sections, from the area of the Don and Murchison oil fields, record the nature of the Brent Delta near the northern limit of its progradation. Here the deltaic deposits of the Brent Group, which regionally constitute a major regressive-transgressive clastic wedge, progressively lose the interbedded sands and muds of the delta plain which characterize the middle part of the succession further S. This leads locally to a maximum sand-to-mud ratio for the Brent Group reservoir interval.

A regressive storm-wave-influenced marine succession, from offshore transition-zone silts and muds up to hummocky cross-stratified nearshore sand, is overlain in turn by stacked distributary channel sands. Distributary mouth deposits are absent. The interbedded delta-plain succession, where it occurs, exhibits some marine influence and records wave reworking of interdistributary bay or lagoonal sediments. The Brent Group succession is capped by transgressive sands which were deposited in a storm-wave-influenced nearshore environment. Delta-plain drowning is also evident in the gradual marine incursions into distributary channels at the northern limit of the study area.

The character of the delta locally is compared with the nature of the wave-dominated Brent Delta elsewhere on its progradational path. High energy nearshore deposits in both regressive and transgressive phases are widely distributed, but there is a contrast between the dominance of fluvial processes in the study area and the development elsewhere of a barrier island complex in front of a coastal plain.
Tectonic controls on boninite genesis
Authors: Murton B. J.
Published: April 2008

Boninites are rare high-MgO and compatible-element-rich basalts that are depleted in incompatible trace elements. Whilst their high magnesium number indicates equilibrium with upper-mantle peridotite, their depleted trace-element geochemistry indicates a highly refractory and depleted source. Many models account for boninite magmatism in destructive margin environments but none has so far explained their erratic spatial and temporal occurrence. Recent studies along the southern Troodos margin have revealed a history of boninite magmatism within a complex supra-subduction zone transform fault. The transform fault boninites form a suite that cross-cuts a less trace-element-depleted magmatic sequence forming the Troodos oceanic crust that was generated at a constructive plate margin.

Melting models demonstrate that the transform fault boninites were probably produced by a 10% melt of the upper mantle which had already been depleted by the extraction of the Troodos crust and mid-ocean ridge basalts (MORB). In contrast with the Troodos crustal sequence, however, the boninites are also enriched in two separate components, selected high field strength elements and large-ion lithophile elements. Such incompatible-trace-element enrichments are unlikely to have survived the earlier source depletion events. Therefore the enrichment of the transform boninites must have been added to their source immediately prior to or during their genesis within the supra-subduction transform environment.

The identification of rapid dilation and transtensional transform displacement along the southern Troodos margin suggests important tectonic factors in the formation and emplacement of boninite magmas. The requirement of an extensional regime and rifting of trace-element-depleted oceanic crust formed above a subduction zone (
eg
extension and rifting of an extended fore-arc region) explains the erratic and rare nature of boninite magmatism.
Geodynamic model for Alpine intra-plate compressional deformation in Western and Central Europe
Authors: Ziegler P. A.
Published: April 2008

The Mesozoic grabens and wrench induced troughs of Western and Central Europe developed in response to Triassic to Early Cretaceous intra-plate tensional stresses which affected the Arctic — North Atlantic and the Tethys borderlands during the rifting phases preceding the opening of the respective oceans. During the Alpine orogeny, collision-related compressive stresses exerted on the foreland induced the reactiviation of pre-existing fracture systems, and caused the inversion of Mesozoic grabens and the uplifting of major basement blocks at distances up to 1300 km to the north of the present Alpine deformation front. The structural style of these inversion structures is indicative of compression and transpression. Total crustal shortening is unlikely to exceed a few tens of kilometres. These displacements require a coupling between the foreland and the orogen at the Alpine A-subduction zones and, within the foreland, a decoupling at intra-crustal levels between the crust and mantle and/or at deeper lithospheric levels.
Rift basin evolution in Africa: the influence of reactivated steep basement shear zones
Authors: Daly M. C. , Chorowicz J. , Fairhead J. D.
Published: April 2008

Phanerozoic rift basin evolution in Africa has been strongly influenced by the pre-existing basement structure inherited from Precambrian tectonics. Two modes of basement reactivation may be identified; the largely dip-slip reactivation of gently dipping shear zone and thrust structures and the largely strike-slip reactivation of steep shear zones and faults. In particular, crustal scale steep basement shear zones have exercised a profound control on rift basin formation on a regional scale. Such shear zone structures are seen to govern the location, trend and structural style of rifts and appear to constrain the orientation of displacements generating the rifts. The shear zones are relatively narrow, high strain zones formed during ductile deformation. They occur as steep, crustal scale ramps that define a pronounced mechanical anisotropy within the African continental crust. These structures are difficult to reactivate by dip-slip movement and thus tend to act as major strike-slip transfer zones during later extensional events. This paper discusses examples of the steep shear zone-rift relationship from three of the rift systems of Africa: the Upper Palaeozoic (Karoo) rifts of Central Africa, the Mesozoic rifts of the Sudan and the Cenozoic rifts of East Africa. These discussions indicate that steep shear zones are sites of rift nucleation and play a fundamental role in the mechanisms of rift propagation and subsequent continental breakup.