bluuebunny/crossref_metadata_2025 huggingface dataset:
Alpine tectonics — an overview
Authors: Coward Mike , Dietrich Dorothee
Published: April 2008

This overview summarizes aspects of 150 years of research in Alpine tectonics and in particular introduces the tectonic setting for the more detailed papers in this volume.

The Alpine Mesozoic ocean, Tethys, formed as a large elongate pull-apart basin in the Jurassic, as a consequence of the opening of the Atlantic and of the movement of Africa towards the east relative to a fixed Europe. The NNE trending Tethys was bounded by WNW trending transforms, by the European/Iberian margin in the W and by the Adriatic promontory of Africa in the E, and its shape determined the present day configuration of the arcs of the Alpine chain. The closing of this ocean and the collision tectonics began during the Cretaceous, as Africa moved to the NE relative to Europe and as the N Atlantic gradually opened, to drive Iberia and the southern part of the European plate to the E. Subduction of oceanic crust and adjacent continental crust led to high pressure metamorphism of Cretaceous age. Ophiolites were obducted over the southern continental margin, but after collision the shear sense reversed so that the Austro-Alpine nappes of the African Adriatic promontory overthrust Europe in a WNW direction. During the main Tertiary deformation the overall anticlockwise rotation of Africa led to a change-over from N to NW and WNW-directed collisional structures. The E-W striking sector of the Alps in Switzerland and Austria is therefore a diffuse transpressive dextral shear belt, approximately reworking the northern transform boundary of Tethys, modifying it by compression related to the rotation of the African Adriatic promontory.

Approximately 250 km of European lithosphere were involved in the building of the western Alps. As Alpine nappes consist largely of rock material confined to the upper crust, a large amount of lower crust and lithospheric mantle of the two continental blocks must be duplicated and/or subducted during the Alpine collision history.
Delamination and asymmetric lithospheric thickening in the development of the Tyrrhenian Rift
Authors: Channell J. E. T. , Mareschal J. C.
Published: April 2008

Late tectonic extensional basins are superimposed on the continental collision suture at certain points along the Alpine belt. The onset of extension in these basins (Alboran, Tyrrhenian, Pannonian and Aegean) postdates the continental collision by only a few million years. In the case of the Tyrrhenian region, the rifting of the collision suture is accompanied by contemporaneous shortening in the Sicilian/southern Apennine thrust belt. We use finite element models to test whether the rapid evolution of the collision suture into a zone of rifting, and the development of closely juxtaposed regions of compression and extension, can be explained by a mechanism involving collision-induced delamination of continental mantle lithosphere. With delamination underway, the models predict the development of a region of contraction flanked by regions of extension, but no appreciable migration of the delamination front. Investigation of various initial lithospheric geometries shows that a uniformly thickened lithosphere will not readily delaminate, and that delamination is triggered if crustal and lithospheric thickening are offset. Asymmetric, or offset, lithospheric thickening produces an asymmetry in underlying mantle flow which gives rise to extension which is largely restricted to one side of the region of compression. The juxtaposition of extension and shortening in the Tyrrhenian/Calabrian region, as well as the greater degree of extension in the Tyrrhenian relative to the Ionian basin, may be a result of asymmetric lithospheric thickening in the initial collision suture.
Oolitic ironstones as event-stratigraphical marker beds within the Upper Devonian of the Ardenno-Rhenish Massif
Authors: Dreesen Roland
Published: April 2008

Seven stratigraphically distinct, Clinton-type oolitic ironstones occur in the Upper Devonian of the Ardenne shelf, south of the London-Brabant Massif. They occur at the lithofacies boundary of succeeding lithological units and represent condensed deposits associated with hardgrounds and shoaling upward sequences. Microfacies analysis points to storm-generated concentration, removal and subsequent transport of the ferruginized allochems. The ironstones represent excellent marker beds which can be traced over several tens of kilometres on the palaeoshelf. Conodonts provide further evidence for the allochtonous character of the allochems and have enabled precise dating and correlation of each ironstone level with synsedimentary volcanic and tectonic events in the Rhenish Massif. Although the source of the iron is still a matter of discussion, lateritic weathering is not accepted here; instead, volcanism and evapotranspiration processes might represent potential sources for the iron in the studied ironstones.
Ironstones in the Mesozoic passive margin sequence of the Tethys Himalaya (Zanskar, Northern India): sedimentology and metamorphism
Authors: Garzanti E. , Haas R. , Jadoul F.
Published: April 2008

The sedimentary succession of the Zanskar continental terrace comprises Triassic (Quartzite Series), Jurassic (Ferruginous Oolite) and Cretaceous (Giumal Sandstone) shelf siliciclastic units, all of which contain either chamositic ironstones or glauconitic greensands associated with reworked phosphorites. Ironstones are found above major unconformities at the top of shoaling arenaceous sequences, and generally mark the rapid transition from shallow-marine sands to highstand offshore pelites or pelagic foraminiferal limestones. Petrographical and sedimentological features of condensed intervals indicate deposition by transgressive fronts. During sea-level rise, high-energy waves mixed ferruginous and phosphatic grains formed in the course of earlier starved stages at low sedimentation rates, with detritus from shoreline and paralic sources reworked during stepwise coastal retreat. The timing of ironstone deposition apparently coincides with breakup stages affecting the Indian continental margin.

Ironstones are also important metamorphic markers in the Tethyan sedimentary zone, which has undergone intense fold-thrust deformation at very low to low metamorphic grade during the Tertiary Himalayan orogeny. In the Triassic iron oolites of central Zanskar, the occurrence of stilpnomelane suggests upper anchimetamorphic conditions (
c.
300° C). In the Cretaceous greensands, however, K-rich glauconite is only peripherally replaced by incipient stilpnomelane growth, pointing to metamorphic conditions comparable to lower prehnite-pumpellyite facies (
c.
260° C).
Cadomian magmatism in the North Armorican Massif
Authors: Brown Michael , Power G. M. , Topley C. G. , D’Lemos R. S.
Published: April 2008

This review concerns intrusive magmatism related to the Cadomian orogeny in the northeastern part of the Armorican Massif of France. The Cadomian orogeny is interpreted to represent tectono-thermal activity in a continental margin above a subduction zone. The North Armorican Shear Zone (NASZ) separates two major Cadomian terranes, the North Armorican Composite Terrane (NACT) and the Central Armorican Terrane (CAT). The amalgamation of three terranes within the NACT (the St Brieuc Terrane (SBT), the St Malo Terrane (SMT) and the Mancellian Terrane (MT)) around 540 Ma by sinistral transpression is a key element in the Cadomian orogeny. Cadomian plutonic complexes intrude the
c.
2000 Ma Icartian Gneisses and form the local basement to, and are emplaced into the Brioverian succession. Cadomian magmatism spans approximately 275 Ma, from
c.
700 Ma to
c.
425 Ma.

Early Cadomian foliated plutonic complexes occur within the Guernsey-La Hague structural block of the SBT; they represent volcanic arc granites derived in a subduction zone environment. Late Cadomian post-tectonic plutonic complexes within the Guernsey—La Hague structural block were emplaced
c.
500 Ma, exhibit a variety of ‘mixed magma’ features and have a normal to mature continental arc geochemistry. To the southeast, the Jersey structural block within the SBT represents a higher structural level as well as a greater distance away from the probable trench site. Here, the late Cadomian posttectonic plutonic complexes were emplaced during Cambrian and Ordovician-Silurian times and represent mature continental arc activity, probably associated with the decay of the Cadomian subduction zone system after transpressional terrane accretion.

The St Malo Terrane, to the southeast, represents an inverted intra- or behind-arc basin. The migmatite belts within this terrane developed by crustal anatexis. Emplacement of homogeneous diatexites and leucogranites synkinematically with sinistral strike-slip movement along shear belts generated by the transpressive terrane accretion allows this event to be dated at
c.
540 Ma.

The Mancellian Terrane, further to the southeast, comprises low-grade Brioverian metasediments into which has been emplaced the Mancellian Batholith, a late Cadomian post-tectonic series of predominantly granite complexes. These complexes have the geochemical features of volcanic arc granites.

Overall, Cadomian magmatism represents
c.
275 Ma of plutonic activity broadly related to subduction beneath a continental margin. The early part of the Cadomian orogenic cycle includes volcanic-arc plutonic suites and basaltic volcanic rocks with associated feeder dykes. The peak of the Cadomian orogeny involves intra- or behind-arc basin inversion with concomitant mid-crustal anatexis and sinistrally transpressive terrane accretion
c.
540 Ma. The late evolution begins with andesite-rhyolite volcanism and calcalkaline plutonism occurs over a period of
c.
100 Ma during the decay of the Cadomian subduction zone system.
Sedimentation dynamics and redox iron-cycling: controlling factors for the apatite—glauconite association on the East Australian continental margin
Authors: O’Brien G. W. , Milnes A. R. , Veeh H. H. , Heggie D. T. , Riggs S. R. , Cullen D. J. , Marshall J. F. , Cook P. J.
Published: April 2008

Detailed sedimentological and geochemical studies of phosphorites and sediments from the East Australian continental margin have shown that both apatite and glauconite are forming at a transition zone between relict, iron oxyhydroxide-rich, organic-poor (TOC<0.3%) outer shelf (200–350 m) sediments and relatively rapidly accumulating, iron oxyhydroxide-deficient, organic-rich (TOC>0.8%) deep water (460–650 m) sediments. The interaction between sediment mixing and Fe-P cycling processes (between the pore waters and the solid phase) appear critical to the formation of modern phosphorites in this area. The phosphate nodules form within the anoxic zone in the sediments at depths of approximately 10–18 cm below the sediment-seawater interface. Nodules which remain in the sediment mixed layer after they form continue to accumulate both P and Fe for up to 60 ka; during this time their apatite and iron oxyhydroxide contents more than double and the nodules become denser and more lithified. Apatite and glauconite formation are favoured by periods of high sea-level and low current velocities, as these conditions allow a relatively high organic carbon input to the sediments and thereby the maintenance of anoxia at shallow depths within the sediments. During periods of low sea-level and high current velocities, the carbon flux into the sediments decreases and the sediments become oxic. Consequently the Fe-cycling processes cease and apatite and glauconite formation stops: the glauconite is progressively transformed to goethite, and phosphorite nodules are concentrated into lag deposits and ferruginized. Alternations of high and low sea-level cycles eventually result in the formation of the massive ferruginous Neogene phosphorites that mantle much of the outer shelf. The iron enrichment processes observed in the modern to Neogene phosphorites on the East Australian continental margin provide explanations for many of the features seen in ferruginous Neogene deposits in the world’s oceans.
Quartzite rheology under geological conditions
Authors: Paterson M. S. , Luan F. C.
Published: April 2008

The problems associated with extrapolating laboratory measurements on quartzite rheology to geological conditions are discussed, with special reference to the question of equilibration with respect to the effect of water. Some new results on synthetic specimens crystallized from wet amorphous silica are presented in the expectation that water equilibration is more closely attained in these than in natural quartz specimens. These results and earlier ones from the literature are collated and used to arrive at limits on quartzite flow strength under geological conditions.
Deformation mechanics in analogue models of extensional fault systems
Authors: McClay K. R.
Published: April 2008

Scaled analogue models of fault structures are powerful tools for investigating the progressive development of extensional deformation. The results of a wide ranging programme of extensional modelling are briefly reviewed and are analysed in terms of the deformation mechanics and rheological behaviour of the modelling materials. Extensional models have been constructed using homogeneous dry quartz sand with isotropic behaviour; sand and dry china clay mixtures; with behaviour controlled by competency; and sand with dry vermiculite mica flakes to simulate anisotropic systems. Rheological tests show that these materials deform by Navier-Coulomb failure with friction angles between 30° and 35°. Extensional faults developed within the models are dilatant granular shear zones, whose widths are grain-size dependent, being widest in the sand models and narrowest in faults cutting the fine-grained clay layers. Initial fault-bedding cutoff angles are high, 60°–70°, but during progressive deformation shear strains may change this, depending upon the boundary conditions of the model. Measurements of angular shear strains within domino fault arrays show significant zones of shear either parallel to the basal detachment or parallel to bounding faults. Small bulk dilations are found in most models as a result of grain packing rearrangements during extension. The primary extensional architecture of the models is controlled by the underlying detachment configuration. Despite limitations to the models, it is believed that the rheologies, the modelling materials and the deformation mechanics within the models realistically simulate brittle deformation of sedimentary rocks in the upper crust.
Evidence for global (glacial-eustatic) control over upper Carboniferous (Pennsylvanian) cyclothems in midcontinent North America
Authors: Heckel Philip H.
Published: April 2008

At least 20 Middle/Upper Pennsylvanian (Westphalian D/Stephanian A, B) major marine cyclothems, each consisting of a thin transgressive limestone, thin offshore dark phosphatic shale, and thick regressive limestone member, extend along outcrop for 600 km from the northern shelf region of Iowa, Missouri and Kansas, to the basinal region of central Oklahoma. Each is correlated lithostratigraphically throughout a gridwork of long cores in the northern region beneath Pleistocene cover, and in good exposures southward. This correlation is confirmed biostratigraphically, along the entire outcrop belt using a combination of conodonts, fusulinids and ammonoids. Most of the cyclothems are separated by thin terrestrial deposits with paleosols, but only rare deltas, for about the northern half of the outcrop distance, which rules out delta shifting as a general control for the vertical alternation of terrestrial and marine deposits across the broad shelf. On the northern shelf, all major cyclothems are traced across the cratonic Forest City basin and over the adjacent Nemaha uplift with little change, which rules out local differential tectonics as a general cause. Presence of Gondwanan glacial deposits at this time, in conjunction with the estimated frequencies of these Pennsylvanian cycles within the Milankovitch band of Earth’s orbital parameters (which control variation in solar heating of the mid-latitudes), indicate that glacial eustasy, rather than distant orogenic movements, was the main control over the cyclothems.
Carboniferous geology of the Southern North Sea Basin and controls on hydrocarbon prospectivity
Authors: Leeder Michael R. , Hardman Martin
Published: April 2008

The thick sequences of Upper Palaeozoic strata present in the Southern North Sea Basin were probably deposited on a basement of Caledonian low-grade metamorphic rocks intruded by numerous late-Caledonian granitoids which form part of the subsurface extension of the Mid-European Caledonides linking the English Lake District and Northern Pennines to the Ardennes. This basement, along with the better known onshore areas of the British Isles and Ireland was intensely fragmented during late Devonian to mid-Carboniferous times by a major phase of extensional tectonics.

Active fault-bounded tilted blocks developed in the Dinantian with deposition of fluviatile redbeds, marine carbonate/deltaic clastic cycles and deltaic coastal plain facies. These are penetrated by a number of wells on the southern flanks of the Mid-North Sea High, but details of basin geometries and facies distributions remain unclear.

Crustal stretching appears to have given way to thermal subsidence by early to mid-Namurian times. This was coincident with a major climatic change, probably forced by glacial expansion in Gondwanaland. The major drainage system that resulted dominated sedimentation in the SNSCB, punctuated by numerous marine transgressions of undoubted glacio-eustatic origins. Within this broad depositional framework, Namurian facies were closely controlled by basement tilt block topography, with good evidence for a gradual southerly migration of the active fairways of fluviodeltaic sandbodies. Westphalian facies were dominated by the interplay between glacio-eustatic and sedimentary processes on a vast low-gradient alluvial plain with much less pronounced tectonic control.

By late-Westphalian B times a phase of basin inversion caused the gradual elevation of the area as a retro-arc foreland basin. Vadose diagenesis and pedogenesis occurred in the fluviatile facies of the so-called Barren Red Measures (BRM). With continued compressive deformation a series of over 50 NW-SE orientated folds, some associated with thrusts, developed. Significant erosion and truncation of these inversion anticlines was accompanied by the deposition of the BRM facies in characteristic growth fold basins, with marked onlap around the basin margins. The Carboniferous play in the SNSCB has been greatly influenced by the subsequent Mesozoic/Cenozoic geological history of the basin, including periods of subsidence and uplift. We demonstrate that variations in original detrital mineralogy and various diagenetic processes have exerted significant controls upon Carboniferous reservoir quality.
Late Jurassic half-graben control on the siting and structure of hydrocarbon accumulations: UK/Norwegian Central Graben
Authors: Roberts Alan M. , Price John D. , Olsen Terkel Svava
Published: April 2008

The late Jurassic structure of the Central Graben is discussed, both in terms of the extensional features developed at this time and their subsequent controls on younger inversion structures. The regional kinematics of the North Sea rift suggest that during the late Jurassic the Central Graben opened approximately orthogonally to the main NNW—SSE fault-trend within the basin. Some evidence that Triassic extensional faults were reactivated at this time is seen towards the basin margins, and a significant Triassic extensional history is inferred. The late Jurassic basin comprises the axial
c.
100 km of the Central Graben (
s.l.
). The basin axis is offset progressively to the SE by both transfer zones and discrete transfer faults. Footwall uplift of the basin margins during extension provided the source for the Upper Jurassic reservoir sands encountered by exploration drilling. These sands were probably preferentially fed into the basin at transfer zones in the margins. Salt structures are ubiquitous within the Central Graben, and their siting is everywhere controlled by the underlying fault-block topography. Salt structures form preferentially towards the crests of half-graben or buttressed against their boundary faults. Inversion structures in the Norwegian sector reactivated, with a reverse sense of displacement, late Jurassic extensional faults. A model is proposed which shows the two main inversion structures (Feda Graben, Norway; Tail End Graben, Denmark) to be detached on antithetic reverse faults riding in the salt. These faults terminate at tip-folds within major salt walls. The geometry of the inversion structures can be understood only when considered together with the geometry of the underlying late Jurassic half-graben.
Deformation around basin-margin faults in the North Sea/mid-Norway rift
Authors: Roberts Alan M. , Yielding Graham
Published: April 2008

It has been demonstrated previously that the rigid-domino model of extensional faulting can account well for the uplift/subsidence patterns observed within particular areas of the North Sea rift. This model, however, provides an unsatisfactory solution to deformation occurring at the basin margins.

It is suggested here that the domino model is an elegant, geometric simplification of the more complete flexural-isostatic solution to fault displacements. Application of a flexural model allows a unified treatment of the basin and its margins.

The basin-margins to the North Sea/Mid-Norway rift are all shown to have responded to extensional faulting by experiencing isostatic uplift in the footwalls to the marginal faults. Uplift magnitude varies from a few hundred metres adjacent to small faults to perhaps 5 km adjacent to the largest faults. Uplift patterns can be modelled or predicted by use of the flexural-cantilever basin model.

Recognition of marginal uplift throughout the rift means that geometric section balancing techniques, all of which require the footwall to be rigid during extension, are inapplicable as a method for analysis of large, basement faults within the North Sea.

Marginal, fault-related uplift is considered to have been a primary source of syn-rift clastic detritus. The precise locus of deposition for material eroded from emergent basin margins will depend on local drainage patterns, but deposition in the hangingwall basin and on the footwall platform may both be anticipated.
Seismic reflections from normal faults in the northern North Sea
Authors: Yielding G. , Badley M. E. , Freeman B.
Published: April 2008

Fault-plane reflections from gravity-driven, listric faults in thick sedimentary sequences are well known. However, reflections from normal faults associated with crustal extension are much rarer and less well studied. We discuss a number of examples of such reflections, present on commercial seismic reflection data from the northern North Sea. Some of these examples have previously been described as reflected refractions, but we show that this requires unrealistic velocities within the sedimentary column. Instead, we consider it more likely that these events are true fault-plane reflections. Depth migration and image-ray migration are used to determine the probable geometry of the faults in both cross section and map view. Estimates of the true geological dip remain sensitive to the velocity structure used, particularly that applied to the Triassic half-graben fill. Despite this uncertainty, the depth-migrations show that these faults are composed of two near-linear segments in cross section. The upper parts (in the Jurassic and Upper Triassic section) dip at 40–50°, but the deeper parts (Triassic hangingwall against basement in the footwall) typically dip at 30–35°. Occasionally, reflections indicate that faults continue into the basement with a 30–35° dip. We interpret this fault geometry as originating from two phases of fault movement. Triassic extension caused rotation of sets of initially steep planar faults. Following Late-Triassic-to-Middle-Jurassic thermal subsidence, renewed extension in the Late Jurassic caused the faults to cut up through the overburden at a steeper angle. Without recognition of a two-phase extension history, such faults might be mistakenly interpreted as forming with a listric geometry.
Geochemical studies of detrital heavy minerals and their application to provenance research
Authors: Morton Andrew C.
Published: April 2008

Although heavy mineral analysis is a sensitive and well-proven technique for determining the provenance of clastic sediments, the interpretation of the data is considerably enhanced by determining the composition of individual detrital grains. Many heavy mineral species, including pyroxene, amphibole, epidote, staurolite, garnet, tourmaline, monazite, chloritoid and spinel, show significant variations in composition that are related to the conditions under which their parent rocks were formed. Thus, as well as giving greater confidence in identification, geochemical analysis of detrital minerals adds precision to the evaluation of the relative contributions of potential source lithologies. Furthermore, by concentrating on stable minerals, geochemical studies avoid, or at least minimize, the problems caused by diagenetic and hydraulic processes.
Palaeoenvironments of the Nordenskjöld Formation: an Antarctic Late Jurassic—Early Cretaceous black shale-tuff sequence
Authors: Doyle Peter , Whitham Andrew G.
Published: April 2008

This study documents the palaeoenvironments of a Jurassic-Cretaceous boundary black shale-tuff sequence exposed on the east coast of the Antarctic Peninsula. Sedimentological and palaeontological indices show that, during the deposition of the lower part of the succession, bottom waters were predominantly anaerobic with brief spells of exaerobic or ‘episodically dysaerobic’ conditions. During the deposition of the upper part of the succession, dysaerobic conditions prevailed with mean levels of dissolved oxygen increasing through time. The upward changes seen in the Nordenskjöld Formation are thought to be related to tectonic uplift of the sea floor, through a pycnocline.
Are numerical models useful in reconstructing the migration of hydrocarbons? A discussion based on the Northern Viking Graben
Authors: Burrus J. , Kuhfuss A. , Doligez B. , Ungerer P.
Published: April 2008

The 2D-finite-difference code TEMISPACK is used to discuss the history of hydrocarbon (HC) generation, expulsion and migration along a 160 km long transect across the Northern Viking Graben. In the first section, HC generation is discussed based on hypotheses on the crustal heat flow; a kinetic model is used to calculate the maturity. Observations are shown to be consistent with a 67 mW m
−2
heat flow, possibly 60 mW m
−2
in the graben axis. The precision of the calibrated heat flow is not more than 10%, which results in an uncertainty of the timing of HC generation of 5–10 Ma. The development of overpressures, observed today in the deep Jurassic reservoirs is shown to be essentially controlled by compaction, with a minor contribution due to gas generation; the model also shows that faults related to the Jurassic rifting are most certainly impermeable. In the second section, the previous overpressures and maturity reconstructions form the framework in which expulsion and migration of HC are modelled, based on the two-phase (water, hydrocarbon) Darcy law. The computation explains the filling of the tilted blocks located in the Tampen Spur zone as well as the presence of gas at Troll. A sensitivity analysis shows the importance of poorly constrained parameters such as the relative permeability curves, capillary pressures, or the properties of the petroleum fluids.
Geochromatography in petroleum migration: a review
Authors: Krooss B. M. , Brothers L. , Engel M. H.
Published: April 2008

Numerous works have been published during recent years discussing the occurrence and effects of chromatographic processes with respect to petroleum migration. ‘Geochromatography’ has been, and still remains, a controversial topic. Based on the principles and conventions of separation science a definition of geochromatography is proposed, followed by a discussion of the relevance and potential occurrence of chromatographic type fractionation effects during primary and secondary petroleum migration. The evaluation of the relevant literature suggests that chromatographic processes in natural systems involving liquid mobile phase can result only in compound class fractionation (saturates, aromatics, polars) whereas molecular fractionation may be expected to occur with gaseous mobile phases. Problems associated with the recognition, experimental verification and quantification of geochromatography are discussed in detail.
Jurassic and Early Cretaceous platform margin configuration and evolution, central Oman Mountains
Authors: Pratt Brian R. , Smewing John D.
Published: April 2008

The Hajar Supergroup is a Middle Permian to middle Cretaceous sequence dominantly composed of shallow-water carbonate rocks which veneer the eastern edge of the Arabian Platform in the Oman Mountains. The Jurassic and Early Cretaceous components of this package were deposited on the passive margin of Neo-Tethys which developed after initial rifting in the Late Permian and Triassic. Strata studied in Jebel Akhdar, Saih Hatat and neighbouring uplifts document low- and high-energy, shallow-water depositional environments punctuated by two drowning events when deeper water, slope conditions migrated onto the shelf margin. The mid-Jurassic event may have been precipitated by synsedimentary down-faulting of the Saih Hatat area as rifting continued. The latest Jurassic, earliest Cretaceous event, depositing the Rayda Formation, is ascribed to a regional decrease in carbonate production because of widespread evaporitic conditions in the Arabian Platform interior.

The configuration of the Mesozoic platform margin in northern Oman conforms to a series of rift and transform segments inherited from the underlying crust when it separated in the Early Jurassic. The transform segments became the focus of major structural lineaments during subsequent thrusting and obduction. This provides an especially clearcut example of the structural control of passive margin geometry, as well as its effect on collision-related deformation. A new Jurassic lithostratigraphic unit, the Saih Hatat Formation, is formally described.
The Alpine deformation of the Central Oman Mountains
Authors: Hanna Samir S.
Published: April 2008

The Saih Hatat, Jebel Nakhl, Jebel Akhdar and the Hawasina Window structures of the Oman Mountains developed as culminations above a major sole thrust associated with late stage emplacement of the ophiolite from the northeast onto the Arabian margin. The outcrop of a pre-Permian sequence in the cores of Jebel Akhdar, Jebel Nakhl and Saih Hatat implies that the major sole thrust is located within the pre-Permian succession. This thrust is emergent in the southern foothills of the mountains south of the Jebel Salakh Range. In the Hawasina Window, however, the thrust is at a higher level and is located within the slope and oceanic sediments. An extensional regime due to the gravity sliding of the Semail ophiolite is recognized from the occurrence of surge zones, extensional duplexes, extensional cleavage and gravity folds. The development of the extensional faults, which occur in the frontal, dorsal, lateral and oblique walls of the above culminations, may account for the absence and locally thinning of the Tethyan thrust sheets around the flanks of these structures. The extensional tectonic regime is also believed to be responsible for the final extensional nature of the ‘Semail Thrust’. The Oman Mountains foreland fold and thrust belt mostly consists of a series of thrust sheets of oceanic sediments (Hawasina) together with widespread exposures of syntectonic (Aruma) deposits. The belt is dominated by imbricate stacks of both Hawasina and Aruma units. These imbricate stacks have been folded around major culminations arranged in a characteristic
en echelon
pattern. Late Alpine (Tertiary) deformation is largely attributed to an extensional regime of culmination collapse. Folds and thrusts were developed at the leading edge of the extensional faults (surge zones). Tertiary folding and thrusting may reactivate the pre-existing Early Alpine (Late Cretaceous) structures.
The post-Campanian tectonic evolution of the Central Oman Mountains: Tertiary extension of the Eastern Arabian Margin
Authors: Mann A. , Hanna S. S. , Nolan S. C. , Mann A. , Hanna S. S.
Published: April 2008

This paper documents the structural history of the central Oman Mountains after the obduction of the Semail ophiolite onto the eastern margin of the Arabian continent during the Late Cretaceous (Santonian/Campanian). The post-obduction history is recorded by a Late Campanian/Maastrichian-Tertiary sedimentary cover to the mountain belt. These sediments have been deformed within an extensional tectonic regime which is continuous northeastwards into the Gulf of Oman hinterland basin and southwestwards away from the SW foothills.

Large-scale down-to-the-basin normal faults have been recognized on the Batinah Coastal Plain NE of the mountain belt. Localized block-faulting and roll-over folds are associated with major faults that separate the Late Campanian to Tertiary sediments from older basement rocks of the Oman Mountains and with Late Cretaceous thrusts reactivated as normal faults. To the SW of the mountains, large NW-SE striking folds are thought to be gravity structures developed above a decollement in Late Cretaceous shales.

Syn-sedimentary tectonic influences are summarized and integrated with the structural analysis to identify the style and timing of tectonism on the S margin of the Gulf of Oman from the Late Campanian/Maastrichtian to present. There is evidence to suggest that penecontemporaneous tectonics controlled rates of subsidence and facies development during this time. A major E-W trending palaeo-high running close to the present-day shoreline affected Palaeogene sedimentation.

Unlike the northern Oman Mountains (Musandam Peninsula), there is no unequivocal evidence of mid-Tertiary (Zagros) compressional structures in the central Oman Mountains. Structural development of the cover sediments in the central mountain belt is believed to be controlled by underlying Late Cretaceous basement structures. In particular the Saih Hatat culmination was a positive structure throughout the Late Cretaceous and Early Tertiary, affecting both the stratigraphic and structural history of adjacent areas. The northern end of the Nakhl culmination seems to have had little effect on sedimentation but did influence deformation in the Tertiary.
Salt removal and sedimentation in Southern Oman
Authors: Heward A. P.
Published: April 2008

On the Eastern Flank of the South Oman Salt Basin lies a Cambrian to Recent sequence much affected by the withdrawal and dissolution of underlying Infracambrian salt. Salt withdrawal began after burial by only a few hundreds of metres of Cambrian continental sands, and salt dissolution has taken place periodically up to the present, initially near-surface and later in the subsurface. Salt removal has (1) resulted in the collapse of masses of dolomite and shale formerly interbedded with the salt, (2) influenced the large-scale geometries of sedimentary bodies within the overlying succession, (3) affected the distribution of sedimentary facies when rates of salt removal balanced or exceeded sedimentation, (4) allowed the preservation of parts of the succession otherwise eroded at this basin margin, and (5) caused folding and fracturing of Cambrian to Recent sediments.

More than 2 × 109m3of oil (12 × 109bbls) is estimated to be present in four types of hydrocarbon trap related to salt removal. In terms of importance these are: large turtleback anticlines resulting from the inversion of former peripheral synclines, truncation traps rimming eroded turtle-back anticlines, small anticlines draped over residual masses of dolomite and shale, and porous and fractured dolomites within such residual masses.

Comparison of salt removal phenomena from the Eastern Flank of the South Oman Salt Basin, the North Sea, western Canada and the USA suggests that the south Oman features may be typical of the margins of basins where a thick mobile salt sequence is overlain by clastic sediments.
Late Cretaceous and early Tertiary evolution of Jebel Ja’alan and adjacent areas, NE Oman
Authors: Filbrandt J. B. , Nolan S. C. , Ries A. C.
Published: April 2008

Jebel Ja’alan forms the northernmost extremity of the NNE-SSW trending Huqf-Haushi Uplift, a structural high which runs parallel to the SE continental margin of Oman. On Jebel Ja’alan Precambrian rocks, exposed in the core of this structural high, are overlain by a cover sequence of shallow water carbonate and clastic sediments of Maastrichtian to Eocene age. These cover sediments, and the structures which affect them, record a sequence of tectonic events leading to the uplift and inversion of Jebel Ja’alan in post middle Eocene times. In the Maastrichtian an extensional regime resulted in subsidence and the accumulation of fluvial clastic deposits, the Qahlah Formation, overlain by shallow marine limestones, the Simsima Formation, of Maastrichtian age. A post-Maastrichtian normal fault, the East Ja’alan Fault, downthrows Simsima Formation to the east against Precambrian basement to the west and controls the deposition of the Palaeocene/early Eocene Hasad Beds, which form debris fans distributed along the fault plane. The East Ja’alan Fault may have been linked to early extensional movement on the WNW-ESE trending North Ja’alan Fault which partly controlled deposition of the Eocene fluvial/marginal marine Rusayl Formation. A mid-Eocene transgression resulted in the deposition of the Seeb Formation shallow water limestones. All these sediments were deformed by a post middle Eocene compressional event which resulted in the formation of north-south trending asymmetric anticlines, forming a pop-up structure; the latter was controlled by a shallow level detachment dipping east which led to the detachment and removal of the Maastrichtian-Eocene sediments from Jebel Ja’alan. An associated deeper level basement ramp resulted in uplift and inversion of the Precambrian basement of Jebel Ja’alan. These compressional structures are displaced by later left-lateral strike-slip on the North Ja’alan Fault Zone.
The Arabian continental margin in Iran during the Late Cretaceous
Authors: Stoneley R.
Published: April 2008

The events which affected the Iranian sector of the Southern Tethyan margin of the Arabian continent during the Cretaceous differed from those that took place in Oman, although they were clearly related to them. Extensional faulting, parallel to the shelf margin, commenced locally in the Cenomanian and culminated later in the Cretaceous with the expulsion of mantle material to the surface. This engulfed the shelf edge before gliding southwestwards over a complementary intra-shelf basin. These events took place progressively later towards the north-west, and are interpreted as an unsuccessful attempt to develop a spreading axis along the continental margin. They were followed by a return to shelf sedimentation at the southern edge of Tethys, which closed finally in the early Miocene. It is suggested that the Oman and Iranian sectors of the Arabian margin were offset some 300 km by a north-south transform fault, and that the differences between the two regions were therefore due to attempted spreading in different tectonic environments in the late Cretaceous.
Thermal effect of the Tertiary Cuillins Intrusive Complex in the Jurassic of the Hebrides: an organic geochemical study
Authors: Thrasher Jane
Published: April 2008

The Tertiary Cuillins Intrusive Complex is partially located within the Mesozoic depositional basins of the Minch and intrudes a Jurassic sequence including a number of moderately organic-rich horizons. The thermal effect of the central intrusive activity, and related minor intrusions, on the Jurassic sediments has been assessed using organic geochemical molecular maturity parameters. Analysis of more than 60 samples, mainly from the Middle Jurassic Dun Caan and Cullaidh Shales, shows that the thermal effect of the igneous complex is very localized and that Jurassic sediments more than 15 km from the complex margin are extremely immature, except in the immediate vicinity of minor intrusions. Igneous activity similar to the Tertiary of western Britain should therefore not be invoked as an alternative maturation route for shallow-buried sediments.
Aspects of the geology of the Malin Sea area
Authors: Dobson M. R. , Whittington R. J.
Published: April 2008

A complex of sediment-filled graben and half-graben, separated by acoustic basement, occupy much of the northwest shelf of the British Isles. Some were established in the Late Carboniferous, but most were formed during a post-Hercynian tensional phase that adopted Caledonoid-trending structures in the basement. A major graben complex occurs in the Malin Sea area; flanked by the Coll-Tiree-Skerryvore-Stanton Banks basement ridge to the north and by the Islay-Donegal Platform to the south. Rock samples obtained by divers from the two basement ridges confirm their Precambrian character. Limited commercial multichannel reflection seismic data together with BIRPS WIRE lines and structural information supplied by Texaco suggest that the graben complex consists of three major depocentres: the Inner Hebrides Trough, the Malin Basin and the South Donegal Basin. To the west lies the Main Donegal Basin. This basin, in concert with the South Donegal Basin, evolved during the Carboniferous as a result of dextral strike slip movement between the Stanton Banks in the north and the Islay-Donegal Platform to the south. Sea bed core and dredge material and well data recovered from the graben complex indicate that the basin sediments include Upper Carboniferous, Permo-Triassic, Lower Jurassic, Middle Jurassic, with thick Cretaceous and Lower Tertiary in the far west. Periods of uplift and erosion restricted sedimentation during the late Mesozoic, whilst uplift and erosion in the early Tertiary was accompanied by extensive igneous activity, with the development of two plutonic centres and twelve other smaller intrusive igneous bodies. Tertiary sediments are inferred for the Blackstones Basin by analogy with similar sequences elsewhere in the Hebridean area. A thick Quaternary section, dominated by glaciomarine sediments, contains three of the five glacial cycles that constitute the Pleistocene in this area.
Identification of tectonic rotations in boreholes by the integration of core information with Formation MicroScanner and Borehole Televiewer images
Authors: MacLeod C. J. , Parson L. M. , Sager W. W. ,   the ODP Leg 135 Scientific Party
Published: April 2008

Palaeomagnetic studies on land have shown that block rotations on a variety of scales are commonplace in many tectonic regimes. In the oceans, however, analogous rotations have rarely been documented. This is due principally to the difficulty of their measurement rather than to their likely absence. Palaeomagnetic measurements can be, and are, readily made on borehole cores. However, the difficulty in reliably reorienting these cores, and hence their magnetization vectors, back to geographical coordinates severely hampers their use for the purposes of addressing tectonic rotations. Several techniques for orienting cores are commonly in use, none of which is without its limitations for palaeomagnetic purposes. A technique is outlined here that potentially allows the reliable orientation of sections of core by matching distinctive inclined planar features measured on the core with their images on Formation MicroScanner and Borehole Televiewer wireline logs. Its methodology is described and an example, from Ocean Drilling Program Leg 135, presented to illustrate its application.
Fluid salinity and dynamics in the North Sea and Haltenbanken basins derived from well log data
Authors: Gran Kjetil , Bjørlykke Knut , Aagaard Per
Published: April 2008

The salinities of porewaters in sandstones and carbonate reservoir rocks from the North Sea and Haltenbanken basins have been calculated using resistivity logs. Applying Archie’s equation, hydrocarbon containing intervals were avoided and a 100% water saturation assumed in the calculations of the formation water resistivity. The salinities obtained vary from 20 000 to 300 000 ppm NaCl equivalents. The highest salinities, which approach saturation with halite, are found in the Southern North Sea, close to the Upper Permian evaporite sediments. There is a marked upwards decrease in the salinity values away from the evaporites. The salinities seem to be a function of the vertical distance to the evaporites and are independent of stratigraphic horizon. In the Northern North Sea, where Permian salt is absent, the salinity values are much lower (20 000–50 000 ppm NaCl eq.). This salinity stratification strongly indicates that large scale vertical mixing of porewater by convection or compaction driven pore water flow has not taken place. We can therefore conclude that saline pore water has not been transported from the deeper parts of the basin to the shallower parts. This also implies that there is no transport of carbonates or silicates in solution over the same distances. Calculation of salinities from well logs is subject to considerable uncertainties. However, the values obtained in this study agree well with reported values from formation water analyses. Mapping the distribution of salinity in the pore waters of sedimentary basins is very important and the information obtained can be used to constrain our models for porewater flow.
Tectonic and bathymetric controls on stratigraphic sequences within evolving half-graben
Authors: Roberts Alan M. , Yielding Graham , Badley Michael E.
Published: April 2008

A modification of the domino fault block model is presented, which can be used to evaluate possible tectonic controls on the development of stratigraphic sequences within an evolving half-graben. The model allows average half-graben load to vary with time and allows the rate of faulting to be varied relative to any background component of basin subsidence.

The basic domino model predicts only transgressive sequences at the crests of tilted fault blocks. It is found with the modified model that a decreasing average half-graben load (increasing bathymetry) or an increasing strain-rate may induce offlap and regression at the fault block crest. The model is used here successfully to model the syn-rift stratigraphy of a tilted fault block in the Viking Graben (North Sea). It is believed to have general implications for evaluating the syn-rift and early post-rift history of extensional basins.

The internal geometry of condensed and unconformable sequences at fault block crests may lie below the limit of seismic resolution. This calls into question the ability of the seismic stratigraphic technique to address successfully the full history of an evolving half-graben.