bluuebunny/crossref_metadata_2025 huggingface dataset:
The Cenomanian-Turonian Oceanic Anoxic Event, II. Palaeoceanographic controls on organic-matter production and preservation
Authors: Arthur M. A. , Schlanger S. O. , Jenkyns H. C.
Published: April 2008

Correlation of the
δ
13
C spike with the well dated occurrences of strata rich in organic carbon detailed in Schlanger
et al.
(this volume), indicates that a global episode of intense organic carbon (
org
C) burial took place during the latest Cenomanian-earliest Turonian ‘Oceanic Anoxic Event’ (OAE) (
A. plenus
through
I. labiatus
macrofossil zones and upper
R. cushmani
TRZ through
W. archecretacea
PRZ foraminiferal zones) over a period of no more than 1 million years (m.y.). The shape of the
δ
13
C curve indicates that rates of
org
C burial gradually increased in the early part of the late Cenomanian, increased more rapidly in the later Cenomanian, and levelled off at peak values in latest Cenomanian-early Turonian time during the maximum rate of
org
C burial. The δ
13
C values decreased nearly to pre-late Cenomanian levels in the early to middle Turonian. The decrease in
δ
13
C reflects decreasing rates of
org
C burial following the Cenomanian-Turonian ‘oceanic anoxic event’ as well as the probable oxidation and return of significant amounts of
org
C to the oceans following regression and re-oxygenation of much of the deeper water masses in contact with the seafloor.

The Cenomanian-Turonian OAE coincided with a maximum sea level highstand. We suggest that sea level, which may be responding to some volcano-tectonic event, is the common link and ultimately the driving force for
org
C deposition in globally distributed basins under different climatic and ocean circulation regimes. The rate of production of warm, saline deep water may have been proportional to the area of shelf flooding such that the maximum occurred near the Cenomanian-Turonian boundary. As rates of deep-water formation increased, rates of upwelling of deeper oceanic water masses must also have increased thereby increasing sea-surface fertility and productivity. In somewhat restricted higher latitude basins, such as the Cretaceous Interior Seaway of North America, periodic high rates of freshwater runoff coupled with deepening seas during the transgression created periodic salinity stratification, oxygen depletion in bottom waters, and resultant enhanced
org
C preservation.

The disappearance of some types of keeled planktonic formainifers and ammonites at the Cenomanian-Turonian boundary is probably due to the rather sudden but short-term disappearance of suitable shallow midwater habitats because of widespread severe oxygen depletion in these levels. This interpretation is strengthened by the occurrence of benthic-free zones or depauperate benthic faunas near the Cenomanian-Turonian boundary in many localities.
The causes of the high-grade metamorphism of the Scourie complex, NW Scotland
Authors: Barnicoat A. C.
Published: April 2008

A number of previous studies of the possible causes of Archaean high-grade metamorphism have suggested that the heat source was large bodies of (intermediate) magma emplaced within pre-existing crust. Geochronological data for the Lewisian indicate that there was probably a gap of
c.
200 Ma between the cessation of significant igneous activity and the peak of high-grade metamorphism. This is sufficiently long to allow conductive dissipation of magmatic heat and to render a model of the metamorphism relying upon heat transported in by intrusions unsatisfactory. The quantity of deep-derived CO
2
that would be required to make it the primary cause of the metamorphism is larger than petrological considerations allow. Alternative models, based on enhanced basal heat-flow in a static crust or heat derived from the decay of radionuclides resident within the crust coupled with a more normal mantle heat-flow and tectonic crustal-thickening have been erected, and reveal that the
P-T
conditions and age data recorded by the Scourian granulites can be satisfied by such models.

Thermal models of this nature, and a consideration of the available petrological and geochronological data, suggest a very slow cooling rate which would allow considerable intercrystalline diffusion to occur. This could give rise to the observed zoning profiles and other forms of re-equilibration found within the granulite-facies rocks of the Lewisian complex.
Petrology of Scourian supracrustal rocks and orthogneisses from Stoer, NW Scotland: implications for the geological evolution of the Lewisian complex
Authors: Cartwright I. , Barnicoat A. C.
Published: April 2008

The Lewisian complex at Stoer comprises a diverse assemblage of paragneisses together with several generations of acid to basic orthogneiss. All lithologies are early-Scourian in age and underwent granulite-facies metamorphism in the Badcallian (
c.
2.7 Ga) event. Petrogenetic modelling coupled with geothermometry yields a
P-T
estimate for the peak of metamorphism of 920–990°C and >11 kb, as well as estimates of conditions encountered during Inverian (
c.
2.6–2.3 Ga) and Laxfordian (<1.7 Ga) retrogression. Certain lithologies, especially those of supracrustal origin, show evidence of partial melting. It is suggested that anatexis, controlled by the internal buffering of metamorphic fluids, may have been an important process in the evolution of the complex.
Early Proterozoic structure and kinematic evolution of the southern mainland Lewisian
Authors: Park R. G. , Crane A. , Niamatullah M.
Published: April 2008

The southern mainland region of the Lewisian is a belt of intense Early Proterozoic deformation with a minimum width of 55 km, in which all the main structures trend NW-SE. Archaean structures are dominant in the central block to the northeast, and in a small enclave (the Ruadh Mheallan block) between Gairloch and Loch Torridon. Major structures within the belt include upright NW-SE folds (the Carnmore antiform, the Letterewe synform, and the Tollie and Torridon antiforms) and the Gairloch and Diabaig shear zones. The structural history is divided into six phases: (1) Inverian (2600–2400 Ma)—the whole belt represents a major steep Inverian shear zone. Structures in areas little affected by subsequent deformation indicate a N-up overthrust movement with a small dextral component (dextral transpressional), (2) Emplacement of dykes and Loch Maree Group (2400–1900 Ma)—evidence of dextral shear during dyke emplacement indicates a change to a dextral transtensional regime, (3) Laxfordian D
1
and, (4) D
2
deformation (1900–1800 Ma)—oblique dip-slip shears and asymmetric shear folds suggest a continuation of the previous dextral transtensional movements on inclined lateral ramps, connected with major gently-inclined shear zone ‘flats’, (5) Laxfordian D
3
deformation (
c.
1600 Ma)—upright folding of previously sub-horizontal fabrics and a reversal of movement to N-up on the Gairloch shear zone indicate a change to a dextral transpressional regime, which is accompanied by retrogressive metamorphism, (6) Laxfordian D
4
deformation (
c.
1400 or
c.
1000 Ma)—steeply plunging large and small-scale asymmetric folds and crush belts indicate a change to a sinistral strike-slip regime. We suggest that these phases of movement represent major changes in relative plate movement direction probably separated by relatively long periods of comparative stability.
Physical properties of Lewisian rocks: implications for deep crustal structure
Authors: Hall J.
Published: April 2008

Data on the densities, seismic velocities, magnetization, and permeability of Lewisian rocks are summarized. These properties have been used to estimate the shallow-depth extension of exposed boundaries within the Lewisian, and between the Lewisian and other rocks; and also to predict downward changes in porosity and electrical conductivity. Geophysical surveys of the deep crust can be interpreted in terms of subsurface distributions of Lewisian, or Lewisian-like, rocks.

Among the more significant conclusions relevant to the interpretation of surface structure of the Lewisian is that the Kylesku Group rocks of the central belt in Sutherland may be isolated from deeper crustal rocks of similar composition which are characterized by
P
-wave velocities of 6.4 km s
−1
and over. Such rocks may form the mid crust below NW Scotland, shallowing north of the mainland possibly with a sharp discontinuity upwards into more acid basement rocks. West of the mainland direct evidence of the 6.4 km s
−1
rocks is missing; they appear to be absent from the upper crust and buried rather deeply, perhaps with a transitional top, in the middle or lower crust.

Granulite-facies rocks occurring in the upper crust west of the mainland are likely to be more acid than average Kylesku Group rocks. The layered basic rocks of the central belt provide an analogue of the deepest crust in NW Britain.

Permeability/porosity studies allow estimates of electrical conductivity to be made in an attempt to explain the high conductivity of the lower crust in terms of trapped saline fluids. Porosities in the deep crust may be high enough to account for deep crustal seismic reflections in areas where porous layers are sub-horizontal.
Deep seismic reflection profiling of the Lewisian foreland
Authors: Smythe D. K.
Published: April 2008

A number of marine deep seismic reflection profiles cross the Hebridean shelf, the Lewisian foreland to the Caledonian orogen. They include the BIRPS MOIST, WINCH and DRUM profiles, several recent commercial deep lines, and many old and new conventional commercial exploration profiles. Basement, comprising the upper crust, is largely devoid of coherent seismic reflections. In contrast, the mid-crust contains many reflectors which may be relics of early Palaeozoic, Caledonian (or earlier Grenvillian) eastward-dipping thrust zones, which pass into an acoustically strongly layered lower crust.

The Outer Isles Thrust is mapped from the surface to the mid-crust, and tied into its land outcrop on north Lewis. Reactivation of this thrust as a normal fault caused the formation of the Sea of the Hebrides, Minch and North Lewis Basins. The Moho is defined by a strong band of reflections at a rather uniform 27 km depth. The eastward-dipping Flannan Thrust can be mapped into the upper mantle from about 15 to 45 km depth.

The Moine Thrust, which carries rocks of the orogen over Lewisian foreland, dips at 20–25° to the east on MOIST, and is either the westernmost of a series of easterly-dipping reflections (thrusts) which flatten or terminate at 17–20 km depth, or a more easterly thrust which structurally overlies these easterly-dipping reflectors. In neither case are the easterly-dipping reflectors themselves likely to be simply ‘Caledonised’ Lewisian foreland.
Mineralogy, petrology and geochemistry of the Scourie dykes: petrogenesis and crystallization processes in dykes intruded at depth
Authors: Tarney J. , Weaver B. L.
Published: April 2008

The mineral chemistry, petrology and geochemistry of representative members of the Scourie dyke swarm are described, and discussed in relation to crystallization processes in deep-seated dykes and to their petrogenesis. Four types of dykes can be defined on petrological and geochemical grounds: bronzite-picrites, norites, olivine-gabbros and quartz dolerites. The latter constitute by far the most abundant dyke-type. In the central Scourian granulite zone in particular the dykes were emplaced at depth into hot country rocks; this enables the through-flow behaviour of different magma types in dyke conduits to be compared. Whereas the dolerites and norites have chilled margins, the bronzite-picrites and olivine-gabbros have coarse grained orthopyroxene-rich and augite-rich margins respectively. This probably reflects turbulent-flow conditions in the less viscous, more ultramafic magmas which permitted rapid crystallization of pyroxenes on the dyke walls during intrusion. The picrite dykes, and to a lesser extent the olivine-gabbro dykes, display additional across-dyke modal and textural variations. These are not accompanied by significant variations in mineral composition, and can largely be attributed to crystal settling in variably inclined dyke sheets.

All dykes typically show a ‘continental’ trace element signature (enrichment in light rare-earth elements and large-ion-lithophile elements), but this must have been inherited from the sub-continental lithosphere because contamination by Lewisian gneisses would not generate the observed trace element characteristics. Both trace element and mineral chemical data indicate that the dykes were derived from at least two distinct mantle sources, that supplying the picrite and norite dykes being more refractory with respect to major elements, but showing a greater relative enrichment in light REE and LIL elements.

The emplacement of the Scourie dyke swarm represents considerable crustal extension, but also poses a thermal problem in the generation of high temperature picritic magmas. It is suggested that the dykes and their compositional characteristics reflect the processes of growth and evolution of the sub-continental lithosphere, and that the extensive retrogression of the granulites, which occurred penecontemporaneously with dyke intrusion, may also be linked with these processes.
The East Greenland Nagssugtoqidian mobile belt compared with the Lewisian complex
Authors: Myers J. S.
Published: April 2008

The rocks, structures and geological history of the Lewisian complex are similar to those of the Nagssugtoqidian mobile belt in East Greenland. Both regions comprise a heterogeneous, mainly tonalitic, gneiss complex which formed 2900–2700 Ma. Both gneiss complexes were intruded by basic dykes and intensely deformed and metamorphosed during two major episodes. The first episode, called Inverian in Scotland, is broadly equivalent to the Nag. 1 episode in Greenland dominated by major dextral strike-slip shear zones about 2600–2300 Ma. The second episode, the Laxfordian of Scotland, is broadly equivalent to the Nag. 2 episode of Greenland associated with the southward translation of sheets of rock along northward-dipping thrusts and shear zones about 1900–1700 Ma. Both gneiss complexes were raised to a high crustal level by 1600–1400 Ma and extensively eroded.
High-grade terrains in and around the Yilgarn Block of Western Australia
Authors: Myers J. S.
Published: April 2008

Two kinds of high-grade terrain occur in the western part of the Archaean Yilgarn Block. One consists of repeatedly deformed and metamorphosed granites (3650 and 3500 Ma), layered basic rocks (3750 Ma), and siliceous metasedimentary rocks, which were metamorphosed to granulite facies at 3300 Ma. The other consists of much less deformed granites and some basic rocks which form deeper, granulite-facies equivalents of the adjacent high-level 3000–2600 Ma granite-greenstone terrain.

The Yilgarn Block is bounded by Proterozoic belts of intense deformation and high-grade metamorphism. These belts were active between 2000 and 1600 Ma and comprise both reworked Archaean and early Proterozoic rocks. Two belts were mobile again and metamorphosed to granulite or amphibolite facies between 1300 and 1000 Ma, and one of these belts was also intruded by plutonic rocks, deformed and metamorphosed to granulite facies at about 650 Ma.
The Archaean geological evolution of Enderby Land, Antarctica
Authors: Harley S. L. , Black L. P.
Published: April 2008

The Napier Complex of Enderby Land, Antarctica, is a major terrain exhibiting Archaean granulite-facies metamorphism at extreme
PT
conditions. Combined Rb-Sr, Sm-Nd and U-Pb zircon isotope systematics define an age of 3070 Ma for initial granulite-facies metamorphism and intense D
1
deformation. Orthogeneissic sheets intruded into the tectonically thickened crust at times up to 3070 Ma resulted in the attainment of peak regional metamorphic conditions of 950°C and 7–10 kb subsequent to D
1
. These high-grade conditions prevailed through a second, less intense, deformation (D
2
) after which the Napier Complex became a stable craton for some 500 Ma. The presently exposed granulites record a near-isobaric cooling path over this time interval prior to reactivation at 700–650°C and 5–8 kb in a deformation event (D
3
) at
c.
2450 Ma. The D
3
event resulted in open dome-and-basin folding of the pre-existing recumbent- and isoclinally-folded pile. Protolith ages for some granulite-facies gneisses, obtained from conventional and ion-microprobe investigation of zircons and deduced from Sm-Nd modelling, are in the range 3750–3950 Ma, some 700–900 Ma prior to the first recognized metamorphism.
Development of extension and mixed-mode sedimentary basins
Authors: Gibbs A.
Published: April 2008

Analysis of onshore and offshore basins using a variety of techniques including deep seismic reflection profiles has shown that basins develop above linked fault systems. These fault systems comprise both steep and gently dipping faults which can have either dip- or strike-slip displacements. Whilst linked fault systems are common to all basins they are developed to different degrees and hence produce a variety of structural expressions within, and between the basins. This allows basins to be classified either in terms of their dominant structural elements or seen as a mechanical and geological continuum.

As the basins grow, changes in crustal thickness in the upper brittle crust can take place, on staircase arrays of low-angle listric faults, on steeper planar (domino) faults or commonly on a combination of both linked by crustally conservative strike-slip systems.

The development of the controlling basement elements affects the distribution and pattern of internal deformation in the evolving sedimentary fill resulting in a characteristic basin architecture. This resulting architecture has profound implications for models of the stratigraphy and sedimentology of basins, and provides an insight into the occurrence of certain facies and stratigraphic changes.

A general model for the evolution of such basin architectures is presented in this paper including both the basement structural elements and the overlying carapace of sediments with both stratigraphic and structural components.
Physical models of extensional tectonics at various scales
Authors: Vendeville B. , Cobbold P. R. , Davy P. , Choukroune P. , Brun J. P.
Published: April 2008

In a preliminary series of experiments, using physical models mechanical processes of extensional tectonics have been investigated at various scales. By a suitable choice of model materials, experiments were performed at low cost in a natural gravity field. Upper layers of the lithosphere were modelled using sand; lower layers, using silicone putties of two different densities; the mantle asthenosphere was modelled using honey. The models deformed under their own weight or under absolute horizontal tension. Rates of extension were controlled using a stepper motor. Surface deformation and faulting were monitored using 35 mm time-lapse photography. Lower lithosphere topography was photographed through the transparent asthenosphere. Fault patterns in models with lithosphere only, were observed by serial sectioning. Otherwise, the brittle-ductile interface was observed after suctioning off the sand.

Simple experiments with uniformly extended sand layers only show that; (i) spacing of normal faults is a measure of the layer thickness; (ii) the length of fault trace increases with the amount of downthrow; and (iii) faults tend to form domino domains. Some experiments with a brittle layer on a ductile substrate show a mechanism of passive rifting where; (i) major faults occur in conjugate pairs, defining rift valleys; (ii) minor faults localize additional extension in rift-valley floors; and (iii) isostatic uplift of the viscous substrate causes uplift and tilting of rift rims. In freely floating continents, gravitational spreading leads to: (i) highly localized extension and thinning at continental margins and (ii) internal rifting.
Analogue models of extensional fault geometries
Authors: McClay K. R. , Ellis P. G.
Published: April 2008

The progressive development of extensional fault geometries in a sedimentary cover sequence above a rigid or deforming basement has been experimentally investigated using analogue models. Quartz sand (700 μm) was used as a modelling material. The experiments were recorded using time-delay 16 mm cine photography and 35 mm photography. Final models were impregnated with resin and serially sectioned to investigate the three-dimensional fault geometry. Four series of experiments were carried out: (i) extension above a linear basement dislocation; (ii) extension above a uniformly stretching basement; (iii) extension controlled by a planar fault; and (iv) extension controlled by a listric fault.

Extension above a linear basement dislocation produced a single graben structure in which initial high-angle bounding faults were cut by later listric faults. Fault nucleation occurred into the hanging wall of the graben and only minor rotation occurred in the fault blocks within the graben. Extension above a uniformly extending basement produced a variety of fault structures. Heterogeneous nucleation of faults occurred with initial planar geometries giving way to more listric faults. Significant fault-block rotation was observed. In some instances rotation of pre-existing fault planes produced a negative (convex upward) listric-fault geometry. Extension controlled by a planar fault produced a single graben structure in which new faults developed into the graben. Little rotation of fault blocks occurred. Extension controlled by a predetermined listric fault showed the progressive development of a rollover anticline. The crest of the anticline collapsed producing a second-order crestal collapse graben. The nucleation of new faults was consistently in the hanging wall above the major detachment faults.

In all of the experiments carried out to date, once a major fault had developed the second-order fault nucleation was consistently in the hanging wall fault block. Footwall collapse and hence fault migration into the footwall was not significant. Listric faults produced considerable rotation of hanging wall blocks. In some instances heterogeneous rotation of preexisting faults generated negative listric-fault shapes. In rollover structures collapse of the crestal region of the fold produced a second-order graben structure.
Brittle modes of foreland extension
Authors: Hancock P. L. , Bevan T. G.
Published: April 2008

Complex systems of mesoscopic brittle fractures striking transversley to orogenic margins are characteristic upper-crustal structures in forelands (and hinterlands) that have experienced extension as a consequence of continental collision. They are especially well developed in foreland peripheries and some forelands lacking major grabens and strike-slip faults, the best-known expressions of horizontal extension. Despite the contribution of the mesofractures to foreland strain being less than that achieved by macrofault zones they are important structures because: (i) they are widespread and pervasive at outcrop scale, (ii) they influence the location of some major topographic lineaments and (iii) they permit regionally significant extension directions to be determined where macrostructures are absent.

The principal styles of mesoscopic brittle structures are planar normal faults, vertical extension joints and veins, and steeply inclined, conjugate hybrid and shear joints. Commonly, the strike of fracture sets is uniform throughout large areas and although it is generally normal to a deformation front, this simple relationship is not present everywhere, particularly distant from some markedly arcuate fronts.

Two examples of parts of forelands containing brittle extensional structures approximately coeval with late-Alpide events are the European platform in S England and N France, and the Arabian platform neighbouring the Gulf coast. In both regions, the fractures are superimposed obliquely on older Tertiary structures and comprise dominant sets of extension fractures accompanied locally by normal faults and hybrid joints. The dominance of extension and hybrid fractures among the failure modes indicates that differential stresses remained low throughout large areas of the forelands and that the effective minimum stress was tensile.
Tectonic heredity and the layered lower crust in the Basin and Range Province, western United States
Authors: Allmendinger Richard W. , Oliver Jack , Hauge Thomas A. , Hauser Ernest C. , Potter Christopher J.
Published: April 2008

COCORP deep seismic reflection data were collected in a transect at 40°N latitude across the entire Basin and Range extensional province and its boundaries, the Sierra Nevada Mountains and the Colorado Plateau. In general, the data display little of the province-wide symmetry of the modern Basin and Range, and instead can be more directly related to the pre-17 Ma asymmetric orogens of the western United States, particularly in the eastern part (112–115°W) of the region. No one model of intracontinental extension is applicable to the entire province: it may be that pre-existing structure of the continental crust predisposes it to a particular mode of extension unless thermally activated processes render the crust mechanically isotropic. Such processes (e.g. enhanced ductility, magmatism, etc.) may be responsible for the development of a highly layered lower crust and pronounced reflection Moho; features not commonly seen in reflection profiles from cratonic regions.
Topography and origin of the southern Rocky Mountains and Alvarado Ridge
Authors: Eaton Gordon P.
Published: April 2008

The southern Rocky Mountains of the western United States and their structural continuation southward to the Mexican border represent the crest of a bilaterally symmetrical, continental feature of large dimensions, the Alvarado Ridge. It is characterized by long, gentle topographic rises with systematic, concave-upward slopes on which elevation declines in a quasi-exponential manner. The rises were originally blanketed with clastic sediments, a few tens to hundreds of metres thick; their erosional source being at the ridge crest. The blanket on the E rise is well preserved and has been undisturbed for nearly 5 Ma, save for regional Neogene warping and local, near-crest faulting associated with uplift of the ridge.

A comparative study of variations in regional elevation, gravity and crustal thickness suggest that the Alvarado Ridge and its rises are isostatically compensated and that almost none of the compensation involves an Airy crustal root. The flexural rigidity of the lithosphere is likewise believed to play but a minor role in the origin of the regional topography. Instead, the data are interpreted as confirming the hypothesis of distributed lithospheric thinning continuous and non-continuous in nature, and related thermotectonic uplift. Elevation of the ridge crest took place above the axis of an elongate, rapidly developing, asthenospheric bulge along which extensional strain in the shallow crust was limited to a central corridor only 150–200 km wide. The relatively low density and geometrical configuration of that bulge supports the topography. Heat-flow data in the region are in accord with this model.
Extension and its influence on Canadian Cordilleran passive-margin evolution
Authors: Thompson Bob , Mercier Eric , Roots Charlie
Published: April 2008

The proto-Pacific margin of North America remained passive for the main part of 1.4 Ga. The stratigraphy that accumulated can be split into four major time-rock sequences; each may be considered a separate passive-margin sequence. The margin, initiated at about 1500 Ma, was remodelled by extensional (rift) episodes. Geological evidence for a 780 Ma extension event is documented here; subsidence analysis supports a 550 to 600 Ma separation event (Bond & Kominz), and stratigraphic and structural evidence from the northern Canadian Cordillera suggests 370 Ma extension. Each event is interpreted as the precursor to, and initiator of, passive-margin sedimentation.

Fold- and thrust-belt trends reflect passive-margin geometry. In the Ogilvie Mountains an abrupt right-angle bend in structural trend was inherited from 775 Ma extension faults.
Timing and style of crustal extension N of the Scottish mainland
Authors: Kirton S. R. , Hitchen K.
Published: April 2008

The region immediately to the N of the Scottish mainland and W of Orkney and Shetland may be divided into two areas on grounds of structural style. The two areas are separated by a well-defined NE-SW-trending horst, the Solan Bank High. The northwestern area is dominated by listric normal faults downthrowing to the NW. These faults were initiated at two main stages: late, or post Permo-Triassic (but pre-middle Jurassic) and middle Jurassic. A well-defined NW-SE-trending transfer (strike-slip) fault, termed here the Judd fault, is associated with the middle Jurassic faults. The southeastern area is dominated by listric normal faults down-throwing to the SE. These faults were initially Caledonian thrusts which subsequently relaxed during the Permo-Triassic.
Sedimentary structures associated with extensional fault movement from the Westphalian of NE England
Authors: Fielding C. R. , Johnson G. A. L.
Published: April 2008

Although differential subsidence and extension has been well established as a control on sedimentation for the Dinantian of N England, the continuation of this tectonic regime into Silesian times is less well demonstrated.

The Westphalian (Upper Carboniferous) Coal Measures of the Durham coalfield in NE England, which were deposited on an essentially flat, deltaic plain, display abundant evidence of a subtle structural control on sedimentation. Medium-scale (hundreds of square kilometres) patterns of sedimentation, notably the disposition of major fluvial channel belts, were at times strongly controlled by active, ENE-WSW- and E-W-trending fault lines. The faults represent a response to a broadly N-S directed tension and many resulted from the reactivation of earlier, Caledonide crustal weaknesses.

The effects of such tectonic activity may be seen spectacularly at field-outcrop scale. At an opencast coal mine in southern Co. Durham close to the major Butterknowle fault, an exposed Upper Westphalian A sequence displays a plethora of tectonically associated structures. These include gravity slides, various dewatering structures, claystone dykes, convoluted and internally chaotic sandstone beds, desiccation cracks and signs of vertical drainage in an originally waterlogged paleosol profile, and abrupt bed thickness changes. The existence and orientation of two minor channels on the ESE side of the fault line, and other palaeoflow data, support the hypothesis that the Butterknowle fault displaced downward the area to its ESE at this time.

Fractures across the area of the opencast site were found to contain high concentrations of galena, sphalerite, pyrite/marcasite and other minerals which were apparently introduced at the time of formation of the N Pennine orefield (Carboniferous-Permian).
The thermal and mechanical development of the Wessex Basin, southern England
Authors: Karner Garry D. , Lake Stuart D. , Dewey John F.
Published: April 2008

The Wessex Basin of southern England is an E-W-trending Late Palaeozoic to Tertiary extensional basin formed on Early Palaeozoic Variscan crust. Extension occurred over a protracted period from Late Carboniferous to Cretaceous times resulting in the evolution of four main depocentres or sub-basins; the Channel basin, Winterborne-Kingston Trough, Vale of Pewsey, and Weald basins. Each sub-basin was controlled fundamentally by the normal reactivation of basement thrust faults and wrench or transfer faults. The basement faults represent primary structures associated with the fold-and-thrust terrain of the Devonian-Carboniferous Variscan Orogeny. Subsequent, but synchronous, reactivation of both thrust and transfer faults tended to compartmentalize the basement, leading to discrete depocentres and the production of general, rhomboidal-shaped, sub-basins (characteristic of pull-apart basins in strike-slip terrains).

To investigate the thermo-mechanical properties of the lithosphere during basin formation, 50 boreholes within the Wessex Basin were back-stripped to isolate the driving tectonic subsidence responsible for basin formation. The resultant driving subsidence is characterized by a gentle, sometimes negative, exponential form, with superimposed rapid, discrete, subsidence events. As the geological development of the Wessex Basin was characterized by polyphase extension and the utilization of crustal detachments, the lithospheric model used to determine
δ
(crustal extension above the detachment) and
β
(stretching below the detachment) included the effects of depth-dependent stretching and finite re-rifting. From the back-stripping analysis, the basin-wide-averaged upper-crustal extension and lower-crustal/sub-crustal lithospheric stretching were 11 and 5% respectively. The distribution of
δ
is strongly influenced by the location of growth faults, and hence basement thrust-fault reactivation, suggesting that
δ
is reflecting the brittle failure and collapse of hanging wall blocks. Repeated slip and block collapse resulted in the polyphase extension responsible for renewed basin subsidence. In contrast,
β
is uniformly distributed across the basin, suggestive of a ductile failure of the lower crust and responsible for the low-magnitude thermal subsidence. Strain compatibility between the upper-crust and lower-crust/sub-crustal lithosphere is maintained partly within the basin (5%), and partly external to the basin. As the Variscan basal thrust was the fundamental relay (in the sense of Wernicke) responsible for the Wessex Basin formation, we postulate that strain balancing induced a cumulative (essentially thermal) isostatic uplift (of ≈ 60 m) under the northern margin of the Armorican Massif.
The Devonian basins of western Norway: tectonics and kinematics of an extending crust
Authors: Seranne Michel , Seguret Michel
Published: April 2008

The Devonian basins of western Norway represent shallow to deep exposures of a synthetic extensional sedimentary basin and provide field evidence for ductile extensional deformation within the basin fill and for the evolution of a brittle low-angle fault and ductile shear zone along the basal contact. The motion along this low-angle (5–25°) detachment is synchronous with both deposition and tilting (25°) of the huge (up to 25-km thick) overlapping coarse detrital Middle Devonian series. Such a geometry requires a minimum dip-slip offset of 50 km.

The structural data are consistent with fault-rock associations along the basal contact and with the prograde greenschist metamorphism observed in the southern basin: deeper and deeper levels are observed from N to S. Except along the highly sheared and retrogressed basal shear zone, the footwall remained unaffected by deformation during basin development.

We discuss three crustal models for basin development and propose that the displacement along the basal contact of the basins is transformed into pervasive ductile flow within the lower crust both at some distance to the side of the basin and beneath the basin.
Deformation processes in unconsolidated sands
Authors: Owen G.
Published: April 2008

Deformation in unconsolidated sands requires the action of a deformation mechanism to reduce sediment strength and a driving force to induce deformation. Deformation mechanisms include liquefaction and fluidization and are reflected in the style of deformation and grain orientation fabrics. They are initiated by a trigger, including groundwater movements, wave action and seismic shaking. Driving forces include gravitational body force, unevenly distributed loads, unstable density gradients and shear forces, and are reflected in the geometry of deformation. These components are combined to produce a genetic classification of soft-sediment deformation processes and structures.
Non-destructive determination of Young’s modulus and its relationship with compressive strength, porosity and density
Authors: Allison R. J.
Published: April 2008

The Grindosonic apparatus, new equipment capable of indirectly determining rock compressive strength, is discussed. This utilizes the principle that elasticity theory can be applied to rock masses (Attewell & Farmer 1976; Selby 1982) and directly measures the fundamental vibration frequency of a rock sample of regular dimensions following shock excitation. Dynamic Young’s modulus and a variety of other parameters can be established.

Samples of Upper Cretaceous Chalk and Upper Jurassic Portland Limestone are used to demonstrate the apparatus and its application. Test specimens were prepared and analysis conducted on material extracted at a number of locations throughout the Isle of Purbeck in Dorset, UK. Samples suitable for deformation in triaxial compression were also prepared and correlations drawn between compressive strength, dynamic Young’s modulus, porosity and density.
Morphology and microstructures of hydroplastic slickensides in sandstone
Authors: Petit Jean-Pierre , Laville Edgard
Published: April 2008

The High Atlas Mountains (Morocco) provide local basins with good Triassic exposures. Basin formation was induced by normal and strike-slip synsedimentary faults, either sealed by Mesozoic and Cenozoic rocks, or reactivated during Alpine orogeny. Thus the Triassic outcrops in these fault zones show not only slickensides obviously due to rupture and friction mechanisms in brittle material (reverse and strike-slip faulting during Alpine orogeny), but also ‘hydroplastic’ slickensides due to faulting in incompletely lithified sediments (normal and strike-slip faulting during Mesozoic pull-apart basin formation).

Microstructural and morphological comparison between these two types of slickensides enable us to describe various specific criteria for ‘hydroplastic slickensides’.
Soft-sediment microfaulting related to compaction within the fluviodeltaic infill of the Soria strike-slip basin (northern Spain)
Authors: Guiraud Michel , Séguret Michel
Published: April 2008

As much as 8 km of fluvio-deltaic strata accumulated in the Sorio strikeslip basin during the Late Jurassic-Early Cretaceous (Wealdian). The strata were bent in a huge syn-sedimentary syncline related to the development of an extensional half-graben in the basement. The alluvial plain siltstones and clays are cut by a large number of microfaults of a special type in regard to their geometry and their morphoscopic characters. The fault surfaces concave upward dip at 10°–40° to bedding with a strong dispersion of fault plane strikes, providing a characteristic circular pattern. They are closely spaced and cut the material into small bi-pyramidal cone-shaped units with axes normal to bedding.

The fault surfaces are glossy with a metallic glint and a fine striation without crystallization. From electron microscopic observations, this is due to phyllite re-orientation. Along the fault planes the displacement is always dip-slip and slight.

Analysis of fault striation populations by a computer-aided method gives a unique stress tensor responsible for the striation with the maximum compressive stress σ
1
normal to the bedding plane along the whole section of the synsedimentary syncline and a
R
ratio (
R
= (σ
2
− σ
3
)/(σ
1
− σ
3
)) close to 0.

The orientation of the stress tensor, the
R
= 0, i.e. σ
2
= σ
3
and the morphological aspect of the fault surfaces similar to shear planes affecting present cohesive soils lead us to conclude that this deformation is related to compaction in cohesive clays contemporaneous with water escape structures in sandstones. This deformation occurs in the evolution of the basin fill from soft sediments to metamorphosed rocks.
Comparative studies of gravity tectonics in Quaternary sediments and sedimentary rocks related to fold belts
Authors: Pedersen S.A. Schack
Published: April 2008

Two different modes of gravity tectonics have been active as deformation mechanisms in the Quaternary of Denmark, namely ‘gravity gliding’ and ‘gravity spreading’. Examples of gravity gliding can be found in the fairly slowly moving coherent landslides which characterize many coastal cliffs with lithologies intercalated with Tertiary marine clay or Quaternary interglacial clay. The gravity spreading mechanism generated folds and dislocation structures characteristic of glacial tectonics, mainly during the Weichselian glacier advances of the Pleistocene glaciation. Imbricate dislocation structures formed by diapiric rise of water-saturated clay into sand occur. This structure type is proposed to be characteristic for the gravity spreading model.

Analysis of these gravity deformed Quaternary deposits serve as a model for structures occurring in large-scale tectonic settings. Among the megascopic tectonic settings mentioned, two regions are discussed. Firstly, the transverse thin-skinned thrust fault belt in the southern part of the North Greenland Fold Belt is demonstrated to contain the characteristics of a gravity gliding system. Secondly, the gravity spreading structures of the glacio-dynamic deformations in Denmark are suggested to have a megascopic equivalent in the plate-tectonic convergence zone of the Barbados Ridge.
Genesis and diagnostic value of large-scale gravity-induced penecontemporaneous deformation horizons in Quaternary sediments of the Kleszczów Graben (central Poland)
Authors: Brodzikowski Krzysztof , Hałuszczak Andrzej , Krzyszkowski Dariusz , Van Loon Antonius J.
Published: April 2008

Gravity-induced soft-sediment deformations are quite common throughout the Tertiary and Quaternary infilling of the Kleszczów Graben. Only a limited number of horizons, however, are characterized by the abundant occurrence of large-scale load casts, ball-and-pillow structures, convolute lamination and related structures. It appears that the structures started to develop at a metadepositional stage and that the process was favoured by unstable density gradients. The further development was rather complicated, but endogenic activity in the graben must have played a role, though mainly as a trigger mechanism.
Mantle metasomatism—perspective and prospect
Authors: Bailey D. K.
Published: April 2008

Mantle replenishment in lithophile elements has been discerned in the patterns of trace elements and isotopes in lavas. One replenishment process is identified as metasomatic replacement, seen in ultramafic xenoliths brought up in high-velocity alkaline eruptions. Thus alkaline magmatism provides the best
prima facie
evidence of metasomatism and open-system conditions in the upper mantle. The list of added lithophile elements includes the following: H, C, F, Na, Al, P, S, Cl, K, Ca, Ti, Fe, Rb, Y, Zr, Nb, Ba and rare earths. Some metasomatism may be due to wall-rock alteration near magma bodies, but the evidence for metasomatism prior to melting opens the possibility that the process is a precursor to alkaline magmatism, giving the necessary source enrichment in lithophile elements. In some igneous provinces the metasomatism is widespread, intensive and pervasive; in others it appears as veining of variable intensity. Metasomatism as a large-scale process is best indicated by the widespread distribution of alkaline magmatism in space and time: volatile flux through the lithosphere would then be the necessary precursor of metasomatism and magmatism.

Volatile activity, metasomatism and melt enrichment clearly widen the scope for mafic magma generation in the mantle, but some long-standing problems of the alkaline associations (and indeed the calc-alkaline) also call for re-examination in terms of volatile activity in the lithosphere mantle. These include the diversity of magmas, the generation of large felsic volumes, and composition gaps in magma series. Experiments show that felsic minerals are stable to 30 kb, indicating the possibility of felsic-melt generation in the upper part of the mantle. A combination of volatile flux and melt percolation along geotherms that intersect the solidus at depths of less than 80 km would lead to enrichment and metasomatism, providing distinct mantle sources for felsic magmas. Initial (or residual) melts from such a region, as distinct from those from greater depths, would be constrained by equilibria involving felsic minerals. Thus an igneous cycle could generate a bimodal association, with felsic melts forming in the upper regime and the mafic melts originating at depths below the range of felsic-mineral stabilities. Such a magma system is consistent with the observed eruptive characteristics, explains the typical ultramafic nodule and megacryst suites in the alkali olivine-basalt association and is free of difficulties with relative volumes of melts, with eruption timing and with rapid changes in erupted compositions.
The genesis of alkaline magmas with emphasis on their source regions: inferences from experimental studies
Authors: Edgar A. D.
Published: April 2008

Experimental studies bearing on the genesis of alkaline magmas are reviewed, with emphasis on alkaline mafic-ultramafic magmas and the nature of their mantle sources. In contrast with previous experiments on simplified systems mainly at low pressures, more recent experiments under mantle pressures have resolved some of the problems of the earlier work. Inadequacies of petrogenetic schemes based on the model pyrolite mantle source for the generation of alkaline magma are also partly resolved by recent studies which suggest that with increasing alkalinity the possibility of deriving mafic-ultramafic magmas from a dry lherzolitic mantle source, or one with H
2
O as the only volatile, becomes increasingly difficult. The addition of CO
2
and H
2
O to the source regions allows generation of magmas such as olivine melilitites, but few magmas of the olivine leucitite kindred can be generated from lherzolitic sources. Such magmas appear to be partial melts of clinopyroxenite or orthopyroxenite sources containing both volatiles. Preliminary experimental modelling of mantle metasomatism suggest that this process may readily occur.

Experiments on felsic alkaline rocks with emphasis on liquid immiscibility and halogens and experiments aimed at solving specific problems are briefly considered.