bluuebunny/crossref_metadata_2025 huggingface dataset:
Cross-faults and differential stretching of hanging walls in regions of low-angle normal faulting: examples from western Turkey
Authors: Şengör A. M. C.
Published: April 2008

Large oblique-throw hinge faults orientated at high angles to the strike of major breakaway faults in W Turkey are examined. They exhibit variable displacements which commonly increase towards the breakaway fault with which they seem to be associated and bound cross-blocks that in places have rotated around vertical axes by differing amounts and senses, and are interpreted in terms of the possibility that hanging walls of major low-angle breakaway faults (extensional allochthons), may exhibit differential internal extension accommodated by transverse faults that may or may not rotate around vertical axes. Such transverse faults, here called
accommodation faults
, are different from transfer faults in that they need not offset the main breakaway fault and may accomplish significant map-view distortions of extensional allochthons.

Irrotational accommodation faults may give rise to horst and graben complexes or may create the image of domino-style, uniformly tilted normal and/or thrust faults at high angles to the strike of major breakaways, thus giving rise to the misleading image of extension in two directions in an extensional zone or shortening at high angles to regional extension. Although strike-slip offsets along irrotational accommodation faults need not bear any relation to the magnitude of regional extension, their strikes must at least be sub-parallel to the orientation of regional extension. Real shortening at high angles to regional extension may give rise to rotational accommodation faults. Neither the strike-slip offset along such faults nor their azimuth need bear any simple relationship to the magnitude and orientation of regional extension. Rotational accommodation faults bring about an equivalent rotation of all structures that form along them, such as pull-apart basins, and the hanging wall blocks they delimit, together with their pre-existing structures.

All three kinds of cross-faults, i.e. transfer faults, irrotational and rotational accommodation faults may coexist in any one extensional region and may grade into one another both in space and in time creating a very complicated structural picture and causing irregular subsidence patterns. It is stressed that cross-faults have an extremely important bearing on our understanding of the geometry and evolution of extensional zones, here called
taphrogens
, and that only a three-dimensional consideration of the structure of taphrogens is likely to lead to a satisfactory understanding of their nature.
Deformation, diagenesis and the mechanical behaviour of chalk
Authors: Clayton C. R. I. , Matthews M. C.
Published: April 2008

The chalk of England is a highly variable material resulting from numerous deformation processes of which the most important are diagenetic or tectonic in nature. The engineering behaviour of the chalk is influenced greatly by these processes thus giving rise to the necessity of carrying out studies of physical properties to aid the prediction of engineering performance. The results of these studies are of value when examining deformation mechanisms associated with diagenesis and tectonism. In this paper use is made of results obtained from engineering laboratory index tests to study deformation mechanisms associated with early diagenesis and the influence of overburden and tectonism.
A laboratory technique for investigating the deformation microstructures of water-rich sediments
Authors: Maltman Alex J.
Published: April 2008

Experimental deformation is a particularly valuable means of investigating the mechanical behaviour of water-rich sediments. Techniques have been developed for preparing and deforming wet sediments in the laboratory and petrographically examining the resulting microstructures. Clays marketed for the ceramic industry are more useful than natural clays because of their consistent properties. They are mixed with sea-water, aged, and consolidated to the desired value either by self-weight or in a simple levered oedometer. Argillaceous sediments with water contents of about 50% and lower are routinely produced. Wetter materials are extremely delicate and vulnerable to non-testing artefacts. In an attempt to circumvent this problem a device has been designed which enables slurries to be sedimented directly on to the base of a triaxial compression cell.

The prepared specimens are tested in a conventional ‘direct’ shear box, or in ‘distributed’ shear which subjects the whole block to shear displacement, or in triaxial compression. The latter tests can be undrained, or partially/intermittently drained, from the top and/or the bottom of the sample.

After testing, slabs are cut from the specimen with a wire-saw and impregnated with Carbowax P.E.G. 8000 to allow thin-section preparation and examination.
Shear zones in argillaceous sediments—an experimental study
Authors: Maltman Alex. J.
Published: April 2008

Experimental deformation of water-rich argillaceous sediments has shown that they deform not by pervasive homogeneous flow, as has sometimes been surmised in the past, but by intense slippage within very narrow, discrete zones of shear. These shear zones enable large total strains to be accomplished whilst leaving the bulk of the material undisturbed. Macroscopically the zones are shiny, finely-grooved planes which may be stepped where sub-structures intersect them. Under the microscope the zones are seen to result from pronounced particle reorientation into sub-parallelism with the zone margins, presumably by slippage at the grain scale. The zones commonly curve and anastomose, and various sub-fabrics may be discernible. Although the details vary, the shear zones are strikingly consistent throughout the range of experimental conditions.

In specimens with 15% water content, the shear zones lack cohesion and are analogous to shear fractures in brittle rocks. Between approximately 15–45% water content the same overall geometry persists, but the shear zones maintain cohesion. At greater water contents, up to at least 60%, the clay sediments are extremely weak and although they may appear to be undergoing pervasive flow, microscopic examination reveals that even here arrays of short, narrow zones of concentrated displacement are being generated.
Sediment deformation structures and the palaeotectonic analysis of sedimentary basins, with a case-study from the Carboniferous of northern England
Authors: Leeder Michael
Published: April 2008

There is a close link between syn-sedimentary faulting caused by earthquakes of magnitude >5 and accompanying ‘soft’ sediment intrafolial deformation in active neotectonic settings. Certain of these intrafolial structures may also be produced by purely sedimentary processes and are termed ‘autokinetic’. Those caused by earthquake-induced stresses are termed ‘allokinetic’. Although certain structures may be caused by either mechanism, it appears that large amplitude dewatering pipes and recumbent-folded cross stratification are likely to be predominantly allokinetic in origin. These structures may be used to define the Allokinetic Deformation Number which can be systematically mapped out in particular lithofacies over outcrops in a sedimentary basin. Results of this mapping are then combined with data on lithofacies thickening and stacking (tectonically-controlled architecture), hangingwall rollover deformation etc., to establish an interdisciplinary outline of palaeotectonic evolution. This methodology of palaeotectonics is tested in the suspected extensional terrain of the Northumberland Basin with encouraging results.
Syn-diagenetic deformation of a turbiditic succession related to submarine gravity nappe emplacement, Autapie Nappe, French Alps
Authors: Labaume Pierre
Published: April 2008

The Autapie Nappe (French Alps) consists of a Late Cretaceous ‘Helminthoid Flysch’ turbiditic succession. It was emplaced in the external Alps during the Late Eocene under the form of a gravity submarine nappe sliding in a turbiditic foreland basin. The syn-diagenetic gravity spreading of the Helminthoid Flysch succession resulted in a pervasive extensional disruption of the sandstone and mudstone layers related to decollements in the marlstone and claystone layers. The deformation regimes and mechanisms testify to: (i) the incomplete and inhomogeneous lithification of the sediments, and (ii) the occurrence of pore pressure gradients and, possibly, excess pore pressure during the deformation. In particular, the critical state of lithification of the sandstone allowed the occurrence of an unusual chronology of deformation mechanisms, the ‘brecciation’ resulting from a partial loss of cohesion of the sandstone in high speed escaping pore-water (ductile, ‘soft-sediment’ behaviour) post-dating calcite-filled veins (brittle, ‘rock’ behaviour). Similar deformation style might be expected to occur at modern convergent margins.
Nephelinites and carbonatites
Authors: Le Bas M. J.
Published: April 2008

Carbonatites found in strongly alkaline intra-plate petrographic volcanic provinces are associated with olivine-poor nephelinites and with phonolites. Olivine-rich nephelinites occur in basanitic and alkali basalt provinces, normally without carbonatites. These igneous provinces are marked by epeirogenic crustal uplift.

Nephelinites, ijolites and carbonatites form discrete magmatic events within individual complexes and correspond to the silicate-carbonate conjugate immiscible liquids observed in the laboratory. Carbonate liquids of variable alkali content can separate from both nephelinitic and phonolitic liquids. The silicate liquids give rise to pyroxenites, ijolites, nepheline syenites and nephelinitic pyroclast-rich strato-volcanoes. The carbonate liquids lose alkalis and fractionate to sövite, alvikite and ferrocarbonatite, each with increasing incompatible-element content. Further fractionation of carbonatite magma can produce mineralizing fluids rich in rare-earth elements, F, Ba, U and Th. Dolomite carbonatite forms only in the deeper parts of carbonatitic complexes, perhaps at depths greater than 2 km. Explosive carbonatite volcanism can occur giving widespread carbonate tuffs, rarely with lavas.

Fenitization characterizes ijolite-carbonatite intrusive complexes. Syenitic fenites containing alkali feldspars, sometimes perthitic, are formed as aureoles 500 m wide around ijolites in granitic terranes, with nepheline syenite commonly formed as a contact-reaction rock. The feldspar-rich syneitic fenite aureoles which develop around the early carbonatites usually contain pure orthoclase or pure albite.
Hawaiian alkaline volcanism
Authors: Clague David A.
Published: April 2008

Hawaiian volcanoes erupt alkalic lavas during three of four eruptive stages. Stage 1 is characterized by submarine eruption of alkalic basalt and basanite followed by tholeiitic basalt which is exclusively erupted during stage 2. Stage 3 consists of a post-caldera-collapse capping of alkalic basalt and related differentiates. Stage 4 consists of post-erosional eruptions of alkalic basalt, basanite, nephelinite and melilitite. Eruption rates and volumes increase from stage 1 to stage 2 and then decline through stages 3 and 4. Stage 1 and 4 lavas contain deep-seated lherzolite xenoliths (even some garnet-bearing xenoliths in stage 4 lavas) whereas stage 2 lavas contain only very shallow cumulates, veins, dykes and sills; stage 3 lavas contain mainly dunite, wehrlite and various cumulates. The xenolith assemblages reflect the development of magma storage reservoirs which act as hydraulic filters in removing any xenoliths entrained below the magma storage reservoirs. The development of the magma storage system is a direct response to varying magma supply rates during the four eruptive stages.

Trace-element, isotopic and rare-gas data all indicate that Hawaiian lavas are generated from heterogeneous sources consisting of at least three distinct compositions. Mixing of the sources or of partial melts derived from these sources is inevitable; however, the characteristics of the mixing end-members are not well constrained. In a general way, early and late lavas are generated by small degrees of melting of sources enriched with less incompatible elements, whereas the voluminous shield tholeiites are generated by large degrees of melting of sources enriched with more incompatible elements.
The petrology, chemistry and crystallization history of the Velasco alkaline province, eastern Bolivia
Authors: Fletcher C. J. N. , Beddoe-Stephens B.
Published: April 2008

The Velasco alkaline province consists of an early volcanic suite, a series of 14 interfering circular and elliptical alkaline plutons, numerous dykes and a silicified carbonatitic complex. The alkaline rocks, excluding the dykes, are restricted to a narrow NE-trending belt that has been traced for nearly 80 km.

The plutons can be divided into two groups defined by the silica-oversaturated or silica-undersaturated character of the rock units within each pluton. It has been possible to deduce from textural relationships and whole-rock and mineral chemistry that the two groups can be derived from a single pulaskitic magma type, in turn producible by minor fractionation of a mantle-derived trachytic parent. Sr isotopic data indicate that the trend from pulaskite through quartz syenite to granite was developed by progressive crustal contamination due to slow rates of emplacement. In contrast, the undersaturated trend towards foyaite shows no evidence of crustal influence, the plutons probably being emplaced at a greater rate. These plutons exhibit marked sub-vertical crystal layering parallel to the intrusion walls suggesting that crystal-fluid processes were responsible for the fractionation trend.

A separate strongly undersaturated magma batch is represented in the Velasco province and occurs as a melasyenitic dyke-swarm. Some dykes are strongly enriched in rare-earth elements (REE), concentrated in the mineral britholite, and indicate interaction with a hydrous phase enriched in REE.
Tertiary alkaline magmatism in Trans-Pecos Texas
Authors: Barker Daniel S.
Published: April 2008

Alkaline magmatism in far W Texas extended from 48 to 16 Ma ago, peaking in volume during the interval 37-26 Ma when all the felsic rocks were emplaced. Primitive magmas are rare, if not entirely lacking, but
ne-
and {ol+hy}-normative hawaiite and mugearite liquids (Ni <120 ppm; 100Mg/(Mg+Fe)<66) erupted through the entire time span. These evolved through
ne
-normative and
q
-normative trachytes to phonolite and rhyolite, including peralkaline varieties of both. Mafic
q
-normative rocks are distinctly subordinate.

Transition from a contractional to an extensional regime occurred between 32 and 30 Ma ago, after which Trans-Pecos magmatism was almost exclusively mafic. Major- and trace-element compositions show gradation south-westward to the calc-alkaline rocks of the Sierra Madre Occidental in western Mexico, which were erupted from 35 to 28 Ma ago. This compositional gradient suggests that the alkaline and calc-alkaline rocks were all related to subduction of the Farallon plate. The Trans-Pecos province is not a continental rift, and probably is not a product of back-arc spreading. Peralkaline phonolites and rhyolites in Texas and calc-alkaline rhyolites in Mexico were erupted at the same time and probably in the same stress regime, and their compositional differences must reflect differences in source rocks and conditions of melting.
Tertiary and Quaternary volcanism in the Massif Central, France
Authors: Downes Hilary
Published: April 2008

Continental alkaline volcanics of the French Massif Central range in age from 65 Ma to 3450 years BP and can be divided by geography and age into 20 separate areas. There is no correlation between the age of an area and the geographic locality.

Two magma series are present: a silica-undersaturated basanite-tephrite-phonolite series and an alkali basalt-trachyandesite-trachyte-rhyolite series, the evolved members of which are silica-saturated. Rare nephelinites are also found. Primitive magmas of both main series may be derived from 2%–20% partial melting of a mantle enriched with light rare-earth elements with variable amounts of garnet in the residue; nephelinites were formed from a source which was isotopically and chemically distinct.

Fractional crystallization of observed phases accounts for the diversity of intermediate and evolved products. Amphibole fractionation in basalts at depth causes the trend towards silica saturation, while alkali feldspar fractionation dominates the final stages of crystallization. Removal of small quantities of sphene and zircon explains trace-element depletions in trachytes, phonolites and rhyolites.

Up to 30% crustal contamination has occurred in the evolved magmas but contamination is generally absent from their basaltic parents. Sr, Nd and Pb isotope data indicate that the most probable contaminant is undepleted meta-sedimentary granulite-facies lower crust. Variations in Sr and Nd isotope ratios also constrain petrogenetic hypotheses such as amphibole fractionation and magma mixing.

Mineralogical disequilibrium, compositional banding and emulsification textures indicate extensive magma mixing. Some flows are homogeneous hybrid lavas in which only the unusual composition and rare mineral disequilibria indicate the mode of formation.
Geochemistry and Tectonic Setting of Early Proterozoic Volcanic Rocks in Northern Ostrobothnia, Finland
Authors: Honkamo M.
Published: April 2008

In northern Ostrobothnia, the Proterozoic Karelidic supracrustal rocks overlie Archaean rocks of the Prekarelidic basement complex. The Karelidic rocks are marine metasediments and marine metavolcanic rocks. The metasedimentary units of the schist area include arkose-quartzites, conglomerates and turbiditic metagreywackes. The associated metavolcanic rocks are mafic lavas with carbonate and chert intercalations.

Despite regional metamorphism of amphibolite facies the chemical composition of the metalavas is well within the limits of modern volcanic suites. Chemically, the metalavas are basaltic tholeiites. On discrimination diagrams based on major element contents the metalavas fall in fields of low-potassium tholeiites (LKT) and oceanic tholeiites. Trace element patterns are intermediate between LKT and mid-ocean ridge basalts (MORB). Field and chemical data permit different interpretations of the plate-tectonic setting of the northern Ostrobothnia basin. There is evidence suggesting marginal basin setting as well as an initial intra-continental rifting environment setting.
Intra-Arc Rifting and Massive Sulphide Mineralization in an Early Proterozoic Volcanic Arc, Skellefte District, Northern Sweden
Authors: Vivallo Waldo , Claesson Lars-Åke
Published: April 2008

The Skellefte district in northern Sweden consists mainly of pyrite-rich massive sulphide deposits intercalated in an early Proterozoic volcano-sedimentary sequence. The volcanic activity was predominantly submarine, and gave rise to abundant felsic rocks with minor mafic volcanic rocks and rare andesitic volcanism. A few dykes and volcanic flows of high Mg-Cr basalts, probably komatiitic, are also found within the volcanic pile.

The geochemistry of the volcanic rocks is of subalkaline character and displays slightly tholeiitic to calcalkaline differentiation trends. The Ti, Zr, Y, Cr and rare-earth contents of the basaltic and andesitic rocks are consistent with a subduction related volcanic arc setting.

The bimodal nature of the volcanic suite, the presence of highly primitive basaltic rocks and the abundance of felsic pyroclastic material suggests that rifting occurred within the volcanic arc. Long periods of tensional stress predominated and favoured the extrusion of large volumes of felsic pyroclastics and the formation of tectono-volcanic depressions. The convergence in time and space of a thick volcanic sequence with high permeability, a high geothermal gradient related to the rising of basaltic magma, and synvolcanic faulting resulted in extensive hydrothermal activity. The hydrothermal activity caused regional alteration of the volcanic rocks and locally deposition of pyrite-rich massive sulphide deposits within local, tectonically controlled, depressions.
Volcanic Geochemistry of the Northern Segments of the Circum-Superior Belt of the Canadian Shield
Authors: Baragar W. R. A. , Scoates R. F. J.
Published: April 2008

The northern segment of the early Proterozoic Circum-Superior Belt extending from the Ungava region of Quebec (Cape Smith Belt) through the islands of eastern Hudson Bay to northeastern Manitoba (Fox River and Thompson Nickel Belts) is distinguished by an extraordinary magmatic assemblage composed of two principal suites; komatiitic and tholeiitic. The older tholeiitic suite comprising basaltic flows and dolerite sills associated with a stable-shelf sedimentary sequence lies next to the Superior Craton in the Ungava-Hudson Bay region and is adjoined on its outboard side by the komatiitic suite of predominantly pillowed lavas. Their relationships are obscured by faulting but the presence of sills of the komatiitic suite at a number of levels in, and folded with, the tholeiite-sedimentary assemblage is evidence of its younger age. In northeastern Manitoba a discrete tholeiitic suite is absent and komatiitic flows and sills, in part differentiated to tholeiitic compositions, are associated directly with siliceous sediments of apparent continental provenance.

The komatiitic suite differs from its Archaean counterparts in that magma compositions rarely exceed about 16–18% MgO, yet spinifex texture (albeit clinopyroxene-olivine) like that of Archaean komatiites is one of its distinctive features. It is typically developed in the upper parts of certain layered (differentiated) flows that are otherwise little distinguished from the majority of layered flows common to the komatiitic sequence.

The tholeiitic suite is little differentiated but the komatiitic suite ranges from olivine- to clinopyroxene- and in places to plagioclase-phyric lavas through both long- and superimposed short-range stratigraphic intervals. Differentiation in the komatiitic suite is effectively represented by declining MgO/CaO ratios through the olivine fractionation stage (to about 1) then by increasing contents of oxides of the less refractory minerals, notably A1
2
O
3
, through clinopyroxene and clinopyroxene-plagioclase fractionation. The short-range variation is attributable to fractionation in the layered flows which are interpreted as principal surface distributors of lava and the long-range variations to major high-level sills as represented by those exposed in the Fox River Belt.
Proterozoic Volcanism in the Flin Flon Greenstone Belt, East-Central Saskatchewan, Canada
Authors: Gaskarth J. W. , Parslow G. R.
Published: April 2008

Proterozoic basaltic and rhyolitic volcanics, and associated gabbros and ultramafic bodies from the Saskatchewan part of the Flin Flon-Snow Lake greenstone belt are all chemically tholeiitic. Both within-plate-MORB and volcanic arc tectonomagmatic environments are indicated by ‘discriminant’ plots (TiO
2
-Zr, Cr-Y, Zr-Ti/100-3Y) and MORB-normalized diagram patterns. Rocks with the various environmental ‘signatures’ are closely associated and there appears to be a continuum of compositions between the various types. The geochemistry is compatible with an origin from a single source within a small marginal basin.

Massive, Cu-Zn-Pb, sulphide deposits occur within volcanics with an arc-type ‘signature’ and associated rhyolitic bodies. Gold appears to have some geometrical association with the boundary between arc-type and within-plate-type volcanics though much is in later fractures. The concentration of mineral deposits in this part of the greenstone belt where there are volcanics showing transitional tectonomagmatic characteristics is thought to be significant.
Two Stage Felsic Volcanism in the Lower Proterozoic Upper Aillik Group, Labrador, Canada: Its Relationship to Syn- and Post-Kinematic Plutonism
Authors: Gower Charles F. , Ryan Bruce
Published: April 2008

The Aillik Group, situated in the Lower Proterozoic Makkovik Province, Labrador, Canada, is a bimodal volcanic suite consisting of a lower mafic volcanicsedimentary sequence and an upper felsic volcanic-volcaniclastic association. These rocks have been intruded by syn-and postkinematic granitoid rocks.

Previous mapping and stratigraphic interpretations suggest that the upper Aillik Group is itself divisible into ‘early’ and ‘late’ parts separated by a mafic volcanic/tuffaceous unit. Both involve dacitic to rhyolitic volcanism, but the earlier one has a much higher proportion of metasedimentary material and the later one is dominated by rhyolitic volcanic rocks. Major and trace element data suggest that these two sequences are compositionally distinct. The late sequence is compositionally similar to the postkinematic granites, but data are inadequate to establish a link between the early sequence and synkinematic granites, yet do not preclude it.

Although geochronological data suggest
c.
1800 Ma and 1660 Ma events in both the upper Aillik Group and granitoid plutons, a straightforward model linking early upper Aillik Group with synkinematic granites and late upper Aillik Group with postkinematic granites is complicated by
c.
1800 Ma ages from some felsic volcanic rocks assigned as late upper Aillik Group. Inadequate stratigraphic control, insufficient petrochemical data and (or) inherited ages could account for the contradictions; or the model may be invalid. Detailed stratigraphic analysis and careful geochemical studies are required to resolve the conflicts.
The Orange River Group: a Major Proterozoic Calcalkaline Volcanic Belt in the Western Namaqua Province, Southern Africa
Authors: Reid D. L. , Erlank A. J. , Welke H. J. , Moyes A.
Published: April 2008

The Orange River Group (ORG) consists of a thick volcano-sedimentary pile (> 8 km) preserved as a subprovince within the Namaqua Province of southern Africa. Volcanic activity during deposition of the ORG was predominantly subaerial, and involved the eruption of an andesite dominated calcalkaline suite ranging from basalt through to rhyolite. Agreement between Rb-Sr, Th-Pb, Pb-Pb and Sm-Nd isochron ages indicate extrusion of the ORG around 2000 Ma ago. Compared with modern arc suites, the ORG is characterized by high K and Mg, together with related trace elements (Rb, Sr, Ba, U, Th, LREE; Ni, Cr). Basic lavas differ from many modern calcalkaline types in having lower A1 contents reminiscent of tholeiites. When normalized to N-type MORB, the ORG basic lavas show enrichments in K, Rb, Ba, Th, and the LREE, and depletions in Nb, Ta, Zr, Hf, Ti, Y and the HREE). Metamorphic overprinting associated with the ∼ 1000 Ma Namaqua tectonothermal event are evident to varying degrees throughout the subprovince, and precludes precise monitoring of initial isotope ratios at time of eruption. Evidence from the rocks least affected by metamorphic overprinting suggests common initial Sr, Pb and Nd isotopic compositions for the entire basalt to rhyolite suite in this particular area, which would support a fractional crystallization model and rule out major involvement of old Archaean crust to produce the voluminous andesites. Initial isotopic data translate to ɛ
sr
(T) + 10), ɛNd(T)∼0, and an apparent high μ value, which, when taken together with the geochemical features described previously, indicate that the source region to the mafic parental magmas of the ORG shows patterns similar to those in the mantle wedge overlying subduction zones. The calcalkaline ORG volcanic suite may therefore represent the remnants of a 2000 Ma old Proterozoic arc complex. The high K character, the intimate association of comagmatic granitoids, the development of copper porphyries, and the lack of any direct or indirect evidence for underlying Archaean crust all point to a mature island arc environment. Active continental margins such as the Andes show similar geochemical and isotopic features, but it is argued that the high elevation of such margins will cause them to be preferentially eroded and lack representation in ancient terrains.
Geochemical Patterns of Metabasites in the Southern Part of the Damara Orogen, SWA/Namibia: Applicability to the Recognition of Tectonic Environment
Authors: Breitkopf J. H. , Maiden K. J.
Published: April 2008

In the southern part of the late Precambrian Damara Orogen, mafic schist and amphibolite represent metamorphic equivalents of tholeiitic basalt and gabbro. The metabasites are extensively developed in several stratigraphic units and extend laterally along strike over more than 300 km. In the Southern Margin Zone, they are interbedded with pelitic schist, quartzite, diamictite and minor marble, whereas in the Southern Zone (Trough Zone) the Matchless Amphibolite Belt occurs within a sequence of predominantly quartz-mica schist.
The Geochemistry of the Lower Proterozoic Willyama Complex Volcanics, Broken Hill Block, New South Wales
Authors: James S. D. , Pearce J. A. , Oliver R. A.
Published: April 2008

The major and trace element geochemistry of the early to middle Proterozoic meta-igneous rocks of the Willyama Complex in the Broken Hill Block indicates that the basic rocks originated by partial melting of a depleted mantle source, followed by fractional crystallization along a trend of strong iron enrichment coupled with some crustal assimilation. At least some of the intermediate-acid rocks were derived from fractionation of these basic parents, but major crustal involvement may be necessary to explain the large quantities of felsic volcanics in the lower part of the Complex. The volcanic rocks in the Broken Hill mine area show an unusually high degree of primary iron enrichment (up to 25 wt % total iron as Fe
2
O
3
) and a lesser degree of bimodality than elsewhere. By analogy with Recent ferrobasalt provinces, this extreme iron enrichment may indicate an origin in a propagating rift where small, high-level magma chambers separated from the magma source. If so, the consequent high geothermal gradient and high fracture permeability may have provided the necessary conditions for genesis of the Broken Hill ore deposit.
The Petrology and Geochemistry of Alteration Assemblages in the Eastern Creek Volcanics, as a Guide to Copper and Uranium Mobility Associated with Regional Metamorphism and Deformation, Mount Isa, Queensland
Authors: Wyborn L. A. I.
Published: April 2008

At least four alteration types are identified in the basaltic Eastern Creek Volcanics of the Mount Isa Inlier. The earliest alteration type, found in predominantly massive rocks, contains albite + actinolite with lesser chlorite + epidote + sulphides + magnetite. These basalts are enriched in K
2
O and S, and Cu has been locally redistributed within the lava flows. The remaining three alteration types are only found in deformed rocks. Two are closely related and are dominated by either epidote + sphene, or calcite + magnetite, with the mineralogy of the alteration type controlled by the concentration of CO
2
in the fluid phase. Both assemblages are depleted in S, K
2
O, Cu, Rb, Ba, and Sr, whilst Zr, V, Y and Cr are enriched, and these samples also have the highest ratio of Fe
2
O
3
/FeO. These two alteration types are associated with major regional metamorphism, and some uranium deposits in the western Mount Isa Inlier and possibly some small Cu deposits are related to them. The final alteration was the most profound, producing rocks dominated by albite + chlorite + rutile, and strongly depleted in CaO, MnO, Fe
2
O
3
, Cu, Sr, Ba, and Ni, and enriched in MgO and SiO
2
. This last alteration is believed to be linked to the formation of the Mount Isa copper ore body.
The Trace Element Geochemistry of Metavolcanics and Dykes From the Central Metasedimentary Belt of the Grenville Province, Southeastern Ontario, Canada
Authors: Smith T. E. , Holm P. E.
Published: April 2008

The trace element chemistry, including the rare earth elements, of two Proterozoic metavolcanic sequences in the Central Metasedimentary Belt of the Grenville Province is presented. The older Tudor suite is a bimodal mafic-silicic sequence comprising dominant tholeiitic basalts with minor rhyodacites. The younger Turriff metavolcanic sequence, is also a bimodal mafic-silicic series made up predominantly of tholeiitic basalts with some calcalkaline andesites and minor rhyolites. In each suite the tholeiitic basalts and the felsic rocks were formed by fractional crystallization of mafic magmas formed by partial melting of enriched (spinel) lherzolite source materials. The Turriff andesites are considered to have formed by fractional crystallization of a parental basaltic andesite produced by partial melting of an eclogite source.

Both suites are enriched in light rare earth elements (LREE), heavy rare earth elements (HREE), and high field strength elements (HFSE). This enrichment is interpreted as resulting largely from metasomatism of the sub-continental lithosphere sources. The two sequences are interpreted as having erupted in a marginal basin formed by rifting of the continental crust. Comparisons are made with similar Proterozoic volcanic sequences from the southwestern USA and with Mesozoic and Quaternary suites formed in marginal basins in central and southern Chile.
Dynamic basin analysis: an integrated approach with large data bases
Authors: Hitchon Brian , Bachu S. , Sauveplane C. M. , Lytviak A. T.
Published: April 2008

Basin analysis must include an appreciation of both the rocks and the processes affecting their contained fluids. Therefore the objective of dynamic basin analysis is to synthesize and evaluate the present geological, hydrogeological, thermal and geochemical situation of sedimentary basins as well as their past history. For sedimentary basins such as that in western Canada, where petroleum exploration has resulted in a vast amount of information, synthesis and evaluation can only be achieved through an integrated multidisciplinary computer-oriented approach. This requires the development of a data-base management system and specialized data-processing techniques. These have been developed by the Basin Analysis Group of the Alberta Geological Survey for geological, hydrogeological, thermal and hydrochemical parameters. The synthesized information can then be integrated into a descriptive steady-state model which forms the basis of the numerical simulation of various processes and phenomena related to present fluid, heat and mass flows in the subsurface. These techniques have been applied to studies of the baseline hydrogeology of the Cold Lake oil sands deposit area and the deep waste disposal site at Swan Hills, Alberta, Canada. They are also applicable to petroleum exploration,
in situ
oil recovery, gas storage, geothermal resource evaluation and mineral exploration. Future research directions are to focus on the inclusion of models for mass transport and on the determination of the palaeohydrogeological regimes in sedimentary basins.
Ancient fluid flow within foreland terrains
Authors: Bradbury Harry J. , Woodwell Grant R.
Published: April 2008

Dewatering of the front Ranges of the Canadian Cordillera in the Campanian and Maastrichtian was achieved by expulsion of connate fluids from thrust sheets during active translation. Each major thrust sheet behaved as a separate hydrodynamic unit. Flow was consistently from hinterland to foreland sections of basal-sheet aquifers. The rate of fluid expulsion exceeded that of rock translation during hydrofracturing. Two flow systems, separated in time, utilized basal-sheet aquifers. Fluid flow at the margin of foreland basins in the Southern Pyrenees in the Oligocene was focused within both the crystalline infrastructure and the sedimentary suprastructure. Ascending metamorphic fluids passed from shear zone conduits in the basement into and through the basin margin, creating ghost vertical and lateral fluid fronts within the sedimentary pile. Connate fluids escaped via cleavage network channelways. Ponding of metamorphic and connate volumes of the fluid budget occurred at high levels close to the leading edge of natural strain within the basin margin, where excess fluid pressures were achieved.
Wind-blown materials and W African soils: an explanation of the ‘ferrallitic soil over loose sandy sediments’ profile
Authors: Vine H.
Published: April 2008

It is suggested that incorporation of dust from the hinterland during episodes in the Pleistocene when the Sahara encroached far S of its present limits accounts for much of the clay and Fe
2
O
3
content of soils of southern Nigeria. The amount of pelagic deposition of clay reported for DSDP Site 366 on the Sierra Leone Rise in the last 1–3 million years is shown to correspond to the anomalous accumulation of clay in deep uniform subsoils of the ‘red ferrallitic soils over loose sandy sediments’. Particle-size analyses of soils at seven sites (showing 10% to 50% clay in the subsoil) are given, with determinations of Fe
2
O
3
in the clay fraction (5% to 9%). Silt-size quartz in the dust is thought to have been lost through solution, and feldspars and clay minerals have weathered to kaolin. Wind systems displaced 600–800 km to the S compared with the situation today would have deposited dust similar to present-day Harmattan fall-out in the N of Nigeria, in the latitude of southern Nigeria and of the Sierra Leone Rise. Creep, mass-movement, faunal activity and eluviation of clay and Fe
2
O
3
have affected the incorporation of dust, in addition to mixing coarser and finer layers of sediments together.
Continental sabkha pans and associated nebkhas in southern Kuwait, Arabian Gulf
Authors: Gunatilaka A. , Mwango S.
Published: April 2008

Small evaporitic pans within a sand-sheet/gravel-plain terrain characterize the coastal margins of southern Kuwait. Sedimentological studies indicate that the pans commenced as ephemeral lakes during a Pleistocene pluvial phase, and later evolved into sabkhas during the Holocene with the changing climatic and hydrological regimes. The lacustrine episode is represented by laminated calcite micrite in the lower parts of the section, grading upward into a diagenetic gypsum facies of the sabkhas, with diagnostic evaporite textures. Dunes are represented only by nebkhas, which occur within the sabkha pans. Their initial build-up is controlled by a halophyte which colonizes the sabkha surface, trapping any moving sand. The nebkhas pass through phases of growth, stabilization and decay that are controlled by fluctuations in the water table. A rise of the water table can waterlog the roots and kill the plants—in which case the trapped sand starts drifting. A continued fall in the water table can lead to deflation of the entire complex. The nebkhas have a poor preservation potential. In this area the aeolian system is highly starved of sand in relation to the available wind energy. Stable isotope data indicate that the groundwaters are derived from old formation waters, with only minor meteoric recharge.
The statistical analysis of ‘mixed’ grain size distributions from aeolian sands in the Libyan Pre-Desert using log skew Laplace models
Authors: Flenley E. C. , Fieller N. R. J. , Gilbertson D. D.
Published: April 2008

This paper describes the statistical analysis and modelling of simple and ‘mixed’ particle mass-size distributions. The particular sediments discussed derive from modern climbing, seif and barchan dunes. The statistical methods considered include calculation of sample moments (mean, sorting, skewness and kurtosis), in particular by semigraphical techniques. Additionally, attention is given to the construction of log normal probability plots and their supposed interpretation in terms of distinct segments representing different components of a ‘mixed’ distribution. Whilst these techniques have been used with some effect in the case of analysing simple or pure sand samples, it is shown that their application to mixed size distributions can be misleading. The approach advocated as an alternative is to model the size distribution directly by one of a family of parametric statistical distributions and use the estimated parameters to characterize the individual samples. The particular family preferred is the three-parameter log skew Laplace, which can be extended readily to a seven-parameter family incorporating mixture densities. This family provides computational advantages over the log hyperbolic family, whilst retaining its geological interpretability and theoretical derivation.
Spring mounds in southern Tunisia
Authors: Roberts C. R. , Mitchell C. W.
Published: April 2008

Chotts Djerid and Fedjadj lie in the arid zone of southern Tunisia, and are chemically active, artesian environments which give rise not only to extensive emergent groundwater gypsum and salt deposits, but also to landforms of more complex architecture. In some geological situations belts of spring mounds up to 25 or 30 m high straddle former high-level late Pleistocene lake shorelines. Spring mounds are circular, gypsiferous or calcareous, volcano-like structures fed by point sources of artesian water rising from aquifers in the Continental Intercalaire and Complexe Terminal aquifer series. Similar mounds have been identified around playas in the USA, Australia, Egypt, Iraq and elsewhere. In some mounds a central well is occupied by a small pond with water conduits in the base, whilst others are dry and degraded. In these latter, water is only present at depth and wind erosion and redeposition are moulding the landforms. The morphology of spring mounds is determined by the balance between aquifer pressure head, output, chemical content, net evaporation, aeolian activity, and plant colonization. Their stratigraphy comprises alternations of crystalline materials, wind-blown gypsiferous sand, wash and splash deposits, and organic layers arranged in overlapping sequences which dip outwards from the water source. Such deposits offer a potentially sensitive index of environmental changes in the late Quaternary and Holocene.
Silicification fabrics from the Cenomanian and basal Turonian of Devon, England: isotopic results
Authors: Carson Greg A.
Published: April 2008

Silicification fabrics within the Cenomanian and basal Turonian of Devon may be classified by their morphology and petrological relationships. The replacement of bioclasts by silica was often a two stage process; the early stage has δ
18
O values ranging from 30.0 to 32.4‰ SMOW. The palaeotemperatures calculated from these are close to values for Cenomanian sea water and thus the early silicification took place in marine porewaters and before significant burial. The consistently lighter values for the later replacement (ranging from 27.2 to 29.4‰) cannot be explained by an increase in burial depth. Partial recrystallization in meteoric waters is believed to be the mechanism responsible. Isotopic data suggest that the formation of siliceous nodules (chalcedony cemented sands) within the arenaceous lower members of the Cenomanian was an early process, although petrographic evidence points to possible (later?) recrystallization. The formation of late-stage drusy quartz occurred entirely in meteoric porewaters.
Mixed-water dolomitization in a transgressive beach-ridge system, Eocene Catalan Basin, NE Spain
Authors: Taberner C. , Santisteban C.
Published: April 2008

Dolomitization occurs at the top of alluvial fan deposits in the SE margin of the Eocene Catalan Basin, NE Spain. The dolomitized rocks occur just below erosion terraces which show a step-like disposition on the continental sediments and on which shallow marine facies (beach-ridge systems and reefs) are developed. Petrography shows that diagenesis in the continental sediments mainly originated under meteoric water influence, while in beach-ridge systems and reef carbonates there is evidence of early diagenesis in a marine environment. All the nearshore sediments (from alluvial fan to reefs) record a distinct geochemical pattern where a progressively more marine influence parallels the regional trend in facies across the ancient shoreline. Petrographic and geochemical data suggest that mixing of meteoric and marine waters in the sediments representing the continental-to-marine transition caused dolomitization at the top of the alluvial fan deposits. Unlike most mixing-zone dolomites, in this case mixing took place when shallow marine sedimentation took place on erosional surfaces causing the inflow of marine waters that mixed with meteoric porewaters.
Wilcox sandstone diagenesis, Texas Gulf Coast: a regional isotopic comparison with the Frio Formation
Authors: Land Lynton S. , Fisher R. Stephen
Published: April 2008

The burial diagenesis of Eocene Wilcox sandstones differs significantly from the burial diagenesis of other Gulf Coast terrigenous clastic formations. Differences between the onshore Wilcox and Oligocene-Miocene Frio Formations are larger than regional variations within either formation. Younger, offshore units have undergone less diagenetic alteration than either of the older, onshore units, whereas Mesozoic sandstones are generally more extensively altered.

Cementation by quartz and calcite was the first diagenetic modification of volumetric significance to affect either the Wilcox or Frio Formations. The average δ
18
O of Wilcox quartz cement is approximately +25‰ SMOW, in contrast to + 31‰ for the Frio. In both formations, quartz cement is more abundant in the overpressured zone. Calcite has an average δ
18
O of −10.8‰ PDB in the Wilcox Formation (compared to −7.2‰ for the Frio).
87
Sr/
86
Sr values suggest that calcite was derived from coeval nannofossils, carbonate rock fragments of Mesozoic age, or by local input of fluids from underlying Mesozoic carbonates. Most calcite cement (and therefore quartz which pre-dates calcite cement) was apparently emplaced prior to extensive alteration of detrital silicates which would have released strontium with a high
87
Sr/
86
Sr ratio.

The most common carbonate cement in the Wilcox Formation is ankerite which replaces calcite with increasing depth. The δ
13
C of ankerite is essentially identical to that of calcite at the same depth, and is also similar to Frio and younger carbonate cements. In contrast, δ
18
O values of Wilcox ankerite (average −9.8‰) indicate emplacement at higher temperature or from more depleted water than was true of Wilcox calcite.
87
Sr/
86
Sr values for Wilcox ankerite are very radiogenic (>0.7100), indicating that ankerite emplacement occurred during or after active silicate diagenesis. Both ankerite and dolomite are very uncommon phases in Frio and younger sandstones despite massive smectite stabilization, suggesting that the conversion of smectite to illite was not the source of iron and magnesium for the late cements.

The volume of secondary porosity is similar in both Wilcox and Frio sandstones. Albitization and K-feldspar removal from both formations are essentially complete below 3000 m and essentially no unaltered detrital feldspar occurs below that depth.

Differences between the Gulf Coast formations are attributed to different geothermal gradients, differences in the basinal sediments over which the units prograded, and to the changing nature of connate fluids in the units themselves. Diagenesis of the Wilcox Formation, like other Gulf Coast terrigenous wedges, is understandable only in terms of interaction with underlying units during the large-scale evolution, in both time and in space, of the Gulf Coast diagenetic system.