bluuebunny/crossref_metadata_2025 huggingface dataset:
An early Pennsylvanian waterhole deposit and its fossil biota in a dryland alluvial plain setting, Joggins, Nova Scotia
Authors: Falcon-Lang H.J. , Rygel M.C. , Calder J.H. , Gibling M.R.
Published: December 2007

The terrestrial ecology of Pennsylvanian tropical wetlands is understood in detail, but coeval dryland ecosystems remain highly enigmatic. To fill this gap in our knowledge, a Pennsylvanian (Langsettian) continental red-bed succession was studied at the classic Joggins locality, Nova Scotia. These units represent the deposits of seasonally dry, alluvial plains traversed by anastomosed drainage networks. One channel complex informally known as the ‘Hebert beds’ (the focus of this study) contains an unusual fossil assemblage and is interpreted as an alluvial waterhole deposit that formed following drought-induced cessation of channel flow. Adpressed and charred fossil plant remains indicate that the alluvial plain surrounding the waterhole was covered by fire-prone cordaite vegetation, with hydrophilic lycopsids and sphenopsids restricted to waterlogged riparian niches. Gigantic unionoid freshwater bivalves, locally in life position, and occurring in large numbers in the waterhole, were probably infaunal suspension feeders during periods of fluvial activity, but aestivated in channel bottom muds when flow ceased. Abundant terrestrial gastropods found clustered around fossil plant detritus may have been deposit feeders scavenging dry portions of channel floors. Common partially articulated remains of small to medium-sized tetrapods possibly represent animals drawn to the waterhole during drought when surface water was scarce elsewhere. In terms of both sedimentology and biology, the Hebert beds alluvial complex bears a very close similarity to the seasonal drainages and waterholes of present-day central and northern Australia. This unique deposit sheds significant new light on the nature of Pennsylvanian dryland tropical ecology.
The nature of ‘roof thrusts’ in the Moine Thrust Belt, NW Scotland: implications for the structural evolution of thrust belts
Authors: Butler R.W.H.
Published: December 2007

Buried thrust systems, commonly analysed in terms of duplex structures, are often interpreted as forming in a particular sequence. This model considers that only one thrust is active at a time and the resulting structural geometry is well ordered. However, a reappraisal of structural geometries from the Moine Thrust Belt in NW Scotland has shown that many apparent duplex structures are more complex. Although the upper (roof) thrust of some duplexes may be folded and bulged by underlying imbricate slices (in accordance with simple duplex models), the footwall to these roof thrusts cut up and down stratigraphic section (in conflict with the simple duplex models). Examples include the Glencoul Thrust with its underlying imbricate system, and the Foinaven ‘duplex’. Simultaneous slip on an array of imbricate thrusts can bulge duplex roofs (relationships classically used to infer piggy-back sequences of thrust ‘propagation’). However, activity on more hinterland-ward imbricate thrusts can cause the roof onto which they branch to truncate more foreland-ward structures. In this fashion, the ‘duplex roof’ can cut up and down stratigraphic section, mimicking an extensional fault. By varying thrust activity in three dimensions within an imbricate system, local parts of ‘roof thrusts’ can generate domains of overstep behaviour that have only local significance. Existing interpretations of overstep relationships in the Moine Thrust Belt as representing late-orogenic low-angle extensional faults may need revision. The new interpretations of deformation activity, where displacements in thrust arrays happen synchronously rather than sequentially, are consistent with emergent thrust and fold belts, with analogue experiments and with mechanical models of orogenic wedges.
New light on the construction, evolution and correlation of the Langavat Belt (Lewisian Complex), Outer Hebrides, Scotland: field, petrographic and geochronological evidence for an early Proterozoic imbricate zone
Authors: Mason A.J. , Temperley S. , Parrish R.R.
Published: December 2007

The South Harris Complex is a domain of largely Palaeoproterozoic rocks within the late Archaean, tonalite–trondhjemite–granodiorite (TTG)-dominated Lewisian gneisses of the Outer Hebrides, NW Scotland. The complex is distinguished by a high proportion of metasedimentary rocks and distinctive meta-igneous units, in part representing the remnants of a continental volcanic arc. The Langavat Belt defines the NE border zone of the South Harris Complex, separating the latter from the Archaean gneisses further NE, and has been repeatedly interpreted as a discrete supracrustal unit. However, detailed mapping and petrographic analysis reveals that up to 60% of the belt may be composed of highly deformed felsic orthogneiss. The metasedimentary rocks that are present have a late Archaean zircon provenance signature, implying that they are younger than, and possibly derived from the TTG gneisses to the NE. Previous work on zircon populations in the Leverburgh metasediments, towards the southern flank of the South Harris Complex, indicates very different provenance and we repudiate correlation of the two sequences. The disposition of units of metasediment, amphibolite and felsic orthogneiss is not a primary, pre-tectonic feature and is very unlikely to be the result of folding. Although direct evidence in the form of early fabrics has been wiped out by later, penetrative ductile shearing and metamorphic annealing, we propose that the Langavat Belt was assembled as a zone of thrust imbrication during early Proterozoic contraction. After imbrication, ductile shearing occurred in two distinct periods, separated by intrusion of granite pegmatites dated at
c
. 1660 Ma.
Underplating-related finite-strain patterns in the Gran Paradiso massif, Western Alps, Italy: heterogeneous ductile strain superimposed on a nappe stack
Authors: Kassem OsamaK. , Ring Uwe
Published: December 2007

A finite-strain study in the central and northern Gran Paradiso massif of the Italian Western Alps has been carried out to elucidate whether ductile strain shows a relationship to nappe contacts and to shed light on the nature of the subhorizontal foliation typical of the gneiss nappes in the Alps. The
R
f
/φ and Fry methods were used on feldspar porphyroclasts from 98 samples of the Gran Paradiso unit (upper tectonic unit of the Gran Paradiso massif) and 12 samples from the underlying Erfaulet unit (lower unit of the Gran Paradiso massif). Microstructures and thermobarometric data show that feldspar ductility at temperatures higher than
c
. 450 °C occurred only during high-pressure metamorphism, when the rocks were underplated beneath the overriding Adriatic plate. Therefore, the finite-strain data can be related to high-pressure metamorphism in the Alpine subduction zone. The augen gneiss was heterogeneously deformed and axial ratios of the strain ellipse in
xz
sections range from 2.1 to 69.8. The long axes of the finite-strain ellipsoids trend west–WNW and the short axes are subvertical, associated with a subhorizontal foliation. The strain magnitudes do not increase towards the nappe contacts. The data indicate flattening strain type in the Gran Paradiso unit and constrictional strain type in the Erfaulet unit and prove deviations from simple shear. We conclude that nappe stacking occurred early during subduction probably by brittle imbrication and that ductile strain was superimposed on and modified the nappe structure during high-pressure underplating in the Alpine subduction zone. The accumulation of ductile strain during underplating was not by simple shear and involved a component of vertical shortening, which caused the subhorizontal foliation
in the Gran Paradiso massif.
Experimental attachment of sediment particles to invertebrate eggs and the preservation of soft-bodied fossils
Authors: Martin Derek , Briggs Derek E.G. , Parkes R. John
Published: December 2007

Clay minerals can be an important agent in the fossilization of soft tissues, notably in the Ordovician Soom Shale of South Africa and the Cambrian Burgess Shale of Canada. The replication of morphology has been attributed to adsorption of pre-existing clay minerals, or direct precipitation of authigenic clays onto tissues. Attachment of quartz and kaolinite to the surface of lobster eggs demonstrates experimentally for the first time that soft tissues could fossilize in pre-existing minerals. However, the eggs became coated only in the presence of metabolizing bacteria. This experimental approach could be used to explore why Burgess Shale-type preservation declined after the mid-Cambrian.
Base-up growth of ocean crust by multiple phases of magmatism: field evidence from Macquarie Island
Authors: DIJKSTRA ARJAN H. , CAWOOD PETER A.
Published: December 2007

Macquarie Island preserves largely

in-situ

Miocene oceanic crust and mantle formed at a slow-spreading ridge. The crustal section on the island does not conform to a simple ‘layer cake” pseudo-stratigraphy’, but is the result of multiple magmatic episodes. Macquarie Island crust did not grow by top-down cooling, but rather from the base up. Peridotites cooled first and formed the basement into which gabbro plutons were intruded. This was followed by cooling and deformation, and by intrusion of dykes that fed a sheeted dyke–basalt complex. Finally, lava-filled grabens were formed. These relative age relations rule out simple co-genetic relations between rock units.
Evidence for an Early Proterozoic crustal province in the North Atlantic region
Authors: DICKIN A. P.
Published: December 2007

Published Sm-Nd data for the Ketilidian belt of Greenland, the Rockall bank in the North Atlantic, and the Islay-Inishtrahull block of the British Isles are used to suggest the existence of a single Early Proterozoic crustal province in the North Atlantic region with a crustal formation age of
c.
1.96 Ga. The three sub-provinces also have similar metamorphic histories, as documented by Sr and Pb isotope data. They probably represent one or more island arcs which were accreted to the Laurentian craton 100 Ma later, in the 1.85 to 1.75 Ga Ketilidian-Laxfordian orogeny. A line of suture between the Archaean craton and Early Proterozoic accreted terranes is tentatively proposed.
Granite magma generation, ascent and emplacement within a transpressional orogen
Authors: D’LEMOS R. S. , BROWN M. , STRACHAN R. A.
Published: December 2007

Crustal thickening during transpressive orogenesis may produce anatectic granites which may then localize deformation leading to transcurrent movement. Granites may be transported from sites of generation through the mid-crust in dyke-like channelways within relatively narrow strike-slip shear zones which link to major fault zones in the upper crust. Extensional jogs within fault systems provide developing sites for the assembly of plutons from magma arriving from below. The model is based upon observations from the Cadomian belt of NW France which exposes sections through middle and upper levels of the late Precambrian crust within different elements of the orogen. The mechanism provides a favourable alternative to diapirism, and explains the common collocation of granites and shear zone/fault systems within orogenic belts.
Changes in the trace fossil biota across the Proterozoic-Phanerozoic boundary
Authors: CRIMES T. P.
Published: December 2007

Trace fossils became relatively diverse in shallow-water clastic seas in the late Proterozoic (Vendian), with a further significant increase in abundance, diversity and complexity in the Tommotian and Atdabanian. Little change followed in the remainder of the Lower Palaeozoic. Traces typical of deeper-water facies evolved in shallow water during the Vendian and early Cambrian, and may have slowly migrated into the deep ocean during the remainder of the Lower Palaeozoic.

There are a limited number of short-ranging ichnogenera which may be useful for correlation but the first appearance of ichnogenera may be a more satisfactory method. Three trace fossil zones covering the ranges Redkino and Kotlin, Rovno and Tommotian-lower Atdabanian are recognized and may be useful for world-wide correlation.
Magma-mingling and melt modification between granitic pipes and host diorite, Guernsey, Channel Islands
Authors: D’LEMOS R. S.
Published: December 2007

Within the Northern Igneous Complex of Guernsey cylindrical pipe intrusions, 20–40 cm in diameter, developed by buoyant emplacement of granite magma into overlying, denser diorite magma. The high viscosity of the partially crystallized magmas and lack of mechanical stirring inhibited physical mixing, and mesoscopically sharp, well-defined interfaces developed. The granitic pipes are surrounded by aureoles up to a metre in diameter which exhibit changes in bulk chemical composition and partial mineralogical re-equilibration. These formed in response to the modification of interstitial melt in the partially crystallized host resulting from residual melt migration and/or melt interdiffusion across the pipe/host interface. Such processes between partially crystallized magmas may, in part, provide an explanation for textural, mineralogical and chemical compositional features commonly exhibited by mafic enclaves within granites.
The history of the Walls Boundary fault, Shetland: the northward continuation of the Great Glen fault from Scotland
Authors: FLINN DEREK
Published: December 2007

Interpretations of the Walls Boundary fault usually take into account only a small part of the previously published evidence, and some even present unsupported speculative interpretations. In particular, geological accounts rarely mention the geophysical evidence available, and geophysical papers take little account of the geology of the area. This paper reviews all published evidence. Geological and marine geophysical evidence show that the fault passes from close to Fair Isle, through Shetland and continues to the northeast; that a 65 km dextral offset occurred in Jurassic times on a gouge-filled fracture; and that prior to this a net sinistral displacement of the order of 100 km had occurred, including a mylonite-producing offset of unknown sense and distance in Carboniferous times.
A tectonic model for Scandian terrane accretion in the northern Scandinavian Caledonides
Authors: ANDERSON M. W. , BARKER A. J. , BENNETT D. G. , DALLMEYER R. D.
Published: December 2007

Muscovite and hornblende
40
Ar/
39
Ar ages have been obtained from several structural levels within the Caledonian nappe stack of Troms, northern Norway. Meaningful ages range between
c.
425 Ma and 370 Ma, suggesting protracted post-metamorphic cooling from Early Silurian to Late Devonian times. Whilst the possibility of earlier ‘Finnmarkian’ (Late Cambrian-Early Ordovician) histories is not precluded by these data, they provide further evidence of intense Scandian tectonothermal activity in this portion of the Scandinavian Caledonides.

Three structural units within the nappe pile record hornblende
40
Ar/
39
Ar ages ranging from 425 to 421 Ma. One of these records a similar muscovite cooling age, taken to indicate rapid nappe exhumation from peak Scandian metamorphism as Laurentia and Baltoscandia collided in the Late Llandovery and Wenlock. All other
40
Ar/
39
Ar muscovite ages from the nappe pile range between 395 ± 1 and 373 ± 1 Ma. These are thought to track the progression of the 350°C crustal isotherm as it relaxed through the tectonically thickened Caledonian crust. Major internal thrusts appear to have been largely inactive at this time and acted as sites of enhanced fluid flow and retrogression.

Further accretion of the orogenic wedge to the Baltoscandian margin was characterized by piggy-back style translation of the nappes on a low-angle basal decollement. This was accompanied by large-scale basement reactivation and out-of-sequence thrusting, resulting in further interleaving of the nappe stack in the Late Devonian (Frasnian).
Passive margins and their subsidence
Authors: BOTT M. H. P.
Published: December 2007

Three main types of subsidence dominate the vertical tectonics of rifted inter-plume passive margins. These are the isostatic response to (1) stretching and thinning of the crust and lithosphere (syn-rift stage), (2) cooling and thickening of the lithosphere (mainly post-rift stage), and (3) sediment loading (both stages). Additionally, vertical movements occur at plume (hot spot) margins as a result of the anomalously hot underlying upper mantle, and emplacement and cooling of magma. Three examples demonstrate various features of subsidence at inter-plume passive margins. At the North Biscay starved margin, the subsidence history can be mainly explained by syn-rift lithospheric stretching followed by post-rift thermal subsidence. At the US Atlantic margin, the much greater overall subsidence results from the additional effect of sediment loading. The Tucano-Gabon graben system bordering the South Atlantic is an example of strongly asymmetrical extensional tectonics during the syn-rift stage which produced deep rifted basins within the adjacent continental borderland.
Caledonian extension in the north Irish Dalradian: implications for the timing and activation of gravity collapse
Authors: ALSOP G. I. , HUTTON D. H. W.
Published: December 2007

Following Caledonian crustal thickening and prograde metamorphism of the Dalradian rocks on the SE Laurentian margin in mid-Ulster, down-dip (gravity controlled) extension due to orogenic collapse was concentrated in the inverted limbs of the major ESE-facing fold nappes. Structural and metamorphic studies show that this extensional collapse, which can be correlated along strike for at least 100 km, occurred immediately after the crustal thickening episode. Such a relationship is compatible with dynamic wedge models for orogenic belts. Our data also imply that (a) collapse was triggered by the release of fluids during the prograde metamorphism and (b) late stage mobilization of the extensional zone created the gold mineralization in the Sperrin Mountains.
Sr-isotope variations in a composite sill: crystal-liquid processes and the origin of the Skye granites
Authors: BELL B. R. , PANKHURST R. J.
Published: December 2007

Sr-isotope data from the Rubh’an Eireannaich sill, one of a suite of composite (mafic-silicic) minor intrusions associated with the 59 Ma Skye Intrusive Centre, NW Scotland, indicate that both crystal-liquid fractionation and magma-mixing processes have played important roles in its genesis. The data indicate that two distinct magmas were involved in the formation of the sill: a ferrobasaltic andesite magma, generated by contamination and fractional crystallization of regionally available basaltic magma, and a rhyolite magma which has a significant crustal component. Hybrid (intermediate) compositions at the gradational boundaries between the mafic and silicic components of the sill formed by mixing prior to or during emplacement, whilst fractional crystallization of the unmixed silicic magma occurred almost exclusively after mixing. The identification of a significant crustal component within the silicic portion of the sill suggests that the compositionally-related granites of the Skye Intrusive Centre have a similar origin.
New seismic evidence for a major geological boundary at shallow depth, N Devon
Authors: BROOKS M. , HILLIER B. V. , MILIORIZOS M.
Published: December 2007

Recently acquired marine reflection data provide evidence for a major geological boundary at shallow depth under the southern Bristol Channel, in the hangingwall block of the Bristol Channel thrust. The boundary is recognized to equate with the top surface of a high velocity layer previously identified along refraction/wide-angle reflection seismic lines in the north Devon area. Depth estimates from the reflection data and from a new interpretation of the earlier refraction data are combined to produce a contour map of the shallow boundary in the south-central Bristol Channel and north Devon areas. The boundary has a WSW-ENE regional trend and lies at a depth of less than 2 km immediately north of the Devon coast. It is truncated to the north by the Bristol Channel thrust and deepens southwards across north Devon and west Somerset. The boundary is interpreted as the top surface of a major litho-stratigraphic unit older than the outcropping Devonian sequence and in normal stratigraphic contact with it. However, in the absence of regional geological control the nature and age of the underlying high-velocity layer remain uncertain.
Influence of subsidence on the anatomy of marine siliciclastic sequences and on the distribution of shell and bone beds
Authors: KIDWELL SUSAN M.
Published: December 2007

Skeletal concentrations are common foci for palaeontological collecting, yet vary widely in their expected levels of taphonomic bias depending on whether they are comparatively simple event-concentrations, more complex accretionary accumulations of either normal or condensed thickness, or residual lags that mark horizons of significant erosional/corrosional truncation of the record (Table 1). These different types and their intermediates also have different potential for use in basin analysis, for example as marker beds, bathymetric indicators, and clues to hiatuses. The stratigraphy and taphonomy of marine siliciclastic sequences from a spectrum of basin types have been examined to determine the extent to which shell and bone beds are linked to physical stratigraphic features such as discontinuity surfaces and transgressive-regressive cycles at various scales, and thus the extent to which physical processes might influence palaeontological data. This series of basin-scale studies has also provided an opportunity to assess the practical utility of skeletal concentrations in reconstructing basin history and to investigate subsidence-related variation both in skeletal accumulation and in the anatomy of siliciclastic sequences.

Field studies have been restricted to post-Palaeozoic, primarily Neogene sections in warm temperate to subtropical palaeolatitudes and have focused on marine facies tracts developed in shelf and shallower water depths during phases of ‘coastal onlap’. (Sequence stratigraphic terms are used here to describe physical aspects of the record, and do not imply any particular chronostratigraphic value or genetic, i.e. eustatic, origin.) The results of this work indicate that: (a) skeletal concentrations are distributed fairly systematically within marine sequences, both in
Shell beds as tools in basin analysis: the Jurassic of Kachchh, western India
Authors: FÜRSICH F. T. , OSCHMANN W.
Published: December 2007

Skeletal concentrations are ubiquitous in Bathonian–Oxfordian shallow water sediments of the pericratonic basins of Kachchh and Rajasthan, western India. By analysing the biofabric of the concentrations and the taphonomic signatures of individual components, it is possible to distinguish between the final concentration process (recorded by the biofabric) and events taking place prior to final deposition (recorded by taphonomic signatures). The skeletal concentrations are grouped in nine genetic types ranging from fair weather wave concentrations to storm concentrations and condensed concentrations. They mainly reflect biological and hydraulic processes, and in some instances permit estimates of their duration and show a rough zonation along a bathymetric gradient. Plotted against the stratigraphic sections, the skeletal concentrations show, in accordance with other sedimentological data, three orders of bathymetric trends. The first two orders are shallowing–deepening cycles which possibly correspond to eustatic changes in sea level; the third order represents parasequences with a strongly asymmetric sedimentary record. Skeletal concentrations are a useful additional tool in basin analysis, but are best used in combination with other data.
Distinguishing between oxygen and substrate control in fossil benthic assemblages
Authors: WIGNALL P. B.
Published: December 2007

Low diversity benthic shelly assemblages occur under conditions of both low oxygen and very soft substrates. It is therefore important to be able to distinguish between these two environmental parameters. A declining oxygen gradient causes benthic diversity (including epibionts on hard benthic islands) to decline to a zero point. A transition from firm to soft substrates is also associated with a decline in overall benthic faunal diversity but, importantly, this has little affect upon epibionts. Functional morphological studies may also provide information about substrates, for example, thin-shelled bivalves of the ‘paper pecten’ morphology may represent super-soft substrate strategists.
The Kap Gustav Holm Tertiary Plutonic Centre, East Greenland
Authors: MYERS J. S. , GILL R. C. O. , REX D. C. , CHARNLEY N. R.
Published: December 2007

The plutonic centre is exposed on the narrow headland of Kap Gustav Holm in East Greenland. It was emplaced into the hinge zone of a major crustal flexure that developed during initial rifting between Greenland and northwest Europe.

The earliest intrusion is a funnel-shaped tholeiitic gabbro that was emplaced into the developing flexure, probably about 55 Ma ago. The intrusion comprises a Basal Contact Zone of gabbro 100m thick, showing syn-plutonic deformation and recrystallization; a Lower Zone of massive olivine gabbro 250–1500 m thick; and an Upper Zone of layered gabbro with a minimum thickness of 500–700 m. All are cut by sheets of gabbro pegmatite. Abrupt reversals in cryptic and phase layering suggest that the gabbro intrusion is the result of several injections of magma.

The gabbro intrusion was tilted steeply seawards before emplacement of plutons and sheets of monzonite, syenite and granite about 50 Ma ago. The main monzonite body forms a vertical cylinder 2.5 km in diameter, with saucer-shaped mineral-graded layering and abundant volcanic inclusions. Regular stratigraphic variation in the composition of plagioclase and augite with structural height is interrupted by an abrupt reversal, suggesting emplacement of a new pulse of magma.

Syenite forms ring dykes and partial ring dykes of two distinct compositions: an older subalkaline syenite related to the monzonite and gabbro, and a younger mildly peralkaline syenite. Granite forms a small composite pluton. Abundant syn-plutonic bodies of microdioite are associated with monzonite, syenite and granite intrusions.
Early Miocene continental extension-related basaltic magmatism at Walton Peak, northwest Colorado: further evidence on continental basalt genesis
Authors: THOMPSON R. N. , GIBSON S. A. , LEAT P. T. , MITCHELL J. G. , MORRISON M. A. , HENDRY G. L. , DICKIN A. P.
Published: December 2007

The Walton Peak lavas erupted directly onto
c.
1.8 Ga basement and were interbedded with subaerial sediments. Four new K-Ar dates for the lavas average 22.8 ± 0.3 Ma, associating them unambiguously with the earliest large-scale extension-related magmatism in NW Colorado. The lavas are basalts, trachybasalts and shoshonites. There is also one 100 m thick composite flow of trachydacite containing pillow-like basic masses (up to tens of metres in size) with chilled margins. Elemental data and Sr and Nd isotopic ratios suggest that the trachydacite evolved from the basalt type forming most of the pillows, by a combination of fractional crystallization and crustal assimilation. Nevertheless, post-evolution magma mixing, during and immediately before extrusion of the flow, has obscured the details of this process. A proportion of the pillow-like basic masses in the composite flow are shoshonites that did not take part in the magma mixing. They are relatively rich in incompatible minor and trace elements; e.g. Nb = 41–46 ppm in the shoshonites and 20–25 ppm in all other Walton Peak basic lavas. The Sr and Nd isotopic ratios of the Nb-rich and Nb-poor compositions overlap. Open-system processes in a long-lived pre-existing magma chamber are considered unlikely to be the main cause of the diverse basic magmas because the Walton Peak lavas directly overlie Precambrian basement. Likewise, crustal contamination models using either upper or lower crustal rock types of this region do not give satisfactory mechanisms to relate the Nb-rich and Nb-poor mafic liquids. A genetic model invoking variable degrees of partial melting of lithospheric mantle containing hydrous minerals does not explain why the Nb-rich and Nb-poor compositions are bimodal and not a range. A variety of sparse ultrapotassic magmas (minettes to lamproites) accompanied the basalt-dominated Neogene volcanism of NW Colorado and surrounding area. Their elemental and isotopic compositions support the view that the ultrapotassic liquids originated by fusion of hydrous and halogen-rich metasomatized zones within the subcontinental lithospheric mantle. Addition of about 15% of lamproitic melt to the low-Nb Walton Peak basic magmas reproduces the main geochemical and isotopic characteristics of the high-Nb magmas. Such a process seems to have been widespread throughout NW Colorado Neogene igneous activity. Although wholly lithospheric mantle sources for all the mafic magmas cannot be ruled out, there is evidence that both subduction-related calcalkaline and ocean-island basanitic melts of asthenospheric origin formed the dominant components of the NW Colorado volcanics. Addition of as little as 10% of lamproitic melt, during their uprise through the subcontinental lithospheric mantle, was sufficient to imprint on them ‘continental’ incompatible element and radiogenic isotope characteristics.
Strike-slip motion in the Minches, NW Scotland, deduced from the trends of the Scourie Dyke swarm
Authors: LISLE R. J.
Published: December 2007

In areas of low Laxfordian strain on the Scottish Mainland Lewisian, the strike of Scourie Dykes varies regionally; trending almost westerly in the north at Assynt and trending northwesterly in the south around Loch Torridon. These divergent trends allow an imaginary point to be constructed on the map at the intersection of these directions. In the Outer Hebrides a similarly variable arrangement of dyke trends is observed but the intersection point of the dyke trends does not coincide with that constructed for the Scottish Mainland. The non-coincidence of these intersection points is interpreted in terms of fault displacement and estimates are made of the combined slip on all faults in the Minch region (including the Minch and Outer Isles faults). Although the data are insensitive to possible dip-slip motions, they indicate a sinistral strike-slip component of between 34–145 km (90% confidence estimates) with a best estimate of 84 km. These results are in good agreement with those determined from the offset across the Minch of correlated metamorphic zones and those measured by the displacement of palaeomagnetic zones.
The changing face of late Precambrian and early Palaeozoic reconstructions
Authors: STOREY B.C.
Published: December 2007

The SWEAT hypothesis, linking the Southwest US and East Antarctica as conjugate rift margins of a Neoproterozoic continent has lead to major revisions of late Precambrian and early Cambrian reconstructions. Geological evidence summarized here supports separation of Antarctica from Laurentia
c.
750 Ma ago. A possible link between the break-up of a Neoproterozoic supercontinent and amalgamation of Gondwana during Pan-African times suggests that the Neoproterozoic was a time of rapid tectonic change. This conflicts with those theories that predict regular supercontinental cycles of 500 Ma from the time of disintegration of one supercontinent to its eventual reassembly.
The Caledonian evolution of the Laurentian margin in western Ireland
Authors: HARRIS D. H. M.
Published: December 2007

The Highland Border–Clew Bay–Fair Head–Baie Verte–Brompton Line, which probably marks the Laurentian margin, passes through Achill Island in western Ireland as a zone of ductile transpression within which the Clew Bay Complex and Dalradian Supergroup are juxtaposed. Dextral greenschist facies transpression fabrics in the Clew Bay Complex record the northwest directed emplacement of terranes, bearing high
P
–low
T
mineral assemblages, against Laurentia, causing the Dalradian continental margin sediments to be thrust westward. There are significant differences in timing, structural geometry and regional facing between W Ireland and the Scottish Highland Border Zone.
Structure, geochronology and tectonic significance of the northern Suguta Valley (Gregory Rift), Kenya
Authors: BOSWORTH W. , MAURIN ANDRÉ
Published: December 2007

The Suguta Valley occupies the present axis of the Gregory Rift south of Lake Turkana. Since the Pliocene, faulting has been focused along a 35 km wide belt between the Ng’iro basement uplift and the Loriu Plateau, producing an eastward thickening asymmetric basin. The oldest exposed volcanic rocks are basalts, tuffs and ashes dated 4.2–3.8 Ma. These were followed by a shift to trachytic volcanism, starting at about 3.8 Ma and continuing at least to 2.7–2.6 Ma. Feeder dykes to the trachytes strike NW–SE, suggesting a NE–SW least horizontal stress direction in the Early Pliocene. The trachytes were followed by a return to basaltic and alkali basaltic volcanism, which continues to the present. The structural evolution of the Suguta rift was similar to parts of the central and southern Gregory Rift, with early, broad basin development followed by constriction of deformation to an axial zone of grid faulting. This later structuring rotated flows and strata by as much as 35° in the past 3.8 Ma, or 9° per Ma. Rotational rates of this magnitude are extremely high in comparison to other continental rift settings. Formation of the Suguta basin also resulted in renewed uplift of the Ng’iro footwall and large-scale monoclinal flexuring of volcanic flows in the vicinity of the basin border faults. The intense basement structuring and accompanying footwall uplift led to the formation of large-scale land slips that carried large volumes of rock into the basin axis. Patterns of fluvial and deltaic sedimentation similarly reflect direct control by rift structures.
Morphology and significance of super-mature calcretes from the Upper Old Red Sandstone of Scotland
Authors: WRIGHT V.P. , TURNER M.S. , ANDREWS J.E. , SPIRO B.
Published: December 2007

Thick calcrete profiles occur in the Late Devonian–early Carboniferous Kinnesswood Formation of the Midland Valley of Scotland. These profiles consist of highly brecciated peloidal and coated grain fabrics with laminar calcrete horizons and linings to fractures. The profiles resemble supermature calcretes associated with late Cenozoic surfaces in the southwestern United States and reflect pedogenesis over periods of several hundred thousand years or more. The possible origins of the highly complex fabrics seen are reviewed and the polygenetic nature of such profiles is stressed. It is suggested that brecciation takes place co-incident with subaerial exposure of the calcrete. Mature profiles may be predisposed to brecciation and surficial weathering because of their susceptibility to soil erosion as a result of changes in vegetation cover. The calcretes underwent phreatic cementation, burial dolomitization and silicification.
Palaeogene peraluminous magmatism, crustal melting and continental breakup: the Erlend complex, Faeroe-Shetland Basin, NE Atlantic
Authors: KANARIS-SOTIRIOU R. , MORTON A. C. , TAYLOR P. N.
Published: December 2007

Well 209/3-1, located close to the centre of the Tertiary Erlend complex, north of the Shetland Isles, contains an unusual volcanic sequence in which thick basalts of N- to T-type MORB affinity overlie highly peraluminous, cordierite-bearing dacites. The dacites from the Erlend complex are almost identical in terms of their petrography and major-element, trace-element and isotope geochemistry to those of dacites from two previously-described localities in the North Atlantic, and indicate the formation of the dacites by anatexis of aluminous upper crustal sediments, which were probably pelitic in character. The discovery of graphite in the Erlend dacites suggests that carbonaceous shales may be implicated as parent material. The geological settings of the Erlend volcanic sequence, and the other examples in the Rockall Trough and on the Vøring Plateau, correspond to intra-continental, pre-break-up rift systems underlain by thinned continental crust, occurring close to the continental margin. A possible petrogenesis for the dacites involves the partial melting of upper crustal sediments as a result of the extensive contemporaneous basic magmatism documented to have occurred within a wide area of the North Atlantic continental margins during the initial stages of continental break-up. This mechanism suggests that the observed volcanic sequence may be more common than is obvious from the present, very limited, drilling evidence.
Geological evidence for a global tectonic polarity
Authors: DOGLIONI C.
Published: December 2007

Inferences about plate motions over the past 40 Ma, based on structural and geodetic data, suggest that there is a coherent movement direction on a global scale. Moreover a ‘westward’ drift of the lithosphere relative to the asthenosphere is indicated by plate motions in the hot-spot reference frame. Independent geological observations such as the different styles of deformation in thrust belts associated with subduction following or opposing the relative ‘eastward or northeastward’ mantle flow also support this global tectonic polarity. Unfortunately most hot-spot reference frames are based on assumptions which strongly limit their validity. For example, many combine magmatic sources originating from different depths in the mantle, including hot-spots located along oceanic ridges which are not fixed relative to one another. Using a hot-spot reference frame filtered to exclude shallow hot-spots and anomalous ‘wet-spots’, we would predict a much higher relative ‘westward’ drift of the lithosphere. Plate motions are not random, rather that they are controlled by the relative motion between lithosphere and asthenosphere, and by gradients in lithospheric thickness and composition.Triangular shaped plates may experience rotations resulting from differential drag at the base of the lithosphere with respect to the asthenosphere (e.g. the clockwise rotation of South America). Plates appear to follow an undulatory pattern of motion on a global scale and their poles of rotation fall into a main cone located near the Earth’s rotation poles. Plate rotations induced by triangular shape, differential basal drag or lithospheric anisotropies along subduction zones may generate the scattered distribution of Euler poles. This might have inhibited the recognition of a coherent global tectonic pattern.
Deep structure of the East Mendip Hills from gravity, aeromagnetic and seismic reflection data
Authors: ATES A. , KEAREY P.
Published: December 2007

Beacon Hill pericline is the most easterly pericline of the Mendip Hills, whose folds and thrusts were formed during the Variscan Orogeny by earth movements directed from the south. The Silurian volcanic rocks making up the core of the pericline are shown to be responsible for the large, linear magnetic anomalies present over eastern Mendip. Magnetic anomalies within the Radstock Coalfield to the north, however, are related to volcanic horizons in the Carboniferous Limestone. A three-dimensional model shows the presence of Silurian rocks beneath the spine of the eastern Mendip Hills.

Seismic reflection sections are used to control two-dimensional interpretations of potential field data from Beacon Hill pericline. Although the magnetic anomalies are adequately explained by the seismic model, the gravity anomalies require the presence of an additional low density body beneath the pericline's southern flank. This body is interpreted as a thrust wedge of Old Red Sandstone emplaced in its present position by a previously unrecognized post-Variscan normal fault. The considerable lateral extent of the structure is demonstrated by the trend of the maxima of the horizontal gradient of the gravity anomalies.
Diachronous and independent histories of plutonism and mineralization in the Cornubian Batholith, southwest England
Authors: CHEN Y. , CLARK A. H. , FARRAR E. , WASTENEYS H. A. H. P. , HODGSON M. J. , BROMLEY A. V.
Published: December 2007

Concordant U-Pb dates of magmatic monazites in the dominant coarse-grained biotite (±muscovite) granites of the late Variscan Cornubian batholith demonstrate that the emplacement of the six main plutons was diachronous, ranging from 293.1 ±1.3 Ma (Carnmenellis) to 274.5 ± 1.4 Ma (Land's End, southern lobe).
40
Ar-
39
Ar age spectra for magmatic muscovites confirm that cooling to about 320 °C at the present level of exposure was accomplished within 4 to 5 Ma in each intrusion. Moreover, age spectra for hydrothermal muscovites show that development of the major Sn (-Cu) lodes of the South Crofty mine commenced before 286 Ma, only 2 to 3 Ma after cooling of the hosting Cam Brea granite, a satellite of the Carnmenellis pluton. The older plutons had therefore approached ambient temperatures, and had even experienced intense hydrothermal activity, before emplacement of the youngest.
Tectonic and climatic implications of alluvial fan sequences along the Batinah coast, Oman
Authors: ABRAMS M. J. , CHADWICK O. H.
Published: December 2007

Along the Batinah coast in northern Oman, seven levels of alluvial fan remnants stretching from the Oman Mountains to the Gulf of Oman were considered evidence for Quaternary uplift by previous workers. We suggest that these regionally extensive fan remnants were instead caused by Quaternary climate fluctuations acting on the Oman Mountain headwaters and the major wadi systems. The gradients of these wadis were oversteepened twice: by mid-Tertiary uplift of the mountains and by a major Plio-Pleistocene alluvial aggradation event. Wadi channels are still being cut through this massive deposit. The episodic nature of climatically induced downcutting has produced the fan-terrace levels. The southern Batinah coast is a hinge zone about which the Arabian plate is tilting as it collides with the Eurasian plate. To the north, the Musandam peninsula and the associated fans are subsiding in response to subduction. To the south, uplifted marine shorelines suggest an apparent rigid plate response to the northward subduction.
Oxford Clay studies
Authors: HUDSON J. D.
Published: December 2007

Thematic set assembled by J. D. Hudson & D. M. Martill.

The Oxford Clay Formation is one of the classical units in British Jurassic stratigraphy. It crops out almost continuously in an arc running from the Dorset coast to Oxford, through the Midlands to Yorkshire. It was traditionally subdivided into Lower, Middle and Upper divisions, now formalized as the Peterborough, Stewartby and Weymouth Members, respectively (Cox
et al.
1993). The Peterborough Member is organic-rich, the Stewartby and Weymouth Members more calcareous and generally organic-poor. Excellent ammonite faunas throughout the sequence are important in the international bio- and chrono-stratigraphy of the Callovian and Oxfordian Stages, and in correlation between the Tethyan and Boreal fauna1 realms. The Peterborough Member—‘the clay that burns’—supports Britain's largest brick industry.

Why study the Oxford Clay? Most obviously for its superb fossils, to be seen in museums the world over (Martill & Hudson 1991). The Peterborough Member also serves as an example of a potential petroleum source-rock, besides being an important current industrial resource. It has not yielded petroleum, because it has never been deeply buried. This means that the rocks and fossils preserve much of their original chemistry and thus help bridge the gap between studies of modern sediments and of conventional source rocks. In the papers in this thematic set we have tried to make use of these opportunities to show what modern methods and concepts can add to classical geological interpretation. We have studied mainly central and eastern England, the area of least burial, simplest structure, best fossil preservation, and, because of the
The stable isotopic records of fossils from the Peterborough Member, Oxford Clay Formation (Jurassic), UK: palaeoenvironmental implications
Authors: ANDERSON T. F. , POPP B. N. , WILLIAMS A. C. , HO L.-Z. , HUDSON J. D.
Published: December 2007

Carbon and oxygen isotopic compositions of calcareous nekton (aragonite
Kosmoceras
and calcite belemnites) and benthos (aragonite nuculacean bivalves and calcite
Gryphaea)
from the Peterborough Member of the Oxford Clay Formation vary considerably at any one horizon but show no obvious trend with time. This suggests that the temperature and isotopic composition of ambient seawater may have varied considerably over short time intervals but did not change systematically during deposition of the unit. Thus, short-term fluctuations were superimposed on longer-term 'stability' in this shallow-shelf environment. The overlap in carbon isotopic compositions between calcareous nekton and benthos suggest a well-mixed water column. Oxygen isotopic palaeotemperatures suggest thermal stratification, provided that
Kosmoceras
(16–28 °C) inhabited near-surface waters and belemnites (12–19°C, indistinguishable from the range for benthic bivalves) were nektobenthic. Oxygen isotopic compositions of phosphate from both shallow- and possible deeper-dwelling vertebrates (reptiles, bony fish and sharks) yield the same range of palaeotemperatures (20–29 °C), suggesting that all vertebrate taxa may have lived predominantly in warm, shallow waters.
Southern Menderes Massif: an incipient metamorphic core complex in western Anatolia, Turkey
Authors: BOZKURT E. , PARK L R.G.
Published: December 2007

In the southern sector of the Menderes Massif, north of Selimiye (Milas) augen gneisses interpreted as a deformed peraluminous granite have been dynamothermally metamorphosed and are surrounded by and intrude a regionally metamorphosed Palaeozoic ‘envelope’. The granitic rocks exhibit a moderately-dipping mylonitic foliation and NNE-SSW- trending mineral elongation lineation. The progressive deformation of the granitic rocks produces a structural sequence typical of an extensional shear zone marked, from bottom to top, by a very thick extensive zone of mylonites followed in turn by brecciated mylonite and cataclasite. The kinematic indicators exhibit a top-to-the south, down-dip sense of shear.

These structures are attributed to exhumation of the granitic rocks of the massif along a major, south-dipping, normal-sense shear zone that accommodated crustal extension during Late Oligocene collapse of the orogen in western Turkey. Thus, the southern Menderes Massif may be interpreted as the exhumed footwall of a major extensional shear zone, and possibly as an incipient core complex.