bluuebunny/crossref_metadata_2025 huggingface dataset:
Northern margins of the Variscides in the North Atlantic region: comments on the tectonic context of the problem
Authors: Matthews S. C.
Published: September 2007

Some of the misconceptions which have figured in discussions of Variscan geology are analysed. Subduction (but not in the obvious sense), strike-slip movement and sole thrusts may all have a part to play, but none of these alone explains the Variscides. Kossmat’s scheme of tectonic zones still deserves attention. So, too, do questions concerning the pre-Variscan geology, especially the prevalence of rift-structures, in central and western Europe. The problem of integrating the pattern of Palaeozoic tectonism in Europe with that in eastern North America is briefly explored. It is suggested that Caledonian and Variscan be treated as parts of one chapter of tectonism and that much of the early tectonic history of central and western Europe be regarded as Pan-African. It is argued that western Europe is unlikely to produce evidence of structural transpositions on the scale recently revealed by seismic reflection profiling in the southern Appalachians. It is also suggested that the idea of a Variscan front is largely illustory.
Thrust nappe tectonics in the Devonian of south Cornwall and the western English Channel
Authors: Leveridge B. E. , Holder M. T. , Day G. A.
Published: September 2007

A review of stratigraphical data from south Cornwall, considered in conjunction with information obtained during a current re-survey by the British Geological Survey of 1:50,000 Sheet 352 (Falmouth), indicates the presence of a major thrust nappe in the Devonian rocks of the district. Seismic records from the western English Channel contain a prominent event that is interpreted as the offshore extension of the thrust. A model of the thrust nappe system of the area is proposed which is consistent with obduction from a small oceanic basin to the south.
Thin-skinned tectonics, basement control and the Variscan front
Authors: Shackleton Robert M.
Published: September 2007

The thin-skinned interpretation of Variscan structures in southern England, Wales and Ireland, based on structural and seismic evidence, is used to define the Variscan front as the northern limit of the thin-skinned structures. The original front was where displacement on the décollement surface diminished to zero. The present front, eroded back from its original line, is near the line previously accepted as the Variscan front. Caledonian (and in places older) structures control the Variscan deformation north of the front. They can also be traced south of it, as post-Variscan structures which have propagated up through the décollement. A zone of ENE-WSW structures in south Cornwall is thought to reflect the influence of Cadomian basement where the basal thrust has cut down into it.
Variscan tectonics of the Alston block, northern England
Authors: Critchley M. F.
Published: September 2007

Several ‘blocks’ can be recognized in the Carboniferous of northern England. These acted as rigid units and were only affected by the last phases of Variscan deformation.

The Alston block provides an example of the deformational history of block regions during the Variscan orogeny. A pre-Carboniferous granite provided resistance to deformation of the block until late Carboniferous times, and fracturing of the Carboniferous sediments above cupolas of the granite is less intense than elsewhere. Away from the cupolas, where a thick succession of Lower Palaeozoic rocks underlie Carboniferous strata, there is a more random and denser distribution of faults and vein fractures.

There were three tectonic phases to the structural evolution of the Alston block during the Variscan orogeny. The first phase, occurring prior to the intrusion of the Stephanian Whin sill, resulted in local folding of the block sediments and the formation of early shear joints under E-W compression. The second tectonic phase was a period of uplift of the Alston block, and resulted in a reticular pattern of ENE and NNW normal faults and extension joints. The third phase was characterized by renewed E-W compression. This resulted in the formation of ESE sinistral strike-slip faults, and horizontal reactivations along existing fractures.
Structural variation across the northern margin of the Variscides in NW Europe
Authors: Sanderson David J.
Published: September 2007

Four areas which cross the northern margin of the Variscan fold belt are examined and models of their structural evolution, from basin development to folding etc., are compared.

Foreland Britain is characterized by heterogeneous deformation localized in basins, whose development accompanied E-W dextral shear. The Munster basin in Ireland contains steep folds and cleavage. A variety of minor structures indicates an important dextral shear component to the deformation, and basin inversion by transpression is proposed. SW England is dominated by thrust tectonics, including the obduction of the Lizard ophiolite. Deformation was preceded by south to north migration of basin development. Brittany is characterized by major dextral shear zones delimiting areas of variable ductile strain.

The structural geometry and history of these areas is interpreted in terms of regional dextral transpression. The common transport direction is to the NNW or NW and oblique to the belt. Variable amounts of thrusting and dextral shear produced the different structural patterns. A case is made for an essentially ‘thick-skinned’ style of heterogeneous deformation produced in a dextral, oblique collision orogen, which involved microplates and a small back-arc basin.
Structural style, shortening estimates and the thrust front of the Irish Variscides
Authors: Cooper M. A. , Collins D. , Ford M. , Murphy F. X. , Trayner P. M.
Published: September 2007

Previous work in the Irish Variscan orogen has emphasized the significance of the major folds in the overall structure. A series of three balanced sections through the orogen reveals that thrusts are an equally important element of the orogenic structure. The ‘thrust front’ is not a fundamental structural line delimiting different deformational zones as previously proposed. Major thrusts occur along the ‘thrust front’ between Killarney and Mallow, but are absent east of Mallow. This change in character of the ‘thrust front’ is a function of a change in the stratigraphic level of the sole thrust to the orogen and the current erosion level. The deeper level of the sole thrust produces the numerous Silurian cored inliers to the NE of Mallow. The balanced sections show that the overall bulk shortening in the orogen increases from 33% in the east, to 42% in the central areas. This increase may be related to the arcuate nature of the orogen.
Does the Variscan front in Ireland follow a dextral shear zone?
Authors: Max M. D. , Lefort J. P.
Published: September 2007

The line commonly taken in Ireland as marking the ‘Variscan front’ passes in a gentle arc from Dungarven Bay in the east to Dingle Bay in the west. This is the most convenient course for the northern margin of the Rheno-Hercynian zone through Ireland and marks a primary Variscan structural line. The line follows thrusts and steep faults in the west and a series of less well seen strike faults along the southern limb of a tight syncline in the east from Mallow to Dungarvan. Although the line is thus structurally composite, and stratigraphic evidence shows that it was locally of indirect influence during deposition of the Upper Palaeozoic sediments, both structural and geophysical evidence suggests that a dextral strike-slip component was associated with this line.

Strike-slip movement along with N-S compression is shown by widespread E-W horizontal strain markers, strike faults and rotated fold traces which formed at the same time as the cleavage. Basement control of structures to the south of the line is not discernible while to the north rejuvenation of Lower Palaeozoic structures in the basement is common.
Variscan structures in the Upper Palaeozoic rocks of west central Ireland
Authors: Coller D. W.
Published: September 2007

The western half of central Ireland is part of a low strain zone on the northern margin of the Variscan orogen in which the structures in the Upper Palaeozoic ‘cover’ rocks appear largely to be controlled by ENE-WSW and NNE-SSW trending structures in the Caledonian, and possibly the pre-Caledonian basement.

Preceding the main ductile phase of Variscan deformation, there was an important period of mid-Dinantian faulting which is closely associated with major base metal mineralization. New structural evidence suggests that the main movement on these faults involved an important component of dextral transcurrent shear.

The main ductile deformation, between post-Westphalian and pre-Upper Permian in age, is characterized by kilometric scale open folds and mainly ENE-WSW trending vertical transcurrent ductile-brittle shear zones. The model of deformation is considered to be one of regional, heterogeneous transpression where the main compression, or pure shear component was approximately N-S, and the simple shear component was an ENE-WSW dextral shear. A major NNE-SSW trending sinistral shear zone, the Fergus shear zone, appears to be the western limit of the dextral shear component.
Geophysical evidence for the extension of the Variscan front on to the Canadian continental margin: geodynamic and palaeogeographic consequences
Authors: Lefort Jean-Pierre , Haworth Richard T.
Published: September 2007

Dextral transcurrent movement and thrusting induced by transcurrent movement have been identified along a roughly east-west line across the continental shelf NE of Newfoundland at approximately 52°N. The location of that boundary correlates well on a pre-Mesozoic fit of the North Atlantic with the terminus of the Variscan front on the European margin. Seismic reflection, refraction, gravity and magnetic data show a structural succession across the boundary that correlates with that across the front in Europe. This North American continuity of the Variscan front has subsequently acted as the locus for Mesozoic tectonics.
A revised nomenclature for Upper Permian strata in eastern England
Authors: Smith D. B. , Harwood G. M. , Pattison J. , Pettigrew T. H.
Published: September 2007

Revised names of some Upper Permian strata in eastern England are proposed and defined, together with new names for some strata previously without formal names; the new and revised names are based on nominated type localities and are intended to replace existing names that were confusing or inappropriate. We propose that the names Lower and Upper Magnesian Limestone in the Yorkshire Province be replaced by the new names Cadeby (Magnesian Limestone) Formation and Brotherton (Magnesian Limestone) Formation respectively and that the Lower and Middle Magnesian Limestone in the Durham Province should become the Raisby (Magnesian Limestone) and Ford (Magnesian Limestone) formations respectively. The Middle Marls are renamed the Edlington Formation in both provinces and the new names Sherburn (Anhydrite) Formation, Sneaton (Halite) Formation and Littlebeck (Anhydrite) Formation are proposed for the Upper Anhydrite, Upper Halite and Top Anhydrite formations. The term Roxby Formation is proposed to replace the present name Upper (or Saliferous) Marls.
Sedimentology of sandwaves in an oolite shoal complex in the Cadeby (Magnesian Limestone) Formation (Upper Permian) of eastern England
Authors: Kaldi J.
Published: September 2007

Four depositional facies are recognizable in the Sprotbrough Member of the Cadeby Formation (formerly the Lower Magnesian Limestone) in eastern England; they are sabkha, semi-restricted lagoon, oolite shoal complex and open marine shelf. Of these, the oolite shoal complex occupies most of the present outcrop in the Yorkshire Province, i.e. south of the Cleveland High. It comprises large-scale cross-stratified oolite bedforms that are interpreted as sandwaves from their size, type of sediment and internal features. The sandwaves probably originated from the concentration of wave energy by eminences on the floor of the Zechstein Sea. Biogenic structures in the sandwaves show that they were inhabited by marine organisms.

A complex pattern of palaeocurrent styles and azimuths is present within the sandwaves. At many exposures, the long axes of the sandwaves trend NW-SE, suggesting predominant currents normal to this direction. A predominant current from the NE is further indicated by the preferential abutment of SW-dipping beds on the NE flanks of bedforms; this was caused by sandwaves that climbed the backs of others. Occasional storms from the SE produced spillover lobes oriented towards the NW. Storms were also probably responsible for the formation of internal structures such as hummocky cross-stratification, fan-shaped bedding and contemporaneous erosion surfaces.
Mineralization in Upper Permian carbonates at outcrop in eastern England
Authors: Harwood G. M. , Smith F. W.
Published: September 2007

Upper Permian carbonates in eastern England at outcrop feature subeconomic syngenetic and epigenetic Pb-Zn-Cu-Fe-Ba-F mineralization. Pb-Zn-Fe sulphides occur in vugs and breccias of sedimentary origin whereas much barite replaces the host carbonate. Vein mineralization is rare. The majority of occurrences are within the Cadeby Formation (Lower Magnesian Limestone; EZ1 Ca).

More than 70 occurrences of mineralization have been recorded, clustered on centres at Farnham, Bramham, Edlington and Mansfield. Host carbonates at the first three of these centres overlie major cross-cutting structures in pre-Permian strata; these structures were possible flow channels for mineralizing solutions. Mineralization in the Mansfield area can be explained by the upwards leakage, on a broad scale, of formation fluids from Carboniferous strata that overlie the South Pennine basement high.
The Trow Point Bed—a deposit of Upper Permian marine oncoids, peloids and columnar stromatolites in the Zechstein of NE England
Authors: Smith D. B.
Published: September 2007

An extensive thin bed of oncoids, peloids and columnar stromatolites occurs at the top of the Cycle 1 carbonate of the English Zechstein sequence in NE England; here termed the Trow Point Bed, it lies on the lower part of the basin-margin slope, basinward of a linear shelf-edge reef in the Ford Formation. The bed is generally 0.05–0.20 m thick but locally ranges to 0.60 m; it rests on an uneven surface that has a local relief of up to 15 m and slopes exceeding 35° in places. It contains a sparse marine fauna of foraminifera, ostracods and bivalves; there are only a few traces of current action and sorting, and terrigenous sediment nowhere comprises more than 2% of the bed.

The Trow Point Bed is generally similar to deposits described at a comparable stratigraphical position in Germany and Poland and although environmental indications are partly mutually contradictory, it seems likely that the bed accumulated in oxic conditions on the basin floor under water perhaps 25–100 m deep.
Subsidence and foundering of strata caused by the dissolution of Permian gypsum in the Ripon and Bedale areas, North Yorkshire
Authors: Cooper A. H.
Published: September 2007

Underground dissolution of thick gypsum beds in the Edlington Formation and Roxby Formation of the Zechstein sequence in North Yorkshire, England, has resulted in a 3 km-wide and 100 km-long belt of ground susceptible to foundering. Within this belt a large subsidence depression at Snape Mires, near Bedale, was largely filled with lacustrine deposits in the later part of the Late Devensian and during the Flandrian. South of Snape Mires the Nosterfield-Ripon-Bishop Monkton area has suffered about 40 episodes of subsidence in the past 150 years, and the presence of several hundred other subsidence hollows indicates considerable activity from the later part of the Devensian onwards. The linear and grid-like arrangement of these subsidence hollows indicates collapse at intersections in a joint-controlled cave system. Linear subsidence features at Snape Mires are also joint-controlled. The transition from anhydrite at depth to secondary gypsum near surface marks the down-dip limit of the subsidence-prone belt. Cavities are propagated upwards by roof collapse of caverns in the gypsum, leading to the formation of breccia pipes. Choking of the pipes can reduce the surface expression of the underground collapse, but the larger cavities are liable to produce pipes that reach the surface even at the eastern boundary of the 3 km-wide belt described. Further subsidence in the Ripon area is predicted and some suggestions for remedial measures are given.
Stratigraphy of the Lower Werra Cycle (Z1) in West Germany (preliminary results)
Authors: Paul J.
Published: September 2007

The Zechstein Limestone (Cal) of Western Germany has a clear stratigraphical subdivision. A lower unit (subcycle a) is separated from an upper unit (subcycle b) by a discontinuity, which is best exposed at the Solhops-Berg at the Harz Mountains and is here termed the ‘Solhops Discontinuity’. The discontinuity was caused by an extensive regression of the Zechstein Sea which led to subaerial exposure of large areas of the Zechstein basin. Based on this discontinuity, a preliminary stratigraphy of the Lower Werra cycle in Germany is established and compared with that of the English Zechstein sequence.
On the Zechstein Copper Shale lithofacies and palaeoenvironments in SW Poland
Authors: Oszczepalski S.
Published: September 2007

Microfacies and geochemical studies on the Copper Shale in the Sudetic Foreland of SW Poland show the strong influence of palaeorelief on the distribution of lithofacies. Spatial variability in microlithofacies and lithofacies indicate sedimentation in the following palaeoenvironments: (1) offshore—deep shelf, characterized by the black shale lithofacies (30–50 cm thick) and shallow shelf, where the Copper Shale is either absent or is represented by red shale lithofacies (up to 60 cm thick), and (2) nearshore, characterized by grey shale lithofacies (40–100 cm thick or more). Shorewards the Copper Shale is replaced by carbonate and/or terrigenous rocks of the first Zechstein cycle.
Chronostratigraphical and lithostratigraphical correlations of the Zechstein Limestone in Central Europe
Authors: Peryt T. M.
Published: September 2007

A glacioeustatic mechanism governed the sedimentation of the PZ1 Zechstein Limestone and made it possible to correlate Zechstein Limestone deposits in different areas chronostratigraphically. The chronohorizons are surfaces (uncommon) or layers up to several centimetres thick that originated in a subaerial or shallow water environment and overlie deposits formed in a subtidal environment. These chronohorizons divide the Zechstein Limestone sequences into three parts or sub-cycles. The boundary between the previously established lower and upper lithostratigraphical units of the Zechstein Limestone corresponds with that between the first and the second sub-cycles or that between the second and the third sub-cycles, or both, depending on the local palaeogeography.
Macrofauna of the Polish Zechstein: its occurrence and stratigraphy
Authors: Karwowski L. , Klapcinski J.
Published: September 2007

This paper presents the characteristics of the Zechstein macrofauna of western and northern Poland. On the basis of the macrofauna and its variability, four biozones* are distinguished within the Zechstein succession. These are biostratigraphic units which correspond to both the basal clastics and the carbonate units of the succession. The faunal assemblages are characteristic of the individual carbonate horizons, and those assemblages typical of various palaeogeographical areas within the basin are reviewed. The macrofauna of the Polish Zechstein strata is also briefly compared with macrofauna recorded from the English Zechstein.
Gravity glide and plate tectonics
Authors: Price Neville J. , Price Geoffrey D. , Price Sarah L.
Published: September 2007

We briefly discuss the three groups of mechanisms which are generally held to be responsible for plate motions, namely basal traction, slab pull, and ridge push. We conclude that individually, or in combination, these mechanisms are insufficient to account for known plate behaviour. Instead, we reintroduce and expand upon an idea proposed by Hales (1969) that the driving force for plate motion is the body weight of the lithosphere, which gives rise to gravity glide on the inclined interface between the lithosphere and asthenosphere. It is shown that this mechanism is capable of producing an average horizontal stress of as much as 6 kbar and is able to explain (1) continental splitting, (2) the initiation of subduction, and (3) observable plate motions, as well as a variety of effects requiring high lateral stresses. This is the first model to be proposed that successfully provides a quantitatively and physically viable mechanism to explain plate-tectonic processes and the behaviour of the Earth’s lithosphere.
Lithospheric stress, deformation, and tectonic cycles: the disruption of Pangaea and the closure of Tethys
Authors: Dewey J. F.
Published: September 2007

Intra-plate stress is governed on a global scale by plate boundary configuration and the disposition of continents. When continents are assembled into Pangaea-type masses, they are in deviatoric tension and predisposed to rifting and splitting along weak zones and where tension is locally enhanced by compensated uplifts. When continents are dispersed, they are, mainly, in deviatoric compression when local and minor thrusting and strike-slip faulting occur. Cyclic assembly, disruption, separation and reassembly of continental fragments causes a globally episodic, perhaps even periodic, repetition and synchroneity of tectonic events, alkaline and mafic magmatism and sea-level changes.
Origin and assembly of south-east Asian continental terranes
Authors: Metcalfe I.
Published: September 2007

The pre-Tertiary continental core of south-east Asia is a composite of at least four tectonostratigraphic terranes; Sibumasu (Shan States of Burma, north-west Thailand, Peninsular Burma and Thailand, Western Malaya, and north-western Sumatra), Indo-China (east Thailand, Laos, Vietnam south-west of the Song Ma-Song Da zone and Kapuchea), East Malaya (including south-east Sumatra and Natuna) and south-west Borneo. These terranes are separated from each other by sutures, and are now joined to South China along the Song Ma-Song Da zone.

Stratigraphic, palaeontological, and palaeomagnetic evidence suggests that all of these crustal blocks probably had their origin on the north-eastern margin of Gondwanaland where they formed part of a complex continental margin.

The Indo-China and East Malaya terranes both have typical late Palaeozoic Cathaysian floras, which indicate that they were close to South China in the Permian. Stratigraphic studies in northeast Vietnam suggest that Indo-China and South China may well have sutured as early as the late Devonian to early Carboniferous. Carboniferous and Permian faunas and floras of these blocks do not appear to be related to those of north-eastern Gondwanaland, indicating that they rifted away at some earlier time.

The Sibumasu block remained on the north-western Australian margin of Gondwanaland until the late Lower Permian, and was in continuity with the Tibetan Lhasa and Changtang blocks (south of the Langcangjiang Fracture Zone) and the proto-Kreios microcontinent further to the west. Late Carboniferous and Lower Permian sediments of the Sibumasu, Lhasa, and Changtang blocks, and the Iranian and Afghan portions of proto-Kreios include extensive glacial-marine deposits. Early Permian shallow-marine faunas were of cold-water type until a major change occurred in late Lower Permian times, with Middle Permian faunas becoming typically warm-water Tethyan in aspect. It seems most likely that Sibumasu (still attached to the Lhasa and Changtang blocks and proto-Kreios) rifted from Gondwanaland in the late Lower Permian.

Amalgamation of the south-east Asian blocks was probably completed by the end of the Triassic. This is indicated by terrestrial vertebrate biogeography and palaeomagnetism. It is still a matter of debate whether Indo-China sutured to South China in the Devonian-Carboniferous along the Song Ma line, or in the late Triassic along the Song Da zone.

Evidence seems stronger for a late Triassic suturing of Sibumasu to the other ‘Cathaysian’ southeast Asian blocks, although a Permian closure of ‘palaeo-Tethys’ has recently been proposed for the Shan-Thai-Indo-China region.

The south-west Borneo block probably rifted from the margin of Indo-China in the Cretaceous, and travelled south to its present position during the opening of the proto-South China Sea.
Lower Palaeozoic facies and faunas around Gondwana
Authors: Cocks L. R. M. , Fortey R. A.
Published: September 2007

A new base map for the Lower Palaeozoic of the Gondwanan continental plate incorporates various modifications to older reconstructions. An analysis is attempted of faunas at various times through the Lower Palaeozoic, in particular the Lower Ordovician (Arenig-Llanvirn), the middle and late Ordovician (Caradoc-Ashgill) and the early Silurian (Llandovery). Regressions and transgressions are identified and ‘western’ and ‘eastern’ Gondwanan platform faunas identified in contrast to the deeper-water peri-Gondwanan marginal faunas. The effects of climatic variation, both with time and between contemporary areas, are reviewed, and are seen to be the chief controlling factor on platform faunas, with a cline from Boreal to tropical faunas alongside the Gondwanan palaeocontinent. The biogeographical relationships of deeper-water faunas were more complex: certain distinctive taxa may be found distributed widely around the perimeters of the continent, and other faunas have taxa in common with marginal faunas around other contemporary palaeocontinents. Some older-described faunas, particularly brachiopods, trilobites and graptolites, are freshly reviewed, and new faunal data from Libya, Turkey, and Iran are presented and figured.
Fossil plants as indicators of late Palaeozoic plate positions
Authors: Chaloner W. G. , Creber G. T.
Published: September 2007

Fossil plant records from a late Carboniferous to early Permian time interval are reviewed for the data that they give on connections and migration routes between floristic provinces, and their palaeolatitudinal positions. Caution is urged in using fragmentary leaf remains as a basis for recognizing affinity between floristic provinces, as illustrated by reference to the genera
Phyllotheca
and
Glossopteris
. The distribution of some 38 plant genera from a range of late Palaeozoic areas and localities is made the basis for ordinating them in the form of a dendrogram. An early Permian plate reassembly, based on that of Lin
et al.
1985, is assessed in terms of its compatibility with the palaeobotanical evidence.
Middle Jurassic ammonite biogeography supports ambi-Tethyan origin of Tibet
Authors: Westermann Gerd E. G.
Published: September 2007

In contrast to the latest Jurassic-earliest Cretaceous ammonite faunas of the Spiti Shales in the Tethyan Himalaya—which were early recognized to be closely akin to Indo-West Pacific faunas of south-eastern Tethys—the scarce Middle Jurassic subjacent faunas were previously identified with mostly European or late Jurassic taxa. Current revision of all known Middle Jurassic ammonites from Tibet, however, indicates that Bajocian and Callovian ammonites of the Tethyan Himalaya are closely affiliated to, firstly, northern (and western) Australasia, and secondly, to western India, Madagascar and Kenya, i.e. they belonged to the ecotone of Himalayan and Ethiopian provinces (or subprovinces of the Indo-West Pacific province). In contrast, the few species known from the Lhasa block of South Tibet and the Qamdo block of North Tibet appear to be typically western Eurasian. This supports current plate-tectonic theory, which holds that the Jurassic location of the Qamdo and Lhasa terranes was, respectively, at and in proximity to Eurasia, while the Tethyan Himalaya belonged to the Indian Shield, then at the northern margin of Gondwana.
Mesozoic Turkey as part of Europe
Authors: Ager Derek V.
Published: September 2007

Evidence from shallow-water, sessile benthos, mainly brachiopods, suggests that Mesozoic Turkey should be placed on the northern side of Tethys in palaeogeographical reconstructions, not on the southern side. Triassic, Jurassic, and Cretaceous faunas, whether from north or south of the supposed Central Anatolian suture, are wholly ‘European’ in character and include no obvious ‘African’ elements. The faunas also contrast markedly with those of the Middle East and (to a lesser extent) with those of the Adriatic Microplate.
The trans-Pacific spread of equatorial shallow-marine benthos in the Cretaceous
Authors: Skelton P. W.
Published: September 2007

From Aptian to Maastrichtian times, the development of broad tracts of anomalously shallow ocean floor in the Pacific, studded with reef-rimmed seamounts and islands, allowed some Caribbean benthos to spread westwards by larval drift between staging posts in Tethyan/Atlantic waters debouching between the Americas. There is as yet no compelling evidence that the Pacific was completely traversed in this manner during the first and major phase of intra-plate volcanothermal activity in the Aptian-Albian: evidently there still existed a sufficiently wide region of Pacific abyss to the west to serve as a barrier to larval dispersal. However, clear evidence for the ‘landfall’ of Caribbean-derived species on eastern Tethyan shores in Campanian to Maastrichtian times is provided by several examples of disjunct endemics from the two regions. Such taxa include rudist bivalves, gastropods, larger foraminifera, and calcareous algae. This episode coincides with a second, smaller phase of mid-plate volcanothermal activity in the Pacific, which, in conjunction with the relict topography from the earlier phase, evidently furnished a temporarily continuous chain of staging posts for the trans-Pacific spread of the shallow-marine benthos.
Phytogeography of the eastern end of Tethys
Authors: Whitmore T. C.
Published: September 2007

Central to biogeography is the classification of geographical distribution patterns and analysis of their origins. Here we analyse angiosperm ranges in the Malay Archipelago (Malesia) mainly against landmass changes since the late Mesozoic. The interpretation is complicated by continual evolution and extinction, by range alterations due to climatic changes, and by the scanty fossil record.

Malesia has the greatest concentration of primitive angiosperms, and was postulated by Takhtajan (1969) to be the place of origin of the group. This hypothesis later became untenable, after the realization that Malesia was only formed in about mid-Miocene times by collision of Gondwana with Laurasia. The sharp discontinuities in the ranges of many animal and plant taxa in central Malesia, at the biogeographical boundary called Wallace’s Line are readily interpreted to be results of this collision. But a few plant taxa, including the ‘primitives’, have puzzling ranges demanding land before the mid-Miocene—not empty ocean. These enigmas spurred further geological enquiry, which led to the discovery of Gondwanan fragments that have drifted northwestwards before the main collision and which could have been the locus of angiosperm evolution. The early history of angiosperms in Australia is not incompatible with this scenario.

Every class of evidence is incomplete, and all kinds must be considered together. Fossils and palaeogeography are less equivocal than distribution patterns. For the future, more fossil evidence should be sought, detailed taxonomic study and phylogenetic reconstruction of major plant groups is also needed: reinvestigation of the conifers could be very rewarding.
Crustal and upper mantle structure of the Appalachian-Caledonide orogen from seismic results
Authors: Cook F. A. , Matthews D. H. , Jacob A. W. B.
Published: September 2007

The Appalachian-Caledonide orogen extending from the southeastern USA to northeastern Canada and Britain now has more crustal-scale seismic reflection profiles recorded across it than any other orogen in the world. From SW to NE these include the COCORP Ouachita traverse, the COCORP southern Appalachian traverse, the COCORP northern Appalachian traverse, the BIRPS Western Isles-North Channel (WINCH) offshore profile, the BIRPS Moine-Outer Isles Seismic Traverse (MOIST) profile and the BIRPS SW Approaches Traverse (SWAT) profile. In addition there are several seismic profiles in the eastern USA and Canada with data to mid-crustal (15–20 km) depth within the crystalline ‘core’ of the Appalachians. In eastern N America, where the Caledonian (Taconian) deformation is overprinted by Acadian and Alleghanian deformation, the seismic reflection data clearly indicate large-scale (several hundred kilometres) cratonward thrusting of shelf, off-shelf and basinal facies sediments and metasediments. The eastern limit of undeformed Precambrian (Grenville) basement is probably represented by a zone of E-dipping reflectors which, in New England, can be correlated with surface exposures of faults and metasediments. Two profiles offshore from Scotland cross the Caledonide orogenic front. The BIRPS MOIST profile indicates that the Moine thrust zone is also characterized by E-dipping reflectors which extend to mid-crustal depths. In contrast with the data from the Appalachians, the BIRPS MOIST, WINCH and SWAT data show remarkably continuous reflections from the crust-mantle transition, as identified on seismic refraction data both across strike (LISPB) and along strike (ICSSP and CSSP). The Moho is at a depth of about 30 km throughout much of Britain and is commonly overlain by a thick (4–5 s travel time or about 12–15 km) layered lower crust at depths between 15 and 30 km. In some areas the data offshore from Scotland show deep reflections which extend into the mantle. Additionally, many of the profiles in eastern N America and Britain provide strong evidence that late normal faults (Mesozoic) may be reactivated older compressional structures (Caledonian and Variscan in Britain; Alleghanian in the southern Appalachians).
Palaeomagnetic constraints on the evolution of the Caledonian-Appalachian orogen
Authors: Briden J. C. , Kent D. V. , Lapointe P. L. , Roy J. L. , Livermore R. A. , Smith A. G. , Seguin M. K. , Van der Voo R. , Watts D. R.
Published: September 2007

Late Proterozoic and Palaeozoic (pre-Permian) palaeomagnetic data from all regions involved in, or adjacent to, the Caledonian-Appalachian orogenic belt are reviewed. Between about 1100 and about 800 Ma the Laurentian and Baltic shields were close together, prior to the opening phase of the Caledonian-Appalachian Wilson cycle. The problems of tectonic interpretation of Palaeozoic palaeomagnetic data from within and around the belt derive mostly from differences of typically 10°–20° between the pole positions. These can variously be interpreted in terms of (i) relative displacements between different continents or terranes, (ii) differences in ages of remanence and (iii) aberrations due to inadequacy of data or geomagnetic complexity, and it is not always easy to discriminate between these alternatives. If the Pangaea A2 reassembly of continents around the northern and central Atlantic is taken as the end-product of Caledonian-Appalachian orogenesis, the following conclusions can be drawn.

1

Lower Palaeozoic palaeolatitude differences between the N American and British-Scandinavian margins of the Caledonides are small; hence any convergence must have been mainly E-W.

2

There are additional differences which could be due to major pre-Carboniferous strike-slip (more than 1000 km), although later strike-slip on this scale is no longer considered likely.

3

The Lower Palaeozoic apparent polar wander paths for Northern Scotland and N America disagree on face value, but must be reconciled if their conventionally assumed geographic relation is correct.

4

Lower Old Red Sandstone data from Britain and Norway disagree, but this is more likely to be due to magnetic overprinting in the Norwegian rocks than to remnant oceans between the regions of Old Red Sandstone facies.

5

Armorica seems to have been far to the S, adjacent to Gondwana, in Ordovician time. The latest view is that it collided with Euramerica in early Devonian time to form the Old Red Continent.

6

The timing of Gondwana’s collision with the Old Red Continent is controversial; it is within either the late Devonian or the Carboniferous. If it occurred early in that time range, much of Hercynian-Alleghanian orogeny post-dated it.
Geophysical evidence for the location of the NW boundary of Gondwanaland and its relationship with two older satellite sutures
Authors: Lefort J. P. , Max M. D. , Roussel J.
Published: September 2007

The boundary between Gondwanaland and Laurussia can be traced from Portugal to southern New England using gravity and magnetic data. Further positional control using geochemistry and palaeomagnetism on rocks near the geophysical suture confirms its position. From Long Island to Florida the suture lies beneath the Coastal Plain and our interpretation relies only on geophysical data. This suture is marked by rift-associated rocks, back-arc intrusives, volcanic-arc rocks or remnants of oceanic crust. Onshore, between Cape Ann (near Boston) and Long Island Sound (near New York), the suture possibly follows the Clinton-Newbury fault. The arc-like swing to the W of the Chesapeake Bay region is probably the result of an indentation of the older Appalachian belts by the Mauritanian Reguibat prong during Carboniferous time. There are two major transform faults associated with the suture zone: N of Florida and W of Spain.

Two satellite sutures can be identified on the eastern side of the Atlantic. In northeastern Senegal and Mauritania the major gravity high which parallels the Mauritanian orogenic belt is interpreted as locating the Taconic? collision between Gondwanaland and a western microplate. S of Brittany the Spanish microplate collided with Laurussia during Acadian time. These two satellite sutures were rejuvenated during the main late Palaeozoic compression between Gondwanaland and Laurussia.
The age and distribution of basement rocks in the Caledonide orogen of the N Atlantic
Authors: Powell Derek , Andersen T. B. , Drake A. A. , Hall Leo , Keppie J. D.
Published: September 2007

Available isotopic data suggest that the Precambrian basement elements of the northern parts of the Caledonide orogen in N America, Britain, Ireland, Greenland and Scandinavia comprise largely Grenvillian and Svecofennian continental crust. Archaean precursors have been identified in Greenland, Norway and Britain, but not positively in the Appalachians. Early Caledonian (Grampian-Finnmarkian) orogenic activity gave rise, in the Scandinavian and British-Irish sectors, to an early Palaeozoic basement which, in Norway, is overlain by Ordovician to Silurian supracrustal sequences. In contrast, a late Precambrian-early Cambrian (Cadomian-Avalonian and older) basement is present in southern parts of the orogen.

The distribution of these basement elements constrains any plate-tectonic reconstructions of the N Atlantic Caledonide orogen and suggests that the orogen was tripartite during its early development. Further, it suggests that transcurrent movements as well as obduction may have been important tectonic processes during Grampian-Finnmarkian tectonism.
Pre-Arenig evolution of the Appalachian-Caledonide orogen: sedimentation and stratigraphy
Authors: Schwab Frederic L. , Nystuen Johan P. , Gunderson Linda
Published: September 2007

The Vendian and Cambrian-Tremadoc stratigraphic record documents the initial plate-tectonic evolution of the Appalachian-Caledonide (Iapetus) orogen. On the western margin several stratigraphic units indicate an episode of late Precambrian continental rifting. Triassic-like intra-cratonic basins were developed within the continental interior. Aulacogens and marginal ensialic shelf basins were located along a subsiding zigzagging continental edge marked by high scarp-bounded mountainous relief. The early Palaeozoic Atlantic-type margin that followed consisted of a landward easterly-thickening continental shelf wedge (basal quartz sandstone and an overlying carbonate bank blanket), bank-edge breccia and, oceanward, a continental rise prism.

The eastern margin, at least in Scandinavia, is very similar in lithology and depositional setting to the western margin. However, S of Scandinavia, in continental Europe and across the British Isles, and farther S and W in N America, lay the Avalon terrane. This regionally extensive strip of apparently sialic crust was criss-crossed in late Precambrian time by a series of grabens that filled with rift-basin deposits (coarse clastic sedimentary rocks interbedded with basic to alkalic volcanic rocks, all deformed by Pan-African-type orogeny). In the early Palaeozoic this Avalon ‘microcontinent’ was a platform (transgressive quartzite, sandstone and mudrock) bordered to the NW by a continental rise. Farther E and S lay the western margin of France-Iberia-Africa with a late Precambrian history of rifting, followed by the early Palaeozoic development of an Atlantic-type continental shelf-rise margin.
Precambrian to Tremadoc biotas in the Caledonides
Authors: Conway Morris S. , Rushton A. W. A.
Published: September 2007

The early Phanerozoic faunas of the Caledonides (late Precambrian to Tremadoc) are reviewed; they reflect the early history of the Iapetus Ocean. The long-established distinction between ‘Pacific’ and ‘Atlantic’ faunas is justified, but the latter are divisible into Avalonian and Baltic sub-faunas. Putative deep-water faunas have a more cosmopolitan distribution. Five time segments are considered: late Precambrian (Vendian), early, middle and late Cambrian, and Tremadoc. In the late Precambrian, consideration is given to the distribution of soft-bodied Ediacaran faunas. In the Cambrian and Tremadoc, interpretations are based primarily on trilobite distributions, although other fossils have biogeographical importance. Supposed transgressive-regressive cycles, which are connected with eustatic sea-level changes, influence faunal distributions. The perturbations giving rise to the biomeres recognized around the N American craton also appear to be recognizable elsewhere in the Caledonides; they may reflect eustatic changes of sea level.
Plutonism and volcanism related to the pre-Arenig evolution of the Caledonide-Appalachian orogen
Authors: Rankin D. W. , Furnes Harald , Bishop A. C. , Cabanis B. , Milton D. J. , O’Brien S. J. , Thorpe R. S.
Published: September 2007

Igneous activity younger than 1 Ga related to crustal extension characterizes the western part of the orogen from N America to Scotland in the Laurentian plate and the eastern part of the orogen in Scandinavia in the Baltic plate. Sub-aerial tholeiites, locally with alkalic rhyolites at embayments, and accompanying mafic dyke-swarms indicate late Proterozoic continental rifting; oceanic tholeiites followed. Iapetus oceanic crust is preserved in ophiolites obducted to the W in Laurentia and to the E in Baltica. Ensimatic island arcs developed locally in late Cambrian and early Ordovician in Scandinavia were broadly coeval with plutonic rocks intruding continental-shelf sediments during the late Finnmarkian deformation in northern Norway. E of the Iapetus suture in N America, southern Britain and northern France many of the igneous rocks of the accreted terranes underwent or were generated during a late Proterozoic orogeny (Avalonian or Cadomian). Locally fossiliferous lower Cambrian strata rest unconformably on calc-alkaline Avalonian or Cadomian granites.

Type I, or cratonal, terranes formed the rifted margins of the Iapetus Ocean. Type II, or accreted, terranes appear not to have been part of the Iapetus rifting cycle and are not now tied stratigraphically to any large cratonal masses. Type II terranes terminate northward against the projection of Tornquist’s line which may have originated in the late Proterozoic as the southern rifted margin of Baltica.
Arenig to Llandovery faunal distributions in the Caledonides
Authors: Fortey R. A. , Cocks L. R. M.
Published: September 2007

Faunal distributions in the Arenig to Llandovery period can be seen to be due to an interplay of the following factors: (i) the biofacies and community distributions on the shelves and slopes of continental margins; (ii) the distribution of former continental masses relative to palaeolatitude; (iii) the contemporary transgressive-regressive cycles which cause diachronous shifts of biofacies belts (with a major crisis at the Ordovician-Silurian boundary); (iv) differential dispersal methods and rates of the different invertebrate groups. The distribution of the common brachiopods, trilobites, graptolites and conodonts exemplify these various influences, and these distributions are considered from both the margins and the central areas of the Caledonides. Palaeogeography cannot be reliably reconstructed without due consideration of all these factors which give results which are relatively independent of structural studies.
Palaeontological evidence bearing on the Arenig-Caradoc development of the Iapetus Ocean basin
Authors: Neuman Robert B.
Published: September 2007

Ophiolitic rocks of the Dunnage-Dundee terranes and deep-water sedimentary rocks in the Gander-Greenore terranes indicate the position of the Iapetus Ocean in the Appalachian-Caledonide orogen during the Late Precambrian and Early Ordovician. The few occurrences of fossiliferous Cambrian to Tremadocian peri-insular limestones in the Dunnage terrane suggest a relatively narrow ocean basin within low palaeolatitudes. Late Arenig-early Llanvirn islands in Dunnage and Gander terranes and their equivalents were more abundant; most have little limestone and distinctive Celtic faunal assemblages of middle to high palaeolatitudes, but two (Glensaul-Tourmakeady, central western Ireland, and Hølonda, central western Norway) have abundant limestone and Scoto-Appalachian fossils of low palaeolatitudes. During the obduction of Iapetus Ocean crust, Celtic faunas were replaced by late Llanvirn-Llandeilo warm-water assemblages, indicating movement of these terranes into low palaeolatitudes.
Ordovician to Silurian volcanism in the Appalachian—Caledonian orogen
Authors: Stillman C. J.
Published: September 2007

Ordovician to Silurian volcanism in the orogen was primarily associated with the change from a spreading to a closing ocean basin and with the converging margins which involved the development of volcanic arcs, marginal basins and active continental margins as arc-trench systems approached and in some cases were subducted beneath the continental crust. Volcanism was controlled by plate-margin dynamics which differed according to the margin concerned but show remarkable similarities along the length of the orogen.

Arenig to Wenlock volcanism in N America, the British Isles, Europe and Scandinavia are surveyed in this paper in the framework of current plate-tectonic models, and the value of volcanic rocks in tectono-stratigraphic compilation is demonstrated.

Extensive destruction of oceanic crust and margins took place during major Ordovician orogenic events but, although parallel developments on both margins of Iapetus are noted, these events were not synchronous. Similarly, whilst subsequent volcanism was produced by both subduction-related and within-plate activity in most regions, the differences in the timing of this activity lend support to the concept of terrane accretion for the assembly of crustal blocks which make up the Caledonide orogen in its entirety.