bluuebunny/crossref_metadata_2025 huggingface dataset:
Faunal evidence for oceanic separations in the Palaeozoic of Britain
Authors: Cocks L. R. M. , Fortey R. A.
Published: December 2007

The faunal criteria for the recognition of the relative separation of old continents are: 1, the contrast between shallow- (not deep-) water faunas off opposite shorelines; 2, the recognition of deeper-water facies surrounding the edges of continents; and 3, the recognition of the disposition of ancient climatic belts. These criteria are defined and used to assess the relative position of the various parts of what is now Britain in the Palaeozoic. The Iapetus Ocean is well characterized by faunal differences from the Cambrian until the lower Ordovician, but from Caradoc times onwards these differences dwindle, until by the latest Silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds. In the early Ordovician, S British faunas are comparable with those from Bohemia, France and elsewhere to the S, indicating connection with Gondwanaland, and these faunas differ from those in the Baltic area, a distribution which we attribute to a true ocean, Tornquist's Sea, between N and S Europe. Tornquist's Sea probably closed in the upper Ordovician, and from Ashgill times onwards the S British and Baltic faunas remained similar. During the middle and late Silurian there is evidence of increasing faunal distinction between Britain, the Baltic area and North America on the one hand and southern Europe and Gondwanaland on the other, indicating an opening Rheic Ocean which appears to have persisted until late Carboniferous times.
Chemical variation in Hercynian basalts relative to plate tectonics
Authors: Floyd P. A.
Published: December 2007

The geochemistry of Hercynian basalts is reviewed with special reference to SW England. Hercynian basalts have the following chemical features: (1) relative enrichment in incompatible elements, (2) fractionated rare earth element (REE) patterns ranging from mild to strongly light-REE enriched, and (3) generally low large-ion-lithophile (LIL)/high field strength (HFS) element ratios. High and variable alkali contents and K/Rb ratios are indicative of low-grade alteration. Basalts are predominantly tholeiites, although alkali basalts may be developed on some trough margins. Basalts from different sedimentary troughs within the main Hercynian tectonic zones are characterized by specific incompatible element abundances and ratios. These variations are interpreted in terms of variable fractional crystallization, partial melting and mantle heterogeneity.

Hercynian volcanism was typically bimodal with basic and acid products characterizing the Rheno-Hercynian and Saxo-Thuringian zones. Activity in the Moldanubian zone was more variable with a high proportion of calc-alkali andesites. Two systematic chemical changes throughout the Devonian and Carboniferous have been recognized, with both light/heavy REE and LIL/HFS element ratios increasing with time. These features may relate to the introduction of LIL-enriched fluids derived by dehydration of subducted lithosphere that caused progressive metasomatism of the overlying mantle wedge.

Chemical discrimination of tectonic setting demonstrates that most of the basalts typify the continental intra-plate environment, although early Devonian basalts produced in the initial stages of continent rifting have some of the chemical features of incompatible element enriched oceanic basalts. An ensialic back-arc basin model underlain by a shallow northward-dipping subduction zone provides a possible explanation for the spatial and temporal variation in the Hercynian volcanic rocks.
The structural development of the Wessex Basin
Authors: Stoneley R.
Published: December 2007

The Mesozoic and Cenozoic structures of southern England form part of a system of disturbances which extends across the Channel into northern France. They are reviewed in the light of published data from south Dorset. Basin development started with Permo-Triassic rifting, and is believed to have continued through the Jurassic and early Cretaceous with intermittent growth on deep-seated listric normal faults, probably associated with roll-over anticlines. This regime had ceased by the Aptian and the basin passed through a period of relative stability in the late Cretaceous, followed by tectonic inversion and, in the Miocene, by northwards compressive movement along the former normal faults. This partially restored the earlier separative displacement at depth, and probably accentuated pre-existing roll-over anticlines: it was expressed in the surficial Upper Cretaceous-Palaeogene sequence as monoclinal flexuring. The early rifting and the change from overall N-S crustal stretching to compression may reflect plate tectonic events in the North Atlantic region.
Regional structure and syntectonic granite intrusion in the Dalradian of the Gweebarra Bay area, Donegal
Authors: Meneilly A.W.
Published: December 2007

6 phases of deformation are recognized in the Appin Group (Lower Dalradian) metasediments around Gweebarra Bay. The major early folds are F
2
in age (including the Crohy and Maghery Anticlines) facing up to the N or NE. S
2
transposes an earlier fabric (S
1
) which is only rarely preserved. No major or minor F
1
folds are recognized and the facing of S
1
is unknown. D
3
produces structures verging S on S
2
but does not produce major folds comparable with D
2
.

The Maghery Anticline, previously interpreted as changing from an open upright fold in the SE to overturned in the NW, is shown to be overturned throughout. Previous interpretation involving the re-folding of the Maas Syncline by the upright Mulnamin Anticline is not supported and both major folds are now interpreted as D
4
, in age, with the axial plane of the Mulnamin Anticline dipping moderately or gently S. D
4
, is a regional deformation phase synchronous with the intrusion of the Ardara Granite. D
5
, structures are upright or face to the N. The development of a major NE–SW trending sinistral shear zone adjacent to the SW end of the Main Donegal Granite is synchronous with D
5
, deformation, F
5
, folds rotating into sub-parallelism with the shear planes.
Late Mesozoic sedimentation in the northern Antarctic Peninsula and its relationship to the southern Andes
Authors: Farquharson G. W.
Published: December 2007

Two distinct phases of island-are evolution are recognized in late Mesozoic sedimentary rocks of the northern Antarctic Peninsula. During late Jurassic (Kimmeridgian-Tithonian) times, alternating radiolaria-rich mudstones and ash-fall tuffs were deposited under shallow marine euxinic conditions in association with restricted volcanic centres. The name Nordenskjöld Formation is proposed for this lithostratigraphic unit. Subsequently, during the early Cretaceous, a major episode of volcanism and uplift led to the construction of an emergent are-terrane. Simultaneous development of a retro-arc basin resulted in the accumulation of coarse volcaniclastic detritus along the eastern side of the arc. Retro-are sedimentation and intermittent volcanism continued into the late Cretaceous and early Tertiary. Although the pattern of sedimentation in the northern Antarctic Peninsula is broadly comparable to that of the southern Andes, an active marginal basin has not been recognized in the former area. Interbedded mudstones and tuffs, identical to those of the Nordenskjöld Formation, were deposited in the southern Andes and South Georgia during the late Jurassic - early Cretaceous.
Post-tectonic uplift of the Cordillera Darwin orogenic core complex: evidence from fission track geochronology and closing temperature–time relationships
Authors: Nelson E. P.
Published: December 2007

The Cordillera Darwin orogenic core complex is a structural and topographic culmination in the southern Andes, which underwent mid-Cretaceous penetrative deformation and metamorphism associated with the deformation and incipient obduction of a partly ophiolitic back-are basin (Rocas Verdes) block along its southern boundary. Whereas structural evidence suggests uplift of the Rocas Verdes terrane relative to the Cordillera Darwin during this event, metamorphic assemblages reflect a greater net uplift of the opposite sense. New fission track ages together with existing K-Ar and Rb-Sr ages were used to estimate cooling and uplift histories and their relationships to foreland sedimentation and present-day topographic relief. The results suggest that, whereas initiation of uplift in the Cordillera Darwin and Rocas Verdes blocks may have accompanied mid-Cretaceous deformation, the major, rapid uplift (around 0.5–1.5 mm/y) occurred after the deformation. Also, uplift occurred earlier in the Rocas Verdes block than in the Cordillera Darwin block, the latter having experienced a greater net uplift since the mid-Cretaceous. The major, post-tectonic uplift of the Cordillera Darwin occurred at a slower rate (0.05–0.20 mm/y) and coincides with the deposition of thick flysch and conglomerates in the foreland basin
Plants and the palaeoatmosphere
Authors: Moore P. D.
Published: December 2007

The effects of the development of terrestrial vegetation during Devonian and Carboniferous times is considered and it is concluded that the maximum possible global biomass in Carboniferous times was equivalent to about 1.148 × 10
15
g C, which is in excess of the current Earth's biomass (560–860 × 10
I5
g C). Global primary productivity, however, estimated at about 73 × 10
15
g C a
−1
is of the same order as that of the present day. The amount of carbon actually becoming stored as 'fossil fuel' was a very small proportion of this productivity, estimated here at only 0.00002% over 65 × 10
6
a. Thus, the vegetation of Carboniferous times cannot be regarded as a significant carbon sink.

The atmosphere of the Mesozoic is discussed in relation to the evolution of angiosperms. In particular, the photosynthetic modifications in so-called C
4
and CAM plants are considered in relation to atmospheric and other environmental stresses. Taxonomic evidence indicates that the C
3
system is primitive in the angiosperms and that the modified systems have a polyphyletic origin. Physiological considerations suggest that their development relates more clearly to the demands of water balance rather than to lowered atmospheric CO
2
or high atmospheric levels of oxygen. It is not considered likely that the evolution of these mechanisms, or of the angiosperms in general, has had any profound effect upon the carbon content of the atmosphere.
The Glen Kyllachy Granite and its bearing on the nature of the Caledonian Orogeny in Scotland
Authors: van Breemen O. , Piasecki M. A. J.
Published: December 2007

The Tomatin (Findhorn) Granite in the NW Grampian Highlands has been separated into two distinct complexes: a late-tectonic Glen Kyllachy Granite (tectonically foliated) and a post-tectonic Findhorn Granite (flow foliated). For the Glen Kyllachy complex, Rb-Sr analyses of muscovites from the granite and from an associated suite of cross-foliated pegmatites yield an emplacement age of 443
+5
−15
Ma. Whole-rock Rb-Sr data support field and textural evidence that the pegmatite and granite emplacement was late-tectonic to the last (F3) phase of Caledonian folding. Initial granite
87
Sr/
86
Sr of 0.7176 supports field and geochemical evidence of derivation from upper crustal metasediments first metamorphosed during the Grenville event. For the Findhorn Granite, concordant Rb-Sr and K-Ar mineral data establish an age of 413± 5 Ma, and an initial
87
Sr/
86
Sr of c. 0.706 indicates a lower crustal and/or mantle source. This age and isotopic contrast between these granites is characteristic of the whole Grampian region, in which a plutonic hiatus between
c
. 440 and 415 Ma coincides with the peak of sedimentary accretion in the Southern Uplands, and may be explained in terms of the lack of hydrous materials passing down the associated subduction zone.

Structural, metamorphic and radiometric evidence suggests (a) that the late-F3 Glen Kyllachy pegmatites are comparable with the 442 ± 7 Ma old, syn-F3 pegmatites in the N Highlands—both pegmatite suites are situated in the axial zone of the metamorphic Caledonides displaced by the Great Glen Fault; and (b) that the Caledonian,
c
. 454 Ma old ‘peak’ tectonic-metamorphic event (D2) previously documented in the N Highlands coincides with a Caradocian basin closure along the Highland Boundary Fault.

For Silurian events in the N Highlands we propose a separate subduction history; in this region, the peak of calc-alkaline plutonism was accompanied by continuous folding and westward thrusting at the same time as a plutonic hiatus in the Grampian Highlands. Current evidence suggests that the
c
. 160 km or greater sinistral movements on the Great Glen Fault were not initiated until late Silurian–early Devonian time.
Younging directions in the Dawros peridotite, Connemara
Authors: Bennett M. C. , Gibb F. G. F.
Published: December 2007

The syn-D2 Dawros peridotite comprises a sequence of layered harzburgites (associated with massive orthopyroxenites) which pass upwards
via
a transition marked by the first appearance of chromitite layers into layered lherzolites. Microprobe analyses reveal that the ultramafic sequence exhibits a range in olivine composition from Fo
91.9
to Fo
82.3
and a range in orthopyroxene from En
91.9
to En
84 3
. A transgressive metagabbro lens in the central outcrop is interpreted as a syn-D2 intrusion into the ultramafic sequence and contains olivine (Fo
80 4
) and a range in orthopyroxene from En
81.5
to En
76.0
. Mg/Fe ratios in coexisting olivines and pyroxenes are used with new structural data to elucidate younging directions in the layered parts of the peridotite. The exposed sequence youngs consistently NE but is repeated by faulting and locally reversed by F4 folding. The peridotite and its immediate hornfels became detached from the surrounding metasediments during D3 and now lie on the SW limb of a recumbent downward facing F3 antiform which plunges NW. Consideration of the structure in adjacent areas suggests that the Dawros and Currywongaun-Doughruagh intrusions lie within a single major F2 fold limb.
Magma and sediment—II Problems of interpreting palaeovolcanics buried in the stratigraphic column
Authors: Francis E. H.
Published: December 2007

President's anniversary address 1982

The preservation of volcanoes—never more than partial—involves a complex interaction of processes and these operate over periods of time which are often undervalued. From a review of the relative rates of these processes it seems that short-lived structures such as Surtseyan ash-rings may be preserved, albeit in modified form, around their whole circumference. For larger composite volcanoes, however, growth and remoulding by activity is balanced by penecontemporaneous and later erosion in such a way that the central and most of the proximal zones are likely to be removed before they can be buried either by transported volcaniclastic or regional epiclastic sediments. Even the preservation of distal zones may be asymmetrical. A one-sided hinge-line subsidence-sedimentation model is therefore suggested to explain the elongate lens-shaped aspects of the many extrusive piles known to lie approximately concordantly within stratigraphic sequences formed in both continental and littoral environments.
Timing of tectonic and magmatic events in the Central Andes of Peru
Authors: Bussell M. A.
Published: December 2007

The study of fault displacements in a region of well-dated sequential granitic intrusions suggests a close time relationship between two peaks of deformation and intrusive activity. Mantle-derived basic magmas were associated with both episodes, which are dated as mid-Cretaceous and early Palaeocene. It is concluded that in each case a period of tensile tectonics with mafic dyke injection preceded the intense compressive deformation which accompanied the generation and rise of granitoid magmas. These events may be related to alternations of slow and rapid subduction.
The petrology and palaeoenvironment of the Sortino Group (Miocene) of SE Sicily: evidence for periodic emergence
Authors: Pedley H. M.
Published: December 2007

The Sortino Group of carbonates consists of three formations which developed on a shallow marine shelf during Tortonian and lower Messinian times. The two lower formations contain three bioherms and are intercalated with thick volcaniclastic sequences. The youngest formation is dominated by shoal water, lagoonal and peri-tidal carbonates. Early submarine lithification is well-developed within the bioherms and consists of construction by both algae and invertebrates, the development of peloidal micrites and isopachous fringe growth. Superimposed upon these are a series of both meteoric phreatic and vadose textures related to episodic emergence of the bioherms. These features include bladed cements showing cathodoluminescence, pendent cements, brecciation, alveolar texture and palaeosols. The Sicilian strata may be directly correlated with the Maltese Islands sequence, and collectively exhibit the effects of fluctuating Mediterranean sea levels at an early stage in the late Miocene salinity crisis.
Temperate, meteoric diagenesis of Pliocene skeletal carbonates from eastern England
Authors: Balson P. S.
Published: December 2007

The Crags of eastern England include marine skeletal carbonates which were deposited and lithified under temperate conditions. The Coralline Crag (Pliocene), in particular, exhibits a variety of carbonate cement types which indicate exposure to meteoric groundwaters during a regressive episode.

The Coralline Crag has been divided into an upper division of dominantly well sorted skeletal calcarenites and calcirudites and a lower, poorly sorted silty calcarenite division. ‘Dogtooth’ sparcement and rudimentary syntaxial cement overgrowths on echinoderm fragments are characteristic of the sediments of the upper division, which has undergone extensive aragonite dissolution. This division is consequently relatively evenly cemented. In the underlying sediments aragonite dissolution has not occurred and the sediments have remained largely uncemented, with cement restricted to irregular patches where growth of intra- and inter-particle equant spar has effectively eliminated porosity. In contrast with the upper leached sediments, syntaxial echinoderm overgrowths show a greater development but only where interference from surrounding sediment and sparry cement was absent. The elongate shape of the Coralline Crag outcrop may be the result of preferential cementation of the upper, coarser, well sorted calcarenites originally deposited as an elongate sediment body, possibly as an offshore sandbank.
The Waitakere Limestone, a temperate algal carbonate in the lower Tertiary of New Zealand
Authors: MacGregor A. R.
Published: December 2007

The Waitakere Limestone is a shallow-water temperate algal carbonate ranging from upper Eocene (Runangan) to lower Oligocene (Whaingaroan) in age. The principal components of the rock are coralline algae 37–72%, benthonic foraminifera 1–9.2%, bryozoa 1–10% and echinoids 1–4%, set in a spar matrix 13–32%. Glauconite l–9% is ubiquitous.

Surface temperatures may have been as high as 20°C, but possibly as low as 12°C; water depth was 1–12 m, with adjacent sea grass probable. However, no evidence was obtained for the widely recognized temperature drop at the end of the Eocene. known elsewhere in New Zealand. The limestone fits well into the cooler-water foramol type, and within this the bryozoan assemblage.
Fluid inclusions in high-grade anatectic metamorphites
Authors: Touret J. , Dietvorst P.
Published: December 2007

Fluid inclusions have been studied, by microthermometry, in metapelites and metabasites which have undergone partial melting in the transition zone from amphibolite to granulite facies. Mobilizates contain far more abundant and larger fluid inclusions than non-mobilized or restite layers, and these inclusions occur as several generations of well-defined and chemically contrasted fluids: carbonic (CO
2
± CH
4
and hydrocarbon), aqueous (H
2
O ± NaCl) and N
2
.

A garnet-bearing mobilizate in amphibolite contains 4 generations of relatively pure fluids, most of them trapped after peak metamorphic conditions in the following
P
-
T
domains: CO
2
and N
2
.
P
= 2–8 kbar,
T
= 550–850°C; CH
4
.,
P
= 1–2 kbar,
T
= 400–550°C; H
2
O,
P
< 1 kbar,
T
= 120–400° C.
Isotopic evidence for the age and origin of pitchstones and felsites, Isle of Eigg, NW Scotland
Authors: Dickin A. P. , Jones N. W.
Published: December 2007

An 8 point whole-rock and mineral Rb-Sr isochron for the Sgurr of Eigg yields an age of 52.1 ± 1.0 Ma (2σ). This is the youngest accurate date yet reported for igneous activity in the British Tertiary Province, and indicates that sub-acid rocks of the Small Isles represent one of the latest episodes of Hebridean magmatism. Pb-isotope compositions of these rocks yield an apparent age of 2660 ± 75 Ma (2σ), interpreted as a mixing line between Lewisian, Torridonian and mantle-derived Pb, and indicating the presence of Lewisian basement under Eigg. Pb, Sr and Nd isotopic evidence points to contamination of basaltic differentiates and granulite-facies Lewisian melts by Torridonian sandstone, but precludes significant involvement of amphibolite-facies Lewisian, which may have been largely removed by erosion before deposition of the Torridonian sediments in the Small Isles.
Structural dating of a Precambrian pegmatite in Moine rocks of northern Scotland and its bearing on the status of the 'Morarian Orogeny'
Authors: Powell D. , Brook M. , Baird A. W.
Published: December 2007

A cross-cutting pegmatite emplaced into low grade, non-migmatitic, Proterozoic, Moine metasediments on the western sea-board of Inverness-shire, Scotland, gives Rb-Sr muscovite ages of 746–776 Ma which are interpreted as minimum estimates for the age of formation of the pegmatite. Intrusion followed at least one phase of folding and schistosity production in the country rocks and an early phase of syn-kinematic garnet growth, correlated with the regional D
2
tectono-metamorphic event. The pegmatite was folded and suffered considerable grain size reduction under metamorphic conditions during the regional D
3
deformation episode.

The dilational character and low grade context of the pegmatite show that it was not a product of melting in place during metamorphism of the adjacent Moine Schists. Its lack of initial, syn-kinematic crystallization textures and fabrics, together with the absence of evidence that might relate its emplacement to any regional phase of compressive ductile deformation, would be compatible with a considerable time lapse between its intrusion and the earlier deformation and metamorphism of the host rocks. We have, however, no direct evidence for the extent of this interval.

The relationship of the pegmatite to the earlier deformation and metamorphism of the host rocks provides clear evidence for substantial Precambrian orogenic activity but the age of this activity, whether 'Morarian' and, or, 'Grenvillian', remains a matter of interpretation.
The geochemistry of ophiolitic mafic rocks from the polymetamorphic Ordenes Complex, Spain
Authors: Williams P. J.
Published: December 2007

Selected aspects of the geochemistry of 395 meta-igneous rocks from the polymetamorphic Ordenes Complex are described in an examination of recently proposed ophiolite models for their genesis. The metabasites are sub-alkaline, tholeiitic and geochemically indistinguishable from mafic rocks of oceanic suites and ophiolites. The geochemistry is consistent with a proposed division of the protoliths into gabbros, including ferrogabbros, and submarine volcanic rocks. The metamorphosed equivalents of highly sodic acidic igneous rocks also occur. These data provide strong support for the ophiolite model. High Ti fractionation trends suggest that ophiolite generation occurred either at a mid-ocean ridge or in a partially evolved back-arc basin.
Eocene deformation on the continental margin SW of the British Isles
Authors: Masson D. G. , Parson L. M.
Published: December 2007

Two narrow E-W trending zones of late Eocene compressional deformation, one on the northern margin of the Bay of Biscay, the other in the southern Porcupine Seabight, are described. Both appear to follow pre-existing geological lineaments. The deformation is related to the NW movement of Iberia between latest Cretaceous and Late Eocene time.
The mechanism of Surtseyan volcanism
Authors: Kokelaar B. P.
Published: December 2007

During eruption, the vent of a Surtseyan volcano is occupied by a highly mobile slurry of tephra, hyaloclastite and water. As magma ascends rapidly through the slurry, mixing occurs due to velocity shear, acceleration of the fluid-fluid contacts and mass incorporation. Consequent expansion of the admixed water causes the jetting and continuous up-rush activity that characterizes Surtseyan volcanism. There is no evidence of the widely accepted fuel-coolant interaction mechanism.
Hot or cold emplacement of the Lizard Complex?
Authors: Barnes R. P. , Andrews J. R.
Published: December 2007

Regional prograde pumpellyite actinolite facies metamorphism (Ml) has affected the Gramscatho Group sediments and volcanics structurally underlying the Lizard Complex. Metamorphism took place synchronously with the main penetrative deformation (Dl). There is no evidence of any increase in the regional gradient towards the Lizard Complex or of any local thermal overprinting. It is concluded that the Lizard Complex was emplaced in a cooled thrust sheet during Dl deformation during a regional heat flow which produced very low grade metamorphism. Temperatures of 250–350°C and a pressure of several kilobars are estimated.
Geochemical characteristics and comparison of the basic rocks of the Lizard Complex and the basaltic lavas within the Hercynian troughs of SW England
Authors: Floyd P. A.
Published: December 2007

The S Cornish basaltic rocks constitute a separate chemical province relative to the rest of the SW England Hercynian volcanic suites. In particular they are characterized by (a) lower incompatible element abundances, (b) generally lower large-ion-lithophile/high field strength element ratios, and (c) being mildly enriched in the light REE. Within this province some of the basalts have chemical features akin to incompatible element enriched MORB.

Tectonically the S Cornish province can be divided into (a) the S Cornish mélange zone (N Lizard-Roseland area), and (b) the Mylor sedimentary trough (Penwith-Camborne area), both of which contain localized basaltic lavas. These basalts are compared with the Lizard ophiolite and associated hornblende schists which have compositions ranging from depleted to enriched MORB. The exotic volcanic megaclasts within the mélange zone are not chemically related to the ophiolite and as such do not represent the dismembered volcanic portion of the Lizard ophiolite.

If the Lizard ophiolite and the hornblende schists represent different segments of a Palaeozoic oceanic area lying to the S, the ocean-floor type Hercynian basalts of S Cornwall are interpreted as an abortive attempt to produce oceanic crust in a rifted continental margin during the early Devonian.
The structure of SW Cornwall and its bearing on the emplacement of the Lizard Complex
Authors: Rattey P. R. , Sanderson D. J.
Published: December 2007

Six phases of deformation can be recognized in the Devonian metasediments to the N of the Lizard Complex. The first two phases (D
1
& D
2
) show a variety of orientations and styles, but with a general N or NW vergence. These phases, while respresenting regional deformation events, are considerably modified by the emplacement of the Lizard Complex. There is a general increase in intensity of both D
1
and D
2
structures towards the Lizard boundary. This is accompanied by variation in the orientation of both early phases of deformation, leading to a variety of interference patterns. Domains of oblique F
1
folds, with axes trending N-S, are developed in the rocks immediately to the NE and NW of the Lizard, and are considered to have formed by differential, NNW-directed thrusting of slabs of the complex during the early regional deformation. D
2
produced discrete shear zones ahead and below the Lizard rocks and may have resulted from the gravitational collapse of the D
1
nappes. The D
3
deformation post-dates the Lizard emplacement, being related to the Cornubian batholith. Subsequent phases represent essentially post-batholith, and hence post-Lizard emplacement, deformation.
Post-Permian events in the Zangbo 'suture' zone, Tibet
Authors: Mitchell A. H. G.
Published: December 2007

The Mesozoic geology of the Zangbo zone in Tibet is closely comparable to that of the zone's continuation through western Burma. Deposition of Triassic turbidites and locally Jurassic sediments on and immediately oceanward of the passive Asian margin was followed by imbrication above subducting ocean floor in the Early Cretaceous. The subducting slab became detached from the downgoing lithosphere and accreted to Asia in the late Lower Cretaceous. Emplacement of the detached slab by southward thrusting over its Triassic flysch cover to form the Zangbo and western Burma ophiolite took place respectively in or after the Cenomanian and before the Albian. Subsidence and deposition of Albian limestones and mudstones in Burma and of the Xigatse flysch in a fore-arc basin in Tibet were followed in both regions by pre-Campanian uplift and Campanian to early Maastrichtian marine deposition. Maastrichtian olistostromes resulted from underthrusting and tectonic burial of poorly consolidated Campanian sediments, their extrusion together with blocks of adjacent older rock, and downslope flow of the extruded melange. Collision of India with Asia in the Eocene was not associated with ophiolite emplacement and in the Zangbo region resulted chiefly in northward-directed thrusting of the Triassic flysch and ophiolite over the Xigatse flysch, with consequent narrowing of the fore-arc basin.
Remnants of the Hercynian orogen along the 'Calabrian-Peloritan arc', southern Italy: a review
Authors: Atzori P. , Ferla P. , Paglionico A. , Piccarreta G. , Rottura A.
Published: December 2007

Remnants of the Hercynian orogen, disturbed by Alpine tectonics, occur widely along the ‘Calabrian-Peloritan arc’. Indications of former oceanic conditions, for example Palaeozoic ophiolites and original deep water sediments, are lacking. Metamorphism developed under intermediate to low pressure conditions; the post-metamorphic plutonism appears to be more compatible with intracontinental orogenic processes than with an active continental margin. The possible evolution of this segment of the Hercynian orogen is discussed.
Lower Proterozoic red beds, evaporites and secondary sedimentary uranium deposits from the East Arm, Great Slave Lake, Canada
Authors: Stanworth C. W. , Badham J. P. N.
Published: December 2007

The East Arm graben contains a thick sequence of well-preserved, unmetamorphosed sedimentary rocks. Three units have been recognized: the Wilson Island Group (
c
. 2400–2300 Ma); the Great Slave Supergroup (
c.
2300–1790 Ma), and the Et-Then Group (
c.
1700–1600 Ma). The Wilson Island Group contains red beds and overlies an apparently normal regolith formed on Archaean granites. The Great Slave Supergroup contains a normal regolith, numerous red beds, glauconitic horizions, biogenic hydrocarbons and sedimentary uranium deposits. There is also abundant evidence for the former presence of evaporites in that part of the sequence that was deposited within 30° of the palaeoequator.

The uranium deposits originated during low temperature diagenesis by precipitation from oxidizing waters in areas of early accumulated hydrocarbons. These observations suggest that the Lower Proterozoic climate, atmosphere and hydrosphere were similar to modern ones and that uranium prospectors in such old terrains should look for chemical deposits as well as the classic placer types.
Deformation produced by the subduction of a Palaeozoic turbidite sequence in northern Chile
Authors: Bell C. M.
Published: December 2007

A deep-sea basin-plain turbidite succession of Ordovician to Devonian age is exposed between 25°30' and 27°S in the coastal region of northern Chile. In the S these turbidites form a large-scale tectonic mélange. Alkali basalt pillow lavas and tuffs are interstratified with the sediments to the S of 26°20'S. The rocks were metamorphosed to the greenschist facies and subjected to two phases of deformation subsequent to the formation of the mélange. The first phase is characterized by large-scale, tight chevron folds and an axial planar cleavage. The folds of the second phase are smaller and more upright and accompanied by a crenulation cleavage. The fold axes of both phases trend between W–E and NW–SE, oblique to the N–S trending Mesozoic and Cenozoic Andean structures. Folds are predominantly asymmetric and overturned towards the S and SW, suggesting an origin by underthrusting towards the N and NE. The deformed succession is interpreted as a subduction complex produced by oblique underthrusting of an oceanic plate towards the N and NE during late Devonian to Carboniferous times.
Geochemical evolution of St Kitts and Montserrat, Lesser Antilles
Authors: Baker P. E.
Published: December 2007

The volcanic rocks of St Kitts have uniform
87
Sr/
86
Sr ratios (c. 0.7037) and high
143
Nd/
144
Nd ratios (c. 0.5130) in a suite which ranges from basalt to rhyolite. Trace element abundances and the flat chondrite-normalized REE patterns resemble those of island-arc tholeiitic suites. In comparison, the Montserrat lavas have higher concentrations of alkaline elements and are LREE enriched. Geochemical differences may be related to partial displacement of the axis of the volcanic arc about 9 Ma. This would have led to the development of the St Kitts’ suite in an essentially new and immature island-arc environment. Being closer to the junction with the former arc axis, the composition of the Montserrat lavas may reflect previous subduction-related mantle enrichment and/or the influence of a sedimentary component. Isotopic ratios and most trace elements in the Redonda basalt indicate island-arc tholeiite or in some respects MORB affinities, but it has a high Sr content and is LREE-enriched. Despite its distinctive composition, Redonda is situated on the main axis of the volcanic arc and its lavas were evidently contemporaneous with the younger eruptions of St Kitts and Montserrat.
The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources
Authors: Brown G. C. , Thorpe R. S. , Webb P. C.
Published: December 2007

Geochemical variations in modern subduction-related igneous suites with respect to arc ‘maturity’ in time and space are illustrated using data for both volcanic suites (basalt, andesite and dacite) and intrusive granitoid suites (diorite, tonalite/granodiorite and granite) from circum-Pacific arcs. Using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites: (a) subduction-zone enrichment of lithospheric mantle, locally coupled with crustal assimilation allied with fractional crystallization (AFC) in zones of thickened crust, all of which yield magmas with enhanced concentrations of the large-ion lithophile (LIL) elements K, Rb, Th, U, LREEs, etc; (b) with increasing distance from the active trench, contributions from within-plate sub-continental lithosphere producing mantle-derived magmas with enhanced levels of high-field strength (HFS) elements, among which Nb, Ta, Hf and Y are particularly distinctive. Thus, even for the evolved granitoids of intrusive arc series, ratios of HFS/LIL elements not significantly affected by crystal fractionation, such as (Ta, Nb)/(K, Rb, La), may throw some light on the origin of mafic-intermediate precursor magmas. In terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites. 1. Primitive, calcic arc granitoids with low LIL and HFS element abundances. 2. Normal, calc-alkaline continental arc granitoids with enhanced LIL element abundances and low HFS/LIL ratios. 3. Mature alkali-calcic arc granitoids with high levels of LIL and HFS elements and higher HFS/LIL ratios. 4. Back-arc/anorogenic alkaline granitoids with the highest levels of HFS elements.
Inversion of the assimilation and fractional crystallization (AFC) equations; characterization of contaminants from isotope and trace element relationships in volcanic suites
Authors: Powell Roger
Published: December 2007

The assimilation and fractional crystallization (AFC) equations of
DePaolo (1981)
are inverted, with the aim of characterizing the chemistry of contaminants involved in magma evolution from observed isotope and trace element relationships. Equations are derived for contours of the ratio of the rate of assimilation to the rate of fractional crystallization in the isotope or trace element region where the contaminant might be located. Even for volcanic suites in which AFC is thought to have been the dominant process, the characterization of the contaminant is poorly constrained. Although AFC is likely to be a normal process, it leaves a recognizable isotopic signature in volcanics only when magmas assimilate material derived from continental crust. In subduction-related volcanics, this can be the case where the volcanic arc lies on or near continental crust.
The regional geology and evolution of the Toba volcano-tectonic depression, Indonesia
Authors: Aldiss D. T. , Ghazali S. A.
Published: December 2007

The late Quaternary Toba volcano-tectonic depression, in the Sunda Arc, is the largest resurgent cauldron in the world and lies in one of the largest ignimbrite fields, the Toba Tuffs. Although composed of at least 3000 km
3
of acid tuffs, spread over 20000 km
2
of mainland Sumatra, the greater part of the Toba Tuffs is believed to be a single ignimbrite cooling unit, formed about 100,000 years ago. The Toba Tuffs also include a 30000-year-old airfall tuff, which was erupted from a centre just N of the Toba depression.

The Toba depression developed through the same stages as other resurgent cauldrons. It formed after lithification of the Toba Tuffs, by collapse along regional faults crossing an eastwards bulge in the Sumatran geanticline. Resurgent uplift raised lake sediments within the depression by about 500 m. The uplift, which was not confined to the depression, was probably caused by continuing influx of magma and it is possible that the Toba depression is still in the stage of post-resurgence volcanism. The eruption of the Toba Tuffs and of all post-ignimbrite volcanism took place on the line of the western marginal fault of the depression. This marginal fault once extended northwards offshore into a zone of Miocene back-arc rifting.
Crustal growth and late Precambrian-early Palaeozoic plate tectonic evolution of England and Wales
Authors: Thorpe R. S. , Beckinsale R. D. , Patchett P. J. , Piper J. D. A. , Davies G. R. , Evans J. A.
Published: December 2007

Comparison of the basement rocks of Britain reveals major contrasts to the N and S of the Iapetus suture (Solway-Shannon line). North of the suture, polymetamorphic high-grade basement schists and gneisses and igneous rocks date from about 2900 Ma to
c.
400 Ma, the latter age corresponding to closure of the Iapetus ocean. By contrast, S of the suture, in England and Wales, the basement is dominantly low grade metamorphic rock, together with intrusive and extrusive igneous, and sedimentary rocks. There is little geological or geochemical evidence for the presence of basement older than
c.
900 Ma below this area and the predominant expression of crustal growth in England and Wales is indicated by radiometric ages between
c.
700 Ma and 450 Ma, reflecting late Precambrian-Palaeozoic magmatism and metamorphism.

Palaeomagnetic data from Scotland demonstrate that the crust in northern Britain developed as an integral part of the Laurentian Shield during mid-Proterozoic times (1600–1000 Ma). The palaeomagnetic record from the pre-Ordovician rocks of England and Wales shows that this area was unrelated to the Laurentian Shield prior to closure of the Iapetus Ocean. The apparent polar wander (a.p.w.) path is that of a distinct microplate involved in rapid growth of Iapetus during early Cambrian times, and emplaced in its present configuration adjacent to northern Britain as a result of subduction of the ocean basin between Cambrian and Silurian times. We conclude that whereas the basement to the north of the Iapetus suture formed an essentially continental unit from 2900 to 400 Ma, the basement of England and Wales is a result of crustal growth from about 900 Ma to 400 Ma and formed by accretion of island arcs, associated accretionary prisms and fore-arc basin sediments within the Iapetus Ocean.
The geometry of listric growth faults in the Devonian basins of Sunnfjord, W Norway
Authors: Hossack John R.
Published: December 2007

Thick Devonian clastic sequences accumulated in fault-bounded basins in W Norway at the close of the Caledonian orogeny and mark the beginning of a phase of crustal extension. Traditionally, the eastern boundary fault to the Devonian basins has been regarded as a thrust but cross-sections based on the use of a branch-line reveal it to be an extension fault. There is clear omission of structural section across the boundary and by matching up metamorphic rocks in the hanging- and foot-walls of the boundary fault, over 40 km of extensional dip slip can be measured at the N end of the fault. The boundary fault is a flat-lying listric fault with rollover anticlines in the hanging-wall. The fault also cuts up and down section along strike to form lateral ramps parallel to the movement direction. These ramps appear as wrench faults in outcrop. The ramp-flat geometry of the fault may have been inherited from a precursor Caledonian thrust fault which reversed its sense of slip in early Middle Devonian times to form the basins. Over 25 km of onlapping sediment accumulated in the largest basin during faulting in 14 Ma, with an average slip rate on the fault of 3 mm yr
-1
.
Tectonic studies: annual meeting 1982
Authors: Williams Graham
Published: December 2007

Report of a meeting of the Tectonic Studies Group held at University College, Cardiff, 15–17 December 1982. The meeting was organized by Dr G. D. Williams.

On the first day of the conference two field trips were held: the first to the Gower Peninsula led by R. Graham (Swansea) and the second to Pembroke led by G. Williams (Cardiff). In both cases, Variscan structures—thrusts, folds and shear zones—were studied. Also on 15 December, a workshop on techniques and uses of the orthographic net was presented by Declan DePaor (Galway).

The main contributions of the AGM were given on 16 and 17 December. During the first session, theoretical aspects of deformation were considered. J. S. Whalley (Portsmouth) and W. H. Owens (Birmingham) considered errors arising in calculating strain ellipsoids from two dimensional data using the program PASE5. D. DePaor (Galway) showed how the use of rank 3 tensors permits the description of complex structures in a simple way and he illustrated this using a zone of heterogeneous simple shear. Stress risers on a mesocopic scale are commonly observed, and A. Morris and J. Ague (Detroit) argued that features such as sedimentary pebbles, porphyroblasts, quartz veins and bed thickness variations may create a strain-sink effect that initiates and localizes deformation. C. J. Talbot (Dundee) discussed the effect of inverse and counter folding in incompetent layers and illustrated his talk with common examples from the Moines of the Fort William area and basic sheets in the Limpopo Belt. Transpression was
Crustal contamination in northern Andean volcanics
Authors: James David E. , Murcia L. Armando
Published: December 2007

Late Cenozoic andesitic rocks of the Colombian Andes are characterized by small but systematic and well-correlated variations in
87
Sr/
86
Sr,
143
Nd/
144
Nd, and δ
18
O that reflect significant crustal contamination. The range in isotopic ratios measured in northern Andean andesites is most pronounced for oxygen, relatively much more subtle for Sr and Nd. Pb isotopic data, so diagnostic of crustal contamination in central Andean lavas, exhibit minimal variation in the northern Andean volcanics. Trace element trends and phenocryst populations suggest dominantly mafic mineral fractionation, although plagioclase fractionation may be significant in later-stage differentiation. An assimilation-fractional crystallization model is constructed to match measured isotopic data. The results of that modelling indicate that the andesitic magmas of Galeras and Ruiz volcanoes may have assimilated up to 10–20% of crustal material. Correlated variations in measured isotopic ratios can be modelled satisfactorily by assuming a crustal contaminant of composition δ
18
O = 10 per mil,
87
Sr/
86
Sr = 0.710–0.712 (Sr ≈ 90ppm), and
143
Nd/
144
Nd=0.5120–0.5122 (Nd ≈ 20–30 ppm). We estimate that the composition of the primary magma (prior to crustal contamination) was in the range δ
18
O = 6.5 per mil,
87
Sr/
86
Sr = 0.704, and
143
Nd/
144
Nd = 0.51285, values that may suggest an additional crustal component was present in the primary magma. If we assume the added crustal component was derived from partial melting of subducted continental sedimentary material and/or oceanic crust (source contamination), then model calculations suggest that crustal material comprises less than 10 to 15 wt% of the primary magma. This model-dependent effect of source contamination is to shift isotopic ratios to the right on the Sr–Nd plot (see also

Hawkesworth
et al.
1977

), whereas crustal contamination of the magma during ascent through the Andean crust appears to have produced isotopic trajectories that cut downward and through the mantle array.
The infiltration of fine matrices into coarse-grained alluvial sediments and its implications for stratigraphical interpretation
Authors: Frostick Lynne E. , Lucas P. M. , Reid I.
Published: December 2007

Traps designed to simulate the framework gravel of natural streams and installed in a coarse-grained channel allow the identification and differentiation of the processes governing the infiltration of fines into bed interstices. Vertical patterns of matrix distribution reflect the relative importance of variables acting at both the surface of the channel bed and within the bed sediments. Differences in grain size (and therefore pore size) of the framework exercise a strong controlling influence. An armour layer of coarse particles superimposed on comparatively finer sub-surface bed material—common to many coarse-grained rivers—is shown to encourage the clogging of near-surface pores with matrix fines. Below the obstruction, interstices may be protected from incursion and the gravel framework is often left open. This is the reason for patchy concentrations of matrices often reported in the stratigraphical literature. In contrast, a uniform size-grading of the bed sediment, or an upwards fining, leads to a more uniform packing of the interstices with matrix. Flood history is shown to be an important determinant of both the size-distribution and the spatial distribution of matrices, and is useful in providing a dynamic explanation of the variability of matrices observed from place-to-place in ancient deposits.