bluuebunny/crossref_metadata_2025 huggingface dataset:
U–Pb and Rb–Sr geochronology of magmatism and metamorphism in the Dalradian of Connemara, western Ireland
Authors: CLIFF R. A. , YARDLEY B. W. D. , BUSSY F. R.
Published: December 2007

New geochronological data which clarify the timing of syn-orogenic magmatism and regional metamorphism in the Connemara Dalradian are presented. U–Pb zircon data on four intermediate to acid foliated magmatic rocks show important inherited components but the most concordant fractions demonstrate that major magmatism continued until 465 Ma whereas the earliest, basic magmatism has been dated previously at 490 Ma; a fine-grained, fabric-cutting granite contains discordant zircons which also appear to be 465 Ma old. Arc magmatism in Connemara therefore spanned a period of at least 25 Ma. Recent U–Pb data on titanite from central Connemara which gave a peak metamorphic age of 478 Ma are supplemented by U–Pb data on titanite and monazite from metamorphic veins in the east of Connemara which indicate that low-
P,
high-
T
regional metamorphism continued there to 465 Ma, i.e. at least 10 Ma later than in the central region dated previously. New Rb–Sr data on muscovites from coarse-grained segregations in different structural settings range from 475 to 435 Ma: in part this range probably also reflects differences in age from west to east, with three ages close to 455 Ma from the eastern area, which is also the site of the lowest pressure metamorphism. Thermal modelling indicates that at any one locality the duration of metamorphism was probably as little as 1–2 Ma. The new dates emphasize the complexity in the spatial and temporal distribution of high-level regional metamorphism caused by magmatic activity. The relatively simple overall distribution of mineral-appearance isograds revealed by regional mapping masks the complexity of a prolonged but punctuated metamorphic history related to multiple intrusions, primarily in the southern part of Connemara. The later stages of magmatic activity followed progressive uplift and erosion after the onset of magmatism, and were localized in the eastern part of the region.
Palaeogene dyke swarm, NW Wales: evidence for Cenozoic sinistral fault movement
Authors: BEVINS R. E. , HORÁK J. M. , EVANS A. D. , MORGAN R.
Published: December 2007

The Berw and Aber-Dinlle Faults represent the brittle manifestation of a major lineament, the Menai Strait Fault System, which is known to have been active intermittently since late Precambrian times. Documentation of historical seismicity in this area provides information on the most recent activity along this structure, although there is a paucity of field evidence to support conclusively Cenozoic or later movements. This work documents data from three separate aeromagnetic datasets, combined with field relationships, which reveal a well-defined Cenozoic dyke swarm traversing Anglesey and Snowdonia, showing clear sinistral offset (up to 1.5 km) along the Berw Fault, the most northwesterly component of the Menai Strait Fault System. These data provide conclusive evidence of Cenozoic (<65 Ma) strike-slip faulting in North Wales.
Extensional tectonics and magmatism as drivers of convection leading to Iberian Pyrite Belt massive sulphide deposits?
Authors: BOULTER C. A.
Published: December 2007

Contrary to the generally accepted volcanic model for the Iberian Pyrite Belt stratiform massive sulphide deposits, Rio Tinto, the largest deposit, formed in a sill-sediment complex. An appreciation of processes unique to such an environment demonstrates that the mineralization event at Rio Tinto was initiated as exhalation of ore fluids into an evolving sedimentary basin. Ore-fluid circulation was, therefore, first driven by extensional tectonics, which in turn triggered decompressional melting. The resulting magmas are inferred to have driven fluid circulation adjacent to the magma plumbing network, contributing significantly to the total fluid discharge. The major stratiform deposits in the Pyrite Belt display a spectrum of characteristics intermediate between volcanic-hosted and sediment-hosted styles; variable combinations of the two fluid convection models implicated at Rio Tinto are largely capable of explaining this spectrum.
Models of fracture orientation at oblique spreading centres
Authors: TUCKWELL G. W. , BULL J. M. , SANDERSON D. J.
Published: December 2007

Three models are proposed for the orientation of extensional faults and dykes at mid-ocean ridges based on their relationship to the ridge axis and the relative plate separation vector. These models predict four ridge geometries allowing them to be tested by orientation data from 17 different sites within the Earth's oceans. A transtensional model is shown to be generally applicable to the structure of mid-ocean ridges, with special conditions required for a departure from it. Spreading rate influences ridge geometry with intermediate and fast spreading ridges more likely to have an orthogonal spreading geometry, which is an end member case of the transtensional model.
Geochronological evidence for post-Variscan cooling and uplift of the Carnmenellis granite, SW England
Authors: CHEN Y. , ZENTILLI M. A. , CLARK A. H. , FARRAR E. , GRIST A. M. , WILLIS-RICHARDS J.
Published: December 2007

Apatite fission-track (AFT) dates for granitic samples from surface exposures and deep boreholes in the 293.3 ± 1.2 Ma Carnmenellis granite, west Cornwall, decrease radically from 130 ± 28 (2
σ
) Ma at surface to 26 ± 6 Ma at
c.
2500 m depth, a trend accompanied by a decrease in mean track length from 12.99 ± 0.80μm to 9.83 ± 0.62 μm. In contrast,
40
Ar–
39
Ar apparent age plateaux for muscovite decrease only from 288.5 ± 1.0 Ma at surface to 286.1 ± 1.0 Ma at
c.
2500 m depth. The small difference in the muscovite ages may record the presence of a slightly younger intrusive phase at depth, but it is apparent that the preserved upper 2.5 km of the pluton cooled rapidly through
c
. 320°C. However, thermal modelling of the AFT data delimits a distinct fast-cooling event at
c
. 137–155 Ma, exhibiting a satisfactory correlation with a major regional uplift episode in Late Jurassic/Early Cretaceous times recorded by the sedimentary basins flanking the Cornubian massif.
P-T
conditions in the South Wales Coalfield: evidence from coexisting hydrocarbon and aqueous fluid inclusions
Authors: ALDERTON D. H. M. , BEVINS R. E.
Published: December 2007

Siderite nodules in the Carboniferous Coal Measures of South Wales contain cavities which are often infilled with quartz, carbonates, sulphides, and hydrocarbons. The quartz contains a mixture of hydrocarbon and aqueous fluid inclusions. The aqueous fluid inclusions consist of a dilute brine (3 wt % NaCl equivalent) and have homogenization temperatures in the range 97–212 ºC (mean 143 ºC). The hydrocarbon fluid inclusions are dominated by methane with a small component of higher order hydrocarbons; their homogenization temperatures are in the range 35–78 °C (mean 54 °C). It is assumed that the two fluids were trapped simultaneously during growth of the quartz and thus
a P–T
estimate of entrapment can be obtained by graphical intersection of the hydrocarbon isochores and the aqueous fluid bubble point (homogenization) temperatures assuming hydrocarbon saturation. This method gives temperatures between 130 and 160 ºC, and pressures between 40 and 55 MPa. The timing of mineralization is uncertain, but it is suggested that it took place during burial and low grade metamorphism of the subsiding sedimentary basin (i.e. in the Upper Carboniferous). The hydrothermal fluids were probably derived from evolved meteoric or connate waters expelled during subsidence and sediment compaction.
A mid-crustal linked ductile thrust zone, Breaghy Head, NW Donegal, Ireland
Authors: JOLLEY S. J.
Published: December 2007

A discrete syn-metamorphic ductile thrust zone, in NW Donegal is documented. The imbricates link to a common detachment, have well developed ramp-flat geometries, and duplicate a relatively simple well defined stratigraphic package. Similar in scale and geometry to imbricate zones in foreland thrust belts, the ductile thrusts carry regionally significant displacement. Timing of imbrication with respect to development of major penecontem-poraneous ductile thrusts and folds, is consistent with a regional piggy-back northwest-propagating sequence of major syn-metamorphic (D2 age) structures in NW Donegal.
Strike-slip deformation in the Confidence Hills, southern Death Valley fault zone, eastern California, USA
Authors: DOOLEY TIM P. , McCLAY KEN R.
Published: December 2007

The Confidence Hills form a well exposed, composite, positive flower structure developed in Pliocene to Recent lacustrine and alluvial fan sediments. The structure has developed along the current trace of the southern Death Valley dextral strike-slip fault zone. California, USA. NW–SE-striking fault zones bound the Confidence Hills. In 3D, these fault segments are inferred to link at depth to a common basal fault system. The flower structure is formed by doubly plunging anticlines that roughly parallel the bounding fault segments. Fold development was aided by: (a) the presence of a basal salt deposit and numerous detachment horizons within the sedimentary pile and (b) buttressing and uplift along and against the bounding oblique-slip reverse faults. Structural and palaeomagnetic evidence indicates that the anticlines developed with axial surfaces parallel and sub-parallel to the trace of both fault zones. Folding appears to have initially developed adjacent to and above the southeastern fault segment and then propagated outwards and to the northwest. The latest displacements along the current southern Death Valley fault zone are probably only in the orders of hundreds of metres and as young as 0.9 Ma to Recent. This young fault zone is highly segmented along the length of the floor of southern Death Valley: the high proportion of oblique-slip faulting consistent with the strike-slip zone being immature. The Confidence Hills structure displays close geometric similarities with other natural examples of positive flower structures and to the features found in scaled. analogue sandbox experiments of strike-slip faulting.
Active tectonism in the intracontinental Middle Atlas Mountains of Morocco: synchronous crustal shortening and extension
Authors: GOMEZ FRANCISCO , BARAZANGI MUAWIA , BENSAID MOHAMMED
Published: December 2007

Geological field observations are integrated with digital topography, LANDSAT imagery, and earthquake focal mechanisms to investigate the Middle and Late Quaternary tectonism in the intracontinental Middle Atlas mountain belt in northern Morocco. The NE–SW-trending Middle Atlas Mountains, approximately 80 km in width and about 200 km long, are part of the Atlas system of northwestern Africa and represent an inverted rift that developed into an intracratonic mountain system in the foreland of the Alpine collisional zone. The Middle Atlas is composed of two provinces, the Folded and Tabular Middle Atlas, representing the palaeo-rift and a flank of the palaeo-rift, respectively. Evidence for Late Quaternary tectonism is provided by the analysis of stream morphology in addition to geological relations. Kinematic analysis of fault-slip data and earthquake focal mechanisms demonstrate the coexistence of both extensional and compressional deformation in different areas of the Middle Atlas with a common sinistral component of slip along NE–SW-striking fault zones. Compressional features dominate the Folded Middle Atlas, whereas extension predominates in the Tabular Middle Atlas. Extension is also manifested by widespread Middle to Late Quaternary alkali volcanism. The observed kinematic variations appear to correlate with the Mesozoic palaeogeography; one possible model may involve tectonic escape. This suggests that differences in the structures inherited from the Mesozoic and Palaeozoic may influence the responses of the different regions to the Cenozoic Alpine collision between Europe and northwest Africa.
Xenoliths of Iapetus Suture mylonites in County Down lamprophyres, Northern Ireland
Authors: ANDERSON T. B. , OLIVER G. J. H.
Published: December 2007

Two Caledonian lamprophyre dykes, intruding Upper Llandovery greywackes in the Central Belt of the Southern Uplands–Down–Longford Terrane on the coast of Down. carry numerous cognate and incognate xenoliths. The incognate xenolith population is dominated by schistose or mylonitic basic and intermediate metavolcanics, some of which are macroscopically banded and rodded. The geochemistry of one large xenolith is that of a calc-alkaline ensialic magmatic-arc basalt, remarkably similar to the basalts of the Borrowdale Volcanics. The xenolith has undergone pumpellyite–actinolite facies metamorphism in a deep-seated shear zone at depths between 7 and 30 km. These incognate xenoliths apparently provide the first direct geological evidence of the lithological nature of the unexposed basement of the Southern Uplands–Down–Longford Terrane. They offer strong support for the hypothesis that the Southern Uplands was underthrust. along the Iapetus Suture. by a basement of Lake District type. The surface interpreted as the Suture on the WINCH 2 seismic reflection profile inclines to the north to a depth of 21 km below the County Down lamprophyre outcrops. The lamprophyres have sampled both basement and Iapetus mylonites. One of the host lamprophyres has been dated by K–Ar hornblende at 415 ± 12 Ma. This implies a minimum of 30 km of underthrusting on the Suture before the end of the Silurian.
Palaeozoic pre- and post-collision calc-alkaline magmatism in the Qinling orogenic belt, central China, as documented by zircon ages on granitoid rocks
Authors: XUE F. , KRÖNER A. , REISCHMANN T. , LERCH F.
Published: December 2007

Based on large-scale reconnaissance mapping, we identified two calc-alkaline plutonic assemblages from the northern Qinling orogenic belt. central China. The older assemblage of intrusions. closely associated and deformed coevally with their host volcanic arc sequences, seems to represent the fractionation product of basaltic arc magma. It therefore predates the collision of the North China Block with the Central Qinling island-arc system that developed in a SW Pacific-type oceanic domain south of the North China Block. Single-zircon
207
Pb/
206
Pb evaporation dating yielded early to middle Ordovician ages for this assemblage. with a relatively small range from 487.2 ± 1.1 to 470.2 ± 1.3 Ma. Intrusions of the younger assemblage are largely undeformed and truncate structures shown in rocks of the older assemblage. They are interpreted as post-collisional calc-alkaline granitoids. Single zircon dating provided an age of 401.8 ± 0.8 Ma for the younger assemblage. consistent with earlier work that defines an age range from
c.
420 to 395 Ma. Our data favour a tectonic model involving formation and amalgamation of island arc and microcontinent terranes between ca. 490 and 470 Ma ago to create the Central Qinling Zone which subsequently collided with the North China Block prior to
c.
400 Ma ago. A late Precambrian age of 762.0 ± 0.7 Ma for a granitoid gneiss at the northern margin of the Yangtze Block supports a Gondwana affinity for this large continental block.
Dating and rhythmicity from the last deglacial cycle in the British Isles
Authors: McCABE A. MARSHALL
Published: December 2007

In northern Britain glaciotectonically thickened sediment piles represent the sediment flux transferred to ice margins by phases of fast ice flow and drumlin streamlining during the last deglacial cycle. Radiocarbon dating of marine faunas within deglacial muds age-constrain phases of drumlinisation (shaping) to between 19 and 14.5 ka
bp
. Individual phases of drumlinization can be correlated with millennial time scale discharge events documented from the circum-North Atlantic which are probably climate-driven. The overall deglacial cycle records at least five major millennial time-scale ice-margin oscillations and phases of sediment sequence rhythmicity.
3.5 Ga old terranes in the West African Craton, Mauritania
Authors: POTREL ALAIN , PEUCAT JEAN J. , FANNING C. MARK , AUVRAY BERNARD , BURG JEAN P. , CARUBA CHRISTIANE
Published: December 2007

Evidence for early Archaean relicts in the northern part of the West African Craton are provided from ion-microprobe (SHRIMP) U-Pb zircon and Sm-Nd whole-rock ages in the West Reguibat Rise of Mauritania. The U-Pb ages range between 3515 ± 15 Ma and 3422 ± 10 Ma for concordant zircons from an orthogneiss which has also been affected by a 2.7 Ga old metamorphic event. Nd model ages calculated for adjacent metasediments range between 3.9 and 3.5 Ga, suggesting that these supracrustal rocks may be derived from the erosion of a similar early Archaean crust. Zircon ages and Nd depleted-mantle model ages provide the first evidence for an early Archaean crustal growth, at least 3.5–3.6 Ga old, in the West African Craton.
High-precision U–Pb monazite geochronology of the
c.
806 Ma Grampian Shear Zone and the implications for the evolution of the Central Highlands of Scotland
Authors: NOBLE S. R. , HYSLOP E. K. , HIGHTON A. J.
Published: December 2007

Two pegmatites and mylonitic rocks from the Grampian Shear Zone yield U–Pb monazite ages of 806 ± 3 Ma, 808
–9
+11
Ma, and 804
–12
+13
, respectively. These confirm that shearing and pegmatite crystallization was essentially synchronous. Lower intercepts of
c.
440 Ma indicate disturbance during Ordovician reworking. These new data question the concept of a single latest Proterozoic–early Palaeozoic orogeny in the Grampian terrane and the significance of the Great Glen Fault.
The Ochil Fault and the Kincardine basin: key structures in the tectonic evolution of the Midland Valley of Scotland
Authors: RIPPON J. , READ W. A. , PARK R. G.
Published: December 2007

The Ochil Fault, one of the most prominent tectonic features in the Midland Valley of Scotland, juxtaposes Lower Devonian volcanic rocks against late Westphalian strata, implying a possible vertical displacement of up to 4 km. The Kincardine Basin in its hanging-wall, although actively subsiding during the greater part of the Silesian, trends generally N–S, perpendicular to the Ochil Fault, and its Silesian sedimentary record shows little sign of tectonic control by that fault. It is proposed that the Ochil Fault was initiated, possibly as a sinistral strike-slip feature, in the Devonian, but acted as a sidewall fault during the early evolution of the Kincardine Basin, attributed to extension on a hypothetical pre-Brigantian fault along the Bo'ness Line on the east side of the basin until late Namurian times, when active extension ceased. Both faults were probably buried during late Namurian and Westphalian times. Reactivation of the Ochil Fault during end-Carboniferous N–S extension, dated by quartz-dolerite emplacement, was responsible for probably at least 2 km displacement. including the presently visible footwall uplift. It is suggested that the hanging wall may have contained a Permian basin, now removed.
Late Precambrian tectonothermal evolution of the Malverns Complex
Authors: STRACHAN R. A. , NANCE R. D. , DALLMEYER R. D. , D'LEMOS R. S. , MURPHY J. B. , WATT G.R.
Published: December 2007

The Malverns Complex is a variably deformed and metamorphosed late Precambrian calc-alkaline plutonic suite exposed within the Avalon terrane of central England. Published U–Pb zircon and monazite ages indicate emplacement at
c.
680–670 Ma.
40
Ar/
39
Ar mineral cooling ages presented here place constraints on the timing of (1) upper greenschist to low amphibolite facies metamorphism and associated ductile deformation; and (2) late hydrothermal activity and variable thermal reactivation. Hornblende from meta-diorites records
36
Ar/
40
Ar v.
39
Ar/
40
Ar isotope correlation ages of
c.
649 Ma and
c.
652 Ma. These are significantly younger than the crystallization age of the complex and are interpreted to date cooling following metamorphism. They further provide maximum age constraints for the development of mylonitic fabrics that overprint metamorphic textures. Hornblende from a diorite net-veined by late granite pegmatites that are discordant to the mylonitic fabrics records a significantly younger isotope correlation age of
c.
610 Ma. This is interpreted to date a static thermal rejuvenation associated with pegmatite emplacement, and places minimum age constraints on the timing of mylonitization. A plateau age of c. 597 Ma recorded by muscovite from a greisen vein within a granite is interpreted to date closely the hydrothermal alteration of the complex. Detrital muscovite of probable metamorphic origin within a Lower Cambrian sandstone records a plateau age of c. 598 Ma. and may have been derived from local units which were first metamorphosed at
c.
650 Ma and later reheated at
c.
610–600 Ma. The
40
Ar/
39
Ar ages are consistent with episodic tectonothermal activity within the Malverns Complex during the interval
c.
650–600 Ma, and indicate a more complex Precambrian history than previously recognized for the Avalon terrane of southern Britain.
Geochemically unravelling the sedimentary components of Archaean metasediments from Western Australia
Authors: LEAKE BERNARD E.
Published: December 2007

Major and trace elements of metasediments of the granulite facies Narryer and the amphibolite facies Jack Hills metasediments have been used to devise plots which enable the original sedimentary components of these rocks to be unravelled, despite their metamorphic state. These plots overcome the predominant effect of quartz which ranges up to 100% and can be used to study other metasediment suites. Six components controlled the composition of the clastic metasediments (1) kaolinite, (2) illite-muscovite, (3) chlorite–montmorillonite, (4) dolomite, (5) quartz, (6) resistant heavy minerals such as rutile, zircon, chromite and monazite. Identifying the first four of these components enables the trace element composition associated with the components to be estimated. Feldspar was largely destroyed by chemical weathering but there is a suggestion that traces of it formed a minor seventh component in the Narryer rocks. There was little or no mixing of the original kaolinite-bearing and illite–muscovite-bearing samples, indicating two distinct sources, although both sets of samples contain >4 Ga zircons, suggesting a third zircon source. The old zircons may have been derived from re-worked sediment or metasediment as they occur in the matrix and in the quartzite clasts of conglomerates. Using Bhatia's criteria, a passive margin is the likely site of deposition. Calc alkaline tholeiites in the Narryer succession are intrusive and did not contribute to the sediments which were derived from extremely heavily weathered granites, komatiites, schists and reworked sandstones or quartzites.
Sediment recycling on Siluro-Devonian floodplains
Authors: MARRIOTT S. B. , WRIGHT V. P.
Published: December 2007

Some of the red mudstones in the Red Mark (Old Red Sandstone) of Llansteffan, Dyfed, are interpreted as originating as sand-sized mud aggregates (pellets) produced in soils and deposited from bed load in small sinuous channels. By analogy with the Channel Country of Australia such channels may represent periods of low discharge and reduced hydraulic efficiency when the floodplains were locally reworked. In contrast, larger channel-fills, with extraneous sediments, may represent pluvial periods with increased discharge.
Pb-isotope evidence for the role of Mesozoic basins in the genesis of Mississippi Valley-type mineralization in Somerset, UK
Authors: HAGGERTY R. , BUDD P. , ROHL B. , GALE N. H.
Published: December 2007

The Mendip–Bristol veinfield is one of a number of British Mississippi Valley-type deposits, the genesis of which has never been fully explained. The Mendip–Bristol Pb-Zn–Ba vein mineralization is hosted in Carboniferous, Permo-Triassic and Jurassic sediments. The metal sources involved during mineralization were investigated using Pb-isotope analysis of vein galenas.
206
Pb/
206
pb values range from 18 311 to 18.594,
207
Pb/
204
Pb values from 15.601 to 15.688 and
208
Pb/
204
Pb values from 38.257 to 38.565. The data show three isotopically different mineralizing fluids, which it is suggested reflect fluid expulsion from adjacent Mesozoic sedimentary basins, as a result of overpressuring due to rapid subsidence in the Triassic–Jurassic.
A sequence-stratigraphic interpretation of a mudstone-dominated succession: the Lower Jurassic Cleveland Ironstone Formation, UK
Authors: MACQUAKER J. H. S. , TAYLOR K. G.
Published: December 2007

Microfacies analysis (using optical and backscattered electron imagery) has been undertaken on 37 samples from the Cleveland Ironstone Formation, Staithes, Yorkshire, UK. Four lithofacies were encountered: clay-rich mudstones, silt-rich mudstones, sand-rich mudstones and concretionary carbonates (two types: berthierine/siderite-rich ironstones and apatite-rich concretionary horizons). Successive units were found to stack systematically into upward-coarsening units on a small (0.1–1.0 m) scale. Upward-coarsening and upward fining packages were also found on a larger (1.0–3.0 m) scale. The small-scale upward-coarsening units are interpreted to be parasequences; while the large scale upward-fining and upward-coarsening packages are interpreted to be retrogradational and progradational parasequence sets respectively. The ironstones occur at the level where the stacking patterns change from being overall progradational to retrogradational, while the apatite-rich concretionary units occur at the level where the stacking patterns change from being retrogradational to progradational. The concretionary carbonates are interpreted to have formed close to/at major stratal surfaces. The ironstones are interpreted to be either the correlative equivalent of sequence boundaries or part/all, of the forced regressive system tract; while the apatite-rich units are interpreted to have precipitated close to/at maximum flooding surfaces (condensed sections) and/or major transgressive surfaces.
Hydrogeochemistry as an aid to geological interpretation: the Glen Roy area, Scotland
Authors: EDMUNDS W. M. , KEY R. M.
Published: December 2007

The hydrochemistry of first and second order streams, sampled at a density of 1 km
2
, has been used to investigate ways of enhancing information obtained during regional geological mapping. The Glen Roy area of Scotland is a pristine area with large variations in bedrock geology, ranging from Dalradian metasediments (including carbonates), to granite, granodiorite and appinite rocks (coarse-grained lamprophyre) but with relatively minor drift.

Alkalinity (HCO
3
,) is used to determine the extent of water-rock interaction with both silicate and carbonate rocks. The ratio Na/Cl is also used to measure the extent of silicate reaction: dolomitic facies are picked out in areas with carbonate by Mg. Mn, Fe and Y are useful indicators of the weathering of biotite and garnet. A strong and well defined hydrogeochemical anomaly of Ba, Sr and K outline the outcrops of the appinites. Together with HCO
3
, these three elements define an extended NW–SE lineament (Strath Ossian Lineament) which includes possible concealed areas of appinite, the bicarbonate probably reflecting the presence of a carbonate aureole derived from mantle CO
2
. In particular the bedrock characteristics in poorly exposed ground can be inferred and information obtained on the volume of bedrock through which the water has passed.
Glaciation and sea-level change for Ireland and the Irish Sea since Late Devensian/Midlandian time
Authors: LAMBECK KURT
Published: December 2007

The sea-level change around the coast of Ireland and the Irish Sea for the past 20 000 years is primarily the combined result of the glacio-isostatic adjustment of the crust to the removal of ice over the British Isles and the total eustatic change from the global ice sheets. However, the isostatic effects due to the removal of ice from northern Europe and North America and the addition of meltwater into the oceans also make a significant contribution. Predictions of sea-level change, based on glacio-hydro-isostatic models are compared with observations to constrain the ice volume over Ireland at the time of the last glacial maximum and the maximum ice height appears to have been of the order of 600 m. The models predict well the spatial variability in sea-level change observed across the region for Holocene and Lateglacial time, with levels above present being predicted only for northeastern Ireland and north of about Morecambe Bay. The models do not support suggestions that Lateglacial levels along the east coast of Ireland or the coast of Wales were 50–150 m above their present levels. Consistent models that would produce such large Lateglacial highstands are incompatible with all other sea level and glacial evidence for the British Isles. Palaeobathymetry and palaeoshoreline reconstructions for the Irish Sea indicates that a tenuous landbridge between Britain and Ireland developed only across the Celtic Sea, between about 18000 and 14000 years
bp.
Sand body orientation, palaeoslope analysis and basin-fill implications in the Westphalian A–C of Great Britain
Authors: RIPPON J. H.
Published: December 2007

Channel orientation and palaeoflow analyses across the British Westphalian (Carboniferous) coalfields show that three main inflow directions dominated the palaeogeography from the mid-Langsettian to the mid-Bolsovian. In eastern Scotland and NE England, flow was mainly from the N and NE. In the Pennine Basin of north central England, flow was mainly from the W and NW, whilst in South Wales, main flow from the SW was compatible with a westerly system that also branched to give the northerly inflows recorded in the Culm Basin of southwest England. The depositional area included discrete tectonic depocentres, notably the Pennine Basin, which was traversed directly and tangentially by major channel belts. These basins were essentially incidental to the main channel pathways, but sand body characteristics vary according to basin location. A new depositional model for the post-rift Pennine Basin is proposed, relating sand concentrations and connectivities to the depositional and tectonic setting. The description of dominant SW to NE flow across South Wales through much of the Langsettian and Duckmantian, in contrast to the centripetal inflows previously proposed, implies some similar channel/depocentre relationships to those described here for the Pennine Basin.
A quantitative approach to soil occurrence in alluvial deposits and its application to the Old Red Sandstone of Britain
Authors: WRIGHT V. P. , MARRIOTT S. B.
Published: December 2007

Palaeosols (fossil soils) are a major component of alluvial deposits in the geological record. Not only do these typically show considerable variations in their degrees of development within single formations, but they also exhibit highly complex stratigraphic relationships with one another. A simple quantitative approach to these variations is presented which provides a graphical means of assessing differences in soil (palaeosol) development within a sequence with calcic or petrocalcic horizons. To illustrate the approach, a range of palaeosols from the Siluro-Devonian (Old Red Sandstone) of Britain are characterized in terms of the likely frequency and magnitude of depositional events which occurred during their formation. In particular, two alluvial suites from the Manorbier area of South Wales are analysed using this approach. The technique should be widely applicable to other alluvial successions and provides a means of graphicallv presenting relationships between multiple palaeosols, although the wide range of rates of formation of calcic and petrocalcic horizons precludes their use to provide narrow estimates of deposition rates.
SHRIMP constraints on sediment provenance and transport history in the Mesozoic Statfjord Formation, North Sea
Authors: MORTON ANDREW C. , CLAOUÉ-LONG JONATHAN C. , BERGE CATO
Published: December 2007

Detrital zircons in sandstones of the Statfjord Formation (late Triassic-early Jurassic) in the northern North Sea have been dated with the sensitive high resolution ion microprobe (SHRIMP). These sandstones lack detailed biostratigraphic control, but display distinct variations in heavy mineral assemblages which facilitate a three-fold subdivision of the sequence. The lower and middle zones (heavy mineral Zones A and B) have similar characteristics, both displaying a marked upward decline in garnet abundance which has proved difficult to attribute unambiguously to change in either provenance or sedimentation history. The base of the highest zone (heavy mineral Zone C) is marked by changes in several heavy mineral indicators attributable to a shift in ultimate provenance compared with the underlying zones.

The detrital zircon ages are a direct fingerprint of the sediment source terrains, and so resolve ambiguities in interpreting provenance from heavy mineral abundances alone. The age spectra indicate that both low-garnet and high-garnet parts of Zone A have the same ultimate provenance. This strongly suggests that garnet removal was not a function of changing provenance, but took place during the sedimentation process, probably by weathering during periods of prolonged alluvial storage. The source of Zone A is not represented in any presently exposed landmass: the required combination of Caledonian granitoids intruding or in immediate proximity to high-grade Archaean-sourced metasediments is likely to have been either to the west, in the northern part of the Shetland Platform, or to the north, in what is now the Norwegian Sea. Zircons in Zone C have a distinctly different age spectrum consistent with a source in the Western Gneiss Region of southern Scandinavia, where there is the required combination of Proterozoic protoliths and Caledonian reworking.
Pan-African rejuvenation of the
c.
1.1 Ga Natal Metamorphic Province(South Africa):K–Ar muscovite and titanite fission track evidence
Authors: JACOBS J. , THOMAS R. J.
Published: December 2007

K–Ar muscovite and titanite fission track analyses were obtained from selected lithologies from the Grenville-aged (
c.
1.1 Ga) Natal Metamorphic Province, SE Africa in order to constrain the low temperature cooling history of this critical part of Gondwana. The data revealed two periods of basement rejuvenation and exhumation at
c.
900 Ma and
c.
530 Ma respectively. Over large areas of the Natal Metamorphic Province, late-stage skeletal muscovite, dated at
c.
900 Ma. has overgrown the regional Mesoproterozoic fabrics. This indicates that, after magmatism and tectonics ceased at
c.
1000 Ma. The Natal Metamorphic Province underwent an early Neoproterozoic thermal/hydration event. Titanite fission track dates from the southern Natal Metamorphic Province scatter around 530Ma. showing that the basement finally cooled fairly rapidly through the300°C isotherm during Pan-African times. Since the Natal Metamorphic Province is unconformably overlain by clastic sedimentary rocks dated at c. 490 Ma. The basement must have been uplifted and exhumed by
c.
6–10km during the Pan-African event.Two possible uplift processes might best account for this Pan-African exhumation: Pan-African crustal thickening and subsequent uplift during the collision of east-and west-Protogondwana or magmatic underplating from hot mantle plumes. Although a combined collision-plume model cannot be discounted for western Dronning Maud Land, the Natal data favour the collision model, so it is proposed that the wider ‘Mozambique Belt’ of East Africa and East Antarctica generally resulted from classical Wison-cycle processes.
Are granitic intrusions scale invariant?
Authors: McCaffrey K. J. W. , Petford N.
Published: December 2007

Published length
(L)
and thickness
(T)
data on 135 laccolith and 21 granite intrusions define power-law relationships of the form
L=kT
a
typical of systems exhibiting scale invariant (fractal) behaviour. Both data sets are characterised by an exponent
a<
1 (0.88 ±0.1 for laccoliths and 0.80 ± 0.20 for plutons) that reflects an inherent preference for scale invariant tabular-sheet geometries. These power-law size relationships can be explained in mechanical terms by the need for an incoming magma sheet to travel laterally some distance before vertical thickening can occur. Sheet thickness is a function of available magma pressure which for an intrusion fed by a feeder dyke is proportional to the vertical magma transport distance.
Tectonic rotation within the British paratectonic Caledonides and Early Palaeozoic location of the orogen
Authors: PIPER J. D. A.
Published: December 2007

Magnetizations of Late Ordovician (Caradoc-early Ashgill) age in the paratectonic Caledonides of Wales and the Lake District are directed westerly and northerly respectively. They identify
c.
55°) of relative rotation of probable Acadian age. Southward subduction of Iapetus ocean crust occurred in Ordovician times beneath a latitudinally oriented orogen sited in mid-southerly latitudes with the Borrowdale and Snowdonia Volcanic provinces forming parallel fore-arc and back-arc lineaments respectively. Mid-Ordovician to mid-Devonian palaeomagnetic poles from the orogen correlate with contemporaneous Gondwana data to identify a former location sited west of the South American perimeter and not near to North Africa as assumed in previous reconstructions.
The role of basement reactivation in continental deformation
Authors: BUTLER R. W. H. , HOLDSWORTH R. E. , LLOYD G. E.
Published: December 2007

The papers in this thematic set arise from a discussion meeting sponsored by the Tectonic Studies Group and held at Burlington House on 6-7 March 1996. About 170 people heard 19 formal oral presentations and studied 17 posters of which 15 were briefly introduced with five minute talks. Sixteen of these contributions are published in this thematic set.

Pre-existing heterogeneities in the continental lithosphere are thought to influence markedly its response to subsequent deformation. A major reason for this thinking is the long held view (e.g.
Hills 1956
) that fault zones are inherently weak. Virtually all regions of continental lithosphere are riddled with faults and shear zones. Thus any subsequent deformation might be expected to utilize these pre-existing weak zones (as reviewed by

White
et al.
1986

). This type of reactivation, where faulting activity is localized onto pre-existing faults, can be contrasted with more general cases of deformation reworking broader volumes of lithosphere. There need not be complete reactivation of individual faults and shear zones. The distinc­tion is nicely illustrated by the concepts of basin inversion, a topic that has been well aired in recent years (e.g.
Cooper & Williams 1989
;
Coward 1994
). In many sedimentary basins there is evidence of contractional reactivation of segments of originally extensional faults (e.g.
Letouzey 1990
). Yet for the contrasting tectonic setting of collisional mountain belts, recent work has emphasized that, in general, thrusts do not reactivate pre-existing extensional faults (e.g.

Gillchrist
et al.
1987

). However, the sites of almost all the shortened continental
The recognition of reactivation during continental deformation
Authors: HOLDSWORTH R. E. , BUTLER C. A. , ROBERTS A. M.
Published: December 2007

Reactivation involves the accommodation of geologically separable displacement events (intervals >1 Ma) along pre-existing structures. The definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings. In neotectonic environments, reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime. This mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors. Four sets of criteria may be used to recognize reactivation in the geological record: stratigraphic, structural, geochronological and neotectonic. Some structural criteria may not be reliable if used in isolation to identify reactivated structures. Much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend, dip or three-dimensional shape of structures. Numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long-term and may be invoked to explain reactivation. The collage of fault-bounded blocks forming most continents therefore carries a long-term architecture of inheritance which can explain much of the observed complexity of continental deformation zones.
Shear zone reactivation at granulite facies: the importance of plutons in the localization of viscous flow
Authors: HANMER SIMON
Published: December 2007

Localization of flow in natural quartzo-feldspathic shear zones at granulite-facies temperatures does not occur purely by dislocation creep and dynamic recrystallization. From a review of some natural shear zones, initial faulting focused along a pre-existing shear zone, and/or the boundary of a volume of relatively strong crust or lithosphere, guides emplacement of hot mantle-derived melts into the base of the crust, leading to extensive melting. When crystallized, the sub-solidus plutons represent a focused zone of thermally softened material, capable of deforming readily and developing the full range of crystal-plastic microstructures and fabrics associated with mylonites, contrasting with the intrusion-free wall rocks.
Deep crustal and local rheological controls on the siting and reactivation of fault and shear zones, northeastern Newfoundland
Authors: D’LEMOS R. S. , SCHOFIELD D. I. , HOLDSWORTH R. E. , KING T. R.
Published: December 2007

The siting of a
c.
25 km wide, transpressive, high-strain zone at the eastern margin of the Gander Zone of NE Newfoundland corresponds to the trace of a fundamental contact between two Gondwanan basement blocks displaced sinistrally relative to one another during Silurian orogenesis. Changes in plate motion during the Devonian led to kinematic reversal and reactivation of the shear zone and, at high structural levels, the development of a major brittle-ductile fault system. At a local scale within the Silurian ductile high-strain zone, the focus of deformation and shifts in siting of shear were closely related to magma presence. We consider that granite magmas exploited shear zones within the crust to aid ascent, and in doing so enhanced local deformation. Cessation of magma supply and/or cooling of magmas within conduits caused deformation to relocate elsewhere. NE Newfoundland hence provides an example of how fault/shear zone siting and reactivation may be controlled by regional-scale pre-existing basement configuration and more localised processes affecting rheology, notably plutonism.
Direct field evidence for sinistral displacements along the Great Glen Fault Zone: late Caledonian reactivation of a regional basement structure?
Authors: STEWART M. , STRACHAN R. A. , HOLDSWORTH R. E.
Published: December 2007

The Great Glen Fault Zone is a major, subvertical, reactivated fault within the Scottish Caledonides. Post-Caledonian dextral movements of a few tens of kilometres have been demonstrated previously from the displacement of geological markers. Late Caledonian sinistral displacement has been generally inferred, but only by comparison with apparently related structures in the Central and Northern Highlands. Critical outcrops of low-middle greenschist-facies phyllonites, mylonites and cataclasites, particularly at Torcastle near Fort William, provide new evidence for the nature and sense of early displacements along the Great Glen Fault Zone. Shallow to moderately plunging mineral lineations and shear-sense indicators suggest sinistral displacements that are thought to be late Caledonian (c. 425-390 Ma) based on regional considerations. Although the Great Glen Fault Zone may at present separate contrasting basement blocks, there is no evidence that the Caledonian displacements were associated with the reactivation of an older Proterozoic structure.
Basement control on the location of strike-slip shear in the Southern Uplands of Scotland
Authors: STONE P. , KIMBELL G. S. , HENNEY P. J.
Published: December 2007

In the SW part of the Southern Uplands terrane the Moniaive Shear Zone has been superimposed above a geophysically defined basement discontinuity. Deformation was a protracted event, commencing in the early Wenlock and ending with granite intrusion at about 392 Ma. At surface the shear zone separates contrasting suites of granitic plutons. These have age diVerences and isotopic characteristics indicative of a lower crustal and possibly mantle break coincident with the crustal discontinuity deduced from geophysical modelling. It is proposed that sinistral reactivation of the basement structure caused the development of the shear zone within the overlying allochthonous thrust belt. This would require the basement structure to have been established beneath the developing Southern Uplands terrane by the Wenlock.
Late Carboniferous to Triassic reactivation of Variscan basement in the western English Channel: evidence from onshore exposures in south Cornwall
Authors: SHAIL ROBIN K. , ALEXANDER ANDREW C.
Published: December 2007

South Cornwall represents the westernmost part of the Variscan basement massif in SW England. Field-based investigations into the distribution, geometry, kinematics and relative chronology of post-Variscan deformation have been accompanied by preliminary palaeostress analyses. These indicate three episodes of basement reactivation: (1) Stephanian to Early Permian NNW–SSE extension, (2) Early to Late Permian strike-slip deformation, and (3) Late Permian to Triassic ENE–WSW extension. These data have been used to test and refine previously published models, based upon seismic and well data, of late Carboniferous to Triassic basement reactivation and sedimentary basin development within the Western Approaches Trough. We confirm that NNW–SSE regional extension and reactivation of ENE-striking Variscan thrusts had initiated by the latest Carboniferous, and that ENE–WSW regional extension and dip-slip reactivation of NNW-striking high-angle faults occurred during the Triassic. Permian strike-slip deformation recognized onshore may have contributed to the development of widespread unconformities within the oVshore basins.
Variscan compressional structures within the main productive coal-bearing strata of South Wales
Authors: FRODSHAM K. , GAYER R. A.
Published: December 2007

The structure of the main productive Coal Measures within the anthracite zone in the western part of the South Wales coalfield is re-examined using data from recently operational opencast coal sites. Variscan thrusts and/or folds and bed-parallel shear structures are present within all of the sites studied. At East Pit, unaffected by any large-scale zone of disturbance, bed-parallel shear structures are developed along many of the gently dipping coal seams, which form a series of simultaneously active detachments, representing a process of easy-slip thrusting. A NE-SW-striking, fold dominated disturbance at Gilfach Iago is interpreted to overlie a reactivated basement fault. A more complex disturbance zone at Ffos Las combines both E-W-trending folds and more variably oriented thrusts which together shorten the coal bearing strata by up to 2 km and this is interpreted to be the major Variscan frontal thrust ramp in the extreme west of the coalfield. The Variscan compressional structures within South Wales are not consistent with a simple thin-skinned thrust system. The strain induced by deep-seated Variscan thrusts was distributed throughout much of the coal bearing sequence by easy-slip deformation rather than being localized along a single regional detachment, and additional thrusts present around the northern margin of the coalbasin appear to have formed as isolated autochthonous structures. The inherent weakness of coal seams across South Wales is attributed to the presence of greatly increased fluid pressures, which would have been generated both by the maturing coal and by the influx of fluids along deep-seated disturbance zones.