bluuebunny/crossref_metadata_2025 huggingface dataset:
Basin evolution in the arc-arc Izu Collision Zone, Mio-Pliocene Miura Group, central Japan
Authors: SOH WONN , PICKERING KEVIN T. , TAIRA ASAHIKO , TOKUYAMA HIDEKAZU
Published: December 2007

A comparative study of the present collision zone between the Izu–Bonin Ridge (island arc) with mainland Japan (Honshu Arc), and the Mio-Pliocene of onshore SE Japan, suggests that arc—arc collision processes and the resulting stratigraphic successions may be repetitious and predictable. Arc—arc collision has led to the incremental accretion of segments of delaminated Izu—Bonin Arc crust onto the Honshu Arc, associated with the sequential southward migration in jumps of the plate boundary and trench. Prior to accretion of a segment of Izu-Bonin Arc crust, the leading edge underwent uplift to generate an approximately trench-parallel topographic high, the Zenisu Ridge being the present example with the Hayama-Mineoka uplift zone as a Mio-Pliocene example. The ridge separated a northern trench or trough from a southern intra-oceanic arc basin. During collision-accretion, the trench received both Honshu Arc-derived, terrigenous, and Izu-Bonin Arc-derived volcaniclastic, sediments, whereas the arc basin tended to receive only arc deposits. During the final stages of accretion, the arc basin began to receive ever-increasing volumes of terrigenous, Honshu Arc-derived, detritus fed through basement-controlled canyons. The accretionary process was accompanied by intense deformation and the residual deep-marine basin was then infilled above an angular unconformity.
Orogenesis in action: tectonics and processes at the west equatorial Pacific margin
Authors: HALL ROBERT
Published: December 2007

A three-day meeting, convened by R. Hall (University College London), G. J. Nichols (Royal Holloway and Bedford New College) and C. Rangin (Paris), was held at the Geological Society in London on 18–20 April 1990. The aim was to draw together workers from a variety of earth science disciplines who are studying or exploring the boundaries of the Australian and Eurasian Plates with the Pacific Ocean, and other scientists who are looking for modern analogues of older orogenic belts. Of the 124 participants, 58 came from Australia, China, France, Germany, Indonesia, Italy, Japan, Malaysia, Netherlands, Singapore, Taiwan, USA and USSR and 39 from industry. There were some 60 oral and poster presentations.

The first day focused on the Australian margin between Indonesia and New Guinea.
M. G. Audley-Charles
(UCL) discussed some practical problems of active orogenic belts; despite the youth (Pliocene and younger) of collison zones in such areas as Timor, there are still considerable difficulties in identifying sutures. Several speakers drew attention to the changing expression of arc–continent collision along the strike of the collision zone.
A. J. Barber
(RHBNC) described the structure of the Australia-Banda Arc collision zone where seismic records show mud diapirism in the 400 km wide forearc accretionary complex south of Sumba; further west on Timor the equivalent zone narrows to less than 150 km, and Australian margin sedimentary and basement rocks are stacked in a duplex.
R. A. Harris
(West Virginia) suggested that past disagreements in interpretation may be a consequence of a failure
The Ordovician history of the Iapetus Ocean in Britain: new palaeomagnetic constraints
Authors: TORSVIK T. H. , TRENCH A.
Published: December 2007

New late Tremadoc—early Arenig palaeomagnetic results from SW Wales imply that S Britain (part of Eastern Avalonia) occupied a southerly latitude of

c

. 60°S in early Ordovician times. When combined with Scottish Ordoviaan palaneomagnetic data, which indicate a 15°S latitude, the results indicate that the British sector of the Iapetus Ocean reached n latitudinal width of

c

. 5000 km in Tremadoc—Arenig times, which was reduced to

c

. 3000 km by Llanvirn—Llandeilo (mid-Ordovician) times.

The new data resolve two previous controversies in Palaeozoic palaeogeography. First, the high southerly palaeolatitude links Avdonia to Gondwana, marginal to W Africa, thus reconciling codlicting reconstructions based upon either palaeomagnetic or faunal/facies evidence alone. Second, reliable Llanvim palaeomagnetic data imply that Avlonia had rifted northwards by Arenig time, whereas Armorica remained proximal to northern Africa throughout the Ordovician. The combined data therefore establish that Avdonia and Armorica formed separate micro-continents when rifting from Gondwana.
A geological map of the southern Deccan Traps, India and its structural implications
Authors: MITCHELL C. , WIDDOWSON M.
Published: December 2007

A geological map of an area of over 125 000 km
2
within the southern Deccan Traps, India is presented, which is based upon variations in the basalt geochemistry. It represents a synthesis of new data and earlier work. The use of large data sets has allowed stratigraphic boundaries to be established with greater accuracy over a much larger area than before. The geochemical sections demonstrate that the southward overstepping stratigraphic style encountered along the Ghats ridge has an essentially north to south polarity and that there appears to be detectable overstep in an east to west sense. However, the stratigraphy defines a pronounced coastward-dipping monoclinal flexure across the Western Ghats ridge between 16°N and 18°N.

The data are consistent with two models of eruption. The first considers the Deccan eruptive focus to have been fixed throughout the volcanic episode, producing an essentially annular flow pattern. The second considers the flow pattern to have been generated from a southerly migrating eruptive centre, resulting from the northerly drift of India over a fixed hotspot. Both hypotheses are consistent with recent quantitative models which suggest that rapid and voluminous flood basalt eruptions are the result of continental rifting events associated with plume-generated thermal 'highs'. Until a causal link between established plume/hotspot trails and impact sites can be satisfactorily demonstrated, we find it unnecessary to invoke a cataclysmic impact event as an explanation for the Deccan eruptions.
Tethyan ferromanganese oxide deposits from Jurassic rocks in Hungary
Authors: CRONAN D. S. , GALÁCZ A. , MINDSZENTY A. , MOORBY S. A. , POLGARI M.
Published: December 2007

Fossil manganese nodules and encrustations from ‘seamount’ and ‘basin’ localities in the Transdanubian Central Mountains of Hungary are lithologically, mineralogically and chemically similar to some modern marine ferromanganese oxide deposits, and show no evidence of postdepositional changes other than cementation. Five groups of deposits were encountered: Fe/Mn nodules, encrusted shells, pavements, stains, and Fe oxide encrusted intraclasts, the first three of which are specific to the 'seamount' environment and the last to the ‘basins’.

Optical and electron microprobe investigation of the samples shows them to exhibit many similarities with modern ferromanganese oxide deposits, and that many of the nodules are surrounded by a halo of dispersed ferromanganese oxides, strongly suggesting that they continued to accrete metals through the pore waters of unlithified sediments for a period after burial. By contrast, pavements which appear to have grown on hardgrounds at the sea floor show little or no evidence of derivation of metals from underlying sediments.

Geochemical investigations on the deposits show the ‘seamount’ varieties to be closer in composition to most modern nodules and crusts than the ‘basin’ varieties, and that the latter are essentially manganese and trace-element-poor ferruginous deposits. Nevertheless, all can be more or less compositionally equated with modern ferromanganese oxide deposits forming in marginal Atlantic environments, which would be in accord with the proposed depositional environment of the Transdanubian Central Mountains based on other evidence
Chalk porosity and Tertiary uplift, Western Approaches Trough, SW UK and NW French continental shelves
Authors: HILLIS R. R.
Published: December 2007

Porosities have been derived from sonic logs for the Cenomanian–Maastrichtian Chalk of 22 hydrocarbon exploration wells in the Western Approaches Trough. Although the burial depth of the Chalk studied ranges from 40–1500 m below sea-bed, porosities are all within the range 0–45%. The most deeply buried Chalk sequences exhibit close to normal porosity/depth relations for offshore chalks. Abnormally low porosities in the remainder of the wells result from burial at greater depth than that presently observed. Low porosities were preserved during Tertiary, Alpine-related uplift of the area. For a given well the magnitude of uplift not reversed by subsequent subsidence (apparent uplift) equals the displacement of its porosity/depth relation from the normal relation along the depth axis. Apparent uplift in the Western Approaches Trough ranges from 0–1.3 km.

In the Western Approaches Trough, Chalk porosity data provide evidence of more widespread Tertiary uplift than do seismic reflection records. Alpine-related compression and uplift (fault-bounded basin inversion) is apparent only on seismic records in the southern part of the Trough (Brittany and South-West Channel Basins), whereas Chalk porosity data suggest regional uplift occurred throughout the Trough and adjacent highs.

Chalk porosity data were also used to quantify Tertiary uplift in the Cleethorpes-l well of the East Midlands shelf. The resultant value of 1.1km is considerably greater than that previously derived from shale sonic velocities (approximately 0.3 km) and slightly less than that based on apatite fission track analysis (1.3–1.7 km).
Tectonothermal chronology of early Cadomian arc development in Guernsey and Sark, Channel Islands
Authors: DALLMEYER R. D. , D’LEMOS R. S. , STRACHAN R. A. , MUELLER P. A.
Published: December 2007

The Cadomian orogen exposed in NW France and the Channel Islands comprises a collage of variably displaced terranes which record the late Proterozoic evolution of a continental margin within a complex subduction zone. On the islands of Guernsey and Sark, calc-alkaline quartz diorite stocks were emplaced during early stages of the Cadomian orogeny. Zircons from two stocks of deformed quartz diorite display evidence of inheritance in larger and more magnetic fractions. Upper intercept ages for these discordant concentrates are c. 1300 Ma. Sm-Nd whole-rock model ages (
T
DM
) suggest older ages of c. 2000 Ma. Conversely, the smallest and least magnetic zircons exhibit a distinct lead-loss pattern suggesting that the
207
Pb/
206
Pb dates of c. 700 Ma correspond to the time of crystallization of the stocks, and provide a lower age constraint for initiation of subduction-related magmatism in the Cadomian orogen. Hornblende concentrates from deformed quartz diorite stocks exposed on Guernsey and Sark display discordant
40
Ar/
39
Ar age spectra. Five samples record well-defined isotope correlation ages ranging between 596 Ma and 606 Ma. A hornblende concentrate from host orthogneiss (Sark) yields a plateau age of 608 Ma. The hornblende ages are interpreted as dating post-metamorphic cooling following late Proterozoic deformation and metamorphism within the arc system. Both magmatic and cooling ages from this early Cadomian arc contrast with those of younger initial tectonothermal activity (c. 570 Ma and c. 540 Ma) in other Cadomian terrane elements, and suggest palinspastic separation until very late stages of the orogeny. The polyphase nature of this late Proterozoic activity is similar to that of related orogens which probably occupied similar tectonic settings along the margin of Gondwana (e.g. SW Iberia, West Africa).
Ordovician bimodal volcanism in SW Wales: geochemical evidence for petrogenesis of the silicic rocks
Authors: BEVINS R. E. , LEES G. J. , ROACH R. A.
Published: December 2007

Rhyolitic lavas, pyroclastic, and high-level intrusive rocks form an important component of a bimodal tholeiitic basalt-rhyolite association of early Ordovician (Llanvirn) age exposed in North Pembrokeshire, on Ramsey Island, in the Fishguard area, and on Mynydd Preseli.

In the same region, a 400 m thick tholeiitic layered differentiated sill, the St David's Head intrusion, considered to be co-genetic with this association, comprises a series of units ranging from basic (olivine + orthopyroxene-bearing gabbros) through intermediate (granophyric gabbro) to acidic (aplite) compositions. Consideration of the major and trace element concentrations in these various intrusive units suggests that they are related largely through a crystal fractionation process. The principal fractionating phases were clinopyroxene and plagioclase feldspar, with olivine and orthopyroxene in the more mafic compositions, substantial Fe-Ti oxide and minor apatite in the intermediate compositions, and minor apatite and titanite in the most evolved compositions.

A comparison between the composition of a typical aplite from the St David's Head intrusion and compositions of the various silicic rocks from the bimodal suites in North Pembrokeshire suggests these latter rocks have also been derived from a more basic tholeiitic magma via crystal fractionation. Potential parental magmas are the various basaltic lavas and related doleritic intrusions of the Fishguard Volcanic Complex. However, certain silicic lavas and intrusions from the Porth Maen Melyn area appear not to have been derived from a tholeiitic parental magma, but rather have a calc-alkaline type chemistry, similar to older lavas of the Trefgarn Volcanic Group exposed a short distance to the south of Fishguard.

These findings support the contention that large volume silicic units in bimodal associations can be derived by crystal fractionation, despite the obvious geochemical discontinuity, and the apparently anomalous volume relationships commonly present.
Early Dinantian dolomites from East Fife: hydrothermal overprinting of early diagenetic fabrics?
Authors: SEARL A.
Published: December 2007

The dominantly siliciclastic early Dinantian succession of East Fife contains numerous thin carbonate horizons which have been extensively dolomitized. The carbonates are generally bioclastic wackestones with restricted molluscan faunas. The dolomite is fine-grained and homogeneous in backcattered SEM images. It generally contains 7–11 mol% FeCO
3
, and 49–52 mol% CaCO
3
, and has δ
l8
O = –6 to + l% (PDB) and δ
I3
C= –11.5 to 1.5% (PDB). Allochems within, and veins cutting, dolomite horizons are chemically indistinguishable from the matrix dolomite and there is little regional variation in dolomite composition. Sandstones in the study area also contain dolomite as cements which appear to be of similar age to abnormally abundant authigenic pyrite. The pyrite has replaced siliciclastic grains, filled intergranular porosity and also occurs as 2 cm thick crusts on bedding plane and joint surfaces. The pyritization is most pronounced within synclines. There is evidence elsewhere in Fife, that hydrothermal activity, related to Namurian volcanism, overprinted the chemistry of early mixing-zone dolomite in late Dinantian carbonate horizons proximal to exposed vents. It is much harder to prove a similar overprinting in the older carbonate horizons of East Fife, although the chemical similarity of vein and bulk rock dolomites and the massive pyritization of sandstones are compatible with a regional hydrothermal model.
On the trace of the Iapetus suture in Ireland and Britain
Authors: TODD S. P. , MURPHY F. C. , KENNAN P. S.
Published: December 2007

The Iapetus suture in Ireland and Britain is that line which separates Caledonian rocks of the Laurentian and Avalonian continents. The suture is cryptic: nowhere is there an exposed fault-zone containing ophiolite remnants, blue-schist melanges, or trench deposits. Instead a suture line may be traced with varying degrees of confidence through a series of faults that traverse the Iapetus suture zone, which contains two or more tectonostratigraphic terranes.

Several data sets are utilized to constrain this trace. The distribution of faunal provinces caused by oceanic separation can be used most reliably to define early Ordovician terranes. Fauna1 intermingling in the mid-Ordovician reduces provinciality and the confidence of terrane identification using faunal data. The late Ordovician and particularly Silurian tectonostratigraphic histories of terranes either side of the suture suggest that amalgamation of the terranes, elimination of the Iapetus Ocean and development of the suture zone had begun by the Ashgill. Therefore the resolution of the suture trace becomes unreliable where Ordovician rocks do not crop out, because Silurian turbidite fans of Laurentian provenance may have dispersed across a significant width of Avalonian crust. Similarly, the precision of structural identification of the suture zone, by correlating the suture with a major fold-facing confrontation, may be weakened by late overthrusting. Tectonic interleaving of crustal flakes in the zone is supported by geophysical evidence which suggests that some major boundaries in the lower crust do not always coincide with their inferred projections in outcrop. Tectonic and sedimentary mixing of crustal blocks and sediments in the zone is further reflected by Nd and Pb isotopic patterns; the Rb-Sr patterns of granites record a stronger distinction between north and south of the suture.

Despite these difficulties, we depict a possible suture trace in Ireland that departs significantly from the traditionally placed line by following a NE-SW-aligned trace through the Slane fault and the Navan-Tipperary lineament, and thence through a dog-leg to pass north of the Dingle Peninsula.
A mountain front model for the Variscan deformation of the South Wales coalfield
Authors: JONES JULIE A.
Published: December 2007

Access to the considerable geological database of British Coal, together with seismic evidence of subsurface relationships, has enabled the development of a structural model for the South Wales coalfield. Detailed site investigations are used as key examples. The simple broad synclinal nature of the coalfield masks a more complex structural system. Thrusting of Variscan age is largely confined to the Lower and Middle Coal Measures and throughout the central and northern coalfield is dominantly a forethrust system. However, along the south-crop a major backthrust system is developed which in places penetrates the Upper Coal Measures. The south-crop shows features analogous with a mountain front and is interpreted as a triangle zone, in which pre-existing basement faults have facilitated uplift.

The lack of thrusting within the Upper Coal Measures, in contrast to the underlying Lower and Middle Coal Measures, suggests they have acted as a passive roof to the thrust system. Geographical variation in the intensity of the deformation can be related to the proximity of, and buttressing against, the Caledonian massif. Although broadly analogous with current models of foreland basin development and deformation, the South Wales coalfield demonstrates the importance of pre-existing massifs and lineaments in determining the precise evolution of an area.
Assessing the carbon dioxide content of ancient atmospheres using palaeocalcretes: theoretical and empirical contraints
Authors: WRIGHT V. P. , VANSTONE S. D.
Published: December 2007

Recent suggestions that the δ
13
C composition of calcretes can be used to determine the
P
(CO
2
) of ancient atmospheres is critically evaluated. The problem of differentiating pedogenic calcretes from groundwater ones (which could not be used for
P
(CO
2
) determinations) is considered a critical problem. A basic tenet of the use of calcretes for assessing P(CO
2
) is that plants using C
4
or cras-sulacean acid metabolism photosynthetic pathways were unimportant until the Miocene. This view is questioned, and evidence is presented from Carboniferous calcified root mats that non-C
3
plants were probably present at this time, confirming speculations by palaeobotanists that crassulacean acid metabolism or C
4
photosynthesis may have arisen numerous times during the evolution of higher plants.
Tholeiitic dykes in the Seychelles and the original spatial extent of the Deccan
Authors: DEVEY C. W. , STEPHENS W. E.
Published: December 2007

We report geochemical data from
c
. Deccan age tholeiitic dykes from Praslin Island in the Seychelles, a continental fragment which lay adjacent to the coast of India before sea-floor spreading started in the Arabian Sea at 64 Ma. The dykes have compositions very similar to the distinctive Bushe magma-type from the Deccan, and hence constitute the first traces of Deccan activity recognized on the Seychelles.

Fitting the Seychelles and India back together to reconstruct the original extent of the Deccan, we find that the present estimate of 1 million km
2
needs to be revised upwards by
c
. 25% to account for the Deccan magmatism in the Seychelles. If dipping reflectors found on seismic lines to the northwest of the Seychelles are also Deccan age flows, this would increase still further the area and volume estimates.
Evolution of Lower Jurassic coastal plain and fan delta sediments in the Beryl Embayment, North Sea
Authors: RICHARDS PHILIP C.
Published: December 2007

Coastal plain and fan delta sediments accumulated throughout the Sinemurian to late Pliensbachian in the Beryl Embayment area, North Sea. These sediments represent a marginal, clastic fringe along the southern extension of the Boreal Ocean into the northern North Sea. Two informal formations are recognized in these predominantly sandy deposits. The lower formation comprises interbedded sandstones, siltstones and coals of minor fluvial channel and associated coastal floodbasin marsh, lake and sheetflood origin. The overlying formation represents a fan delta system, possibly generated as a result of footwall uplift and associated erosion. This formation is dominated by probable channel sandstones, with marine-influenced debris flow deposits developed at a number of levels indicating an active, coastal fan setting. Fan sediments are capped by a marine sandstone, beds of which interfinger with the overlying offshore siltstones. Thickness variations are related to major normal faults, suggesting at least a limited fault control on the deposition and distribution of facies during a time conventionally regarded as tectonically quiescent in the northern North Sea. There is no clear evidence for a eustatic control on sedimentation, although deposition of post-fan delta shallow marine siltstones may have been principally controlled by eustatic sea level rise recording the acme of the early Jurassic expansion of Boreal waters into the Viking Graben.
The lithostratigraphy, sedimentology and tectonic setting of the Monian Supergroup, western Anglesey, North Wales
Authors: PHILLIPS EMRYS
Published: December 2007

The Monian Supergroup of western Anglesey comprises three major lithostratigraphic units: the basal South Stack Group, the pelitic New Harbour Group, and the overlying Gwna Melange. The South Stack Group comprises a sequence of deformed turbiditic metasandstones, pelites and subordinate quartzites. A dominance of T
abcde
and T
bcde
, turbiditic units, high sandstone to mudstone ratio and the presence of coarsening and thickening upward sequences indicate that the South Stack Group was deposited in a sand-rich fan system. The quartzites form large-scale sand-bodies (10–25 m thick) and are interpreted as major channel-fill sequences that transported mature quartz-arenites from relatively shallow marine environments deeper into the basin, by-passing the inner to mid-fan environments. Whole-rock sedimentary geochemistry indicates that the South Stack Group was deposited in a continental margin setting. The New Harbour Group forms a monotonous sequence of highly deformed pelites, semipelites and subordinate volcaniclastic psammites which exhibit geochemical characteristics of sandstones deposited in a continental island arc setting. Petrographic data indicate that the New Harbour Group records the dissection and unroofing of a pre-existing andesitic arc complex. Limited palaeocurrent data obtained for the South Stack Group and facies changes within the New Harbour Group suggest that the Monian Supergroup was deposited within a NE–SW-trending basin. No evidence was found to suggest that the Monian Supergroup represents part of an accretionary prism, consequently the deposition of the Monian Supergroup within the trench of a Monian subduction system cannot be justified.
Dinoflagellate cyst biostratigraphy of the last 2.3 Ma from the Rockall Plateau, northeast Atlantic Ocean
Authors: HARLAND REX
Published: December 2007

The organic-walled dinoflagellate cyst record for Hole 552A of Leg 81 of the Deep Sea Drilling Project is extended to 2.3 Ma. Peaks of dinoflagellate cyst recovery are noted for isotope stages 19, 23, 25 and two unnumbered stages in Cores 7 and 8.
Impagidinium
cysts are persistently present throughout and especially prominent in the older sediments of Cores 7 and 8 where they are the characteristic component of the oxygen-light units. Younger dinoflagellate cyst assemblages from isotope stages 19, 23 and 25 characteristically contain
Nematosphaeropsis labyrinthus
(Ostenfeld) Reid,
Operculodinium centrocarpum
(Deflandre & Cookson) Wall and
Spiniferites
spp., and are comparable to those recovered from sediments deposited over the last 0.7 Ma. This correlation of dinoflagellate cysts with the oxygen isotope stratigraphy over the last 2.3 Ma may provide a reference section for the eastern North Atlantic Ocean on the western flank of the Rockall Plateau.
Devonian extensional tectonics versus Carboniferous inversion in the northern Orcadian basin
Authors: SERANNE M.
Published: December 2007

The Old Red Sandstone (Middle Devonian) Orcadian basin was formed as a consequence of extensional collapse of the Caledonian orogen. Onshore study of these collapse-basins in Orkney and Shetland provides directions of extension during basin development. The origin of folding of Old Red Sandstone sediments, that has generally been related to a Carboniferous inversion phase, is discussed: syndepositional deformation supports a Devonian age and consequently some of the folds are related to basin formation. Large-scale folding of Devonian strata results from extensional and left-lateral transcurrent faulting of the underlying basement. Spatial variation of extension direction and distribution of extensional and transcurrent tectonics fit with a model of regional releasing overstep within a left-lateral megashear in NW Europe during late-Caledonian extensional collapse.

Later inversion (probably during the Upper Carboniferous) is characterized by E–W to NE–SW contraction. It induced reactivation of extensional faults as thrusts, development of small-scale folds and thrusts, and right lateral transcurrent movement of the major faults such as the Great Glen and Walls Boundary faults.
Evolving alluvial stratigraphy and thrust front development in the West Jaca piggyback basin, Spanish Pyrenees
Authors: TURNER J.P.
Published: December 2007

In the southwestern Pyrenees, distinctive alluvial mega-sequences correlate with the development of a shallow, mountain front thrust structure, the Peña flexure. The flexure evolved during the Oligocene to early Miocene and is interpreted as a duplex marking the limit of thrust front advance. Three main depositional facies, preserved completely across the Peña flexure, are described: a pre-thrust emergence, high-sinuosity fluvial sequence (the
c.
2300 m thick Sheet Sandstone Facies of the Campodarbe Formation); a syn-thrust emergence, conglomeratic alluvial fan sequence (the
c.
1100 m thick Conglomeratic Facies of the Bernués Formation); and a post-thrust emergence, low sinuosity fluvial sequence (the > 700 m thick Ribbon Sandstone Facies of the Uncastillo Formation). Collectively, their continuous exposure across the Peña flexure provides a more or less complete record of the sedimentary evolution of a mountain front. Syn-deformational features in the alluvial fan sequence of the Bernués Formation and marked contrasts in palaeochannel morphology, sandstone body geometry and palaeocurrent patterns between the two fluvial sequences are best understood by invoking a strong tectonic control over their deposition. An increased rate of vertical sediment accretion was a key process in determining fluvial style. The onset of major thrusting led to more rapid basin floor subsidence, thereby promoting a change from lateral to vertical accretion as channels responded to the rising base level.
Expert Systems: can they aid geological interpretation and evaluation?
Authors: WADGE G. , VIDLER P.
Published: December 2007

This was a one-day meeting convened by P. Vidler and G. Wadge of the Geoscience Information Group at Burlington House on 20 February 1991. The aim was to provide a critical look at how valuable was the current and potential contribution of ‘expert system’ technology to geological problems. Ten papers were presented to an audience of 39.

In the first of four papers from oil industry applications,
D. Wolstenholme
(BP Research) gave a real time presentation of
BIOMARKER,
an expert system to evaluate depositional environment. The evaluation is performed from the knowledge of the molecular parameter interpretation technique used by geochemists on organic samples. The system is written in the Prolog language on a Macintosh and performs to 96% accuracy against a test data set. Another system concerned with applying expert knowledge to the interpretation of analytical data was discussed by
L. A. J. Garvie
(Bristol).
INTERSTRAT
aids the identification of interstratified clay minerals using X-ray diffraction data. The ambiguities and uncertainties in the data, with which the expert system approach is designed to cope, arise from poorly crystalline structure, crystal chemical variability and interstratification of structural layers. The knowledge used by the system embodies diffraction parameters for all the clay minerals and their variations with pretreatments.

A. C. Higgins & P. A. Swaby
(
BP
Research) jointly described a much more ambitious system than the previous two, a visual identification expert system
(VIDES)
for microfossils. Developed using the expert system shell
KEE
on a workstation,
VIDES
is intended as both
Does the Birchy Complex of Newfoundland extend into Ireland?
Authors: WINCHESTER J. A. , WILLIAMS H. , MAX M. D. , VAN STAAL C. R.
Published: December 2007

Metabasalts from the Birchy Complex in Newfoundland are chemically similar to the Kill-Callow suite of metabasalts in Co. Mayo, Ireland. The latter, which are also chemically akin to Cambrian amphibolites of the Highland Border Series of Scotland, lie along the shear zone associated with the Fair Head-Clew Bay line, which has been equated with the Baie Verte-Brompton line in Newfoundland. The Birchy Complex occupies an analogous tectonic position along the Baie Verte-Brompton line and hence we postulate that chemical similarities suggest a correlation between the Birchy Complex of Newfoundland and the Kill-Callow suite of Ireland.
Detrital magmatic muscovite from the Lower Carboniferous of southwest Ireland: buried granites uncovered?
Authors: CLIPSTONE D. , ROYCROFT P.
Published: December 2007

Lower Carboniferous limestones from the Kilmaclenine area, Co. Cork, contain magmatic muscovites, individual granitic minerals and granitic fragments. These constitute primary evidence for the presence of buried granites to explain various geological and geophysical observations in southwest Ireland.
Zircon U-Pb ages for the Early Cambrian time-scale
Authors: COMPSTON W. , WILLIAMS I. S. , KIRSCHVINK J. L. , ZICHAO ZHANG , GUOGAN M. A.
Published: December 2007

Single zircons from two Early Cambrian volcanic horizons have been analysed using the SHRIMP ion microprobe. Full details of the analytical procedures and data reduction are given. Zircons from tuff within the Lie de Vin Formation, near Tiout, Morocco, show little spread in U-Pb age and have a mean value of 521 ± 7 Ma (2σ). Those from a bentonite within unit 5 of the Meishucun section near Kunming, southern China, show relatively dispersed U-Pb ages, revealing the presence of both detrital or xenocrystic grains as well as areas within grains that have lost radiogenic Pb. The main population has a mean age of 525 ± 7 Ma, but a mean
207
Pb/
206
Pb age of 539 ± 34 Ma which is a maximum estimate for the bentonite age. These results conflict with previous Rb-Sr whole rock ages of
c.
580 Ma for overlying Cambrian shales at Meishucun, and
c.
570 Ma for Atdabanian shales from the E. Yangtse Gorges area.
Igneous emplacement in a transpressive shear zone: Ox Mountains igneous complex
Authors: MCCAFFREY K. J. W.
Published: December 2007

The central and southwestern Ox Mountains Inlier in western Ireland, comprises amphibolite facies metasediments and metavolcanic rocks which have been deformed by a major sinistral shear zone. Granitoid emplacement occurred in two distinct styles in the Ox Mountains shear zone. The older pluton, the Ox Mountains granodiorite was emplaced syn-kinematically with respect to the main sinistral, transpressional events in its wall rocks. It was emplaced as a series of compositionally distinct sheets in the central part of the shear zone system thus suggesting the formation of transient, dilational sites occurred in response to the transpressive deformation. The younger, Lough Talt and Easky Lough adamellites were emplaced as individual intrusions in dilational cavities produced by reactivated movements on a discrete zone within the Ox Mountains inlier. The different intrusive styles are considered to be a product of the tectonic regimes operating during emplacement. Intrusion into transient dilational cavities may be one method by which granitoid magmas are emplaced into fault zones undergoing active contractional deformation.
Palaeomagnetic analysis of fault gouge and dating fault movement, Anglesey, North Wales
Authors: HAILWOOD E. A. , MADDOCK R. H. , FUNG TING , RUTTER E. H.
Published: December 2007

The palaeomagnetic properties and magnetic fabric of clay-rich fault gouge in a prominent fault at Porth-y-pistyll on the northern coast of Anglesey, North Wales, have been investigated in an attempt to constrain the age of last movement on this fault. The texture and mineralogy of the gouge has been examined by scanning electron microscopy. The Porth-y-pistyll fault is one of a set of E-W-trending faults that cut metasedimentary rocks of the late-Precambrian/early-Cambrian Monian Supergroup on the island of Anglesey. A high-coercivity component of magnetization with a reverse polarity has been identified in the gouge. This is believed to be a chemical remanent magnetization resulting from fluid movement and oxidation which post-dated the last major movement on the fault. This strongly suggests that movement has not occurred during the present normal polarity chron, thereby establishing a
minimum
age of 0.73 Ma for the last movement on the fault. A well-defined planar magnetic fabric is present in the gouge and this corresponds in geometric orientation to a planar tectonic fabric observed from SEM studies. These fabric data provide additional information on the kinematics of motion on the fault.
Rates of delta progradation during highstands: consequences for timing of deposition in deep-marine systems
Authors: BURGESS PETER M. , HOVIUS NIELS
Published: December 2007

Estimated times required for 24 modern river systems to form a shelf-edge delta range from 8.5 ka to 116.5 ka, depending on fluvial sediment supply, delta width, shelf volume and shelf transport rates. These values indicate that transport of sand into deep-marine systems is likely to be significant during third-order highstands of relative sea-level. Factors such as shelf transport dynamics may slow delta progradation while submarine canyons cutting the shelf may reduce the time before deep-marine deposition occurs. Interpreting ancient sand-rich deep-marine strata as lowstand deposits without sufficient palaeogeographic information may not therefore always be appropriate.
Amino acid geochronology of Pleistocene marine sediments in the Wanganui Basin: a New Zealand framework for correlation and dating
Authors: BOWEN D. Q. , PILLANS B. , SYKES G. A. , BEU A. G. , EDWARDS A. R. , KAMP P. J. J. , HULL A. G.
Published: December 2007

Amino acid analysis of indigenous protein preserved in fossil marine bivalves from Plio-Pleistocene sediments in the Wanganui Basin, North Island, New Zealand has provided an aminostratigraphy. D-alloisoleucine/L-isoleucine (D-aIle/L-Ile) ratios were measured in the venerid marine bivalves
Tawera spissa
and
Austrovenus stutchburyi
that show progressively increasing D-aIle/L-Ile ratios with increasing stratigraphical age. When calibrated by magnetostratigraphy, fission-track dating, radiocarbon dating, and coccolith biostratigraphy, these relative D-aIle/L-Ile ages range up to
c.
1 Ma. The aminostratigraphy provides a framework of correlation and dating for other sites in the New Zealand area where other methods of dating are unavailable, as well as facilitating correlation with oxygen isotope stratigraphy. This is the first calibrated aminostratigraphical sequence from a long sequence of sediments that cover a significant proportion of the Pleistocene.
The coastal flexure of Disko (West Greenland), onshore expression of the ‘oblique reflectors’
Authors: GEOFFROY L. , GELARD J. P. , LEPVRIER C. , OLIVIER P.
Published: December 2007

There is a clear analogy between the coastal flexured basalts of the Disko Island (W Greenland) and the offshore oceanward-dipping reflectors otherwise observed in the volcanic margins of the Thulean Province. Our tectonic analysis suggests that the coastal flexure is of both syn-magmatic and tectonic origin. It is contemporaneous with dyke injection parallel to the axis of the flexure and to an increase in the extension across the area during the Palaeocene. The proposition that the coastal flexure of Disko Island is syn-tectonic and syn-magmatic throws new light on its origin. We propose that this flexure and, by extrapolation, part of the plume-related seaward-dipping reflectors elsewhere, may correspond to volcanic layers forming a roll-over anticline overlying a continentward-dipping detachment.
Shale tectonics and deformation associated with active diapirism: the Jerudong Anticline, Brunei Darussalam
Authors: MORLEY C. K. , CREVELLO P. , AHMAD ZULKIFLI HAJI
Published: December 2007

The Jerudong anticline provides an outcrop example of multiple phases of diapir growth and interaction with the country rock. The N–S-trending anticline overlies a high-angled basement fault zone that was episodically active, moving predominantly sinistrally in response to transpressive stresses. The earliest stage of deformation in the study area was mid-Miocene E–W to NE–SW-trending growth faulting and shale-diapir growth. Early normal faults exerted an important influence on the large- and small-scale bedding geometries and facies changes of syn-tectonic shallow-marine, shoreface and tidal strata. In particular there are laterally rapid facies changes and differential rotation of strata across growth faults. Diapir activity is indicated by the presence of shale dykes which commonly intrude normal fault planes, and concordant shale intrusions. Shale intrusions, associated minor thrust faults and normal faults were rotated during a late Miocene–early Pliocene phase of folding related to transpression. Few shale dykes were intruded during this phase, probably because the horizontal principal stresses were relatively large compared with the pore-fluid pressure. At the end of the transpressive phase, shale dykes were intruded into steeply dipping beds as a result of stress relaxation. Continued uplift and erosion elevated overpressured horizons to a point where hydraulic fracturing reached the surface and Holocene-age mud volcanoes were developed.
Embaluh Group turbidites in Kalimantan: evolution of a remnant oceanic basin in Borneo during the Late Cretaceous to Palaeogene
Authors: MOSS STEVE J.
Published: December 2007

Turbidites crop out in large parts of NW Borneo, including Kalimantan and Sarawak. The strata predominantly comprise the Embaluh Group in Kalimantan and lateral equivalents the Rajang Group in Sarawak (collectively known as the Rajang–Embaluh Group). Previous interpretation of these rocks as the product of deep marine deposition and deformation as part of an accretionary prism during Late Cretaceous to Palaeogene subduction implies south-dipping thrusts, northward stratigraphic younging, the existence of a complete arc to trench system, and significant deformation and metamorphism of the turbidites. However new fieldwork described here established consistent southward stratigraphic younging in Kalimantan, no evidence for southward-dipping thrusts and a lack of metamorphism and associated accretionary complex-related deformation features. Furthermore all available palaeontological and radiometric age data indicate that deposition of the bulk of the Rajang–Embaluh Group postdates in-board subduction-related magmatism and thus a complete arc-trench system cannot be demonstrated. Consequently the Rajang–Embaluh Group turbidites are re-interpreted as having formed in either a post-collisional foreland basin or a remnant ocean basin. The lack of a identifiable mountain belt and linked thrust system, and probable oceanic aYnity of crust beneath the Rajang–Embaluh Group basin favour the latter interpretation.

Borneo occupies a pivotal position within southeast Asia. This new model of a remnant oceanic basin setting for extensive turbidite units of Borneo therefore has far-reaching implications for the tectonic evolution of Borneo, the South China Sea and SE Asia. The new model is compared with older, Palaeozoic orogenic belts which also contain thick turbidites sequences.
Evolution of the NW margin of the North Permian Basin, UK North Sea
Authors: CLARK J. A. , STEWART S. A. , CARTWRIGHT J. A.
Published: December 2007

Mapping of depositional zones within the Zechstein Supergroup using semi-regional 2D and 3Dseismic data constrains the geological evolution of the northwestern margin of the North Permian Basinin UK offshore Quadrants 14, 15, 19, 20 and 21. Four depositional zones are identified on the basis of mappable seismic facies, which are assumed to vary with halite content. Well control characterizes the most reflective seismic facies as an interbedded carbonate and anhydrite ‘shelf’ succession which grades laterally through clastic-rich sequences to the least reflective, a thick halite basinal equivalent. The trendsof the depositional zones illustrate the geometry of the North Permian Basin and may have been locally controlled by basement lineaments. Within the North Permian Basin a N–S-trending axis (Devil’s Hole High) characterized by thick reflective facies separates the Central North Sea from the Forth Approaches Basin. Basement fault blocks control facies type and thickness along the northwest margin of the North Permian Basin in the Moray Firth area. Permian tilt of the Zechstein shelf related to regional flexure played a role in precipitating gravity-driven tectonics within the seismically reflective facies on the east flank of the Devil’s Hole High. This Permian phase of salt tectonics resulted in numerous intra-Zechstein structures which can be interpreted as rafted and folded zones of interbedded halite, carbonate, sulphate and clastics. The ‘rafts’ initially slid downdip towards the main halite basin then their evolution became dominated by differential subsidence of the interbeds which forced salt layers into adjacent passive diapirs. This episode predates deposition of the Triassic Smith Bank shales which rest upon the Zechstein with local angular unconformity. The spatial distribution of Zechstein facies influenced the tectonostratigraphy of the Triassic, Jurassic and Cretaceous systems. Boundaries between Zechstein facies correspond to the limit of Triassic halokinesis on the West Central Shelf and the limit of detachment of Jurassic and lower Cretaceous thin-skinned tectonics in Moray Firth half graben.
Granitic magma dynamics
Authors: CLEMENS J. D.
Published: December 2007

Of what importance
is
granite? If we take an imaginary journey through our solar system we can visit terrestrial planets like Venus and Mars, our Moon and various rocky satellites orbiting the outer gas giants. In all of these places there are marvels and ‘geological’ wonders, but none of these worlds has any granite. Earth is unique in our solar system in having these rocks—the fundamental stuff of continents. Another unique aspect of Earth is its combination of active plate tectonics with a 70% covering of liquid H2O. It is also unique (as far as we know) in supporting DNA-based life, notwithstanding recent descriptions of what may be Martian nanobacterial fossils. Is it fantastic to suggest that all of these unique features might be linked in some way? If Earth’s mantle did not convect and there were no spreading ridges, we would not have plate tectonics. If the ridges were not covered in water, there would be no hydrothermal ridge processes; sea-floor basalts would not undergo alteration to hydrous minerals like clays and chlorites. If altered basalts had not formed in the Archaean then the first voluminous granitic magmas could not have formed by partial melting of these materials. If the early granitic rocks had not formed then ‘indestructible’ continental masses would not have grown. Although the distribution and elevation of continents plays a role in climate control, their role in the evolution of life is unclear. However, if granitic continents had not existed, we would not be able
Observations on the origins and ascent mechanisms of granitic magmas
Authors: CLEMENS J. D.
Published: December 2007

This review presents a summary of most current beliefs about the origins of granitic magmas, together with calculations of some important properties of granitic melts and magmas. Turning attention to magma ascent mechanisms, diapirism and dyking represent two competing end-member models. The differences in processes and products between the two are fundamental, and the implications of the differences are profound for the evolution of the continental crust.

The weight of evidence favours dyke ascent for the majority of granitic magmas, and modelling shows that dyke ascent is both rapid and thermally effcient. Furthermore, the structural and petrological features expected in crust through which granitic diapirs have passed have not been discovered in nature. Granites in the upper crust commonly have forms and structures indicating that diapirism could not have been the emplacement mechanism. In contrast, granitic dykes are common, suggesting that it is reasonable to consider the ascent of granitic magmas through fracture systems. Many granitic batholiths are tabular and some have been fed by large dykes. The rate-limit in granitic plutonism is not magma ascent, but either the rate of melting and segregation (near the magma source) or the rate of deformation (at the emplacement site)