bluuebunny/crossref_metadata_2025 huggingface dataset:
Pressure solution in nature, theory and experiment
Authors: Rutter E. H.
Published: December 2007

Some of the features of water enhanced deformation of rocks by diffusive mass transfer (pressure solution) in nature which are pertinent to the rate controlling mechanism of the deformation are reviewed, and it is inferred that (a) the diffusion of matter in an aqueous intergranular film which can support shear stress is an essential part of the process, and (b) the diffusion is driven by stress induced chemical potential gradients, together with gradients due to local chemical reactions.

The theoretical approach to the derivation of constitutive flow laws for creep by diffusive mass transfer is outlined, and a simplified flow law proposed. Crucial to the absolute rate of deformation predicted by the flow law is the estimation of the phenomenological coefficient which links diffusive flux to chemical potential gradient. It is argued that this should be several orders of magnitude less in thin, stressed aqueous films than for solutions of ions in large water volumes.

Some simple experiments are described to address the question of (a) the existence of thin intergranular aqueous films which can support shear stress, and(b) the magnitude of the above phenomenological coefficient. The results obtained are consistent with the inferences made from the study of microstructures in naturally deformed rocks, and this is illustrated by means of extrapolation of theoreticallv derived relationships to conditions of natural rock deformation and sediment compaction by pressure solution.
Clay mineral assemblages of the Townsend Tuff Bed (Lower Old Red Sandstone), South Wales and the Welsh Borders
Authors: Parker A. , Allen J. R. L. , Williams B. P. J.
Published: December 2007

Suites of samples from the Townsend Tuff Bed possess widely varying clay mineralogies, which may be divided into 5 major assemblages showing a general increase in diagenetic grade (from smectite/illite/kaolinite to chlorite/illite) to the W. This trend appears to be related to depth of burial. fold style and proximity to basement. Illite crystallinity closely parallels cleavage development.
Nature and origin of layering in the diorites of SE Jersey, Channel Islands
Authors: Bishop A. C. , Key C. H.
Published: December 2007

The Cadomian diorites of SE Jersey contain two principal types of layering:
primary gabbroic layering
, formed as igneous cumulates in a gabbroic magma, and
differentiated diorite sheet layering
, which developed from gabbroic layering during metasomatic alteration of gabbro to diorite, probably by hydrous fluids associated with several pulses of granite emplacement. Primary gabbroic layering is preserved only in a small area near Le Nez and, though only a small proportion of relict grains of original pyroxene and calcic plagioclase remain, many of the textures and structures typical of primary layering are still preserved. The differentiated diorite sheets have the same orientation as the primary layers and each grades upwards from meladiorite to leucodiorite or quartz diorite. Intrusive structures at sheet contacts indicate that they, like the gabbroic layers, formed essentially horizontally, though they are now inclined NE at up to 65°, probably as a result of block rotation in granite. Chemical variation within the differentiated sheets suggests that there has been considerable modification of the original gabbro. The most basic part of the sheets occurs several tens of centimetres above the base because of reaction across the quartz diorite-meladiorite boundary. The layered diorites, and probably much of the homogeneous diorites as well, have originated by metasomatic alteration of layered gabbro, rather than having formed as cumulates in a primary dioritic magma.
A palaeomagnetic study of Jurassic rocks from the Antarctic Peninsula and its implications
Authors: Longshaw S. K. , Griffiths D. H.
Published: December 2007

Palaeomagnetic measurements have been made on rocks of Jurassic age from the Antarctic Peninsula to determine whether there has been relative movement between the region and East Antarctica. Samples were collected from the Oscar II Coast and the Cole Peninsula. Reliable results have been obtained from Jurassic dykes dated in one area at 174 ± 5 Ma, from an acid pluton of approximately the same age and from lavas of middle Jurassic age. Combining these results a position for the palaeopole was obtained at 48°S 238°E. If a reconstruction of Gondwana consistent with sea-floor spreading, palaeomagnetic and geological data is adopted, such as that of Norton & Sclater (1979), then a well-determined position for the Antarctic Peninsula within the reconstruction is obtained. Space considerations suggest that during the Jurassic the relative positions of the Antarctic Peninsula and the Ellsworth block were not very different from the present day, though palaeomagnetic measurements (Watts & Bramall 1981) indicates that the orientation was different.
The
40
Ar/
39
Ar age spectrum of a rock from Gerrans Bay, Cornwall
Authors: Fitch F. J. , Forster S. C. , Miller J. A.
Published: December 2007

The results of the
40
Ar/
39
Ar step heating study of a folded and cleaved metasiltstone from Malmanare Point, Gerrans Bay, S Cornwall are presented. The rock comes from the mélange of the Roseland area, NE of the Lizard. The age spectrum obtained has two distinct sectors: (i) an older age component giving apparent ages of 521–490 Ma and (ii) a younger component with an apparent age of 424–410 Ma. The petrography suggests that the former is derived from the more resistant parts of large detrital muscovite flakes, while the latter comes from smaller metamorphic micas aligned along the prominent cleavage and also, in part, from original detrital grains partially or totally overprinted during the metamorphic event. The sediment was probably mid- to late-Ordovician in age. The source rocks appear to have been very largely of late Cambrian/early Arenig age. Folding, cleavage and low-grade regional metamorphism occurred as a result of late Caledonian orogenesis between 424–410 Ma. There is no evidence of any Variscan overprint on the age spectrum. Very minor, low energy argon losses appear to have taken place in mid-Mesozoic times (possibly related to regional mineralization) but, apart from this, the absence of any major overprint younger than 410 Ma indicates that since the end of the Silurian the rock has suffered only high-level, cold, non-penetrative deformation.
Geochemical soil surveys as an aid to mapping and interpretation of the Lizard Complex
Authors: Smith K. , Leake R. C.
Published: December 2007

The results of a geochemical soil survey have been used to refine the geological mapping and interpretation of the inland, poorly exposed part of the Lizard Complex. Using cluster analysis, six major groups were recognized within a data set comprising 619 samples analysed for Cr, Co, Ni, Cu, Zn and Pb. These represented ultrabasic rocks, gabbroic rocks, two distinct units of Landewednack hornblende schist, metalliferous mineralization, and soils dominated by exotic material such as löess. The geographical distribution of these groups indicates that a lower Landewednack unit can be recognized in the S and W, separated by a thin transition zone from the main Lizard peridotite. This contrasts with its NE contact which is marked by a wide transition zone of soils suggesting interdigitating gabbroic and ultrabasic compositions, corresponding to the Traboe cumulate complex (Leake & Styles 1984), and a sharp contact with an Upper Landewednack unit. Both soil and deep auger sampling indicate that ultrabasic rocks underlie much of the area previously mapped as a tongue-like SW extension of the Crousa Gabbro.
Displacement-distance methods in the analysis of fold-thrust structures and linked-fault systems
Authors: Chapman Timothy J. , Williams Graham D.
Published: December 2007

A new method for analysing the internal strain in thrust sheets is described. The displacement-distance diagram can be used to quantify the relative shortening (or relative stretch) between the hangingwall and footwall beds of a thrust and is a useful way of analysing commonly occurring fold-thrust structures. The stretch is a function of the relative rates of thrust propagation and thrust slip. The cumulative displacement-distance diagram described here can be used to calculate the absolute shortening or extension, when this is accommodated above a single décollement in a linked-fault system. This method offers a useful way of analysing duplex structures and large-scale, foreland thrust zones, as well as extensional fault systems and can be used as an alternative to, or in conjunction with, balanced cross-sections. Both methods can be used to quantify the variation in shortening above a single detachment zone; this variation may be mechanically significant, reflecting lithological control on thrust or normal fault growth.
The ending of the Caledonian orogeny in Scotland
Authors: Watson Janet
Published: December 2007

The geological evolution of the metamorphic Caledonides during the 100-million-year period from the phase of crustal thickening initiated by the Grampian orogeny to the emplacement of late granites at
c.
400 Ma is reviewed. Shortening and westward transport associated with the Grampian orogeny were diachronous; deformation began in the weak Dalradian basin and extended north-westward into the area of crystalline Moine rocks. An important change in tectonic regime, coinciding roughly with the closing of the Iapetus Ocean, was recorded at
c.
425 Ma by the onset of sinistral transcurrent motions on NE–SW lines parallel to the Caledonian belt. Regional uplift of the thickened crustal welt, accompanied by deep erosion, led to the removal of 20–30 km of overburden from the Highlands, mainly in Ordovician and Silurian times. Known Lower Palaeozoic deposits of Scotland cannot account for the volume of sediment so produced and it is suggested that other basins were located in a major transform zone beneath the northern North Sea.

The onset of late Caledonian magmatism coincided with the start of the post-collisional regime of strike-slip motion. The distribution of granites and appinites was influenced by transverse (NW–SE) dislocations in the lower crust and the emplacement of individual intrusions was often fault-controlled. The deep transverse lineaments acted as conduits for both sub-crustal and crustal melts over periods of up to 25 million years. It is suggested that mantle rocks metasomatized by fluids generated at an underlying subduction zone were the main source of the sub-crustal melts, and that magmatism was triggered from above by vertical and horizontal block movements.

This paper is a contribution to IGCP Project 27.
Continental crustals regimes as factors in the formation of sedimentary ore deposits
Authors: Watson J. V.
Published: December 2007

Although the formation of metalliferous mineral deposits by processes active in sedimentary basins depends ultimately on physical and chemical conditions at the site of deposition, the style, scale and timing of mineralization are influenced by tectonic and magmatic events, which relate to the evolution of the crustal environment as a whole. During the Archaean era, when geothermal gradients were high and igneous activity widespread, sedimentary processes played little part in mineralization. The development of cratonic crustal massifs at the beginning of the Proterozoic era was followed by an increase in the variety and size of deposits formed in sedimentary basins. From this time onward the cratonic cover formations have provided important sites for the deposition of metals. These metals entered the basins from a variety of sources, some being derived from weathering and erosion of the basement and others being transferred to surface levels by magmas and volatiles originating in the mantle or lower crust. It is suggested that the timing of mineralization is related to that of the major magmatic and tectonic happenings determined by the global tectonic regime.
Can the Elliot Lake uraninite-bearing quartz pebble conglomerates be used to place limits on the oxygen content of the early Proterozoic atmosphere?
Authors: Robinson Andrew , Spooner Edward T. C.
Published: December 2007

The ways in which evidence derived from the mineralogy of uraninite-bearing quartz pebble orthoconglomerates has hitherto been used in attempts to determine the oxygen content of the early Proterozoic atmosphere are critically reviewed in the light of recent studies of the conglomerates of the Quirke Ore Zone, Elliot Lake, Ontario, Canada (
Robinson 1982
; Robinson & Spooner 1982 Robinson & Spooner in press). The presence of detrital uraninite and pyrite in the ores is not, as once was thought, evidence for a 'reducing' atmosphere because persistence of these minerals as detrital phases has been documented in recent sediments of the Indus fluvial system. Theoretical models of the kinetics of uraninite dissolution are hard to interpret quantitatively in terms of atmospheric partial oxygen pressures because of the large number of assumptions that need to be made. Post-depositional uranium mobility, which has been conclusively documented at Elliot Lake and in Upper Witwatersrand ores, does not necessarily indicate the presence of free-oxygen-rich groundwaters, even if transport was as hexavalent species. In addition, post-depositional uranium mobility was a diagenetic, groundwater-mediated process. Hence, while the chemistry of groundwaters may be inferred from mineralogy, that of surface waters, and by extension that of the atmosphere, may not, because surface and groundwaters are not necessarily in mutual chemical equilibrium. The same restriction applies to the analogous use of the diagenetic mineralogy of any rock type. Sulphur isotope systematics of pyrite concentrates from the Elliot Lake and Upper Witwatersrand ores do not unequivocally demonstrate the involvement of sulphate reducing bacteria or, thus, the presence of aqueous sulphate. The presence of sulphidized limonitic pebbles in Upper Witwatersrand ores can only place an extremely low, lower limit on the oxygen content of the atmosphere at the time.

It is concluded that it is by no means as straightforward a matter as has hitherto been assumed to make deductions concerning the oxygen content of the early Proterozoic atmosphere from evidence provided by either the detrital or Secondary mineralogy of uraninite-bearing quartz pebble orthoconglomerates, and that, to date, the problems involved have not been sufficiently considered.
Early atmospheric oxygen levels: constraints from Archaean photoautotrophy
Authors: Schidlowski Manfred
Published: December 2007

Various independent lines of evidence suggest that photoautotrophic carbon fixation is a very ancient process that had attained control of the terrestrial carbon cycle from at least 3.5 Ga (if not 3.8 Ga). Since it is generally accepted that only biological processes could substantially increase the negligible oxygen pressures sustained on the early Earth by inorganic photochemistry (probably between 10
–8
and 10
–14
present atmospheric level (PAL)), the advent of photoautotrophy would provide a limiting condition for the oxygenation of terrestrial near-surface environments. However, as the size of the free oxygen reservoir is determined by the kinetics of the geochemically relevant oxygen-consuming reactions, we may assume that the start of photosynthetic oxygen production did not result in an instantaneous buildup of modern P
O2
levels. Low atmospheric oxygen pressures generally inferred for the Archaean and Lower Proterozoic Earth would, therefore, indicate extremely rapid processes of oxygen consumption in certain compartments of the ancient exogenic system (notably the hydrosphere) rather than the non-existence of photosynthetic oxygen production at this time.
The geotectonic environment of Zambian copper-cobalt mineralization
Authors: Annels A. E.
Published: December 2007

Suggestions that the Ore-Shale of the Lower Roan Group is part of a series of sedimentary fades produced by the north-easterly transgression of the sea over dominantly arenaceous strata of littoral, deltaic, lacustrine and continental origin have been viewed with scepticism by the author. The lack of proven transition facies from silty carbonaceous argillites, of the type found close to the south-western limits of the Ore-Shale in the Zambian Copperbelt, to dolomites found further to the SW, has tended to accentuate this feeling. The intensity of exploration drilling in these areas must imply that, whatever the cause, the cut-out of the Ore-Shale is rapid. At the same time a 'normal' Lower Roan succession of the Chambishi type suddenly becomes attenuated and dolomites and dolomitic/chloritic schists are found immediately overlying argillaceous gritty sandstones, conglomerates and arkosic quartzites characteristic of Footwall Formations. Locally this contact may be reddened, porous and brecciated. Amphibolite bodies of the type normally found at a much higher stratigraphic level in the Upper Roan Group are present a short distance above this contact. It is thus proposed that this feature represents a hiatus in the succession and that all rocks from the Ore-Shale to the top of the Lower Roan, and possibly into the base of the Upper Roan, are missing due to non-deposition and/or erosion. This interruption of the succession coincides with the western cut-out of the Ore-Shale from the Konkola Dome in the NW, down to the western end of the Roan Basin in the S.

In view of various authors' contention that the Zambian Copperbelt is the site of an ancient aulacogen, the above observations take on an important genetic significance. The present paper thus reviews the location of copper-cobalt mineralization in relation to sedimentary facies changes and cut-outs, structural features and amphibolitic bodies, and proposes a geotectonic model for the Copperbelt which, it is hoped, provides a more satisfactory and comprehensive explanation for the facts. This model also provides a source for the metals which were converted to sulphides by diagenetic processes. It is believed that the complex mineral chemistry of the Ore-Shale associated ores and, in particular, the presence of localized concentrations of Mo, Co, Ag, As, Bi and Te-bearing phases, is best explained by high level seepage of magmatically enriched hydrothermal solutions along Precambrian rift systems and associated fractures. These metals were either dispersed by wave and current action in the sedimentary environment or were concentrated by porosity and physico-chemical and bacteriogenic controls in the diagenetic environment.
The Neogene succession of the South Western Approaches, Great Britain
Authors: Evans C. D. R. , Hughes M. J.
Published: December 2007

The outer continental shelf in the South Western Approaches is underlain by an oceanward-dipping wedge of Neogene strata up to 400 m thick that is divisible into three units: (a) the early to mid-Miocene, parallel-bedded calcilutites of the Jones Formation, which rest with a marked eastward overstep on Palaeogene and Cretaceous strata; (b) the mid- to late Miocene Cockburn Formation, which consists of calcarenites fashioned into a series of linear tidal ridges that are separated locally by an unconformity from the underlying unit; (c) the Plio-Pleistocene Little Sole Formation, which sits with a planar unconformity on the Miocene strata and is divisible into a lower, wedge-like member, restricted to near the shelf edge, and an upper, more widespread, sheet-like member. Palaeo-bathymetric and seismo-stratigraphic evidence indicates that the shelf and adjacent land areas have suffered epeirogenic movement since the early Miocene.
The Upper Jurassic ‘Boulder Beds’ and related deposits: a fault-controlled submarine slope, NE Scotland
Authors: Pickering K. T.
Published: December 2007

During the late Jurassic (Kimmeridgian) rifting in the North Sea, large-scale normal faults developed to form relatively deep marine basins. The north-western margin of one of these basins, defined by the Helmsdale Fault, is described to show the nature of sedimentation associated with a submarine fault scarp. Subsidence to the SE of the Helmsdale Fault led to a fault scarp down which various sediment mass flows moved, including debris flow, rockfall, high and low concentration turbidity currents and possible storm surge wash-over deposits. Redeposition by soft-sediment sliding was important as a slope accretion process. Deep channels were incised into the slope and infilled by coarse-grained elastics: contemporaneous, small-scale submarine fans, located farther off in the present North Sea, may have developed as a result of slope-bypassing sediment flows down such channels. The succession is characterized by extreme sediment heterogeneity compared to many deep water environments and it may be part of an ancient fan delta. The main influences upon slope accretion were the synsedimentary movements on the Helmsdale Fault and the nature of the available sediment input, such that the succession represents structurally controlled and sedimentologically modified slope development.
Evolution of high-Ca, high-Sr C-series basalts from Grenada, Lesser Antilles: the effects of intra-crustal contamination
Authors: Thirlwall M. F. , Graham A. M.
Published: December 2007

Basaltic lavas from Grenada, Lesser Antilles, may be divided into two series on the basis of CaO-MgO relations. The C-series roughly corresponds to the ankaramites or high-Sr series of the earlier literature and may be distinguished from the M-series basalts by higher CaO at a given MgO and by their strongly augite-phyric nature. Compositional variation within the C-series is dominated by fractional crystallization of augite and plagioclase but this was accompanied by assimilation of sedimentary material from the island arc crust. The assimilation-fractional crystallization process is well illustrated by an increase in
206
Pb/
204
Pb from 18.6 to 19.4 with fractionation, and correlated increases in Nd/Sr, Zr/Sm and Rb/Ba. The most primitive members of the C-series can not be generated from erupted M-series near-primary basalts, but instead show evidence for an origin by olivine fractionation from high-Mg basalts never seen at the surface. These appear to have been derived from a source similar to that of MORB, but enriched in
87
Sr and other hydrophile elements by dehydration of subducted lithosphere, and in REE and radiogenic Pb by a small component of subducted sediment.
Sea-level changes and the evolution of benthic marine calcareous algae during the Palaeozoic
Authors: Riding Robert
Published: December 2007

Maximum spreads of epicontinental seas in the area now covered by the USSR correlate closely with major periods of diversification of benthic calcareous algae (especially chlorophytes and rhodophytes) during the early-middle Ordovician and the late Devonian-early Carboniferous. This is consistent with the 'species-area' concept of direct relationship between taxonomic diversity and habitat-size. The shape of the Earth's hypsometric curve makes it likely that continental flooding would enlarge the area habitable by benthic algae more than it would for benthos not limited to the photic zone, thus enhancing the species-area effect for algae at times of sea-level change. In contrast, the first occurrence of calcareous algae near the Precambrian-Cambrian boundary does not seem to correlate with a transgressive maximum. Cyanophytes are the main calcified group appearing in the early Cambrian and it is possible that their occurrence at this level reflects a different factor which resulted in the calcification of pre-existing algal groups.
Carbonatite tuffs and lava clasts of the Tinderet foothills, western Kenya: a study of calcified natrocarbonatites
Authors: Deans T. , Roberts B.
Published: December 2007

Miocene 'limestones' around Tinderet, noted for their mammalian fossils, but recognized as in part carbonatitic, are now shown to be carbonatite tuffs and volcaniclastics. They contain ejectamenta of fenites, sövites and blocks and bombs of carbonatite lava which have quenched trachytoidal and flow-banded textures and consist largely (90%) of calcite, plus apatite and magnetite. The principal primary constituent is inferred to have been nyerereite
(cf.
the natrocarbonatite of Oldoinyo Lengai) now pseudomorphed by calcite, and accompanied by primary tabular calcite. Phase relations and other occurrences of such rocks are discussed. The tuffs comprise lithic and crystal lapilli, Pele's tear-drop and accretionary lapilli types.

The silicate content of the tuffs and bombs is remarkably low (
c
. 3%), and their eruption
preceded
major flows of nephelinitic agglomerates. We suggest this sequence followed simultaneous separation of immiscible silicate and natrocarbonatite magmas at depth, but the greater energy and mobility of the carbonatite enabled it to surface first. These pyroclastics (and one carbonatite plug) form an outpost of the Homa Bay petrographic province.
On natural deformation mechanisms in the Chalk
Authors: Jones Mervyn E. , Bedford Jonathan , Clayton Chris
Published: December 2007

Chalk exhibits mechanical properties ranging from those akin to a stiff soil to those akin to a strong rock, depending on its diagenetic and deformation history. Two processes are identified which lead to this hardening of chalks: consolidation, which leads to a reduction in porosity to about 40%; and pressure solution/cementation which will occur only after consolidation has increased the effective stress. Both of these processes are restricted by low permeabilities so that during hardening it is necessary for the deformation to maintain permeable pathways to dissipate any excess pore fluid pressure. This is facilitated by fracturing or by the development of open stylolites which permit fluid flow. In the Isle of Purbeck (Dorset) a lateral gradient in chalk hardness across the Purbeck Axis reflects the increasing deformation of the chalk. Both consolidation and pressure solution/cementation have occurred, with complete cementation being restricted to the most highly strained chalk which is also the most fractured. The Purbeck Chalk provides a model for both diagenetic and localized deformation hardening in chalks generally.
Fenites, breccia dykes, albitites, and carbonatitic veins near the Great Glen Fault, Inverness, Scotland
Authors: Garson M. S. , Coats J. S. , Rock N. M. S. , Deans T.
Published: December 2007

In the Abriachan-Dochfour-Moniack, Rosemarkie and Foyers areas near Inverness, Moine metasediments and gneisses, amphibolites, Caledonian granitic rocks, and meta-limestones of uncertain age are veined or replaced by metasomatic assemblages of blue fibrous magnesio-riebeckite (crocidolite), green aegirine-rich pyroxene, albite, calcite, hematite, and anatase. The field characteristics, textures and parageneses of these assemblages duplicate those of classical fenites around igneous carbonatite complexes. In the Abriachan granite, the chemistry of metasomatism also recalls classical fenitization in correlating removal of SiO
2
, K
2
O and Rb with addition of Na
2
O, Fe, MgO, CaO, F, Zn, Zr, Ba, Ce and CO
2
, while sums of cations such as [Si + Na] and fSi + A1 + Fe + Mg + Ca] remain virtually constant. At Moniack and near Rosemarkie, the fenites border calcitic veins, whose high contents—especially of Sr, Nb, Ba, REE, and Mn—ally them with late-stage carbonatites rather than with limestones or hydrothermal carbonates. Associated dykes of ffuidized, often carbonate-rich breccias, and bodies of brick red, sometimes radioactive albitites, also have close analogues in carbonatite complexes. The fenites are distinguished by petrology, field relationships and age from other Scottish alkali metasomatized rocks. All these rocks are believed to have formed broadly coevally, during the emplacment of at least one carbonatite mass deep into the Great Glen Fault zone, most probably during early Devonian (c. 390 Ma) fault movements.
Spatial and temporal variations in the Andean subduction zone
Authors: Wortel M. J. R.
Published: December 2007

A comprehensive global study of the process of subduction of oceanic lithosphere has made it evident that a major role is played by the age of the subducted lithosphere. The age-dependence of subduction is of special interest in an understanding of the late Cenozoic history of the Andean subduction zone. Many of the complexities of this zone result from the peculiar circumstance that the zone is in transition from an early stage of consumption of old lithosphere (>70 Ma, producing deep earthquakes) towards the latest stage of consumption of young lithosphere (without deep earthquakes). This transition has induced an evolutionary pattern with significant temporal variations, notably changes in dip and length of the subducted slab. Moreover, the various segments of the South American subduction zone are in different stages of this evolution. In comparison with the other segments, the northern Chile segment is in the earliest stage and, therefore, shows a much closer resemblance to the ‘classic’ deep subduction zones in the Western Pacific than do other parts of the South American zone. A roughly N–S symmetry in development is found around northern Chile, with the Ecuadorian and southern Chilean (near 40–45°S) segments representing the latest stage. To a large extent the temporal and spatial variations in lithospheric age at the trench originate from the reorganization in plate configuration which started with the break-up of the Farallon plate into the Cocos plate and the Nazca plate at about 25 Ma.
Rb-Sr isotopic ages from late Palaeozoic metamorphic rocks of central Chile
Authors: Hervé F. , Kawashita K. , Munizaga F. , Bassei M.
Published: December 2007

Two metamorphic complexes of different lithology and structure crop out in the coast ranges of central Chile. The Tanumé metamorphics to the E, which belong to a low P/T series derived from a continental platform clastic sedimentary sequence, give a whole rock isochron age of 347 ± 32 (2σ) Ma and the accreted Pichilemu metamorphics to the W, an age of 311 ±10(2σ) Ma. The latter consist of stilpnomelane-bearing cherts, crossite-bearing metabasites and siliceous metapelites. An errorchron of
c.
370 Ma was obtained from sillimanite-cordierite gneisses of the low P/T series. Different provenance of the protoliths is indicated by the higher Sr
87
/Sr
86
initial ratios of the Tanumé rocks (0.7124 ± 0.0007 (2σ) relative to the Pichilemu rocks (0.7060 ± 0.0005
(2
σ
),
which were more affected than the former by the main (D
2
) phase of deformation and metamorphism in the area.
Orogenic development of the Argentinian/Chilean Andes during the Palaeozoic
Authors: Miller Hubert
Published: December 2007

The geological development of the basement of the Argentinian/Chilean Andes during the Palaeozoic is characterized by an almost continuous structural, metamorphic and plutonic mobility. Ultrabasic magmatism occurred only in a few regions; terrigenic psammopelitic sequences are widespread at all times. Metamorphism is mostly of low to intermediate pressure, but of variable temperature; late Palaeozoic high pressure metamorphism only took place near the present Pacific coast. There are about four culminations of orogenic activity, varying in space and time: from the mid-Cambrian to the mid-Ordovician, from the Silurian to the mid-Devonian, during the early Carboniferous, and at the Permian/Triassic boundary. Fold structures strike in variable directions at different times. On the whole, the Pacific margin of Gondwana consolidated continuously from the shield region in the E towards the Pacific during the Palaeozoic.
Deformation of country rock by an intrusion in the Sierra de Moreno, northern Chilean Andes
Authors: Skarmeta J. , Price N. J.
Published: December 2007

The highest sedimentary cover rocks of the Sierra de Moreno are the Tertiary, Sichal Formation. These are flat lying and undeformed, except at one locality adjacent to an igneous intrusion, where the sediments exhibit a spectacular though minor belt of folds that, from the stratigraphy, can be shown to have formed under a cover of 300–500 m. From the orientation and interaction of fibrous veins within the folds it is possible to unravel the history of deformation. We infer that the lateral compression of these rocks first resulted in minor conjugate thrusts. The major phase of deformation which followed gave rise to folds whose geometry and position can be attributed to the existence of gypsum intercalations which underwent the gypsum/anhydrite reaction as a result of the high geothermal gradient associated with the emplacement of an igneous intrusion. This reaction released large quantities of water which enabled kink folds to develop, coupled with the infilling of the pre-existing barren thrusts to form a complex of fibrous veins. A final phase of extensional deformation can be attributed to subsequent shrinkage of the country rock as the intrusion cooled.
Digital image processing of integrated geochemical and geological information
Authors: Green Philip M.
Published: December 2007

Image analysis computer systems, developed for the study of remotely-sensed data, provide a powerful method for handling spatial data in digital-image formats.

Recent developments in image creation techniques, using point and line data, and the digital image processing of the derived images, enable interactive colour mapping and statistical analysis of integrated geological and geochemical data to be performed. The system, developed jointly by the British Geological Survey (BGS) and the Experimental Cartography Unit (ECU), is based on an International Imaging Systems (I
2
S) Model 70 computer and allows the geochemical signatures, or associations of trace elements for particular geological units, to be studied in terms of lithological or stratigraphical groups or subgroups, on a regional or local scale.

This paper describes the techniques used to create the geochemical digital images and geological region-of-interest sub-images for the Moray-Buchan region of Scotland. The application of image processing and image analysis methods available for geochemical mapping and the identification of geochemical signatures of specific geological units are discussed with reference to the Moine-Dalradian geochemistry, mineral exploration, heat production, medical geography (epidemiology) and agricultural studies.
Geothermometry and geobarometry: a discussion
Authors: Powell R.
Published: December 2007

Geothermometric and geobarometric methods are considered under two groupings: directly calibrated methods and internally consistent data set methods. Emphasis is placed on the importance of error propagation calculations. They allow the calculation of
model
uncertainties on temperatures and pressures due to the quantifiable uncertainties involved in the calibration and application of methods. The possibility of
systematic
errors, due to the unquantifiable effects of the assumptions and approximations involved, must be evaluated using the criterion of geological reasonableness.

The advantages of the internally consistent data set approach are expounded and an appropriate error propagation procedure developed.
Uplift patterns and orogenic evolution in the Scottish Dalradian
Authors: Dempster T. J.
Published: December 2007

Rb-Sr and K-Ar mineral ages from the Dalradian rocks of Angus and Perthshire indicate considerable diachronism of cooling, following the Caledonian regional metamorphism. The oldest mineral ages (~515 Ma) are similar to the independent estimates of peak metamorphic age, and hence indicate very rapid cooling; they occur in the highest grade samples of the Angus region and are associated with localized uplift. Regional rapid uplift events at 460–440 Ma and 410–390 Ma may also be recognized and correlated with structural and igneous activity and metamorphic age dates within the Scottish Highlands.

Thermal models show the importance of localized and discontinuous uplift in determining both the timing of metamorphism and the shape of the P–T paths. They suggest that the major diachronous metamorphism (and granite production) in the Scottish Highlands is depth controlled rather than representing major lateral diachronism of thickening or heat sources.

Localized uplift may exercise considerable control on mineral facies variation within orogenic belts. The local uplift in Angus introduces hot, high-pressure Tarfside Nappe rocks into higher level Tay Nappe rocks. Thus, the uniform block uplift models frequently used in the interpretation of metamorphic and structural phenomena in orogenic belts are an oversimplification of the real events.
Petrography and carbonate chemistry of some Dalradian dolomitic metasediments: preservation of diagenetic textures
Authors: Fairchild I. J.
Published: December 2007

The Bonahaven Formation of northern Islay, Scotland, despite having undergone high-pressure greenschist facies metamorphism, contains many relics of its diagenetic history. Early diagenetic dolomite textures, calcite and clay mineral cements are well preserved locally. Burial diagenetic compactional and pressure solution features are recognizable together with quartz and feldspar overgrowths and variable styles of replacement by and recrystallization of dolomite, calcite, pyrite, quartz and albite. The imprint of deformation and metamorphism is variable, probably as a result of differences in the degree of pressure solution and the CO
2
, content of pore fluid. Typically, some solution-transfer phenomena occur, accompanied by growth of cleavage-defining phengite and sporadic phlogopite. Locally, abundant phlogopite or biotite is present in lithologies with fully developed metamorphic textures. Dolomite shows good preservation of pre-metamorphic textures and minor element zonation, but calcite does not. Metamorphic re-equilibration was insufficient to allow the derivation of maximum temperatures by calcite–dolomite geo-thermometry.
The timing of calc-alkaline magmatism in parts of the Alpine–Himalayan collision zone and its relevance to the interpretation of Caledonian magmatism
Authors: Zhou Jiaxin
Published: December 2007

Discussion of the timing of calc-alkaline Caledonian magmatism in Scotland has centred on its tectonic setting: the view that active subduction was responsible for the formation of 415–400 Ma magmatism is in conflict with the palaeogeographical and stratigraphical evidence. In order to shed light on the problem, magmatism in the continent–continent collision zone of modern times (Turkey, Iran and Tibet) is surveyed. The conclusion is reached that the final stage of calc-alkaline magmatism displays a time-lapse and possible space-shift after two continents have collided. From the evidence of magmatism, the timing of the major collision events in the Himalayas is thought to be diachronous, ranging from around or no later than 38–30 Ma for the central part, to later than 30 Ma for the eastern part. This view is supported by the available palaeogeographical and stratigraphical evidence. Evidence from this survey enables the Scottish data to be seen in a new context. By comparing modern collision zones with old ones, such as those in Scotland and the Appalachians, it is also concluded that collision-related magmatism is not volumetrically trivial and is diverse in composition.
Potassium-argon age studies of a low-grade, progressively metamorphosed greywacke sequence, Dansey Pass, South Island, New Zealand
Authors: Adams C. J. , Bishop D. G. , Gabites J. E.
Published: December 2007

39 potassium-argon age determinations from a progressively metamorphosed greywacke-schist sequence in southern New Zealand (prehnite-pumpellyite to greenschist facies), range from 199 to 154 Ma (early to late Jurassic). They show an inverse correlation of age with metamorphic grade. Concordant ages from varied lithologies at all metamorphic grades show that degassing of detrital potassium-bearing minerals was completed during metamorphism. The lowest grade metamorphic rocks, which yield the oldest ages, about 200 Ma, have only just exceeded the argon closure temperature and hence their ages date closely their time of metamorphism in the earliest Jurassic. Thus it is shown that the Jurassic-Cretaceous, Rangitata Orogeny in New Zealand commenced earlier, is more complex, and of longer duration than previously supposed. The remaining ages, from pumpellyite-actinolite and lower greenschist rocks, range uniformly through the Jurassic and are interpreted as cooling ages. From these data, a long-term uplift rate of 0.23 mm/yr is calculated for the Jurassic-Cretaceous.
Storm sedimentation
Authors: Allen P. A. , Pound C. J.
Published: December 2007

Report on a meeting of the British Sedimentological Research Group held on 5 May 1984 at University College, Cardiff. The meeting was organized by P. A. Allen and C. J. Pound.

This meeting was arranged to reflect the recent resurgence of interest in the dynamics and sedimentation of continental shelves. To some extent this has been stimulated by advances in physical oceanography which reveal exceedingly complex patterns of water motion on the continental shelf. Yet there still remains enormous uncertainty as to the response of a mobile substrate to these complex water motions. As if to fill this scientific vacuum, hummocky cross-stratification has become an obsessional focus of attention amongst clastic sedimentologists. It was thought timely to consolidate our knowledge of storm sedimentation as a whole and in particular, first, to evaluate critically the palaeohydraulic constraints on the formation of hummocky cross-stratification (HCS) and second, to document its context within marine sedimentary sequences.

The contributions to the meeting fall into two themes: (1) insights into the genetic significance of storm deposits, particularly HCS, and (2) documentation of the geometry and anatomy of storm-generated sedimentary sequences.

Physical significance of storm deposits

J. R. L. Allen (Reading) provided the general physical implications of storms. Storms cause changes in water level resulting from barometric effects and from wind drag on the water surface creating a water surface slope or set-up. They also induce currents in the affected water body; these currents are created by wind drag, flowing as a surface layer in the
Fault-associated salt flow and mass movement
Authors: Jenyon Malcolm K.
Published: December 2007

It has been suggested by model studies that swell formation may occur in a mobile layer subjected to faulting where the fault throw is less than the mobile layer thickness. In such a case in the real earth, differential overburden loading due to deposition or erosion at a later stage will cause a mobile layer (e.g. salt rock) to flow from the region of higher to the region of lower load stress via the fault zone, resulting in swell formation on the upthrow side of the fault. Two different examples of this effect are illustrated in marine seismic sections from the North Sea. Associated with one of these examples are seismic events which appear to indicate mass movement of sediment down the fault scarp prior to the deposition of the main body of Zechstein halite at this location.
Computer education in geology
Authors: Howarth Richard
Published: December 2007

Report of a joint meeting of the Geological Information and Earth Science Education Methods Groups of the Geological Society, held at the Department of Earth Sciences, University of Cambridge, on 22 May 1984. The meeting was organized by P. Hill.

The aim of the meeting was to examine the requirements of employers of geology graduates and the present provision for the education of undergraduates with respect to computing and computer-based skills and applications. It consisted of presentations on the viewpoints of academic,industrial,and government bodies, plus discussion.

G. Y. Craig opened the meeting with an account of the use of computing and statistical teaching in the Geology Department at the University of Edinburgh. All geology students take a course in statistics as part of the coursework.They are also encouraged to take an optional l-year course in Computer Science.

One major problem theDepartment has found is the rapidly increasing cost of acquiring and maintaining capital equipment. There is not enough money to provide sufficient student workstations at present, and this applies to all departments. The last 10 years have seen the computing costs rise from 10% to 37% of the equipment budget. Annual maintenance charges for computing have been found to be very much higher than that for other major equipment,and paying an hourly rate for maintenance can be cheaper.The problem of where to draw the balance between the costs of data analysis and that of acquisition of hardware with which to obtain original data is an increasing one.

A. D. M. Burwell (BP Research Centre, Sunbury) drew a contrast between the way computers are being used by geologists in the petroleum industry
Submarine silicic volcanism and associated sedimentary and tectonic processes, Ramsey Island, SW Wales
Authors: Kokelaar B. P. , Bevins R. E. , Roach R. A.
Published: December 2007

Lower Ordovician marine strata, formed in an ensialic marginal basin environment, record early episodes of distant explosive silicic volcanism and a later volcanotectonic episode which was a precursor to the development of a wide variety of near-to-source submarine silicic volcanic phenomena. The early episodes are represented by well-bedded turbiditic tuffs. These and older strata, at least 770 m thick in normal stratigraphical succession, are allochthonous, and large-scale wet-sediment sliding with reworking of strata in cohesive debris flows reflects major disturbance of the sea floor during the emplacement of silicic magmas. Volcanotectonic uplift along a N-S fault exceeded 1 km and led to the emergence and erosion of a rhyolitic volcanic island. This was followed closely by submergence, and in water depths perhaps of about 500 m there accumulated welded and non-welded rhyolitic ash-flow tuffs, rhyolitic turbiditic tuffs, rhyolite lavas and a wide variety of debris flow deposits. Shallow intrusions of 'low'-viscosity rhyolite were emplaced contemporaneously. The more or less simultaneous emplacement of rhyolitic tuff with both 'low'-viscosity and high-viscosity rhyolites is attributed to variable water content related perhaps to volatile fluxing in a magma body. The location of the near-to-source extrusive rocks and consanguineous intrusions along the contemporary N-S fault is considered to reflect channelling of magma, initially along a deep crustal fracture.
The distinction between Caledonian burial and regional metamorphism in metapelites from North Wales: an analysis of isocryst patterns
Authors: Roberts B. , Merriman R. J.
Published: December 2007

In Snowdonia and Llŷn three prograde stages in metapelites (Gwna Group to Wenlock) are recognized: Stage I, with white mica crystallinity values >0.43Δ°2
θ
; Stage II, 0.43–0.26Δ°2
θ
; Stage III, <0.26 Δ°2
θ
. Isocrysts at 0.03Δ°2
θ
intervals are drawn separately for pelites above and below the sub-Arenig unconformity. Approximately concordant isocryst patterns characterize the unconformity in the northwestern and southeastern limbs of the Snowdonia Syncline, whereas discordant patterns are developed northwest of the Aber–Dinlle Fault and at the closure of the Ynyscynhaiarn Anticline. The contrasting patterns are the result of a pre-Arenig burial metamorphism. In post-Tremadoc pelites belts of steep metamorphic gradient coincide with belts of high strain, and a broad area of higher grade Caradoc pelites coincides with the core of the Snowdonia Syncline. The later metamorphism is related to Caledonian deformation rather than to burial. Pelite alteration adjacent to small, intermediate to acid plutons suggests emplacement before the Caledonian regional metamorphism. A close correlation exists between metabasite zones and metapelite stages.