bluuebunny/crossref_metadata_2025 huggingface dataset:
Tectonic quiescence punctuated by strike-slip movement: influences on Late Jurassic sedimentation in the Moray Firth and the North Sea region
Authors: Frost R. E. , Rose J. F.
Published: April 2008

Present geological models do not explain all of the observed Upper Jurassic structural and depositional relationships, not only within the Moray Firth but also the entire North Sea region. Observations based on modern 2D seismic reflection sections, well and onshore outcrop data, have led to a significant reappraisal of the Upper Jurassic which may have far-reaching consequences for presently accepted models. Analysis of the Upper Jurassic sediments in the Moray Firth almost invariably indicates an onlapping, non-rotated parallel-bedded sequence. The onlapped topography resulted from a rapid tectonic pulse of probable Callovian age, the loosely-termed ‘mid Cimmerian’ event, which will be redefined. This likely synchronous event terminated the broadly paralic deposition of the Brent Group and equivalents. Extensive seismic reflection data, well and outcrop evidence from the region strongly suggests that the resulting relief was ‘frozen’, i.e. not driven by, nor creating, vertical earth movements. During this generally tectonically quiescent Late Jurassic period, the lows were passively filled with little apparent subsidence. This relatively low-energy environment may explain the deposition of sands proximal to eroded margins, and also the occurrence of parallel-bedded source rocks where tectonic quiescence would have been beneficial to the maintenance and preservation of anoxic conditions. Evidence of earthquake activity based on grain-size distributions suggests that fault movements were local, of probable strike-slip origin and may have been the result of post ‘mid Cimmerian’ strain energy release along contemporaneously eroding margins. These movements resulted in different localised tectono-stratigraphic sand pulses. The recognition of passive infill during what appears to be a regionally tectonically quiescent period, from intra-Callovian to the major Late Ryazanian sea-level rise, suggests that the intra-Callovian (‘mid Cimmerian’) event, which signalled the beginning of the period, was probably synchronous across the North Sea region. It would be unreasonable to expect contemporaneous passive infill and subsidence-generated sedimentation in adjacent basins if this were not the case. The virtual absence of any evidence of syn-depositional rotation of the Upper Jurassic sediments also suggests that the intra-Callovian event was geologically rapid. It should be noted that these observations, involving a generally tectonically quiescent period which was both rapidly formed and ended, are incompatible with the application of the necessarily gradual processes of thermal subsidence models and classical rifting, which are considered inappropriate for the region during the Late Jurassic period.
The Puffin Field: the appraisal of a complex HP-HT gas-condensate accumulation
Authors: Dickinson B.
Published: April 2008

The Puffin Field was discovered by well 29/5a-1, drilled in 1981 to test a Base Cretaceous closure. The well encountered over-pressured gas-condensate-bearing Upper Jurassic shallow marine sandstones below 14,000 ft. The well flowed over 20 × 10
6
SCFGD with 2650 barrels per day of condensate. Appraisal of the discovery, which fell into five phases, involved the acquisition of two 3D seismic surveys and drilling of eight wells. This paper describes the appraisal of Puffin from an historical perspective.

The initial geological model involved an intra-graben horst draped by a relatively uniform blanket of Fulmar Formation sandstones. The first two apprisal wells were disappointing and hinted at previously unsuspected complexities. The second pair of appraisal wells, although proving up further hydrocarbons, penetrated thick Triassic sequences, with the Pentland and Fulmar Formations being absent. These results were incompatible with the existing geological model and a new model was developed. This envisaged the presence of several large Triassic-cored pods, formed in response to salt withdrawal and local tectonism, separated by narrow elongate zones (inter-pods) in which Middle and Upper Jurassic sections were present. Four further appraisal wells were drilled and proved the model to be robust with respect to stratigraphy and structure, although it did not always accurately predict hydrocarbon distribution.

The reservoir geology of the shallow marine Fulmar Formation and fluvial Skagerrak Formation reservoirs will be outlined. The wells indicated considerable differences in hydrocarbon contacts between wells and differences in fluid gradients and pressures between the pods. The geological evolution of the field will be described.
Laminated diatomaceous sediments: controls on formation and strategies for analysis
Authors: Sancetta Constance
Published: April 2008

This paper summarizes observations and ideas developed over a decade studying a variety of laminated sediments. Laminations of alternating lithic (dark) and biogenic (light) particles occur near land masses, where the lithic laminae may be interpreted as a record of precipitation, and the biogenic laminae as a record of primary production resulting from wind stress. The seasons corresponding to each type can be predicted from regional climatology. Biogenic laminae frequently consist of sublaminae representing a sequence of production events, so that several seasons of information may be included. Laminations of two types of biogenic particles are unusual, but have been reported from deep-sea settings. The physical control on these laminations is very indirect, and their occurrence is more probably a result of complex ecologic interactions. Although many laminar couplets are true varves, problems can occur when the normal cycle of alternating conditions is disturbed. Examples include the occurrence of a heavy rainstorm producing a dark lamination within the light lamina of a productive season, juxtaposition of two dark laminae from separate rainy seasons if the intervening productive events did not occur, and continuous rainfall throughout a productive season such that the light biogenic material is masked by the dark lithic material. Such cases, while complicating the use of laminae for time control, may provide useful information on interannual variability if they can be accurately identified. As an alternative to generating very long time series, it is suggested that selected clusters of samples combined with the use of analyses of variance may provide such information. Another strategy is the use of factor analysis to define seasonal proxies from the top part of a section; the proxies may then be projected downcore to derive information on seasonal trends of the past.
Preparation and analysis techniques for studies of laminated sediments
Authors: Pike Jennifer , Kemp Alan E. S.
Published: April 2008

Laminated sediments commonly represent the highest resolution continuous records available of marine and lacustrine variability. To exploit this information effectively careful sampling and preparation procedures must be followed, thus the method of analysis chosen must have sufficient resolution to recover the data. The scanning electron microscope, because of its good spatial resolution, is an ideal tool for analyzing laminated sediment fabric. Backscattered electron imagery produces preserved flux data on intra-annual time-scales, which enables high-resolution fabric studies, palaeoenvironmental reconstructions and compilation of data for time-series analysis. This contribution summarizes the techniques used for laminated sediment fabric investigation and discusses the use of the scanning electron microscope, including the best preparation method for thin sections and the procedure for backscattered electron imagery analysis.
The potential for palaeoclimate records from varved Arctic lake sediments: Baffin Island, Eastern Canadian Arctic
Authors: Hughen Konrad A. , Overpeck Jonathan T. , Anderson Robert F. , Williams Kerstin M.
Published: April 2008

Tidewater lakes on Baffin Island in the eastern Canadian Arctic offer an excellent opportunity to study interannual to century-scale Arctic climatic change. Freeze-cores were analysed from three lakes in southeastern Baffin Island: Upper Soper Lake, Ogac Lake and Winton Bay Lake. The sediment record in each lake consists of massive sediments overlain by an organic-rich, finely laminated section which continues to the surface. The laminae in Ogac Lake were studied in detail and consist of two types. The light layers are composed almost entirely of intact diatom frustules, primarily
Chaetoceros
spp. The darker layers are dominated by clay and silt-sized terrigenous mineral grains, including abundant quartz and feldspars. These couplets are probably deposited as the result of diatom blooms in the late spring/summer growing season followed by settling of grains introduced by summer runoff. Sedimentation rates based on
210
Pb dates agree well with rates based on laminae counts in both Ogac and Winton Bay Lakes, indicating that the laminae couplets are annually deposited varves. Our experience suggests that shallow-silled tidewater lakes with varved sediments may be relatively common along the coast of Baffin Island. It should thus be possible to create a network of sites with annually dated palaeoclimate records.
Varved sediments in Sweden: a brief review
Authors: Petterson Gunilla
Published: April 2008

This paper gives an introduction to studies made on varved (annually laminated) sediments in Sweden. Varve studies began in the late 19th century when Gerard de Geer correlated glacial clay-varves in order to establish a geochronology over the latest deglaciation. Both the chronology and its connection to present sediment deposition has continuously been revised during this century. As well as for glacial clay-varve deposits, annual formation of sediment laminae has also been found in estuarine, marine and lacustrine environments. Varved sediments, of different origins, first mainly used for dating purposes, have today also become an important tool for detailed reconstructions of past environmental changes.
Conflicting indicators of palaeodepth during deposition of the Upper Permian Castile Formation, Texas and New Mexico
Authors: Leslie A. B. , Kendall A. C. , Harwood G. M. , Powers D. W.
Published: April 2008

The Upper Permian Castile Formation of the Delaware Basin, Texas and New Mexico has commonly been interpreted as a deep-basin, deep-water evaporite deposit, based on the lateral continuity of mm-thick, anhydrite-calcite laminations. The identification of pseudomorphs after bottom-growth gypsum crystals throughout the Castile Formation implies that during much of the time, deposition occurred within a partially desiccated basin from supersaturated brines at most 40 m deep. The pseudomorphs are commonly poorly preserved within nodular horizons which were previously interpreted as mesogenetic alterations of former laminae. The nodular horizons themselves contain pseudomorphs after displacive lenticular gypsum crystals that suggest a synsedimentary, shallow-water origin for both bottom-growth and nodular textures. In contrast, structureless beds of sulphate up to 0.5 m, thick which are most common in the eastern part of the basin, appear to have been deposited from density currents as turbidites, implying relatively deep water. The evidence provided by pseudomorphs after gypsum within the Castile Formation suggests that average water depths across the basin were at times less than 40 m and could not have exceeded 200 m. Synsedimentary fault control may have partly accommodated the turbiditic sedimentation in the eastern part of the basin, although the depositional environment would require a remarkable coincidence between deposition and subsidence. The evidence as a whole suggests a delicate balance between subsidence, sedimentation and water depth during deposition of the Castile Formation.
A survey of occurrences of Holocene laminated sediments in California Borderland Basins: products of a variety of depositional processes
Authors: Gorsline D. S. , Nava-Sanchez Enrique , de Nava Janette Murillo
Published: April 2008

Laminated sediments are found in several contemporary depositional settings in basins of the California Continental Borderland. These primary structures are preserved either as a result of anoxia (or near anoxia) in the bottom water when oxygen demand is greater than the oxygenation rate, or as a result of sedimentation rates in excess of bioturbation rates. The controlling processes in the anoxic settings include cyclic seasonal variation in the composition of hemipelagic deposition (hemipelagic laminae), and/or formation and destruction of bacterial mats as a result of periodic bottom water flushing of normally low-oxygen bottom waters (cyclic organic-rich laminae). Non-bioturbated hemipelagic laminations occur in San Pedro, Santa Monica and Santa Barbara Basins. Low-oxygen bottom waters and exclusion of macrobenthos have been characteristic of San Pedro Basin for a few decades. These conditions have existed in Santa Monica Basin for a few centuries and in Santa Barbara Basin for most of the Holocene. Physical sedimentation processes include rapid deposition from turbidity currents. Fine grained contemporary distal turbidites occur in Santa Monica, San Pedro, San Diego, Santa Cruz and San Nicolas Basins. Turbidity current flows into the deep basin floor occur at century scale in Santa Monica Basin and at somewhat longer frequencies in San Pedro Basin. Frequency of such flows is at the kilo-year level in San Diego Trough, Santa Cruz Basin and San Nicolas Basin. Such events are of interest because they show that submarine canyons will remain active through a high sea-level period. In narrow-shelf, active margins, canyons may not be shut off as generally occurs on passive margins when sea level rises rapidly, as is typical of climatically driven sea level fluctuations.
Laminated sediments of Santa Monica Basin, California continental borderland
Authors: Hagadorn James W.
Published: April 2008

Laminated sediments from Santa Monica Basin differ from other laminated sediments of the California Continental Borderland (such as varved sediments of Santa Barbara Basin) because lamina couplets represent a 3–6 year time interval and are affected by distinct local oceanographic and climatic processes. Unfortunately, mechanisms which control the temporal pattern of laminations in this basin are poorly understood. In addition, this laminated sediment record has been underutilized in paleoceanographic and paleoclimatic studies. In an attempt to identify mechanisms which control lamination formation and in order to obtain paleoclimatic and paleoceanographic records from this region, a sediment core from this basin was analysed using a suite of geochemical, sedimentary, and micropaleontologic techniques. This box-core (core DOE 26) was collected from the centre of the basin and was vertically sub-sampled at one millimetre increments, providing a temporal resolution of 1–3 years per sample. A variety of paleoceanographic and paleoclimatic records were constructed via analysis of these samples and include proxy records of: (a) regional rainfall — as expressed by gross variations in downcore density profiles; (b) relative contribution of terrestrial and marine organic carbon to the basin — as expressed by downcore variations in the isotopic composition of total organic carbon (TOC); (c) Primary productivity — as expressed in downcore variations in the carbon isotopic composition of planktonic foraminifera; (d) changes in sea surface temperatures —reflected in downcore variations in the oxygen isotopic composition of
N. pachyderma
(dextral); (e) large-scale changes in bottom-water oxygen conditions — as expressed by downcore changes in abundance of oxygen- sensitive benthic foraminifera. To extract additional information from these proxy records, spectral analysis was also employed in this study. Although ENSO-length signals are present in several records, decadal-length signals appear to dominate records from this basin. This dominance suggests the presence of a longer-term forcing function (other than the ENSO phenomena) which strongly influences this region. Considered together, preliminary results suggest that the flux of organic carbon is a dominant factor affecting lamination variability in Santa Monica Basin. Organic carbon fluxes in this basin are mainly affected by primary productivity, and to a lesser extent, by terrestrial input. Variations in bottom oxygen conditions may not control lamination variability; rather, long-term depression of oxygen levels appears to allow preservation of this organic matter as laminated sediments.
The nature of varved sedimentation in the Cariaco Basin, Venezuela, and its palaeoclimatic significance
Authors: Hughen Konrad A. , Overpeck Jonathan T. , Peterson Larry C. , Anderson Robert F.
Published: April 2008

A laminated sediment record of the last 12 600 radiocarbon years from the anoxic Cariaco Basin provides a rare opportunity to study interannual to millenial-scale climatic change in a marine setting. Sedimentological and radiometric analyses of laminae couplets in the basin sediments indicate that they are annually deposited varves. The varves consist of light and dark laminae which are deposited as the result of the Intertropical Convergence Zone’s annual north-south migration over the tropical North Atlantic, and the impact of this migration on regional upwelling and rainfall patterns. Light laminae contain planktonic fossils (predominantly diatoms) and are an indicator of productivity over the basin during the winter-spring upwelling season. Dark laminae contain terrigenous mineral grains and are an indicator of runoff from northern South America during the late summer-fall rainy season. Varve measurements developed using X-ray images and thin sections indicate that a continuous 14 000 calendar year long varve chronology may be generated. Preliminary measurements of thickness of individual light and dark laminae for the period 12.6 to 9 ka
BP
show similar palaeoclimate signals. Thickness of dark laminae, interpreted as measuring changes in regional runoff, increased rapidly at 10.8 ka
BP
and decreased rapidly at 9.8 ka
BP
, an interval coincident with the Younger Dryas cold period. Thickness of light (diatom-rich) laminae, indicative of productivity and possibly upwelling intensity, also show an increase at 10.8 ka
BP
, and a rapid decrease at 9.8 ka
BP
, simultaneous with the changes in the dark laminae. Current work is focusing on building long multiple-core varve chronologies and using these for palaeoclimatic reconstructions and geochronological research.
Laminated sediments from the oxygen-minimum zone of the northeastern Arabian Sea
Authors: Schulz H. , Von Rad U. , Von Stackelberg U.
Published: April 2008

Holocene and late Pleistocene hemipelagic muds with fine, mm-scale laminations were discovered during the International Indian Ocean Expedition (IIOE) 30 years ago in the northeastern Arabian Sea, where an expanded mid-water oxygen-minimum zone (OMZ) impinges on the continental slope and prevents bioturbation. In 1993, R.V.
Sonne
revisited Pakistani waters in order to study in more detail the complex interactions between surface and deep water hydrography, productivity and terrigenous input which form and preserve organic-rich sediments. Laminated sediments frequently occur along the Indian-Pakistani continental margin between 300 and 900 m water depth. The zone of high organic carbon contents roughly coincides with the depth of the present OMZ (200–1200 m water depth). However, resedimentation processes influence the character of sediment laminae and the distribution of organic carbon in the slope sediments. Microscopic and SEM studies suggest that the lamination may be due to the seasonal changes of surface productivity and to the lateral supply of fine-grained sediment. We infer that the OMZ must have varied considerably in space and time. This is documented by the distinct, possibly cyclic alternations of dark-coloured, organic-carbon-rich, laminated intervals with light-coloured, carbonate-rich, bioturbated sedimentary sequences, reflecting variations between anoxic to suboxic and oxic bottom water conditions.
Detailed volcanic geology of the MARNOK area, Mid-Atlantic Ridge north of Kane transform
Authors: Lawson Kate , Searle Roger C. , Pearce Julian A. , Browning Paul , Kempton Pamela
Published: April 2008

We present the results of a detailed study of the volcanic geology of the median valley floor of the Mid-Atlantic Ridge in two adjacent but contrasting spreading segments immediately north of the Kane transform fault, based on a high-resolution deep-towed sidescan sonar (TOBI) survey, near-bottom photographic traverses, and geochemical analyses from 28 precisely sited sets of dredge samples. This has allowed us to assess the relative importance of small (intersegment) and large (transform) boundaries on magmatic processes at slow spreading ridges.

We find that the petrological effect of the transform (so-called ‘transform fault effect’) is minor compared to the manifestation of the second-order segmentation. The southern, ‘narrowgate’-type segment has a poorly developed neovolcanic zone, which tapers towards its bathymetric centre where continuous faulting causes rapid dismemberment. Flat-topped seamounts are preferentially located at the ends of the segment, particularly near the non-transform offset. There is a striking variation in the degree of fractionation along the length of this segment: primitive lavas were sampled at the centre, while more fractionated basalts, showing a greater range of parental compositions, form discrete volcanic edifices at the ends. In contrast, the northern segment has a wider inner valley, and syn-magmatic faults extend up to 15 km along the crest of its robust axial volcanic ridge. The along-segment trend of increasing MgO towards the bathymetric crest of the axial volcanic ridge is similar to, but less well-defined than, that for the southern segment.

There is no variation in the bulk degree of melting along the segments as determined from the major element geochemistry; hence, melt migration is favoured over focused mantle upwelling as the main cause of the crustal thickness variations which define second-order segmentation. Radigenic isotopes show only very small variations with no systematic pattern emerging within or between the ridge segments. The isotopic irregularities are not related to the variations in incompatible elements, implying that the latter result from dynamic melting processes rather than from long-lived source heterogeneity. A model is developed which relates the volcanology, tectonic style and the geochemical trends to the episodicity of magma supply and eruption.
Ocean-ridge segmentation and vent tubeworms (Vestimentifera) in the NE Pacific
Authors: Southward Eve C. , Tunnicliffe Verena , Black Michael B. , Dixon David R. , Dixon Linda R.J.
Published: April 2008

Vestimentiferan tube worms are important components of the hydrothermal vent and cold seep communities of the Pacific Ocean. The distribution and geographic intraspecies variation of
Ridgeia piscesae
and
Lamellibrachia barhami
were examined in the region of the Explorer (1 site), Juan de Fuca (33 sites) and Gorda (2 sites) Ridges and the nearby Cascadia Subduction zone. Isozyme electrophoresis, DNA restriction fragment length polymorphism (RFLP) and DNA sequencing techniques have been used.
Ridgeia piscesae
is widespread at hot vent sites along the ridges. The transform offset between Explorer and Juan de Fuca does not appear to impede gene flow (isozyme and RFLP data). Hydrographic conditions close to the Juan de Fuca Ridge favour bidirectional along-axis transport of planktonic larvae and some cross-axis transport. Rapid colonization by
R. piscesae
has been observed at a new vent site on CoAxial Segment of the Juan de Fuca Ridge, supporting the idea of a pool of larvae in the water overlying the ridge. The 360 km offset between southern Juan de Fuca and Gorda Ridges is associated with a significant level of genetic differentiation (shown by RFLP) which indicates some interruption to larval dispersal at this scale. The occurrence of the cold-seep species
Lamellibrachia barhami
is confirmed at one hydrothermal site, the sedimented Middle Valley on Juan de Fuca Ridge. Vestimentiferans appear to have a tremendous dispersal, location and adaptation capability. It is only at the thousand kilometre scale, that complete barriers to dispersal occur.
Plate tectonic history and hot vent biogeography
Authors: Tunnicliffe Verena , Fowler C. Mary R. , Mcarthur Andrew G.
Published: April 2008

Since the 1977 discovery of hydrothermal vents and their extraordinary fauna, vents have been discovered along mid-ocean ridges in the eastern Pacific, on the Mid-Atlantic Ridge and on the spreading centres behind western Pacific subduction zones. The dependence of these habitats on plate activity suggests that plate history may play a strong role in their evolution. As few of the species known from vents are found in other marine habitats, an examination of biogeographic patterns with respect to plate history is feasible. An examination of taxon overlap with other sulphide-rich habitats (seeps and whale carcasses) suggests that a minority of faunal elements have common ancestors. Proximity of vent sites to each other does reflect regional similarities but it is the distance along ridge pathways and vicariant events that appear most important. An understanding of the tectonic history of our oceans provides a framework for predicting the composition of vent communities in unexplored areas.
Australian Proterozoic high-temperature, low-pressure metamorphism in the conductive limit
Authors: Sandiford Mike , Hand Martin
Published: April 2008

High-temperature, low-pressure (HTLP) metamorphism often reflects transient advection of heat due to magma ascent. However, the origin of HTLP metamorphism in a number of Australian Proterozoic terranes remains contentious either because of the deficiency of magmatic bodies in the terranes, or because the long time delay (>100 Ma) between magmatism and metamorphism precludes heating by existing magmatic bodies. Furthermore, a number of Australian Proterozoic HTLP terranes (such as the Reynolds Range in central Australia) show evidence of an extended history (
c.
30 Ma) of HTLP mineral growth suggesting metamorphism during a thermal regime dominated by conduction at lithospheric length scales. Australian Proterozoic metamorphic terranes are characterized by both elevated modern-day heat flows (averaging
c.
85 mW m
−2
) and granitic gneisses with anomalously high heat production rates (commonly >5–10 μW m
−3
). We show that the conditions required for HTLP metamorphism may result from conduction if the crustal heat production responsible for modern-day heat flows is concentrated at mid-crustal levels (15–20 km). Importantly, for low-intermediate mantle heat fluxes (10–20 mW m
−2
) and moderate synmetamorphic crustal thicknesses (
c.
45 km), the conductive geotherms attendant with such HTLP metamorphism do not necessarily lead to significant melting of a refractory lower crust. Importantly, the thermal regimes are very sensitive to the depths at which crustal heat production is localized. The strong dependence of the resulting geotherms on the depth of the heat-producing layer has the important consequence that only minor burial may be required to induce HTLP metamorphism, while only minor erosion (
c.
5 km) is necessary to terminate the event.
The implications of Sr-isotope disequilibrium for rates of prograde metamorphism and melt extraction in anatectic terrains
Authors: Harris Nigel , Ayres Michael
Published: April 2008

The preservation of chemical disequilibrium during anatexis can provide constrains on the rates of high-temperature processes provided that, firstly, the appropriate diffusion coefficients are known, and secondly, isotopes in the protolith have not been reset by dynamic recrystallization. In one example of an anatectic migmatite from the western Himalaya, isotopic homogenization has been approached between the leucosome, mesosome and melanosome at
c.
20 Ma, although apatite, with a low Sr diffusivity, remained in strong disequilibrium at that time. Intrusive granites are more likely to preserve evidence of isotopic disequilibrium provided a rapid rate of prograde metamorphism culminates in a rapid rate of melt extraction. Evaluation of the available Sr-isotope database from the Miocene Himalayan granites and their metasedimentary protoliths suggests that any increase of
87
Sr/
86
Sr in the melt relative to its source is less than 0.002. Mass-balance calculations indicate that isotopic exchange between plagioclase and mica in the protolith, and plagioclase and melt prior to extraction, must have occurred over a period in excess of 100–200 ka. Since rapid melt extraction (within <50 ka) is indicated by accessory phase dissolution rates, a comparatively slow rate of prograde metamorphism is indicated by the lack of Sr-isotope disequilibrium. This supports heating from internal heat production in thickened crust rather than from thermal advection or dissipative heating.
Chemical and volume changes during deformation and prograde metamorphism of sediments
Authors: Vernon R. H.
Published: April 2008

A long-standing problem is the extent of selective removal of chemical components and the amount of volume loss during the formation of slaty and crenulation cleavages during prograde metamorphism of sediments. Heterogeneous deformation leads to the formation of strongly deformed phyllosilicate-rich (P) domains and less deformed quartz-rich (Q) domains. The rocks generally show evidence of solution of quartz in P domains and addition of quartz in Q domains. The problem is whether the subtractions and additions balance or whether silica is lost from the system on the metre or broader scales. Structural studies tend to favour volume loss, whereas chemical studies tend to favour no volume loss. This review indicates that it is not yet possible to determine which of these views is correct. Deformation heterogeneity and fluid activity on a range of scales may play an important part in the development of slaty cleavage, suggesting the need for a detailed coordinated study, in which structural geologists and geochemists study identical material from the same sedimentary beds, in different parts of folds.

Local compositional changes may develop adjacent to quartz veins, and some aluminous (‘pseudopelitic’) mineral assemblages may develop in igneous rocks by base-cation leaching before or during metamorphism. However, broad chemical changes with metamorphic grade are difficult to determine, owing to protolith variation on all scales, although much more detailed chemical work with good geological control remains to be done. Marked local protolith variability may also hamper attempts to determine the chemical effects of cleavage formation.

As the density of metamorphic rocks increases generally with increasing grade, inferred conservation of mass on a regional scale implies loss of volume, which conceivably could be accommodated by collapse during deformation. Inferred volume loss implies a mass loss appropriate to the density increase. Inferred volume conservation implies regional mass gain, which is intuitively less probable. Marked chemical and volume changes occur when partial melts are removed at highest metamorphic grades.
The role of microcracking and grain-boundary dilation during retrograde reactions
Authors: Barker Andrew J. , Zhang Xing
Published: April 2008

Under conditions of sub-greenschist and low greenschist facies retrogression, both intragranular microcracking and grain-boundary dilatancy are important processes for facilitating fluid access into domains between fractures in metamorphic rocks. Both processes play a crucial role in promoting retrogression of high-temperature minerals. In a quartz-rich schist from Norway, garnet retrogression is clearly linked to microcracking in the vicinity of a minor fracture. Intense microcracking of quartz extends up to 12 mm from the fracture, and permits fluid infiltration. This causes extensive (70–100%) garnet retrogression to chlorite. At distances >16 mm from the fracture there is no sign of fluid infiltration and garnets are entirely unaltered.

The spatial distribution of grain-boundary reaction products in a scapolite-dominated rock from Scotland provides evidence for the significant role played by transient grain-boundary dilatancy in promoting grain-scale fluid access and resultant retrogression. By applying distinct element modelling (UDEC code) the geometrical differences in distribution of grain-boundary reaction products are directly related to subtle differences in the magnitude and orientation of applied stresses and patterns of transient grain-boundary dilatancy.
Recent experimental progress in the physical description of silicic magma relevant to explosive volcanism
Authors: Dingwell Donald B.
Published: April 2008

Explosive eruptions of silicic magmas are the result of a complex interplay of physico-chemical processes (e.g. decompression, volatile saturation, bubble nucleation and growth, crystallization, foaming, fragmentation and annealing). These processes occur over a relatively wide range of temperature, pressure, stresses and time scales. Furthermore, substantial changes in the chemical and physical properties of the eruptive magmas are induced by decompression and degassing. Numerical modelling of eruptive processes presents us with a picture of a magma column where enormous vertical gradients in the physical state and properties of the continuous magma body in the conduit result from decompression and degassing. Adequate representation of the physico-chemical evolution of eruptive systems therefore requires a relatively detailed description of melt properties in order to be robust and generalizable. The source of property data for silicate melts is experimental investigation. In recent years much effort has been concentrated on rhyolitic melts under conditions relevant to explosive volcanism. Here several aspects of this work are highlighted. (1) The description of hydrous rhyolitic melt properties (density, surface tension, viscosity and thermal conductivity) has been greatly improved. (2) The solubility of water as well as its diffusivity in silicic melts has been investigated in detail at relatively low pressures. (3) Rheological and transport complexities of melts and magmas such as non-Newtonian melt rheology, viscoelasticity, crystal and bubble suspension rheology, and foam deformation, permeability and stability have been explored. (4) Studies of the mechanical strength of magma have been initiated. Some of the insights into the nature of explosive volcanism provided by these studies are also of a qualitative nature (e.g. the degree of equilibrium during degassing, the longevity of textural magmatic states and the mechanism of fragmentation). It is hoped that this description of experimental progress in melt properties will encourage the reader to conpare the assumptions, descriptions and predictions of the modelling of eruptive processes presented in this volume with the physical nature of the magmas involved that is described herein.
Gas loss from magmas through conduit walls during eruption
Authors: Jaupart C.
Published: April 2008

There is ample evidence that the transition from an explosive eruption regime to an effusive regime can be due to magma losing gas to fractured country rock during ascent towards the surface. This is shown by the deuterium: hydrogen isotopic ratios and dissolved water contents of erupted samples, and by various petrological observations. Field studies demonstrate that the walls of an eruption conduit are fractured and penetrated by veins infilled with vesicular pyroclasts and ash. At Mule Creek, New Mexico, USA, a fossil eruption conduit filled with lava can be studied over a height of 300 m. The gas volume fraction exhibits complex variations with height and with horizontal distance from the walls, which is not compatible with closed system degassing. Lava close to the conduit walls is almost devoid of vesicles, showing that gas escape has been efficient. Gas flow through liquid magma may be achieved through fractures or through connected bubbles. Theoretical flow models which account for gas loss through conduit walls show that eruptive behaviour is very sensitive to the eruption rate and to the chamber pressure. A gradual decrease of chamber pressure, due to withdrawal of magma, leads to a transition from explosive to effusive conditions. Conversely, a gradual increase of chamber pressure, due to reinjection from a deeper source of magma, leads to a transition from effusive to explosive conditions. The initiation and propagation of gas-filled fractures during an ongoing eruption may be detected seismically. This chapter puts together several independent pieces of evidence in a coherent framework and includes a discussion of unresolved questions.
Analysis of digital elevation data for the Scottish Highlands and recognition of pre-Quaternary elevated surfaces
Authors: Ringrose Philip S. , Migoń Piotr
Published: April 2008

Remnants of elevated palaeosurfaces in the Scottish Highlands, proposed in numerous earlier studies, have been re-evaluated using digital elevation data from the Ordnance Survey. Histograms of elevation frequency, for areas 5, 10 and 20 km
2
, display multi-modal distributions which can be interpreted in terms of plateau and base-level components. The effects of glaciation, lithology and faulting can be inferred from patterns in the elevation distribution. The histogram modes reveal a series of elevated surfaces which broadly confirm previous, qualitative models of inclined Tertiary palaeosurfaces, but indicate a much more complex pattern. Many of the elevated surfaces are seen to be inclined towards two major faults, the Great Glen Fault and the Ericht-Laidon Fault. However, the highest surfaces may have crossed these fault zones uninterrupted. A model in which an episode of tectonic reactivation imposes a major change on pre-Quaternary geomorphological evolution in this area is proposed.
A pre-Pliocene or Pliocene land surface in County Galway, Ireland
Authors: Coxon Peter , Coxon Catherine
Published: April 2008

This paper describes a site on the Carboniferous limestone of County Galway, Ireland, where a complex of gorges, cave passages and shallow surface depressions is filled with organic silt and clay overlain by white quartz sand. The dating of the biogenic deposits to the Late Pliocene by biostratigraphical means provides a record of this largely undocumented period of Irish geological history. However, the particular importance of this site is that unlike other Irish karst infills, it represents not just the localized preservation of material in a closed depression but evidence of a more widespread cover of sediments suggesting the preservation of a Pliocene or pre-Pliocene land surface. This implies that glacial action throughout the Pleistocene has resulted in relatively little bedrock erosion in this region and raises the possibility that the present day landscape of the western Irish limestone lowlands may retain influences of preglacial karstification.
Palaeosurface palynofloras of the Skye lava field and the age of the British Tertiary volcanic province
Authors: Jolley David W.
Published: April 2008

Collection and palynological analysis of intratrappean boles, mudstones, sands, conglomerates and coals from the Skye lava field has allowed the reconstruction of geomorphological features and vegetation distribution on five palaeosurfaces. These palaeosurfaces show a changing pattern of streamside, swamp and upland vegetation which fall into three forest types: mixed mesophytic, upland Taxodiaceae and montane conifer forest. Comparisons of the Skye palynofloras with others analysed from the intra and intertrappean beds of the Antrim, Mull and Small Isles lavas, gives evidence of three (possibly four) main phases of British Tertiary volcanic province extrusive activity. In comparison to complete palynological records from the Faeroe-Shetland Basin, Skye and Antrim palynofloras can be dated as ranging from 58.23 Ma to 57.99 Ma, while those of the Mull Lavas, and coals below the Faeroe Islands Middle/Upper Basalts indicate initiation and resumption of extrusion after 55.00 Ma. Evidence of the age of palynofloras from the intratrappean beds of Eigg and Muck is limited, and does not dispute the Chron 27r age suggested by isotopic analysis (
c.
62 Ma). Finally palynological evidence is presented showing two major subsidence phases, and one uplift phase during the 0.24 Ma existence of the Skye Lava field. It is suggested that altitudes in excess of 1200 m were experienced during thermal doming related to the emplacement of the Cuillin centre. The relative timing of this and other volcanic events is compared to subsidence patterns in the marine sedimentary record.
Tertiary palaeosurfaces of the SW Deccan, Western India: implications for passive margin uplift
Authors: Widdowson M.
Published: April 2008

Two genetically distinct lateritized palaeosurfaces of different ages are recognized in the southwest Deccan Traps region of Western India using a combination of geochemical, topographical, and satellite image data. The Deccan Traps were erupted at the Cretaceous-Tertiary boundary (c.65 Ma), and comprise a huge area of originally near-horizontal basalt lavas covering much of northwest Peninsular India, and topographically forming the coast-parallel Western Ghats escarpment and elevated Maharashtra plateau to the east.

Remnants of the older, palaeosurface currently exist as a series of isolated, laterite-capped plateaux forming the highest elevations along the Western Ghats (15°30′–18°15′ N). This surface is of late Cretaceous-early Tertiary age, and originally developed upon flows which lay at, or near to, the top of the lava sequence. This lateritization phase was terminated by a period of uplift and extensive erosion in lower- to mid-Tertiary times during which the low-lying, low-relief coastal (Konkan) plain developed through the eastward recession of the Ghats scarpline. A second phase of lateritisation occurred upon this coastal pediplain during mid- to late Tertiary times. Since the earlier uplift had gently deformed the lava pile prior to the development of the pedimented surface, the low-level Konkian laterite lies with marked angular unconformity upon the lava stratigraphy. Both surfaces have been subject to further large-scale distortion resulting from continuing uplift effects.

Development and evolution of these Deccan palaeosurfaces is important since together they provide a record of uplift effects in western India. Moreover, they offer a datum against which the uplift erosional history may be further constrained and demonstrate that uplift effects have acted upon the Indian margin throughout the Tertiary. Since such longevity of uplift is difficult to reconcile with the commonly cited thermal and dynamic post-rift mechanisms known to act upon passive margins, the morphological and structural evolution of the rifted Deccan margin is better described in terms of denudational isostasy.
Topography, palaeosurfaces and denudation over the Karnataka Uplands, southern India
Authors: Gunnell Yanni
Published: April 2008

A synthesis of new information from extensive field reconnaissance and systematic map analysis from southern India is presented. A new interpretation for landscape development in the Karnataka upland region is given. Three phases of geomorphological evolution are identified.

A fundamental, pre-Deccan Trap palaeosurface is recognized. While becoming largely fossilized at 65 Ma by the eruption of the Deccan lavas, its southern continuation, studied here, remained exposed throughout the Tertiary and evolved in response to uplift and crustal flexuring of the Indian margin. Remnants of this surface now occur as the highest summits of the region (c.1900 m above sea level).

A younger, intermediate or post-Deccan Trap palaeosurface may be recognized by a combination of superimposed drainage and morphological mapping techniques. It evolved in response to the loading of the Indian shield by the 1–2 km thick Deccan lava pile.

The third and final phase of landform development is complex since it has not resulted in a single surface of regional extent but rather a series of contemporaneous, but locally restricted, levels at varying altitudes. These are not interpreted as the result of cyclical erosion but as controlled in acyclical fashion by individual river catchments and lithological heterogeneity. The possible causes for this complexity are discussed and confronted with existing schemes of landscape development on other continents.
Archaean crustal growth and tectonic processes: a comparison of the Superior Province, Canada and the Dharwar Craton, India
Authors: Choukroune P. , Ludden J. N. , Chardon D. , Calvert A. J. , Bouhallier H.
Published: April 2008

We present a comparison of the processes involved in the tectonic evolution of two Archaean cratons, the Superior Province of Canada and the Dharwar Craton of India. These two cratons exhibit distinct map patterns, the Superior Province being dominated by elongate belts, while the Dharwar Craton is characterized by dome and basin features.

We suggest that certain tectonic processes operating in the Phanerozoic, such as enhanced mantle plume activity, subduction of young (warm) oceanic crust, and faster than usual accretion of crust, may have been the norm during the Archaean. In the Superior Province rapid crustal growth occurred, largely due to horizontal tectonic forces. Models analagous to modern plate tectonics are applicable, but the rates of convergence and accretion exceeded those normal for the present day. Accreted crust was warm and subject to more ductile deformation than in modern accretionary zones. These accreted arc, ocean-floor and ocean-plateau fragments would have been underlain by a thick refractory, buoyant, warm lithospheric root that was rapidly underplated (or imbricated) below the recently accreted terranes. In the Dharwar craton a major thermal event appears to characterize its evolution at 2.5 Ga. Reheating of the lower and middle crust in response to magmatism and metamorphism, resulted in diapirism and growth of crust in a vertical sense.

The southern Superior Province’s evolution reflects accretion at the margins of a protocraton, while the Dharwar craton’s tectonic environment may reflect plume impact and incipient rifting in the centre of an Archaean craton.
Structure and geodynamics of the Uralian orogen
Authors: Puchkov V. N.
Published: April 2008

The Urals are a Late Palaeozoic orogenic belt. The relicts of earlier orogens are traced in its basement. In particular, the Late Vendian pre-Uralian orogen is reconstructed and identified as a part of the Late Precambrian Cadomian orogen. The Uralian orogeny was preceded by Late Cambrian-Early Ordovician epicontinental rifting and formation of the Paleo-Uralian ocean whose remnants are Palaeozoic ophiolites. Calc-alkaline volcanites and plutons, typical of active margins, are widely developed in the eastern Urals. The Uralian foldbelt results from oblique collision between the East European (Laurussia) passive margin and the active margin on the Kazakhstanian continent. Collision began in the south of the Urals and moved, wave-like, to the north. The eastern and northern parts of the Urals have been affected by the Middle Jurassic Cimmerian intracontinental (intra-Pangaea) shortening. The Uralian-Cimmerian mountain belt was eroded and partially inundated by seas in the Late Jurassic-Early Cretaceous times and has been reactivated since the Oligocene in response to a recent intracontinental shortening.