bluuebunny/crossref_metadata_2025 huggingface dataset:
On cleavage and strain: results of a study in West Germany using tectonically deformed sand dykes
Authors: BORRADAILE GRAHAM J.
Published: December 2007

“Slates” in the Schiefergebirge east of the Rhine contain tectonically deformed sedimentary dykes. The present orientations of the sand dykes relative to cleavage and bedding allow one to identify discrepancies of more than 30° between the cleavage and the
XY-
plane of the macroscopically defined finite strain ellipsoid in some localities. Further, microscopic evidence of a non-coaxial strain history is provided by curving fibre “strain shadows”. Individual surfaces composing the anastamosing cleavage are considered to have been generated in or near local principal plane orientations although the mean cleavage plane deviates from the principal plane of the bulk finite strain ellipsoid. This is because the cleavage has been “frozen in” to the rock fabric as material surfaces which could not keep pace with the rotation of the strain axes and is also due to the cleavage-forming processes having been operative only at an advanced stage in the strain history. The spaced, stylolitic nature of the cleavage, as opposed to the penetrative fabric of classic “slaty” cleavages, is attributed to a relatively thin sedimentary cover at the time of cleavage formation.
Transform fault systems of California and New Zealand: similarities in their tectonic and seismic styles
Authors: SCHOLZ C. H.
Published: December 2007

When the San Andreas fault system of California and the Alpine fault system of New Zealand are compared within the reference frame of their regional slip vectors, their major tectonic elements are seen to occupy similar positions. This argues that a genetic relationship exists between the development of these complexities.

Furthermore, three distinctive modes of slip, or ‘seismic style’, can be recognized for different segments of these major transform fault systems. These are: (1) slip during great
(M
≥ 8) earthquakes separated by long periods of quiescence; (2) slip during more frequent large (6.5 ≤
M ≤
7.5) earthquakes, and (3) aseismic slip (creep). These different seismic styles occur in distinctive tectonic settings and it is argued that they represent fundamental variations in the mechanical properties of the faults. I suggest that style 1 occurs along sections of the fault orientated such that a component of tectonic convergence occurs, leading to a high effective normal stress and hence high frictional strength. Style 2 occurs along sections of faults which strike close to the regional slip vector, and hence have a lower normal stress. Aseismic slip occurs only along sections of faults in central California where anomalously high pore pressures reduce the effective normal stress to a very low value and the fault is entirely within the stable sliding frictional field.
The relationship between Precambrian shear belts and modern fault systems
Authors: GROCOTT J.
Published: December 2007

Deformation in the brittle (seismic) mode is characteristic of the upper parts of fault systems, while at lower levels displacements are predominántly ductile (aseismic). It is suggested that the geometry and products of ductile and brittle displacements can be examined in eroded Precambrian faults. The Ikertôq shear belt in western Greenland is cited as an example of how, due to uplift and erosion, all the deformation products which occur through the vertical extent of fault systems, and the geometry of deformation processes associated with the formation of these products, can be observed at the present erosion level in a Precambrian shear belt. It is argued that this circumstance allows the use of Precambrian faults to infer the various deformation processes occurring in modern, highly active, fault systems.
An experimental investigation of the development of structures in multilayers under the influence of gravity
Authors: BLAY P. , COSGROVE J. W. , SUMMERS J. M.
Published: December 2007

A series of model experiments which were designed to simulate certain aspects of the development of large-scale folds in bedded sedimentary rocks are described and illustrated. The models used were multilayers made up of thin sheets of gelatine. Two types of experiment were carried out. In the first a section of the base of a horizontally layered model was slowly raised to simulate uplift and gravity gliding. In the second a horizontally layered model was laterally compressed by a slow moving piston. The initial development of lateral compressive stress in both types of model was achieved by the propagation of a stress front along the length of a model. In a number of experiments this stress front played an important role in localizing the initiation of folds. The existence and rate of propagation of the stress front were related to the presence of shear stresses along the base of a model. The development of folds in the majority of experiments resulted in well defined anticlines separated by broad, poorly defined synclinal zones. Individual anticlines were initiated at different times during the course of an experiment and this resulted either in the serial development of a train of folds or in the development of isolated anticlines. Asymmetric anticlines were, during the initial stages of amplification, upright symmetrical structures. Asymmetry tended to develop late in the amplification history of such folds. In three dimensions, folds tend to be non-cylindrical and adjacent structures form an array of
en echelon
periclines.
Granite diapirism in Archaean rocks
Authors: STEPHANSSON O.
Published: December 2007

This contribution is an extended abstract of a paper published elsewhere. It is included as an introduction to the published discussion which was stimulated by Professor Stephansson’s lecture.

IN A RECENT PAPER the author suggested a classification of igneous intrusions (
Stephansson 1975
). The term batholith is proposed for a large composite body of granitic diapirs and narrow septa of supracrustal rocks in a region of orogenic deformation. The diapirs may be of the same or different composition and have a characteristic feature of piercing a root of older rocks.

The Svecofennian of central Sweden has been divided into a number of batholiths and subprovinces where metasediments, metavolcanics and migmatites predominate over the intrusions in the subprovinces. The batholiths consist of mostly granitoid intrusions of deep-seated type. The Vdala-Uppland batholith is the largest and most characteristic in form of the batholiths in central Sweden. It has an almost circular outcrop within a belt of synorogenic granites and foliated granitic gneisses which concentrically surround a core of less deformed quartz diorites and granodiorites. The division of central Sweden into batholiths and subprovinces and the distribution of granite diapirs within each batholith is supported by gravity data.

The variation in the tectonic appearance of diapirs in orogenic zones results from a number of factors. Depending on the characteristics of the diapir, e.g. size, structure, contact with surrounding rocks these are divided into epi-, meso- and catatectonic diapirs (
Stephansson 1975
). Most of the intrusions in the Vdala-Uppland batholith are of catatectonic type and their
Inferred isocrymal distribution of Jurassic dasycladacean Algae in Europe, north Africa and southwestern Asia
Authors: ELLIOTT G. F.
Published: December 2007

The distribution of certain dasycladacean algae (Chlorophyceae) in the Jurassic of Europe and adjacent parts of Africa and southwestern Asia is summarized. The isocrymal limits for different groups of living dasycladaceans are indicated. The Middle Jurassic dasycladacean distribution, when plotted on the ‘Pangaea stage of continental displacement 180 Ma B.P.’ (
Owen 1976
) shows that, on the then Earth’s surface, there is a marked line of distribution limitation, now inferred as isocrymal in origin. Beyond this the very limited records available suggest either a more limited temperature tolerance in the Middle Jurassic dasycladacean flora than exists in the living dasycladacean flora, or a distribution due to the effects of increased solar radiation on an Earth of smaller diameter than at present.
Palaeozoic volcanism in Great Britain and Ireland
Authors: STILLMAN C. J.
Published: December 2007

IT IS APPARENT that the study of Palaeozoic volcanism attracts a multi-disciplinary approach involving sedimentologists as closely as volcanologists, and utilizes techniques ranging from field mapping to the statistical handling of geochemical data. This interdependence was revealed by the list of contributions and even more by the discussion stimulated by the papers read at the September 1976 meeting of the Volcanic Studies Group, held in the Geology Department, Trinity College, Dublin. It is also clear that for a fair proportion of workers in both Caledonian and Hercynian volcanism, plate-tectonic models are now being invoked both to explain the petrogenesis of volcanic suites and to account for regional variations in petrochemistry.

Speakers who eschewed this 'armchair' approach were concerned with regional field aspects and emphasized the need for detailed field mapping as a basis for any interpretation. Dr Max described work in progress on the correlation, distribution and deformation of the South Connemara Group volcanics, including the extension of land-based studies by surveys of islands in Galway Bay. Dr Howells demonstrated the relationship between the Bedded Pyroclastic Formation of central Snowdonia, and the Middle Crafnant Volcanic Formation of eastern Snowdonia, and showed that, whilst synchronous, two very different volcanic facies derived their material from different sources––the Bedded Pyroclastics from the Snowdon area and the Middle Crafnants from sources east of Betwsy Coed. In the region of Dolwyddelan a rising submarine ridge is invoked to provide separation between the shallow-water, bedded basaltic tuff sequence of the former and the acid
LISPB–III. Upper crustal structure of northern Britain
Authors: BAMFORD D. , NUNN K. , PRODEHL C. , JACOB B.
Published: December 2007

Interpretation of upper crustal data obtained during the LISPB seismic experiment reveals the velocity structure of the pre-Caledonian basement in northern Britain. Lewisian-like basement with a relatively high seismic velocity (> 6.4 km/s) extends from the Caledonian foreland into the Midland Valley and probably terminates at the Southern Uplands fault. To the south, beneath northern England, the basement has a lower velocity (< 6.3 km/s). We suggest that a horizontal discontinuity may exist in the pre-Caledonian basement between the Southern Uplands fault and the Stublick Line though we cannot yet determine the exact nature of this discontinuity.
The possible mechanics of convolute lamination in graded sand beds
Authors: ALLEN J. R. L.
Published: December 2007

Gravitationally unstable vertical gradients of bulk density can arise during the re-sedimentation of water-logged normally graded sand beds following liquidization, even though the density gradient imposed on the beds by the depositional process was originally stable. Convolute lamination, found chiefly in turbidites, is therefore capable of being formed under a wider range of circumstances than was formerly appreciated, and the presence of an unstable bulk density gradient dating from the time of deposition is not necessary for the manifestation in sediments of this type of Rayleigh–Taylor instability. Although the analysis promoting these conclusions cannot at present be tested experimentally because of practical difficulties, a laboratory study of the analogous sudden heating of a layer of viscous liquid from below affords some evidence in its support.
Pressure solution and Coble creep in rocks
Authors: McCLAY K. R.
Published: December 2007

On 5 November 1976, the Tectonic Studies Group of the Geological Society held a lively discussion meeting at Imperial College on the subject of pressure solution and Coble creep.

Deformation mechanisms may be divided into three broad categories:

(1) Cataclastic processes

(2) Intracrystalline processes involving dislocations

(3) Diffusive mass transfer processes.

We are concerned here with group (3) processes, flow by diffusive mass transfer of matter from grain boundaries which are subjected to high normal stress to less highly stressed grain boundaries. If the diffusion is predominantly through the grain the process is termed Nabarro-Herring creep whereas if it is predominantly around grain boundaries, the flow is termed Coble creep. In general, grain boundary sliding must occur to accommodate the grain shape changes due to diffusion. For these processes to proceed at rates which are detectable, the temperature must be at a large fraction of the absolute melting temperature of the material.

Geologists have long recognized in low grade metamorphic rocks, textures in-dicative of deformation by diffusive mass transfer. Even over the relatively long time available for geological deformations, at the low temperatures concerned, solid state diffusivities would be too slow to account for the observed strains. It is therefore inferred that the rate of diffusive mass transfer is enhanced by the presence of an intergranular fluid film. Hence the use of the term ‘pressure solution’ to describe the process. Thus Coble creep and pressure solution are similar in that they both involve intergranular diffusion.

The meeting was intended to
RRS
Shackleton
in the Indian and south Atlantic oceans
Authors: Cronan D. S.
Published: December 2007

RRS S
hackleton
undertook one of the longest voyages of her career between October 1974 and July 1976. After leaving England, she worked in the equatorial and south Atlantic, prior to rounding the Cape of Good Hope into the Indian Ocean, and then went down to work in the Antarctic. Finally, she undertook some work in the eastern Pacific, before returning home through the Panama Canal. These cruises were not without their moments of excitement, such as the attempted arrest off the Falkland Islands by an Argentinian destroyer. Most of the time, however,
Shackleton
carried out routine work and collected a great deal of valuable material which is being worked on at various centres throughout the country.

The largest single part of
Shackleton’s
voyage was spent in the western Indian Ocean, where she undertook a number of 20-26 day cruises. This region is one which has been visited in the past by British research vessels, most notably by RRS
Discovery
in 1963 and 1967. Some of the work planned for
Shackleton
was to follow up studies made on these two cruises, but the greater part of her work was on the investigation of new problems.

After working with Dr Calvert’s group off SW Africa,
Shackleton
sailed for the Gulf of Aden, where the first of the Indian Ocean geophysical cruises, that with Dr Girdler’s group, was undertaken. This was followed by geophysical work in the Gulf of Oman, with Dr Matthews’ group, and in the northern Arabian Sea, with Dr Whitmarsh’s
Structural style of continental margins: a discussion
Authors: AUDLEY-CHARLES M. G.
Published: December 2007

Publication of the results of continuous seismic reflection surveys around the continental margins (Beck & Lehner 1974, Seely
et al.
1974, Lehner & Ruiter, in press) and discoveries made by drilling in the deep-sea by JOIDES and IPOD (Initial Reports of the Deep Sea Drilling Project) have shown that seismically active continental margins display structures very different from the structures associated with aseismic margins.

Lehner showed that after the initial rifting phase of evolving aseismic margins the main structural features are vertical movements manifested by the development of sedimentary basins at the margin of the new ocean and complementary basins on the craton. Some of these cratonic basins may be associated with aulacogens (Dewey & Burke 1974), others appear to be separated from the continental margin basins by a geanticlinal structure.

The structural style of the present aseismic margins of West Africa, northwest Australia and elsewhere can be summarized as asymmetric in cross section showing a ‘steep, mostly faulted landward flank on continental basement and a gentle seaward flank on oceanic basement' (Lehner).

Evidence is available from parts of the Mediterranean region (Smith) and from northern Australia (Garter
et al.
1976) which shows how within few Ma an aseismic continental margin can become seismically active and be involved in orogenic processes such as major overthrusting.

Recent papers by Fischer (1975) and Bally (1975) have greatly influenced the way we think about the structural style of seismically active continental margins. Fischer’s suggestion that oceanic lithosphere has a maximum life of about
The crust-mantle boundary in space and time
Authors: WINDLEY BRIAN F.
Published: December 2007

This review links the main points in the set of papers in vol. 134, part 2 dealing with reasearch related to the crust-mantle boundary. Whilst it is possible to characterize the properties of the boundary and the rocks above and below it in Mesozoic-Recent oceanic and continental areas (and their modifications during the plate tectonic scheme of events), we are also able to study lower crustal and upper mantle rocks of Precambrian and Palaeozoic age. The history of the boundary is directly related to continental evolution throughout geological time.
Sub-cratonic crust and upper mantle models based on xenolith suites in kimberlite and nephelinitic diatremes
Authors: Dawson J. B.
Published: December 2007

Kimberlite and nephelinitic magmas have sampled the upper mantle and crust beneath the ancient cratons
en route
to the surface from their points of origin within the upper mantle. Study of the xenolith suites at most localities favours the hypothesis that the Moho is a chemical break, with lower crustal granulite facies rocks overlying harzburgitic or dunitic upper mantle rocks. The model for Kansas, USA, however, favours the hypothesis that a broad zone of granulite-facies rocks straddles the Moho. Bulk chemical analyses of garnet-pyroxene granulites from Matsoku, Lesotho and Lashaine, Tanzania, show them to be similar to gabbroic rocks, but certain other features (high K/Rb, low Rb/Sr) suggest that they may arise by ‘basification’ of rock of intermediate composition during high-grade metamorphism with associated anatexis. These lower crustal granulites are not chemically similar to upper mantle eclogites.
Thermal history of excavation of Archaean gneisses from the base of the continental crust
Authors: O'HARA M. J.
Published: December 2007

General considerations of the mineral assemblages likely to be developed at the crust-mantle boundary, the manner in which such rocks are brought to the surface and the criteria for identifying them are followed by a specific model of events in the Scourie gneiss complex.

The Scourie gneisses equilibrated their strontium isotopes to upper mantle ratios 2.8 Ga (1 Ga = 10
9
years) ago at the base of a
c.
50 km thick crust. Conditions of 15 kbar, 1250°C with low water fugacity, imply thick crust
(c.
50 kin) very high crustal geothermal gradient
(c.
25°C/km over 50 km) and probable residual character of the entire complex due to loss of granitic fractions. Recovery history was marked by (i) rapid reduction in pressure to
c.
11 kbar at high temperature (consistent with near adiabatic uplift) followed by (ii) rapid fall in temperature to
c.
450°C at pressures of 11–7 kbar (consistent with rapid cooling of the outer parts of the earth in the late Archaean) prior to dyke intrusions at 2.2–2.4 Ga. Subsequent thermal history until surface exposure 10 Ga ago is poorly marked by mineralogical events.

Closure of different mineral systems to chemical change at several stages allows the thermal history of excavation to be unravelled.
Effects of H
2
O and CO
2
on magma generation in the crust and mantle
Authors: WYLLIE P. J.
Published: December 2007

The solution of H
2
O, CO
2
, and H
2
O–CO
2
mixtures in silicate magmas has many effects. Fusion temperatures are lowered, and the fusion and crystallization sequences may be modified. Hydrous minerals and carbonates may become stable within the fusion interval. Liquid compositions are changed as a function of dissolved volatile content, and as a function of H
2
O/CO
2
. The physical properties of magmas may be influenced markedly by dissolved volatile components. The presence of vapour bubbles arising from exsolution of volatile components during uprise or crystallization of magma may be responsible for physical and geochemical effects. These effects are outlined by examination of the phase relationships in several synthetic systems and rock–H
2
O–CO
2
systems. The results demonstrate that H
2
O is more influential than CO
2
in magma generation at crustal pressures. The presence of CO
2
, however, may influence the evolutionary history of a crustal magma body. In the mantle at depths of 75–80 km, the influence of CO
2
becomes as significant as that of H
2
O because CO
2
reacts with peridotite stabilizing calcic dolomite, and the solubility of CO
2
increases dramatically. The compositions of near-solidus mantle magmas are strongly influenced by H
2
O/CO
2
and by the distribution of carbonate, amphibole and phlogopite in the peridotite. The normal product of crustal anatexis is H
2
O-under-saturated granitic liquid. Tonalite liquids can-not be produced by crustal anatexis during normal regional metamorphism.
The Kannaviou Formation, Cyprus: volcaniclastic sedimentation of a probable late Cretaceous volcanic arc
Authors: ROBERTSON A. H. F.
Published: December 2007

In SW Cyprus the upper pillow lavas of the Troodos Igneous Complex are directly overlain by up to 75° m of volcaniclastic sediments, ranging in age from Lower Campanian to Mid-Maastriehtian. The basal sediments are pre-dominantly non-calcareous bentonitic clays and radiolarian mudstones representative of relatively slow deposition, probably below the carbonate compensation depth. In contrast, stratigraphically higher calcareous bentonitic clays, volcaniclastic siltstones and volcaniclastic sandstones show evidence of much more rapid deposition by a combination of mass flow, traction and minor turbidity currents.

The basal volcaniclastic sandstones were partly derived by submarine erosion of the subjacent Troodos lavas. In contrast, the overlying sequences contain abundant fine-grained pumice, basic to acid volcanics, and also terrigenous material derived predominantly from outside the present area of southern Cyprus. The hetero-geneous provenance of the Kannaviou sediments can be explained by postulating late Cretaceous subduction of Troodos-type oceanic-crust with genesis of a related volcanic arc and eventual tectonic juxtaposition of the Troodos Massif with a Mesozoic continental margin.
Fluid inclusions and mineralization at Cligga Head, Cornwall, England
Authors: N.J.JACKSON  , MOORE J. McM. , RANKIN A. H.
Published: December 2007

Thermometric and salinity data are presented for fluid inclusion populations in samples of hydrothermal vein material (quartz intergrown with wolframite, cassiterite, stannite, arsenopyrite, pyrite/chalcopyrite, hematite and limonite) and in metasomatic quartz over-growths in granite, greisen and kaolinized granite. These data indicate that multistage high temperature (Th 200– 400°C) and single or multistage low temperature (Th 70–150°C) hydrothermal events occurred within the Cligga Head granite stock and its environs. Although fluid inclusion salinities were low (2–12 equiv, wt. % NaCl) throughout the hydrothermal history of the stock, there is a general tendency for the salinity of the vein fluids to decrease with decreasing temperature. The homogenization temperature and salinity of the vein fluids of the wolframite-cassiterite mineralization are similar to those of another Cornish Sn-W deposit at St Michae¾s Mount. Low to moderate salinity fluids in the Th range 300–400°C appear to be a characteristic of the cassiterite-wolframite mineralization in the Cornubian metallogenic province.
Depositional environment and diagenesis of Permian Rotliegendes sandstones in Leman Bank and Sole Pit areas of the UK southern North Sea
Authors: Glennie K. W. , Mudd G. C. , Nagtegaal P. J. C.
Published: December 2007

Early diagenetic features of the Rotliegendes are related to deposition in a desert environment. Wadi sands suffered early calcite cementation as the sediment dried. Aeolian sands were cemented with halite and gypsum where the water table was close to the surface and all grains became coated with ferric oxide and clay after passing below the water table.

Later diagenesis is recognised by the presence of pore-destroying cements (recrystallized clays, authigenic feldspar, quartz, dolomite and anhydrite). Their occurrence is related to deep burial followed by faulting and late Cretaceous uplift. In Leman Bank and Sole Pit, the top Rotliegendes reached depths of about 11000 and 14000 ft (3350 and 4250 m respectively) prior to maximum uplift of 6000 ft (1800 m).
Sandstone-framework instability as a function of burial diagenesis
Authors: Nagtegaal P. J. C.
Published: December 2007

The effect of sandstone-framework instability during diagenesis on porosity and permeability is compared for quartz, arkosic and lithic arenites. Of the cases investigated, quartz frameworks are the most stable and suffer only from mechanical compaction and pressure solution. Arkosic frameworks have variable stability involving potential widespread alteration of feldspars. The least stable are the lithic sandstone frameworks which are suscepti­ble to all four main porosity and permeability-reducing processes: mechanical compaction, plastic deformation, pressure solution, and mineralogical alteration of framework constituents.
Porosity gradients in North Sea oil-bearing sandstones
Authors: Selley R. C.
Published: December 2007

The relationship between porosity and burial depth of sandstone reservoirs is complex. It depends on primary porosity, which is related to grain-size and depositional process and also on the porosity gradient (the change in porosity with depth) which, in most cases, is linear.

This relationship may be expressed as:

φ
d
=φ
p
−

G.D, where

φ
d

is the porosity at depth D below surface,

φ
p

is the primary depositional porosity, and G is the porosity gradient in % porosity per 1000 ft (305 m). Porosity gradients decrease with increasing quartz content, abnormal pressures, and in the presence of hydrocarbons. They increase with increasing geothermal gradient. There are at present insufficient data to quantify these relationships.

Empirically derived porosity gradients for North Sea Rotliegendes, Jurassic and Palaeocene reservoir sandstones are 2.2, 2.6 and 2.9 per cent per 1000 ft (305 m) respectively.

When calibrated with the porosity spectra of the various facies, graphs can be drawn showing the depth: porosity windows within which economic reservoirs may be found.
Dendrites
Authors: Van Straaten L. M. J. U.
Published: December 2007

Two categories of dendrites exist: those in which the shape is mainly determined by crystal structure, and those where this is not the case, the material being amorphous or ultra-finely crystalline. Only dendrites of the second category are dealt with in this paper, and especially those found in the Solnhofen Limestone of southern Germany. These dendrites consist either of Mn- or Fe-(hydr)oxides.

Two classes occur: (1) fissure dendrites, limited to joints and bedding plane fissures, etc.; (2) internal dendrites, branching in three dimensions, and penetrating from dendrites or other Mn-or Fe-accumulations on joint surfaces or bedding planes into the non-fissured rock. Both types form by deposition in pores, not wider than a few μm, thus constituting ramified systems of host rock, cemented by Mn- or Fe-material.

The first generation of Mn-dendrites in the Solnhofen Limestone evolves in two steps: (1) manganiferous solutions, derived from residual loams and alluvial deposits overlying the limestone, penetrate into joints, and from there into bedding plane fissures and unfissured rock; (2) from these solutions solid dendrite material is deposited. The precipitation starts from older deposits of the same composition: specks, lumps, crusts etc., or from newly generated nuclei. In this way either 'tree'- or 'star'-shaped dendrites are produced. 'Tree'-shaped dendrites on bedding planes, starting from joints, grow in directions away from these joints, the material being supplied from the opposite directions. Once formed, renewed supply of Mn-material results in thickening and broadening of stems and branches, the ‘texture’ thereby becoming coarser, but the total size remaining unchanged.The first formation of Fe-dendrites takes place in the same manner, but their further growth is more complicated.

The branching of these dendrites is due to the circumstance that diffusion of the components of the solution is limited to a system of micropores. This inhibits the production of ‘massive’, round patches around the points of origin, because the average distances to be covered by the ions, molecules or sol particles before they are fixed on the growing deposits, are much smaller for branched shapes.

Most dendrites are the result of repeated deposition. The transportation of Mn and Fe to the fissures or other sites of precipitation probably occurs chiefly as dissolved chelates. In most cases the precipitation itself may be due to a rise in pH as a consequence of dissolution of CaCO3.
Computer programmes for the analysis and presentation of orientation data
Authors: Nuttall D. J. H. , Cooper M. A.
Published: December 2007

This letter gives a brief preliminary description of computer programmes for use by structural geologists. Since the GODPP was described at the Tectonic Studies Group meeting at Dundee in December 1976 a number of workers have expressed interest and this letter is intended to make the existence and scope of the package more widely known. Copies of the programmes involved may be obtained from the authors at University of Bristol.

The Geological Orientation Data processing package (GODPP) is a package of computer programmes principally aimed at helping the field geologist to process large quantities of structural and sedimentological orientation data. The programmes have been so written as to be independent of the model of computer used (see Appendix).

The package offers the plotting of orientation data on stereograms, maps, and rose diagrams and also offers reorientation routines.

Stereograms.

This section of the package has facilities for producing Pi diagrams and contoured Beta diagrams with options for a vector mean or a best fit great circle, with the appropriate statistics (
Mardia 1972
). A large number of programmes to plot stereograms have been published (

Robinson
et al.
1963

, Spencer & Clabaugh 1967,
Warner 1969
and Bonyun & Stevens 1970). The majority produce line printer output, although a graph plotter has been utilised for the plotting of Pi diagrams (
LaFountain 1970
), but not apparently for contoured diagrams. GODPP has the advantage of producing Pi diagrams and contoured stereograms on a graph plotter. It also produces line printer output of contoured diagrams which allows
The stratigraphy and structure of the Kamasia Range, Kenya Rift Valley
Authors: Chapman G. R. , Lippard S. J. , Martyn J. E.
Published: December 2007

The Kamasia Range (or Tugen Hills) is an uplifted, faulted and arched block lying between the Elgeyo Escarpment, which forms the western boundary of the Kenya Rift, and the axial Baringo-Suguta trough between 0°15′N and 1°30′N in the northern part of the rift. Metamorphic basement rocks are exposed in the central part of the Kamasia area where they are overlain by 3000 m of Miocene lavas and sediments ranging in age from 16 Ma to 7 Ma. This is the thickest sequence of rocks of this age exposed in the Kenya Rift. Downwarping occurred from the beginning of this period, but the first major fault movements occurred at about 7 Ma (late Miocene) forming the Elgeyo and Kamasia fault-scarps. Later volcanicity was mainly confined to the area east of the Kamasia, although extensive flood trachyte lavas covered practically the whole area east of the Elgeyo at one time. A second period of major fault movement occurred in the late Pliocene-Pleistocene (2–0.5 Ma) uplifting the Kamasia Range, forming the western dip-slope and downfaulting the axial graben of the rift to the east.

The overall structure of the Kamasia is a broad arch cut by large faults, with maximum displacement of about 4000 m, just to the west of its axis, but in detail the structures are complex. The numerous faults are normal dip-slip type and show an
en echelon
and obliquely intersecting pattern with dominant directions 000–040° and 330–340°. Folding on axes perpendicular to the fault planes produced half-domes and basins indicating a secondary compressive stress directed along the rift. It is proposed that the majority of the faults developed as vertical fractures in horizontal rock sequences and were then rotated by continued extension and up-arching to produce complex tilted fault-block systems both synthetic and antithetic to the main rift structure.
Heat flow, radiogenic heat production and crustal temperatures in England and Wales
Authors: Richardson S. W. , Oxburgh E. R.
Published: December 2007

New values of heat flow are reported from thirty-one sites in England and Wales. Thermal conductivity of core material from fourteen of the sites and thermal gradients, obtained over several hundred metres, were used to calculate reliable heat flow values for these localities. At the remaining seventeen sites the value of heat flow is calculated from thermal gradient measurements alone, assigning conductivity on the basis of stratigraphic logs. An average heat flow of 1.40 μcal/cm
2
s is computed for the region, based upon 40 sites. Slightly higher than average heat flow may characterise the Lincolnshire sedimentary basin, within which temperatures may reach 100°C at about 3 km depth.

Surface heat flow is linearly correlated with heat production in local granitic or low-grade metasedimentary basement encountered in ten boreholes. The intercept of the dependence is 0.56±0.07 μcal/cm
2
s; and the high slope, 16.0±1.6 km compared with 5–11 km reported for other areas, is held to reflect the nature of late Precambrian, Lower and Upper Palaeozoic tectonic events in the region.
Metallogenic evolution of a collisional mountain belt in Pakistan: a preliminary analysis
Authors: Sillitoe R. H.
Published: December 2007

Five post-Palaeozoic metallogenic environments are recognized in Pakistan and adjoining regions: 1. Mississippi Valley-type fluorite, barite and barite-galena deposits generated in Jurassic limestones on the northern miogeoclinal margin of the Indian Shield, perhaps during incipient rifting heralding India's separation from Gondwanaland; 2. chromite deposits in serpentinized dunite, manganese-oxide deposits in layer-1 sediments and a cupriferous massive sulphide occurrence in layer-2 basalts generated within an ophiolite complex at a Tethyan oceanic spreading centre; 3. porphyry copper–molybdenum, manto-type copper, Kuroko-type massive sulphide, vein copper, contact–metasomatic copper and iron, and volcanogenic iron deposits generated in Cretaceous-Cenozoic times by intrusive and extrusive calc-alkaline magmatism along the southern edge of the Iran-Afghanistan microcontinental plate during northward subduction; 4. stibnite occurrences emplaced in the intracontinental Chaman transform fault system during post-early Miocene times; and 5. sandstone-type uranium deposits in Siwalik molasse-type sediments shed during post-early Miocene uplift of the Himalaya. Juxtaposition of the miogeoclinal, ophiolitic and Cretaceous calc-alkaline metallogenic provinces resulted from suturing of the Indian plate with the southeastern margin of the Iran-Afghanistan microcontinent, representing final closure of eastern Tethys. Subsequent renewed northward motion of the Indian plate, causing crustal shortening and uplift along its northern edge, produced the transform-fault and molasse-basin provinces and, along the Makran segment of the plate boundary unaffected by continental collision, the post-early Miocene calc-alkaline province.
Metallogenesis of the Caribbean region
Authors: Kesler S. E.
Published: December 2007

The Caribbean region contains four island arcs. The Greater Antilles, on the north, and Venezuelan Antilles, on the south, underwent volcanism from early Cretaceous to Eocene time and the Lesser Antilles and Central America, on the eastern and western sides of the region, respectively, began major arc volcanism largely in early Cenozoic time and are still active. These areas expose three different levels in the metallogenic evolution of island arcs. The most mature arc, metallogenically, is northern Central America, which is underlain by pre-Mesozoic cratonic rocks and contains widespread precious- and base-metal vein mineralization associated with terrestrial silicic volcanic rocks, and smaller deposits of tungsten, antimony and mercury. Next in order of metallogenic maturity is southern Central America, where porphyry copper mineralization is abundant. Precious- and base-metal veins in silicic volcanic rocks are moderately well developed here and tungsten-antimony-mercury mineralization is lacking. Least metallogenically mature of the arcs are the Greater, Lesser and Venezuelan Antilles in which both silicic volcanic-associated vein mineralization and tungsten-antimony-mercury mineralization are lacking and porphyry copper and massive sulphide mineralization are widespread.
‘LUST’—a seismic refraction survey of the Lewisian basement complex in NW Scotland
Authors: Hall J.
Published: December 2007

The ‘Lewisian Units Seismic Traverse’ (LUST) was executed in an attempt to demonstrate the presence of granulite-facies rocks below the amphibolite-facies quartzo-feldspathic gneisses of the northern belt of the Precambrian Lewisian complex in Sutherland, NW Scotland. Shots fired in Badcall Bay, Lochs Laxford and Inchard, and near Durness were detected along the 40 km line by seismometers spaced at about 400 m intervals. Interpretation of the first-arrivals by ray-tracing methods suggests that no laterally-extensive body of granulite-facies rocks, like those of the central belt, exists at depths less than 4 km below the northern belt, though some such rocks may be included in the cores of major antiforms in the area. Gravity modelling, based on the velocity distribution obtained, suggests various alternatives including less dense rocks below the granulites of the central belt and a ‘brown-schist’ succession between deep granulites and surface gneisses in the northern belt.
Pyroxenes in igneous systems
Authors: Pearce J. A.
Published: December 2007

THE PAPERS PRESENTED can be divided into two broad subject areas: the study of pyroxenes in plutonic rocks as a means of tracing the history of magma input and crystallization within a magma chamber; and the study of relict pyroxenes in altered volcanic rocks as a means of 'seeing through' the effects of chemical alteration and thus of identifying any important magmatic characteristics.

The first was covered by studies on the following three plutonic bodies: Fongen–Hyllingen complex in Norway (Wilson
et al),
the Bushveld complex (Buchanan) and the Freetown complex, Sierra Leone (Wells & Bowles). Two important points of caution emerged from these studies. Buchanan examined the relative merits of using focused and defocused beams to probe pyroxenes and found that focused beams only gave analyses representative of the pyroxene as a whole provided exsolution lamellae were below a critical size. Wells & Bowles questioned the generally-held assumption that all layered textures in intrusive bodies have an igneous origin by providing convincing evidence of pyroxenes which grew by a late-stage autometamorphic process.

The implications of pyroxene studies on metabasalts was discussed by Nisbet & Pearce and by Rowbotham & Bevins. This type of application is still in its infancy but it was apparent that it has the potential to provide useful information on the magmatic character of rocks too altered for conventional studies. To be fully effective, more information is needed on the relationship between pyroxene composition and the temperature, composition and cooling history of its host magma. Nevertheless,
Stratigraphy and sedimentation of the Lower Inferior Oolite of the Cotswolds
Authors: Mudge D. C.
Published: December 2007

The Lower Inferior Oolite rocks of the Cotswolds are described in terms of a single formation divided into 11 members. Fades and thickness variations indicate the presence of a well-defined sedimentary basin, the Severn Basin, in the Cotswold area during Lower Inferior Oolite times. This basin contained a complex sequence of shallow-water carbonate facies including oolite shoal, protected lagoonal and open shelf sand. It was bounded to the west by the Malvern Swell and to the east by the Vale of Moreton Swell, and contains minor structural features which possess similar north–south trends. The Malvern and Vale of Moreton Swells were a major influence on Lower Inferior Oolite sedimentation and are marked by rapid thinning of the succession. Facies belts occur parallel to the trend of these structural features.
Archaean and Proterozoic crustal evolution in Lofoten–Vesterålen, N Norway
Authors: Griffin W. L. , Taylor P. N. , Hakkinen J. W. , Heier K. S. , Iden I. K. , Krogh E. J. , Malm O. , Olsen K. I. , Ormaasen D. E. , Tveten E.
Published: December 2007

The region exposes an unusually deep section through the continental crust. Gravity and seismic surveys show a ridge-like NE-SW up-warping of the Moho to within 25 km of the surface. The oldest rocks are migmatitic gneisses of generally intermediate composition, probably largely of supracrustal origin; these were intruded by a granodiorite/granite pluton at about 2600 Ma. Pb isotope data indicate that the migmatites formed
c.
2700 Ma ago, and the isotopic systems in some rocks have been disturbed by Proterozoic granulite-facies metamorphism. A Proterozoic supracrustal series is composed dominantly of felsic metavolcanic gneisses but includes marble, quartzite, graphite schist and iron formations. An Rb-Sr whole-rock isochron age of 1830 ±35 Ma is interpreted as the age of a metamorphism which reached intermediate-pressure granulite facies in the western part of the area. An early low-P, high-T event was followed by PT conditions near 900°C and 10 kbar at the metamorphic maximum. This thermal peak was essentially post-tectonic and coincided with the intrusion of gabbro, anorthosite and huge volumes of mangeritic to charnockitic magma between c. 1800 and c. 1700 Ma ago. A swarm of alkali-olivine dolerites was intruded shortly after the mangerites. The Lefdingen granitic batholith gives an Rb-Sr isochron age of 1380 ±80 Ma and may represent remobilized basement rocks. Pelitic schists of the Leknes Group were tectonically emplaced against the mangerites and gneisses, and metamorphosed in amphibolite facies 1140 ±135 Ma ago. Rb-Sr and K-Ar mineral ages suggest that the extensive retrograde metamorphism observed in the older rocks occurred at this time. Small granitic pegmatites were also emplaced during this episode. The Caledonian orogeny is recorded by some K-Ar mineral ages, the development of local W-dipping thrust zones, and the local intrusion of large pegmatites. Lofoten-Vesteralen was probably part of the Baltic plate during the Caledonian orogeny, but escaped deformation because it lay at a high tectonic level and consisted largely of massive granulite-facies rocks.
Manganese and iron micronodules from a volcanic seamount in the Tyrrhenian Sea
Authors: Kidd R. B. , Ármannson H.
Published: December 2007

A gravelly sand deposit at 103 m depth on the summit region of Mount Palinuro, a seamount approximately 100 km off the W coast of Italy, is made up of a mixture of manganese and iron micronodules with shallow water shell debris. The manganese-rich micronodules have very high Mn: Fe ratios, they are deficient in minor elements and todorokite is their principal manganese oxide phase. The iron-rich concretions are made up of smectite, probably along with amorphous iron oxides and/or hydroxides. The high degree of partition of manganese and iron and the mineralogical variation suggest that the deposit has a hydrothermal origin. The associated shell debris appears to be entirely of recent origin suggesting that Mount Palinuro has been volcanically active in historical times. This places constraints on models for the tectonism of the southern Tyrrhenian Sea.
Strain study of the Caledonides in the Islay region, SW Scotland: implications for strain histories and deformation mechanisms in greenschists
Authors: Borradaile G. J.
Published: December 2007

Metasediments of the NW foreland have a deformation phase older than the first phase in metasediments of the internal part of the Caledonides to the SE. They also suffered constrictive deformation with NE-SW extension of more than 100%. In contrast, rocks in the adjacent internal zone were extended 70 ± 10% upwards in a NW direction and also thrust in the same direction; they were extended less than 10% along strike. An important concealed boundary separates the foreland from the internal zone. Total strains determined for foreland metasediments (Colonsay Group) and for metasediments of the internal zone (Dalradian Supergroup) allow a partial pretectonic reconstruction. Total cross-strike shortening for both zones averages 55 ± 5%. It is not possible completely to unfold the profile through Islay by removing the measured strains. This is partly because those strain analyses based on grain shapes fail to detect the important component of intergranular slip and partly because mechanisms not involving penetrative rock deformation helped produce the folds.

Strain histories for the Dalradian rocks are generally coaxial within the limits of a test applied so that the primary cleavage of the Dalradian rocks and its extension lineation may be mapped as total strain trajectories. The fan of primary cleavage and examples of extreme non-coaxial strain near the hinge of the Islay Anticline may be associated with hinge migration during its growth.

Several lines of evidence favour grain boundary sliding as an important deformation mechanism in these low grade metasediments. It may have been enhanced by elevated pore fluid pressures and three classes of grain boundary sliding are proposed, dependent on the prevailing fluid pressure.
Quaternary transgressions and regressions
Authors: Evans Graham
Published: December 2007

The Quaternary has been a time of both rapid eustatic changes and almost static sea level. Studies of Quaternary sequences of modem coastal plains and continental shelves, which have been produced during these times, provide an opportunity to see the stratigraphical results of the consequent transgressions and regressions under fairly well known conditions. It appears that usually the periods of rising and static sea level have been the times of greatest accumulation of sediments on coastal plains and shelves rather than the periods of failing sea level. Examination of the Quaternary sequences allows a better understanding of the likely rates at which some ancient sequences were deposited and for which the time-control is more limited.
Ordovician and Silurian changes in sea level
Authors: McKerrow W. S.
Published: December 2007

Depth-related brachiopod-dominated benthic communities can provide well-dated indications of
change in relative sea level, especially if they occur in tectonically stable regions. Eustatic
changes in sea level occurred over short (less than 1–3 m.y.) intervals at the end of the
Llandeilo, in the late Ashgill and at the base of the Upper Llandovery; they may be related to
changes in the size of the icecap in Gondwanaland. In the Llandovery and in the late Silurian,
there are periods of more prolonged eustatic changes in sea level. Other changes are shown to be
restricted to a single continent, and suggest vertical movements with very little tilting.