bluuebunny/crossref_metadata_2025 huggingface dataset:
Cadomian geodynamic evolution of the northeastern Armorican Massif (Normandy and Maine)
Authors: Dupret L. , Dissler E. , Doré F , Gresselin F. , Le Gall J.
Published: April 2008

The oldest Precambrian rocks of the northeastern Armorican Massif are represented by the migmatites and gneisses of La Hague which make up the Lower Proterozoic Pentevrian basement. This is overlain by the low-grade Upper Proterozoic rocks of the Brioverian succession which were affected by Cadomian deformation and plutonism.

The Lower Brioverian Group consists of four tholeiitic volcanic formations overlain by terrigenous units which contain some black chert horizons. In the Coutances area this assemblage was affected by a local tectonometamorphic event which led to the formation of the Constantian arc (585 Ma). The uplift and erosion of this arc yielded the Upper Brioverian flysch which includes spilitic rocks at Vassy and sulphide deposits at Rouez.

A plate tectonic model is proposed for the Cadomian orogeny in the northeastern Armorican Massif. Major and trace element geochemical features of the Brioverian igneous rocks allow us to assign it to a ‘Pacific’ tectonic setting. An island-arc system was established on Pentevrian crust remnants and led to magmatic arcs separated by marginal basins floored by MORB. Strain analysis shows that Cadomian thrusts and folds verge northwestwards, synthetic to southeasterly-directed subduction. A second southeastward-dipping subduction zone developed and was responsible for the closure of the back-arc basin and the generation of the Mancellian Batholith. The folded Upper Brioverian rocks were then intruded by these late-tectonic granodiorites at
c.
540 Ma. The Brioverian rocks and Cadomian plutons were then uplifted, eroded and unconformably overlain by the Lower Cambrian cover.

In the northeastern Armorican Massif, this Cadomian evolution corresponds to a major period of crustal growth and represents progressive cratonisation of the Brioverian basin and arc system.
Shear zone pattern and granite emplacement within a Cadomian sinistral wrench zone at St Cast, N. Brittany
Authors: Gapais D. , Balé P.
Published: April 2008

The St Cast shear zone, situated in Northern Brittany, is a crustal-scale sinistral wrench zone of Cadomian age. We describe the associated deformation history on the basis of a detailed analysis of structures, microstructures and quartz fabrics of rocks outcropping at the Pointe de St Cast.

The study area shows rocks of dominantly granodioritic composition which are intruded by a leucogranite (the Roche de St Cast). Strain measurements, foliation trajectories and outcrop-scale shear zones reveal a heterogeneous pattern of plane-strain type, associated with a bulk sinistral transcurrent shearing within both intrusion and country rocks. rocks.

Local shear zone patterns are however different within the two units. Country-rocks show decimetre scale shear zones anastomosed around low strain domains. In contrast, the intrusion shows a more homogeneous structure with a single set of parallel pervasive millimetre scale shear bands (sinistral C-S structures). These differences are interpreted in terms of different local kinematics and bulk thermal history as expected between a magmatic intrusion and its surrounding country-rocks during syntectonic emplacement. The overall structural pattern emphasizes that the Roche de St Cast is a syntectonic granite emplaced within a major zone of Cadomian sinistral wrenching.
Late Precambrian M-type granitoid genesis in the Cadomian belt of NW France
Authors: Graviou Pierrick , Auvray Bernard
Published: April 2008

A subduction model has been proposed for the geodynamic evolution of the North Armorican Massif during Upper Proterozoic times. An early period of plutonism (600 ± 20 Ma) is characterized by M-type granitoids with largely intermediate composition. Based on petrological, geochemical and isotopic (Rb-Sr) data from the best exposed batholith (North Tregor Batholith, NTB), the following major conclusions can be proposed.

(a)

Various rock-types have resulted from fractional crystalization processes occurring in granodioritic parent magmas.

(b)

The Lower Proterozoic basement (Icartian/Pentevrian) has no major role in the genesis of these granitoids.

(c)

The mantle wedge above the subducted oceanic slab constitutes one possible source region for the magmas. Fluids escaping from the downgoing slab induced melting of a garnet lherzolite mantle which was also slightly metasomatized. During the ascent of these primary magmas, fractionation of olivine and spinel produced the parent magmas of the Tregor plutonic suite thus explaining the low transition element abundances of these rocks (e.g. Ni, Co contents).
Contrasting metamorphic terranes in northwest Wales
Authors: Gibbons Wes , Horák Jana
Published: April 2008

The oldest rocks exposed in northwest Wales crop out as a series of fault-bounded slices. Whereas some of these fault-bounded units are clearly part of the same stratigraphic package (e.g. those belonging to the Monian Supergroup), others show little or no affinity with contiguous areas. Three suspect terranes are identified on Anglesey: (1) the Monian Supergroup; (2) the gneisses and granitic rocks of central Anglesey; (3) the Monian blueschists. The metamorphic histories of these terranes are markedly different and
PT
conditions vary from anchizonal/lower greenschist (1) to sillimanite grade (2) to epidote- or lawsonite-bearing blueschists (3). Each terrane has been juxtaposed by faults, the earliest movements along which (where not obscured by brittle reactivation) can often be shown to have been ductile and accompanied by dynamic recrystallization. The most prominent of these ductile fault boundaries occur between terranes 1 and 3 in Anglesey (Berw Shear Zone), and between terrane 1 and the Sarn Complex (terrane 4) in the Llŷn peninsula (Llŷn Shear Zone). A similar ductile mylonite zone exists between the Cullenstown Group (terrane 1?) and the Rosslare gneisses (terrane 2?) in SE Eire. All these shear zones are steep, and kinematic indicators indicate oblique sinistral shear. Radiometric, lithostratigraphic and palaeontological data constrain the ductile fault movement to
c.
550–540 Ma. This latter event is interpreted as recording an important phase of terrane dispersion and subsequent assembly that occurred along the Avalonian margin immediately prior to marine transgression over southern Britain. It is likely that this transcurrent event is recorded inboard from the Monian area by shear zones within the late Precambrian (Avalonian/Cadomian) basement in southern Britain and northwest France.
Kinematic history of the Bass River Complex, Nova Scotia: Cadomian tectonostratigraphic relations in the Avalon terrane of the Canadian Appalachians
Authors: Nance R. Damian , Murphy J. Brendan
Published: April 2008

The Late Proterozoic Bass River Complex of Nova Scotia, Canada, forms part of the Northern Appalachian Avalon terrane and comprises the Great Village River Gneiss, the platformal metasedimentary Gamble Brook Formation, and the mafic metavolcanic Folly River Formation. Three major periods of deformation within the complex are considered to be of late Precambrian age. The earliest (D1) appears to affect only the gneiss. The second additionally affects the Gamble Brook Formation and records polyphase ductile shear (D2a/D2b) along contacts with the gneiss attributed to sinistral transtension at
c.
630 Ma. Locally sheeted, late-kinematic mafic dykes suggest limited crustal extension and may herald the development of basinal conditions in the Folly River Formation. The third deformation (D3a/D3b) records dextral transpression and polyphase, NW-directed thrusting in the Folly River Formation. D3 is tentatively attributed to basin closure during the latest Precambrian and may have accompanied thrust emplacement of late Precambrian bimodal volcanic rocks and turbidites preserved in the nearby Jeffers Group. Regional considerations suggest D2 and D3 kinematics respectively reflect the development and destruction of a late Precambrian volcanic arc rift.
Diagenetic stability of the isotopic composition of phosphate-oxygen: palaeoenvironmental implications
Authors: McArthur J. M. , Herczeg A.
Published: April 2008

Meteoric diagenesis of the Lower Pliocene Varswater Formation phosphate deposit, Cape Province, South Africa, has altered the chemistry of the francolite component. The isotopic composition of the phosphate-oxygen has been changed, as have other chemical and isotopic parameters that are known to be susceptible to meteoric diagenesis. Reduced major axis regressions between δ
18
O

PO
4

and Sr/Ca,
87
Sr/
86
Sr and francolite-δ
13
C

CO
3

, and the correlation of francolite-δ
18
O

CO
3

and francolite-δ
13
C

CO
3

, predict an original isotopic composition of +30.7‰ (SMOW) for francolite-δ
18
O

CO
3

and + 22.7‰ (SMOW) for francolite-δ
18
O

PO
4

. These data fix the temperature of phosphogenesis at 16°C and the δ
18
O of the phosphatizing fluid as +0.3‰ (SMOW).
Episodes of phosphogenesis and phosphorite concretion formation in the North Sea Tertiary
Authors: Balson Peter S.
Published: April 2008

It is well established that certain periods through geological time have been more favourable to the formation of phosphorite deposits than others. These ‘phosphogenic episodes’ have been correlated with periods of eustatic high sea-levels, global warm temperatures and increased marine productivity, amongst other factors.

The development of phosphorite concretions in marine sediments of the southern part of the North Sea Tertiary Basin reflects episodicity corresponding to times of sea-level high-stands. In particular, concretions in Early Eocene marine mudstones make their appearance just above levels which indicate initiation of a more open connection between the Atlantic Ocean and the relatively enclosed North Sea Basin. Similarly, during the Middle Miocene, phosphorite concretions occur associated with indicators of oceanic influence. Elsewhere in the world, remote from the North Sea, phosphate-rich sediments also formed during these stratigraphically brief episodes. Occurrences of relatively sparse diagenetic phosphorite concretions in the North Sea Basin therefore appear to be linked to large scale, possibly global, cycles and not solely to the local diagenetic environment.
Role of microbial processes in the genesis of Jordanian Upper Cretaceous phosphorites
Authors: Abed Abdulkader M. , Fakhouri Khalid
Published: April 2008

Samples representing granular and laminated phosphorites from the main Upper Cretaceous deposits of Jordan, have been examined by SEM for microbial structures. Globular clusters and sheaths, postulated to have formed by bacterial activity or possibly as inorganic precipitates, have been found only within cavities and in the cementing phosphatic material around phosphate grains of the granular phosphorites. These structures are interpreted to have originated in association with percolation of ground-water late in the history of the sediments, rather than as a consequence of primary grain-forming processes. Organic-rich phosphate grains are believed to have acted as a food source for the bacteria and are, therefore, the cause of the microbial structures and not the result of them. On the other hand, empty cylindrical and/or spherical sheaths, interpreted to be of primary, cyanobacterial (blue-green algae) origin, were readily observed in the laminated phosphorites. These phosphorites are interpreted to have been deposited as algal mats in shallow, subtidal to intertidal environments.
The influence of magnesium ions during the formation of stromatolitic phosphorites of Udaipur, Rajasthan, India
Authors: Sisodia M. S. , Chauhan D. S.
Published: April 2008

The stromatolites of the Precambrian Aravalli Supergroup outcropping around Udaipur City are composed of carbonate (dominantly dolomite) and/or phosphate (carbonate-fluorapatite). The phosphate present in these stromatolites occurs in varying proportions, from traces levels to a major constituent. Compositional variation is attributed to the differences in the original magnesium contents stromatolites which reflects their different of the depositional environments, Intertidal stromatolites had primary magnesium calcite mineralogies, while those deposited in subtidal areas were composed of aragonite and/or lower magnesium calcite. The Mg/Ca ratio in the stromatolites appears to have been controlled by microorganisms which, it is argued, also played an important role in the process of phosphate fixation. It is inferred that the high Mg content of intertidal stromatolites prevented extensive phosphatisation in this facies.
On the description and modelling of glacimarine sediments and sedimentation
Authors: Dowdeswell Julian A. , Scourse James D.
Published: April 2008

Models of glacimarine environments fall into two categories: (a) qualitative summaries of the depositional record; and (b) quantitative models of the physical processes of glacimarine sedimentation and their rates of operation. Qualitative approaches include descriptive vertical lithofacies profiles and models derived from observations of two- or three-dimensional sedimentary or seismic facies changes. Qualitative models are based on observations in modern glacimarine environments where the processes and settings are relatively well constrained, or on sedimentary or seismic investigations of Cenozoic and pre-Cenozoic successions. Some represent hypothetical sedimentary facies associations based on intuitive reasoning in the absence of observations (e.g. beneath ice shelves). Quantitative process and rate models can be based on theory or empirical data. The nature and rate of sedimentation beneath ice shelves has been modelled quantitatively. Scaled-down laboratory models, for example of iceberg melting, have also been used experimentally. Process models linking different parts of the glacimarine system are an important future research area.
Glacimarine processes at grounding-line fans and their growth to ice-contact deltas
Authors: Powell Ross D.
Published: April 2008

Grounding-line fans originate from subglacial and basal stream tunnels at grounding lines of glaciers terminating in a marine environment. During melt seasons discharge forms a turbulent jet beyond the efflux. Over the initial zone of flow establishment in the jet, it may remain in contact with the sea floor for up to 13.4
D
(
D
-tunnel diameter) along a runout distance, if velocity is constant. Plug flow in this zone deposits marine outwash sediment commonly having chaotic texture and graded and welded contacts. Texture may vary rapidly because of discharge pulses, and the jet may move laterally to produce sheet or cut-and-fill geometries. Imbricate gravel is deposited near the efflux, then farther out are sheet or weakly channelized finer-grained gravels and sands. In some instances a migrating barchanoid bar forms at the detachment zone and forms large-scale trough cross-beds. Beyond the detachment zone the turbulent jet becomes vertical and sediment cascades from it in a ‘veil’. This sediment is added to by flows from continuous failures of bed load sediment in the detachment zone. Resulting deposits are interstratified sediment gravity flows and thick, coarse end-members of cyclopsams. The jet changes to a plume when inertial forces become less than buoyancy forces. Turbulent vorticies within the jet can re-entrain particles into a buoyant plume. The plume remains vertical, although continuously spreading, until reaching neutral buoyancy. Fan depocentres have been observed to accumulate at over 10
6
m
3
a
−1
in temperate glacial areas. Eventually, at quasi-stable grounding lines, fans may aggrade to sea level to form ice-contact deltas. At incipient deltas when the delta plain is intertidal, coarse sediment is redistributed to the prodelta with each tidal cycle. Intense prodelta rhythmites mark the transition of a fan into a delta.
Suspended sediment transport and deposition of cyclically interlaminated sediment in a temperate glacial fjord, Alaska, U.S.A.
Authors: Cowan Ellen A. , Powell Ross D.
Published: April 2008

Cyclically interlaminated sediment is a distinctive lithofacies within the sediment package of temperate glacial fjords. The tidewater terminus of McBride Glacier is at the head of a small fjord that receives abundant suspended sediment and is a site of rapid accumulation of cyclically interlaminated sediment. Buoyant sediment-laden meltwater rises from a subglacial stream at the base of the glacier and mixes with fjord water to produce a thick, brackish overflow. Peak suspended sediment concentrations occur beneath the surface of the overflow at 3 to 10 m depth. Particle release from the overflow is controlled by semi-diurnal tidal fluctuations. The major vertical flux of suspended sediment is initiated at low tide because of low horizontal current velocities and reduced vertical eddy velocities. Sorting occurs as the particles settle; sand and coarse silt settle as single grains and finer flocculated particles settle as turbid layers at a rate between 2.5 m/h and 10.9 m/h. Each low water produces a couplet of a coarser grained lamina that is sorted as coarser single grains settle, and a finer grained lamina of flocculated particles in turbid layers. Individual couplet thicknesses and particle size decrease with distance from the discharge source. Semi-diurnal tides produce two couplets each day. Interstratified with these tidal rhythmites are coarser and poorly sorted laminae deposited by sediment gravity flows and coarse laminae contributed by peak daily discharge.
Laminated terrigenous sediments from the Antarctic Peninsula: the role of subglacial and marine processes
Authors: Domack Eugene W.
Published: April 2008

Although laminites from the continental slope and rise are known from a number of regions in Antarctica, laminated muds on the continental shelf have not been widely recognized. During USAP-88 cruise III of the RV
Polar Duke
, a number of piston cores were collected from bays and fjords along the Danco Coast of the Antarctic Peninsula. A fine-grained sedimentary sequence of thin-bedded to randomly laminated mud and sand with dropstones is underlain by a rhythmically laminated sequence of silt and clay. The deposits were collected within a submarine valley (450 m deep) which appears to extend beneath the terminus of a tidewater glacier (Cayley Glacier). The glacial regime of the area today is polar to sub-polar with restricted ice surface melting. Fine-scale horizontal layers are observed in the water column and these transport fine-grained sediment at mid-water depths. Such features are not related to surface meltwater processes, but may be caused by tidal pumping of the basal cavity. The random bedding thickness of the sediments is related to the present depositional environment which includes iceberg rafting, bioturbation, pelagic settling of biogenic detritus, and sediment gravity flows. The rhythmic units likely represent subglacial marine deposition within the valley; possibly under the influence of tidal processes.
Glacial geomorphic features in the northern Barents Sea: direct evidence for grounded ice and implications for the pattern of deglaciation and late glacial sedimentation
Authors: Solheim Anders , Russwurm Lars , Elverhøi Anders , Nyland Berg Mona
Published: April 2008

Glacial landforms, in particular flutes and associated transverse ridges, have been identified in an area of roughly 4000 km
2
and between approximately 150 and 340 m water depth in the north-central Barents Sea. The transverse ridges are interpreted to be analogous to De Geer moraines and are observed only in the southern part of the fluted area. The flutes, trending roughly north-south, have a relief of less than one metre, widths on the order of 4–8 m and lengths usually 100–500 m. Although the present data do not allow firm conclusions on the exact genesis of these features, they are compared to analogous terrestrial features.

The existence of these glacigenic morphological features has major implications concerning the late glacial history of the Barents Sea. They give the first direct evidence of a grounded ice sheet in the northern Barents Sea. Ice flow directions during the latest phase before final recession were mainly southwards, but a partly topographically influenced flow pattern indicates that the main banks may have acted as local ice centres. The ice sheet was relatively thin and rested on a deformable bed. It retreated rapidly from the fluted area, as evidenced by the lack of sediment masking the low-relief flutes. The general retreat from the region took place in a stepwise manner, apparently without surging and with deposition of locally thicker glacier-proximal deposits during halts in the recession.
Geometry and genesis of the glacigenic sediments in the southern Barents Sea
Authors: Vorren Tore O. , Lebesbye Erland , Larsen Kjell B.
Published: April 2008

The glacigenic sequence on the continental shelf and shelf break in the Barents Sea has been mapped seismostratigraphically. Four and six major units have been distinguished on the outer and inner shelf, respectively. The outer shelf units increase in thickness towards the shelf break, except for the youngest one. The geometry of these units is determined partly by the morphology of the basement (infill), but to a larger extent by postdepositional erosion. The older, inner shelf units are located in a coast parallel trough. The two youngest units also have a blanketing character. The geometry of the younger units gives indications of source areas, as well as of ice movement direction of subsequent eroding ice sheets. The palaeoenvironment and palaeogeography during the mid-late Weichselian in the Barents Sea is reconstructed.
Sedimentology and micropalaeontology of glacimarine sediments from the Central and Southwestern Celtic Sea
Authors: Scourse J. D. , Austin W. E. N. , Bateman R. M. , Catt J. A. , Evans C. D. R. , Robinson J. E. , Young J. R.
Published: April 2008

Thin discontinuous glacigenic sediments occur at or close to the sea bed as far south as 49°N in the Celtic Sea. The northern samples (facies A) are clast-rich, overconsolidated lodgement tills or proximal glacimarine sediments containing sparse reworked microfaunas. The southern samples (facies B) are distal glacimarine plastic silty clays containing abundant cold water microfaunas. Both these facies are correlated with the Late Devensian Scilly Till, enabling a quantitative reconstruction of ice thicknesses, grounding line, sea-level and shoreline elevations in the Celtic Sea at 19 000 years BP. The ice advance terminated in marine waters towards the shelf edge break, and is likely to have constituted a thin lobate surge over deformable marine sediments.
Marginal-marine glacial sedimentation in the late Precambrian succession of East Greenland
Authors: Moncrieff A. C. M. , Hambrey M. J.
Published: April 2008

The late Precambrian succession of the fjord region of East Greenland is characterized by two glacigenic formations. The lower one, the Ulvesø Formation, rests on different members of the Limestone-Dolomite ‘series’ (Eleonore Bay Group), and comprises diamictite, conglomerate, sandstone, and rhythmite, all with a high dolomite content. Few beds can be traced for more than a few hundred metres, and erosional breaks are sometimes present. Clasts are predominantly intrabasinal. A turbiditic dolomitic sandstone-mudstone succession, the Arena Formation, lies above. The upper glacigenic horizon, the Storeelv Formation, contains a similar range of facies as the Ulvesø Formation, but is less dolomitic and contains exotic gneiss, granite and volcanic clasts, as well as intrabasinal carbonates.

The Ulvesø and Storeelv formations contain facies characteristic of a low-level terrestrial to marine glacial environment, including: basal tillite, waterlain tillite, debris-flow deposits, ice proximal to distal glacimarine sediments, glaciolacustrine and glacimarine rhythmites with ice-rafted material, permafrost features, aeolian sands and evaporites.

The bulk of the glacigenic sediment appears to have been associated with an ice sheet of continental proportions, which in terms of its thermal regime was probably below the pressure melting point but sliding on its bed. Our interpretation of large parts of the succession as waterlain till implies that floating, debris-bearing ice was an important part of the system, probably in the form of outlet glaciers or ice streams, coalescing as an ice shelf in a tectonically controlled marine basin beyond an irregular coastline. Considerable current activity, both beneath the ice shelf, and sedimentation controlled by iceberg drifting in distinct lanes existed. The succession further illustrates the response of glacimarine sedimentation to ice-retreat onto land, with marked changes from facies with obvious glacial affinities to those devoid of them.
Fracture evolution in the upper ocean crust: evidence from DSDP hole 504B
Authors: Agar Susan M.
Published: April 2008

Detailed structural studies of ocean crust specimens from DSDP borehole 504B have been integrated with a well defined alteration history to yield fracture evolution paths during movement away from the mid-ocean ridge. Fracturing was the dominant deformation process in the upper ocean crust of 504B. A broad range of fracture types exists. The type and orientation of most early fractures were related to lithology. Late fracture orientations and intensities were less dependent on lithology and may have been more closely related to tectonic stress conditions. Prolonged deformation histories are evident in the transition zone where non-coaxial deformation overprints successive generations of dilational veins. Changes in the orientation of the local stress field between successive generations of fractures occurred between and within lithological units. This suggests the presence of numerous mechanical discontinuities in the upper ocean crust, particularly in the transition zone. Changes in fracture porosity between alteration stages were often large and, together with changes in the characteristics of fractures, would have influenced local thermal gradients. The upper ocean crust rheology would have been extremely time dependent as a result. Zones of weakness were created by clay alteration products in basalts and were located in the most permeable horizons. Movement on these zones was promoted by elevated pore-fluid pressures with major failure most likely during major upwelling associated with a hydrothermal system. Subsequent precipitation of high temperature minerals from circulating fluids may have strengthened permeable horizons causing migration of detachment surfaces to another level.
Velocity-dependent friction in a large direct shear experiment on gabbro
Authors: Cox S. J. D.
Published: April 2008

Velocity-stepping sliding experiments on a 0.1 m
2
interface between two gabbro blocks under 0.5–2.0 MPa normal stress have shown consistent velocity-weakening behaviour. The parameter values are in a similar range to those measured on other materials in other laboratories. However, simulation of the complete transient behaviour requires a second state-variable which has a characteristic decay distance
c.
100 × longer than the first, and with the opposite sign (velocity-strengthening). The second delayed effect is not sufficient to cancel out the short period effect, and is not expected to control the onset of unstable stick-slip sliding. These experiments extend the observed realm of velocity-weakening behaviour to larger interfaces and lower normal stresses than has been previously reported.
Reaction-enhanced formation of eclogite-facies shear zones in granulite-facies anorthosites
Authors: Klaper Eva M.
Published: April 2008

Infiltration of a dry granulite-facies anorthosite complex by a hydrous fluid phase under high pressure was crucial to start (a) an eclogitization process which led to the breakdown of plagioclase and the formation of either dense or hydrated minerals and (b) dynamic recrystallization of plagioclase in shear zones. Reaction products of the eclogitization process are related spatially to cracks and shear zones. The reduction in grain size during dynamic recrystallization, the foliation development in shear zones and the occurrence of new ‘soft’ and/or dense minerals changed the mechanical properties (e.g. ductility) of the rocks considerably.

The initiation of detachment and thrusting in the lower crust may be an important large scale consequence of such localized processes.
Propagation and localization of stylolites in limestones
Authors: Carrio-Schaffhauser E. , Raynaud S. , Latière H. J. , Mazerolle F.
Published: April 2008

Matrix investigations (X-Ray tomography, porosimetry by mercury injection, SEM analysis) around stylolites revealed major zones that represent different states in the propagation of the pressure solution structure.

Near the stylolite termination, a significant increase of porosity relative to the far-field host rock porosity and variations in the shape of matrix particles are associated with the lateral propagation of the dissolution zone in the plane of the seam. Close to the sides of the seam, this porosity enhancement zone is found again and may be responsible for vertical development of the stylolite style. Above and below the stylolite seam, the rock matrix is less porous than the reference state and this region appears to have been a site of precipitation of diffused solute. These observations imply that the enhanced porosity state around the stylolite tip is a transient one. This zone becomes a site of deposition as the stylolite tip propagates through it.
High-temperature deformation of calcite single crystals by r
+
and f
+
slip
Authors: De Bresser J. H. P. , Spiers C. J.
Published: April 2008

Optical quality calcite single crystals have been uniaxially compressed at constant strain rates and temperatures (
T
) in the range 3 × 10
−4
to 3 × 10
−7
s
−1
and 550–800°C respectively. The tests were performed with the compression direction parallel to [4041], i.e. parallel to the intersection of two cleavage rhombs. At strains above 1% strain, steady state flow was observed at
T
≥ 600°C. The flow stresses at these temperatures were found to be rather insensitive to strain rate, and can be empirically described by a power law creep equation with a stress exponent ranging from
c.
13 at 600°C to
c.
9.5 at 700–800°C. The active glide systems were identified by slip line analysis. The crystals were found to deform by
e
-twinning at
T
< 600°C, and by slip on a single
r
<2021> system plus a single
f
system in the so-called positive sense at
T
≥ 600°C. The effective slip direction on the active
f
-plane was of <1011> type rather than the <0221> type reported previously. The observed creep behaviour in the slip dominated regime has been compared with microphyical models. The deformation behaviour seems best explained by cross slip or glide-controlled creep models, or a combination of these.
Microstructure in hornblende of a mylonitic amphibolite
Authors: Skrotzki Werner
Published: April 2008

The microstructure in a dynamically recrystallized amphibolite of a deep crustal shear zone from the Ivrea Zone, NW Italy, has been investigated by conventional and high resolution transmission electron microscopy. The microstructure in the amphibole phase consists of recrystallized grains, subgrains, free dislocations and various stacking faults bounded by partial dislocations. The Burgers vectors of the dislocations are [001], [100], 1/2 <110> and very likely <101>. Dislocations with [100] Burgers vector and [001] line direction are mainly arranged in low angle tilt boundaries lying parallel to (100) planes. Faceting of these subgrain boundaries is produced by 1/2 <110> edge dislocations dissociated on (010). The free dislocations are mainly non-dissociated [001] and [100] dislocations gliding on (100) and (010) planes, respectively. It is assumed that [001] and [100] dislocations recombined to form <101> dislocations which subsequently dissociate in (010). The stacking faults extended on (010) planes either have pyroxene or sheet silicate character. They act as obstacles for dislocation motion on the main slip system (100)[001]. High resolution imaging of the dislocation cores suggests these areas consist of lower density material providing channelways for fast diffusion which may be responsible for the deformation-induced compositional changes found within amphibolite shear zones.
Albite deformation within a basal ophiolite shear zone
Authors: White Joseph Clancy
Published: April 2008

Albite mylonites at the base of the White Hills Peridotite ophiolite fragment at St. Anthony, Newfoundland formed during obduction at lower crustal
P - T
conditions. Optical microstructures preserve a history of intracrystalline distortion, recovery and cyclic dynamic recrystallization associated with hot-working. The crystallographic fabric is consistent with shear of the polycrystalline aggregate dominated by glide on (010) planes, as is also indicated by TEM observations. Slip systems implied by trace analysis and invisibility experiments using TEM are b = [101](010), b = [001](010) and b = 1/2[112](201). Dissociated dislocations have separations ranging from 8.3–15 nm. These separations are notably smaller than in more calcic plagioclases and indicate a relatively higher ductility for albite in the case of cross-slip controlled creep. The observed dislocation densities relate to a high stress/strain rate pulse imposed on the dominant high-temperature creep textures.
High temperature deformation of octachloropropane: dynamic grain growth and lattice reorientation
Authors: Ree Jin-Han
Published: April 2008

Dynamic grain growth and lattice preferred orientation development in octachloropropane polycrystals simple-sheared at 0.8 Tm using synkinematic microscopy are discussed. Both soft and hard grains are observed to grow. It is suggested that globular grains are not necessarily an indicator of coaxial deformation. Strain heterogeneity is induced by partitioning of deformation into relatively increased components of rigid-body rotation in hard grains and strain in soft grains. With dominant grain rotation, similar lattice preferred orientation to that of a single-slip model is introduced although the unlocking process is different.
Vein structure and the role of pore fluids in early wet-sediment deformation, Late Miocene volcaniclastic rocks, Miura Group, SE Japan
Authors: Pickering Kevin T. , Agar Susan M. , Prior David J.
Published: April 2008

The Misaki Formation, Miura peninsula, SE Honshu, Japan, comprises deep-shelf/slope volcaniclastic rocks deposited on a segment of the Izu-Bonin island arc prior to accretion about 2.5 Ma. Wet-sediment deformation structures occur as gravity-controlled slides, and sediment injections intruded into semi-lithified over-pressured sediments. Vein structures are locally abundant in the Misaki Formation, mainly as 1–10 cm wide layer-parallel zones, with individual veins about 0.1–3 cm apart. The veins are planar and up to 0.1–2.0 mm in width in their central parts. Vein structure occurs within blocks in chaotic horizons, interpreted as early sediment slides on the seafloor. The vein structures have a strike interpreted as forming parallel to the strike of the basin slope and are considered to arise from pervasive creep-related phenomena in unconsolidated sediments at shallow depths of burial. Back scattered electron (BSE) studies of the vein structure suggest high average atomic number contrasts between the infill of the vein structure and the surrounding sediments and, together with microprobe analyses, suggest that this is due to very fine disseminated pyrite. The pore-fluids involved in the vein evolution were derived from trapped sea water in which certain elements such as S, Fe and Ca were preferentially concentrated in the vein structure. Later generations of vein structure tend to have more complex geometry, are larger, show greater spacing, and appear to have a geometry that cannot be related to any regional trends.
Slickenside lineations due to ductile processes
Authors: Wilson C. J. L. , Will Thomas M.
Published: April 2008

Faults with features characteristic of slickensides, but with plastically deformed wallrocks, were propagated in paraffin wax in a shear environment. Flow of the bulk of the sample produced a strong foliation with fabric strengthening in localized zones that preceded the initiation of the fault. The faults contain ridge-in-groove type slickenlines that have excess-lengths with respect to the measured slip displacement on the fault plane. With progressive movement on the fault surface, these surface markings are disrupted by both ductile shear bands and brittle fractures. The slickenside lineations observed on the fault surface are produced while the material behaves in a ductile manner which does not involve brittle processes.
Convergence-related ‘dynamic spreading’ in a mid-crustal ductile thrust zone: a possible orogenic wedge model
Authors: Holdsworth Robert E. , Grant Colin J.
Published: April 2008

Collisional orogenic belts usually form as approximately wedge-shaped dynamic units irrespective of their bulk rheology. Quantitative models suggest that if the boundary conditions and mechanical properties of a wedge remain unchanged, it will undergo internal yielding (shortening or extension) until a stable, unchanging shape (or ‘taper’) is established. It follows that geological processes periodically leading to changes in wedge shape, or mechanical properties, will perturb the system, generating internal deformation as the wedge strives to re-establish a stable taper.

Geological applications of wedge models to upper-crustal features such as accretionary prisms and foreland thrust belts have highlighted the importance of internal, thrust-parallel shortening strains. The Caledonian, mid-crustal ductile thrust zone of Sutherland, N Scotland, displays many geometric and kinematic similarities to shallow-crustal thrust belts. In contrast to such belts, however, deformation fabric studies suggest a predominance of thrust-parallel
extension
strains which developed
synchronously
with large bulk shortening due to thrust telescoping. Using a viscous wedge model, it is suggested that the deformation occurs due to a process of
dynamic spreading
, possibly triggered by intermittent thrust-slice accretion, towards the foreland, at depth (‘underplating’) and/or strain softening in amphibolite-facies mylonite zones along the ductile thrusts. Similar deformation fabrics are recognized in many regions of ductile thrusting which suggests that a dynamic spreading model may be applicable to mid-crustal deformation regimes in collision zones worldwide.
The Precambrian, Caledonian and Variscan framework to NW Europe
Authors: Coward M. P.
Published: April 2008

During the Precambrian and the Palaeozoic, the tectonics of NW Europe were dominated by the sequential accretion of different terrains on to the North American Craton, e.g. old continental crust of the Scandinavian Craton, magmatic arcs of the Avalon-Brabant Massif, Pentevrian continental crust and the Brioverian magmatic arc. Terrains were locally bounded by thrust packages, by NW-SE trending strike-slip — transform faults parallel to the accretion direction, and by NE-SW trending strike-slip faults defining localized oblique collision or, more generally, boundaries to zones of lateral continental extrusion and escape. Close analogies can be made with the Tertiary Makran-Himalaya-Tibet collisional zones. The Laxfordian/Caledonian/Variscan thrusts and more importantly the large-scale strike-slip faults, imposed a complex heterogeneity to the crust which critically influenced the subsequent extension directions and the siting of basin bounding faults and tectonic inversion, from Devonian times to the present day.
The early to mid-Jurassic evolution of the northern North Sea
Authors: Richards Philip C.
Published: April 2008

The early to mid-Jurassic history of the northern North Sea is summarized as a complex transgressive-regressive-transgressive cycle controlled by variations in ‘long-term’ eustatic sea level rise, basin subsidence/margin uplift and sediment supply. The first of the transgressions occurred in the Hettangian when the Boreal Ocean spread southwards during a phase of fluctuating but generally increasing sea level rise. Offshore marine conditions developed in the north of the area during the Hettangian to Pliensbachian, while coastal to non-marine conditions developed around the ocean margins in the Unst Basin, the Beryl Embayment, the central Viking Graben and over parts of the Horda Platform. The coastal systems in the Beryl Embayment and central Viking Graben were drowned in the late Pliensbachian as a result of continued sea level rise. A widespread marine link was established with the Tethys Ocean to the south at this time. Sea level fall and/or basin margin uplift and erosion in the Aalenian led to the shedding of coarse clastic sediment transversely into the East Shetland Basin and the northern end of the Viking Graben. The main reservoir unit of the area, the Middle Jurassic Brent Group, formed in this way. A marine connection was maintained into the central Viking Graben area throughout the transverse progradation further north. Drowning of the progradational clastics and the start of the third major transgression began in the late Bajocian, but offshore conditions did not become fully established throughout the area until the Callovian or early Oxfordian.
Controls on Eocene submarine fan deposition in the Witch Ground Graben
Authors: Harding A. W. , Humphrey T. J. , Latham A. , Lunsford M. K. , Strider M. H.
Published: April 2008

The framework of Eocene sedimentation in the Witch Ground Graben is described using an integrated analysis of seismic stratigraphy, biostratigraphy, and lithofacies mapping. Primary controls on the distribution of Eocene submarine fan sands are (1) Cimmerian tectonic elements, (2) palaeotopography created by deposition of Palaeocene sediments, and (3) variations in sea level.
Subsidence and sedimentation in semi-starved half-graben: a model based on North Sea data
Authors: Barr David
Published: April 2008

By combining simple extensional fault-block geometries with overall tectonic subsidence curves generated by geodynamic mechanisms such as uniform lithospheric stretching, it is possible to generate forward tectono-stratigraphic models of basin evolution. Previous studies have used simplistic assumptions about sedimentation, e.g. that half-graben were filled to sea level or to the crests of their constituent fault blocks. A better appreciation of the range of possible natural geometries can be obtained by inputting subsidence and sedimentation histories derived by observation of a real sedimentary basin.

The central North Sea is a well studied basin which in general terms can be explained by a simple model of uniform lithospheric stretching in the Late Jurassic, followed by thermal subsidence to the present day. Long-period departures from this model principally reflect the waning thermal effect of Permo-Triassic rifting, but short-period complications are shown to result primarily from variations in the externally controlled sediment input rate. Periods of sediment starvation permitted a substantial increase in water depth, and subsequent rapid sediment input resulted in episodes of load-driven subsidence.

Three typical central North Sea subsidence histories have been combined with a simple model of domino faulting with a 10 km initial fault spacing. Contrasting depositional geometries, with or without the emergence of footwall islands, result from relatively subtle variations in sediment input rate within a small area which can reasonably be inferred to have had a consistent tectonic history.
Continental extension on sets of parallel faults: observational evidence and theoretical models
Authors: Westaway Rob
Published: April 2008

Extension of the upper-crustal brittle layer is taken up in many regions on sets of sub-parallel normal faults that have sub-parallel slip vectors and are approximately planar between the earth’s surface and
c.
10 km depth. This style of faulting accommodates different deformation senses in the underlying upper mantle lithosphere in different deforming regions.

In the western North Anatolian fault zone, southernmost Italy, Tunisia and Libya, simple shear and extensional pure shear are distributed over zones up to
c.
200 km wide and are taken up by oblique slip on faults that strike parallel to the simple shear direction. The strain rate tensor in such zones is biaxial, and although vertical vorticity is non-zero, blocks in the brittle layer do not rotate around vertical axes relative to the zone boundaries. In central Italy extension involving uniaxial extensional pure shear is taken up by normal slip on parallel faults.

In the northeastern Basin and Range province, central Greece and western Turkey, single sets of parallel faults take up extension in the interiors of deforming regions with dimensions >
c.
500 km and appear to rotate around vertical axes relative to the boundaries of the regions. Distributed simple shear, oblique to fault strike, combined with extensional pure shear may
a priori
account for this deformation sense. However, it requires faults to strike in a unique direction that depends on the ratio of rates of pure and simple shear, and to have slip vector azimuth perpendicular to the simple shear direction. With this geometry, left-lateral slip takes up clockwise rotation and right-lateral slip anticlockwise rotation around vertical axes. To take up finite extension and rotation this unique geometry must persist over time. It is unlikely that this process explains deformation of any region considered. In contrast, uniaxial extension, whether occurring on its own or combined with rotation around a vertical axis at a non-zero rate, does not require any strong geometrical constraints: fault slip vector azimuths follow the direction of extensional pure shear and the sense of any strike-slip depends on the orientation of this direction relative to fault strike. Blocks in the brittle layer generally rotate around vertical axes at the same rate as elements in the underlying plastic deformation, which may equal half the vertical vorticity in the upper mantle lithosphere. This deformation sense, which accounts for the observed deformation in these regions, minimizes local differences in horizontal velocity between the brittle layer and the underlying upper mantle lithosphere.
Normal fault geometry and fault reactivation in tectonic inversion experiments
Authors: Krantz Robert W.
Published: April 2008

Scaled physical models of extensional structures associated with fault reactivation were achieved in sand by superimposing extensional tectonics on thrust faults created during prior episodes of horizontal shortening. The extension produced three distinct responses: selective reactivation of pre-existing thrusts, favouring the steepest thrusts and those dipping more than a critical value; development of new normal faults that were linked to or developed within the zone of thrust faults; or generation of new normal faults in previously unfaulted domains. Linked normal faults showed a wide range of orientations and most dipped less steeply than the unlinked faults, which had dips clustered about the value predicted from the internal friction coefficient of sand. During extension of a distributed fault model, reactivation of existing thrusts influenced the location, dip direction, and dip magnitude of linked normal fualts. Compared to natural fault systems, the models showed many geometric similarities especially regarding linked normal faults.