bluuebunny/crossref_metadata_2025 huggingface dataset:
Sedimentary environments of the basinal Llandovery of mid-Wales
Authors: Cave R.
Published: May 2008

The position of the Ordovician-Silurian junction in the mid-Wales rock sequence remains elusive. There is no evidence of a break in sedimentation between the periods but conditions of deposition changed abruptly about then; presumed Ashgill rocks, mainly argillaceous and disturbed, are overlain by Llandovery strata with even and regular bedding, yielding
Glyptograptus persculptus
(Salter). The former are barren and accumulated rapidly, near the foot of a slope facing west from the ‘shelf’, synchronously with a marine regression and a glaciation in N Africa. The regresson may therefore have been a sympathetic eustatic fall. Of the succeeding strata, those of lower Llandovery age embody much pelagic mud while the middle and upper Llandovery strata record an invasion by turbidite fans. Current indicators in the turbidites reveal westward (‘lateral’) flow below the Aberystwyth Grits (low
turriculatus
Zone), later augmented or replaced by northwards (longitudinal) flow. This change stemmed probably from a tectonic event to the south.

Characteristic turbidite rhythms have been noted, varieties of which are used to define many of the formations. Such formations are probably highly diachronous.
Early history of the southern margin of the Tywi anticline in the Carmarthen area, South Wales
Authors: Cope John C. W.
Published: May 2008

The area SW of Carmarthen exposes Precambrian, Cambrian, and Ordovician rocks, unconformably overlain by Old Red Sandstone. Frequent depositional breaks and derived fragments of igneous and metamorphic rocks suggest a nearby land area. Periodic movement along the Llandyfaelog disturbance may have controlled sedimentation locally. Complex Caledonian structures in the area involve Lower Old Red Sandstone and may be due to the presence of basement at shallow depth.
South-east Ireland: Lower Palaeozoic stratigraphy and depositional history
Authors: Brück P. M. , Gardiner P. R. R. , Reeves T. J. , Shannon P. M. , Colthurst J. R. J. , Feely M. , Penney S. R. , Smith D. G. , Vanguestaine M.
Published: May 2008

Four lithostratigraphical groups are distinguished in the Lower Palaeozoics of SE Ireland. The sand-dominant Lower-Middle Cambrian Bray Group consists of laterally-derived flysch. The conformably overlying, and perhaps in part time-equivalent, mud-dominant Middle Cambrian-Llandeilo Ribband Group comprises axially-transported flysch with minor volcanics. Due to tilting and subsequent differential erosion of these trough sediments, the overlying mainly Caradoc Duncannon Group rests unconformably on the Ribband Group except in the west where there is a conformable passage. The Duncannon Group is a highly faulted, predominantly volcanic, platform sequence with local basal calcareous beds. In the northwest of the district, the Bray and Duncannon Groups are absent and a continuous period of flysch sedimentation is reflected by the upward passage of lower Ordovician Ribband Group sediments into sand-dominant turbidites of the Kilcullen Group (lower Ordovician to at least Llandovery). In contrast, the Slievenamon inlier in the southwest contains Ribband Group strata of Upper Cambrian age which palynological evidence suggests are separated by a major stratigraphical break from the entirely Silurian Kilcullen Group. In the Comeragh Mountains to the south, the Kilcullen Group is conformably underlain by the Ribband and Duncannon Groups. These features point to large-scale irregularities within the Leinster Basin, the main Lower Palaeozoic palaeogeographical feature of SE Ireland.
Ordovician stratigraphy of the Tramore area, County Waterford, with a revised Ordovician correlation for south-east Ireland
Authors: Carlisle H.
Published: May 2008

The Ordovician succession around Tramore is complicated by contemporaneous and later faulting and by widespread intense volcanicity. Early Ordovician NE-SW faulting divides the rocks of the Ribband Group so that to the east of the area the Tramore Group rests unconformably on the Arenig (?) Tramore Shale Formation and in a basin to the west a sequence of intermediate and basic volcanics and volcaniclastics, here named the Ballynaclogh Volcanic Formation, pass conformably into the Tramore Group. A Llanvirn to lower Llandeilo age is suggested for this formation.

It is proposed that the remaining five formations of the succession, ranging from lower Llandeilo to Longvillian in age, be termed the Tramore Group. The oldest of these, the Tramore Limestone Formation, marks a relatively stable period before the extensive predominantly acidic volcanicity which dominates the remainder of this group. The relatively thick basinal facies of this formation exposed mainly in coastal sections west of Annestown can be divided into two members: the lower, the Dunabrattin Shale Member and the upper, the Dunabrattin Limestone Member. Variations in the rich shelly fauna of this formation are shown to be partly related to facies change. Overlying this is a sequence of subaqueous acidic tuffs, volcaniclastics, and argillaceous rocks for which the name Carrighalia Formation is proposed. The remaining three formations have been named by Williams (1972, fig. 10) but are redefined in the present study.

A brief discussion of the regional correlation of the Tramore Group is included.
Caledonide volcanism in Britain and Ireland
Authors: Stillman C.J. , Francis E.H.
Published: May 2008

This paper reviews the state of knowledge of the volcanic activity which accompanied the development of the Caledonide orogen in Britain and Ireland. Unlike the relative abundance of stratigraphical data, geochemical information is scarce in earlier accounts, but in the last decade or so there has been a surge of interest coincident with the introduction of rapid analytical techniques and with the development of methods for distinguishing primary petrographical and chemical characteristics from those of subsequent alteration. Many of the Caledonide volcanic rocks were erupted into or beneath water, interbedded with sediments and then affected by orogenic deformation and metamorphism, yet this has apparently had less effect than might be expected on the gross geochemistry of the volcanic rocks. In the paratectonic Caledonides, submarine weathering and low-grade hydrous metamorphism have been the principal causes of element mobility, and the effects produced are, in many cases, remarkably similar to those which affect present-day eruptives in oceanic environments.
Late Precambrian igneous activity in Southern Britain
Authors: Thorpe R.S.
Published: May 2008

Late Precambrian igneous activity was widespread in England, Wales, SE Ireland and the Channel Islands. This activity included calc-alkaline intrusive and volcanic complexes which, although showing important regional differences, have many similar features. Recent U-Pb and Rb-Sr isotopic ages indicate that this activity was broadly contemporaneous at ca 600 ± 50 Ma. The initial Sr isotope ratios of these igneous rocks indicate that they largely represent new additions to the continental crust in an island arc-continental margin setting during a major period of crustal growth in south Britain.
Basic volcanic rocks in the Dalradian of Ireland
Authors: Max M.D. , Long C.B.
Published: May 2008

Basic volcanic rocks in the Dalradian Supergroup of Ireland are identified and listed here. They are widespread, though stratigraphically restricted, and provide useful marker horizons locally in Connemara, Achill Island, the Nephin Beg Mountains, the Ox Mountains inlier, and counties Donegal, Tyrone, Londonderry, and Antrim. A major volcanic unit is found in the Argyll Group (Easdale Subgroup) in the west of Ireland, whereas in N Ireland, including Donegal, several units are recorded in or immediately below the Southern Highland Group. Pillow lavas, lavas, agglomerates, hyaloclastites, tuffs and combinations of these occur. Minor striped amphibolites reminiscent of the tuffaceous or lava lithologies of the Lakes Marble Formation (Easdale Subgroup) are present throughout much of the Connemara succession. Incoming of the first substantial thickness of volcanics is associated with a change from stable, shallow-water, continental-shelf deposition to deposition of a deeper water, greywacke facies, associated with rifting and block faulting.
Ordovician volcanicity of the SE Harlech dome
Authors: Dunkley P. N.
Published: May 2008

Acid and basic volcanism took place in the SE Harlech dome from the Arenig to early Caradoc. The chemistry and field relationships of the volcanics suggest that they were generated in a back-arc tensional basin environment.
Early Ordovician volcanism in Dyfed, SW Wales
Authors: Bevins R. E. , Roach R. A.
Published: May 2008

A review of early Ordovician volcanism in SW Dyfed, Wales is presented in the light of recent work. There were a number of major and minor volcanic episodes within the area during Arenig and Llanvirn times. For the most part, submarine volcanism occurred within a restricted basin, in which black muds were deposited and to which coarse terrigenous material had very limited access. Basic and intermediate magmas were generally erupted as flows. Acidic magmas were also erupted quietly, to produce thick, frequently autobrecciated flows and domes. Volcaniclastic rocks resulted by deposition from sediment gravity flows originating on unstable flanks of lava domes. Evidence for occasional episodes of explosive activity is provided by sediment gravity flows carrying a high proportion of pyroclastic debris. Subaerial or shallow water conditions are suggested only for the oldest volcanic rocks on Ramsey Island. Numerous basic and less frequent intermediate and acid high-level intrusive sheets have a close temporal, spatial and genetic relationship with the volcanic rocks.
The Capel Curig Volcanic Formation, Snowdonia, North Wales; variations in ash-flow tuffs related to emplacement environment
Authors: Howells M. F. , Leveridge B. E. , Addison R. , Evans C. D. R. , Nutt M. J. C.
Published: May 2008

A detailed correlation of the tuffs of the Capel Curig Volcanic Formation is presented. The depositional environment of the sediments associated with the tuffs varied from subaerial to submarine. Variation in the ash-flow tuffs are related to this changing environment. Subaerially emplaced tuffs are characterised by even bases, distinctive recrystallisation textures, the absence of reworked tops and sparse siliceous nodules. Tuffs emplaced in a subaqueous environment show irregular undersurfaces with welding to the lower contact, reworked tops and common siliceous nodules.
The age of mineralisation at Parys Mountain, Anglesey
Authors: Nutt M. J. C. , Ineson P. R. , Mitchell J. G.
Published: May 2008

Borehole exploration of the mineralisation in the Ordovician and Silurian rocks of Parys Mountain, Anglesey has produced two hypotheses to explain the emplacement of the ore deposits. Mineralisation was either syngenetic, associated with volcanicity in the Ordovician, or epigenetic, occurring during or after the Caledonian orogeny. An outline of the geological evidence is given together with the results of K-Ar isotope age determinations on thirty-two core samples. Two isotopic events have emerged from the K-Ar determinations, 394±9 Ma and 353±7 Ma; these ages are discussed in the light of the evidence available.
The petrogenesis of Old Red Sandstone volcanic rocks of the western Ochils, Stirlingshire
Authors: French W. J. , Hassan M. D. , Westcott J.
Published: May 2008

Lavas and intrusions ranging from basalt to felsite have been examined. These form a high-K, calc-alkaline series of rocks characterised by acid groundmasses and zoned plagioclase phenocrysts which sometimes have rims of sanidine. The intrusions are generally more acid than the lavas and the differentiation trend seems to relate to subtraction of olivine and feldspar. Formation of the secondary magmas may have been due to removal of olivine, clinopyroxene and garnet from a crystal-melt system generated some 250 km below the contemporary surface. The rocks match other Old Red Sandstone volcanic provinces and are chemically analogous with the Caledonian plutonic series.
Geochemical and geophysical constraints on the origin and evolution of Caledonian granites
Authors: Brown G. C.
Published: May 2008

Most British Caledonian granites, which were emplaced during the period 600–390 Ma, are characterised by temporal progression towards low K/Rb ratios, low total Sr, low initial
87
Sr/
86
Sr ratios and high U contents coupled with increasingly large negative Bouguer gravity anomalies and sometimes large magnetic anomalies. It is proposed that some early granites from N Scotland represent low mobility, migmatitic low volume melts resulting from overthrusting which reached a climax in mid-Ordovician times. Early granites from S Britain and late granites from both the north and south provinces have geochemical characteristics of mantle-derived parental magmas capable of metamorphosing the lower crust to a high grade and ascending to become voluminous high level and possibly sub-volcanic intrusions. Late Caledonian granite emplacement ignores the major lateral break in crustal continuity near the Southern Uplands fault that is identified on seismic, geochemical, aeromagnetic and geological evidence. Seemingly, these granites were produced after Silurian continental plate closure. Yet their characteristics have little in common with the modern Alpine-Himalayan plutonic rocks and a source within sub-continental lithosphere under tension is postulated.
Petrogenetic significance of Rb-Sr and U-Pb isotopic systems in the 400 Ma old British Isles granitoids and their hosts
Authors: Halliday A. N. , Aftalion M. , van Breemen O. , Jocelyn J.
Published: May 2008

Rb-Sr isotopic data for 17 Caledonian granitic plutons in Scotland and northern England define low
87
Sr/
86
Sr
i
(<0.709) for all granitoids emplaced between ∼435 and 390 Ma with the exception of Moy and Aberdeen (which may be older). There is no apparent correlation of
87
Sr/
86
Sr
i
with distance from the proposed Iapetus suture near the Southern Uplands. Zircons from the Moinian and Dalradian rocks and Southern Uplands Ordovician greywacke roughly define U-Pb discordia lines with upper concordia intersections of 1200–1800 Ma and the position along the discordia related to the age of the sedimentary unit. Zircons from the Strontian tonalite and granodiorite and the Loch Doon granite contain little if any Pb isotopic memory, whereas the Strontian central granite contains a marked inherited zircon component. We conclude that, in strong contrast to the
c.
550 Ma and
c.
460 Ma granitoids, the
c.
400 Ma magmas were largely hybrids derived from the mantle and, or, ‘new’ lower crust with varying degrees of incorporation of upper crustal-type material.
Caledonian granites in relation to regional geochemistry in northern Scotland
Authors: Johnstone G. S. , Plant J. , Watson J. V.
Published: May 2008

Regional geochemical maps based on stream sediment sampling provide means of comparing the relative abundances of trace elements in granitic bodies with those characteristic of the enclosing Moine and Dalradian country rocks. Migmatites, vein complexes and pre-Devonian forceful intrusions in northern Scotland differ little from their country rocks on these maps (apart from higher levels of Sr): this observation supports the widely held view that they are derived mainly from remelting of Proterozoic crustal materials. The latest mainly Devonian granites and alkaline complexes tend to show levels of U, Li, Be and Rb well above the background values of the province and from this and other evidence it is suggested that their parent magmas were derived at least in part from material underplated on the crust, and perhaps metasomatically enriched in incompatible elements through mantle processes connected with subduction. The magmas were released during phases of deep fracturing. There are indications of compositional variations related to distance from the Caledonian suture.
The Carn Chuinneag granite and its aureole
Authors: Wilson D. , Shepherd J.
Published: May 2008

The Carn Chuinneag granite comprises a suite of acid and subsidiary basic igneous rocks, intruded into the Moine of Easter Ross. Structural evidence indicates that the granite may have been sheet-like or laccolithic in form, and now occupies the core of a major D
3
synform. Hornfelsing of pelitic rocks in the aureole has preserved a variety of sedimentary structures. The former presence of andalusite and cordierite within the hornfels indicates that the granite was emplaced at a high structural level; their alteration to aggregates containing kyanite, may have resulted from burial of the area under the superincumbent load of the Ross-shire nappe during the second episode of deformation.
The Assynt alkaline suite
Authors: Parsons Ian
Published: May 2008

The alkaline intrusives in Assynt overlap the thrust movements and can be used to put limits on their magnitude and timing. The Ailsh syenite is the earliest igneous body within the thrust belt; the Borralan complex is later and overlapped major movements on the Assynt thrust. Large scale displacements on the Sole and Assynt thrusts ended before movement ceased on the Moine thrust. Pyroxene variation in the Ailsh complex confirms that the leucosyenites form a series fractionated before emplacement, but the pyroxene range is unusual for an igneous series and consistent with a metasomatic origin for pyroxene-rich members.
Caledonian minor intrusions of the N Highlands of Scotland
Authors: Smith D. I.
Published: May 2008

Three major suites of minor intrusions cut the Caledonian rocks north of the Great Glen fault. (1)
The amphibolite suite
was intruded between D
1
and D
2
deformations as a swarm of dolerites (locally gabbroic) trending sub-parallel to the fold belt. (2)
The microdiorite suite
, consisting of a wide variety of rock types, was emplaced during the closing phases of Caledonian tectonism principally in the southern part of the area. Early members were locally involved in the final phase of folding and metamorphism. The majority, however, occur as sheared sheets occupying easterly dipping planes which transgress all the Caledonian fold structures. These sheared intrusions exhibit a variety of internal fabrics, the geometry of which indicates that they were recrystallised under an ESE, subhorizontal maximum stress. Over most of the N Highlands, the recrystallised mineral assemblages belong to the greenschist facies metamorphism, but in the south an area of amphibolite facies has been defined. This is believed to reflect a residual Caledonian temperature gradient. (3)
The minette suite
, centred on the Ratagain granite is a post tectonic dyke swarm which marks the end of igneous activity in the N Highlands Caledonides. Members of the swarm are widely dispersed throughout the Moine and are also present in the foreland rocks west of the Moine thrust. Variations in K-Ar ages from these dykes may relate to differential uplift or to the continuing presence of a local Caledonian thermal high.
Plutonic rocks in the Irish Caledonides
Authors: Kennan Padhraig S.
Published: May 2008

Major occurrences of plutonic rocks are briefly reviewed. The important granites are described with special reference to the features that makes each of them unique. The association of basic intrusives with all of the major granites is noted. Some of the basic intrusions may have had, prior to deformation, a lopolithic or funnel-shaped form. Most, however, occur as dykes or sills or as large numbers of small appinitic bodies associated, in a special way, with certain diapiric granites. Possible controls on the siting and mechanism of intrusion of the granites are briefly examined.
Geochemistry of Irish granites
Authors: Elsdon R. , Kennan P. S.
Published: May 2008

Major, trace and isotopic element data are reviewed, in the light of current views on the petrogenesis of Caledonian granite magmas. The information at present available tends to confirm a crustal anatexis model rather than a mantle origin, but more work is needed to confirm this.
The Newry igneous complex, County Down
Authors: Meighan I. G. , Neeson J. C.
Published: May 2008

New field and petrographic observations indicate that the ultramafic rocks and closely associated meladiorites at the northeast end of the complex are principally clinopyroxene-biotite-apatite (±plagioclase) cumulates from intermediate magmas. The meladiorites grade into more evolved oversaturated dioritic-monzonitic rocks, many of which may also be accumulative, and consequently the main intermediate and ultramafic body is thought to be part of a layered intrusion. New geochemical data for major and trace elements (including R.E.E.) and initial
87
Sr/
86
Sr ratios show compositional similarities between some of the intermediate rocks and the younger granodiorites. The granodiorites are areally most abundant and probably constitute three contiguous plutons formed mainly by crystal fractionation at depth in intermediate magmas of Upper Mantle origin.
The age of the Saltees granite in the Rosslare complex
Authors: Max M. D. , Ploquin A. , Sonet J.
Published: May 2008

A 10 point whole-rock Rb-Sr isochron of 436 ± 7 Ma from this granite probably indicates emplacement age. The granite is thoroughly deformed and its foliation passes into the Rosslare complex to form a composite fabric. A suite of metadolerites, which were intruded before the Saltees granite had fully crystallised, were metamorphosed in the low amphibolite facies even well away from the granite. This Caledonian tectono-thermal event was associated with the emplacement of the granite and the metadolerites and overprints at least two major thermal events seen in the Rosslare complex gneisses.
The Haweswater complex
Authors: Nutt M. J. C.
Published: May 2008

The largest basic intrusive complex of the Lake District cuts Borrowdale Volcanic Group rocks around Haweswater. It is composed mainly of gabbros, dolerites, andesites and microdiorites. These have been altered in places by hydrothermal and tectonic activity as well as by later igneous intrusions. Petrology and geochemistry have established a consanguineous relationship between the Haweswater complex and the coeval Borrowdale volcanic rocks. It is suggested that the complex was intruded into an active continental margin island arc.
What is a Thrust? What is a Nappe?
Authors: Dennis J. G. , Price R. A. , Sales J. K. , Hatcher R. , Bally A. W. , Perry W. J. , Laubscher H. P. , Williams R. E. , Elliott D. , Norris D. K. , Hutton D. W. , Emmett T. , McClay K. R.
Published: May 2008

The definitions of thrusts and nappes are discussed and the criteria used to define these structures are analysed. A ‘thrust fault’ is defined as a map-scale contraction fault. A ‘thrust nappe’ is an allochthonous tectonic sheet which has moved along a thrust fault. Suggested definitions are also given for, a fold nappe; extension fault; contraction fault; listric normal fault; reverse listric fault; klippe; tectonic slide; ramps and flats; duplex structure; blind thrusts.
Thoughts on the tectonics of folded belts
Authors: Bally A. W.
Published: May 2008

Balanced cross sections and their palinspastic reconstruction in structurally simple external zones of folded belts suggest that a limited amount of continental lithosphere was subducted (Ampferer or A-subduction). This process appears to be associated with preceding or synchronous basement remobilization. Therefore, the rigid or ductile nature and the timing of basement deformation remain as some of the most important orogenic problems. Decoupling and ductility contrasts within the lower continental crust and within the overlying sedimentary sequences are responsible for varying structural styles in mountain ranges.

Gravity gliding as an important factor for mountain building is examined in some detail. Soft sediment gravity tectonics on passive continental margins are dominated by listric normal growth faults. This style contrasts with observed styles of deformation in folded belts. Gravity tectonics induced by stretching of the underlying basement area is commonly observed during the rifting phase of passive continental margins and in episutural basins associated with orogenic systems.
Pore pressure, discontinuities, isostasy and overthrusts
Authors: Gretener P. E.
Published: May 2008

Thrusting represents the shifting of large loads. This has the following consequences: (a) in the overridden sequence high pore pressures develop initially due to rapid loading and later due to aquathermal pressuring caused by slower thermal adjustment. This leads to the activation of deeper and more outward emerging thrust planes: (b) the isostatic response to the loading in front and the simultaneous unloading in the back produces the
wandering bulge
.

These two processes tend to make thrust faulting a self-perpetuating process once it has been initiated.
Gravitational gliding in deltas
Authors: Mandl G. , Crans W.
Published: May 2008

Many cases of faulting in prograding delta slopes can be genetically attributed to submarine gravity gliding (Terzaghi 1956) on slope-parallel slip planes. Essential in this ‘thin-skinned’ model of gravitational faulting is the overpressuring of the pore fluid, which not only permits the formation of very gently dipping slip planes, but also determines the listric shapes of normal and thrust faults at the head and toe of the gliding path.

The development of the gliding process and the associated primary and secondary structures (antithetic faults, ‘roll-over’ anticlines) are described for an infinitely long slope and for somewhat idealized pore-pressure profiles, a main parameter being the distance of the overpressure top from the (rising) sediment surface.

Realistic deviations from the idealizing assumptions are discussed in qualitative terms.

The gliding process is ‘slow’ and controlled by sediment compaction in the slope-parallel direction. Continued sedimentation gives rise to changes in effective stresses and these, in turn, cause active faults to adjust their shapes. Examples of this adjustment are discussed.
Thrust sheet deformation at a ramp: summary and extensions of an earlier model
Authors: Wiltschko D. V.
Published: May 2008

A previous model for the equilibrium of a linearly viscous thrust sheet at a ramp shows that fault-zone drag is the most important energy sink, though bending can be an important resisting force and gravity can be a moderate driving force. The model was developed for the Pine Mountain block of the southern Appalachians, though it is general and may be adapted to other geometries.

Increasing the viscosities, ramp dip and thickness of the sheet each impedes forward motion whereas increasing the dip of the erosional surface and fraction of incompetent material and including the effect of an overriding thrust sheet aid movement. Doubling the viscosities doubles the bending resistance. Doubling the block thickness increases bending resistance as well, but since the force required to overcome this added resistance acts over twice the surface area, the net traction is reduced. Doubling the ramp dip greatly increases bending resistance, but if the length of the ramp is adjusted to maintain the same stratigraphic climb, this increase is lessened. The dip of the erosional surface is linearly related to the traction due to gravity in the ‘glacier analogy’ model. However, gravity cannot overcome all other sources of resistance for a sheet of the dimensions of the Pine Mountain block unless the surface slopes at 16° in the direction of transport. Increasing the fraction of incompetent material decreases bending stresses and therefore reduces the net traction necessary for movement. Finally, in the case of an overriding thrust sheet, if the upper fault zone is equal in shear strength to the lower fault zones, the resistance to movement of the lower one is cancelled; only bending resists motion. Successively overriding thrust sheets is a documented kinematic scheme in major thrust belts and the resulting applied shear stresses may be one mechanism by which stress and displacement are transferred from higher to lower sheets.
Deformation and secondary faulting near the leading edge of a thrust fault
Authors: Rodgers D. A. , Rizer W. D.
Published: May 2008

Elastic dislocation models are used to calculate the displacements of the ground surface produced by a thrust fault climbing section. The models show that the surface uplift is asymmetrical with the steepest slope above and ahead of the fault. The gradient of the surface uplift becomes steeper and the magnitude of the uplift increases as: (1) the fault dip increases; (2) the fault becomes shallower; and (3) the shear modulus of the material above the fault decreases relative to the shear modulus of the material containing the fault. Elastic dislocation, photoelastic, and clay models are used to predict the orientation of secondary faults which might be produced by a thrust fault climbing section. In general, the secondary faults strike parallel to the master fault, but the dislocation models show that there are regions in both the hanging and foot walls where the secondary fault strikes are not parallel to the master fault. The models suggest that a thrust fault in a homogeneous medium is likely to extend itself with about the same dip, while a thrust fault moving from a stronger material into a weaker material is likely to flatten and become layer-parallel. High-angle antithetic faults or back thrusts in the hanging wall are associated with a thrust fault propagating through a homogeneous medium, but layer-parallel antithetic faults are more likely than high-angle antithetic faults in the hanging-wall when a thrust fault is propagating from a stronger to a weaker medium.
Mechanical model of thrust sheet gliding and imbrication
Authors: Mandl G. , Shippam G. K.
Published: May 2008

A competent thrust sheet being pushed over incompetent substrata is idealized as an elastic block overriding a stack of perfectly plastic substrata. The stress fields induced in both the plastic substrata and the thrust sheet have been analysed for various thrust sheet dimensions and boundary conditions.

In the case of the plastic substrata the stress analysis accounts for the formation and location of a basal slip plane, its stepping-up to a higher gliding horizon, and for secondary faulting.

For the competent thrust sheets, finite-element analyses have located a well-defined spot where a push force of sufficient magnitude is likely to induce fracturing and, consequently, may control the formation of an imbrication thrust, assuming that faulting is not preceded by compressional folding. Some further conclusions are drawn with respect to the development and spacing of subsequent imbrication thrusts.
The rock mechanics of thrust and nappe formation
Authors: Murrell S. A. F.
Published: May 2008

Three models of thrust formation have been proposed: sliding opposed by friction, movement by flow in a thin layer, and spreading by more general flow. The field evidence suggests that the shear stresses involved must be low. Experimental and theoretical rock mechanics provide criteria which may be able to distinguish the applicability of the three models, with the constraint of low shear stresses, and the present paper investigates this possibility.
Subduction and coeval thrust belts, with particular reference to North America
Authors: Smith A. G.
Published: May 2008

Although thrust belts at present-day subduction zones are sub-parallel to such zones, several ancient thrust belts dip in the opposite direction to the inferred dip of the contemporaneous subduction zone. Gravity spreading, gravity sliding or compressional stresses transmitted from the subduction zone are unlikely causes of such thrust belts. The driving force is attributed instead to the hydrostatic head of a fluid-like rock welt. If the welt is hot—as in a granitic/metamorphic belt—foreland thrusts may form.

The emplacement stresses are similar in magnitude to those of gravity spreading, but the thrust wedge need not be weak. The essential requirements of a thrust belt created by a fluid welt are a downhill surface slope in the direction of thrust transport and weak decollement horizon(s).

In the Mesozoic foreland thrust belts of western North America, the volume of rock pushed onto the foreland is comparable to the volume of the batholiths in the orogenic core and to the volume of new material added by subduction to the overriding American Plate.

After collision with another continent or an island arc the recognition of thrust belts emplaced by fluid welts is more difficult, particularly when ophiolites are present.
The role of gravity in orogenic belts
Authors: Ramberg H.
Published: May 2008

The diapiric and nappe structures of the Caledonides in Scandanavia are briefly discussed and compared with similar structures in other orogenic belts. The buoyancy of sialic basement is discussed and a theoretical treatment of basement buoyancy is outlined. The gravitational spreading of nappes is then considered. Two theoretical models are presented: the first considers the condition of free slip on the base and the second deals with the more realistic situation when the base exhibits coherence.
Dynamic analysis of a small imbricate thrust and related structures, Front Ranges, Southern Canadian Rocky Mountains
Authors: Spang J. H. , Brown S. P.
Published: May 2008

The thrust is a hanging-wall imbricate of the Lac des Arcs Thrust within the McConnell Thrust Plate. The small imbricate terminates vertically into a cylindrical, macroscopic kink band and laterally by merging with the Lac des Arcs Thrust. Numerical dynamic analysis of calcite and dolomite twin lamellae gives a near layer parallel maximum principal compression and an extension at a high angle to layering, both in the transport direction. The twinning is considered to be late stage. Reorientation of the principal axes occurs near folds and other minor thrusts in the area.
The microfabric of calcite tectonites from the Helvetic Nappes (Swiss Alps)
Authors: Schmid S. M. , Casey M. , Starkey J.
Published: May 2008

The crystallographic orientations of calcite in tectonites from the Morcles Nappe and the Glarus Overthrust have been measured by X-ray texture goniometry. The orientation data are represented by pole figures, inverse pole figures and as orientation distribution functions.

The patterns of preferred orientation observed in the specimens from the Morcles Nappe are correlated with intracrystalline slip mechanisms. The patterns exhibit quasiaxial symmetry and in some cases the axis of symmetry is oblique to the symmetry of the macroscopic fabric. This obliquity is interpreted as the result of a rotational strain path and may be used to infer direction and sense of shear.

Some specimens of the Lochseiten mylonite from the Glarus Thrust show no strong preferred crystallographic orientation. This suggests that grain boundary sliding was the major deformation mechanism.
Very low grade metamorphism with a reverse gradient induced by an overthrust in Haute-Savoie (France)
Authors: Aprahamian J. , Pairis J.-L.
Published: May 2008

The observed distribution of metamorphic transformations, using: (1) the metamorphic facies of Taveyanne Sandstones, (2) the mineralogical composition of the argillaceous fraction (which shows index minerals like paragonite, corrensite and allevardite), (3) the crystallinity indices of the illites, enables us to show that a reverse gradient (the maximum of which must be located in the thrust plane) is superimposed on the already known gradient which decreases both towards the top of the series and towards the external part of the chain.

To account for the observed relationships, we propose that heat was produced by friction along the thrust plane; the theoretical implications of such a model are discussed.

It is pointed out that such a reversed metamorphic gradient will only be observed following favourable circumstances of nappe thickness, rate of motion, thermal properties of the rocks and rate of erosion.
Saline horizons acting as thrust planes along the southern margin of the Damara Orogen (Namibia/SW-Africa)
Authors: Behr H. J. , Ahrendt H. , Schmidt A. , Weber K.
Published: May 2008

At the S margin and at the base of the Naukluft Nappes of the late Proterozoic-early Palaeozoic Damara Orogen in Namibia so-called ‘Unconformity Dolomites’ are developed along thrust planes of nappes and within imbricate structures. Micro-probe analyses, phase-petrological relationships of the unusual mineral composition of the ‘Dolomite’, the salinity and composition of fluid inclusions as well as other geochemical data prove a saline origin of the ‘Unconformity Dolomite’. These data combined with observations on the relationship of deformation and mineral growth, calculated geothermal gradients and radiometric datings lead to the conception that the ‘Unconformity Dolomite’ is a hot saline intrusion which has already intruded a preexisting nappe pile. After intrusion and rapid crystallization, the rocks directly below the rigid dolomite plate acted as a thrust plane for last nappe movements which brought the Naukluft Nappes into their present position.
Sliding and other deformation mechanisms in a glacier of salt, S Iran
Authors: Talbot C. J.
Published: May 2008

One of the salt glaciers in S Iran is interpreted here as an analogue of a gravity-driven, thrust nappe complex which has overridden its own debris. Fabrics from different parts of the body are interpreted in terms of grain boundary diffusion and dynamic recrystallization, together with tensional separation and sliding along the cubic cleavages of porphyroclasts of clear halite.

Slides generate within the salt as ramps, where the salt glacier slows and thickens to surmount the bedrock obstructions to its general downslope flow. Zones of plastic flow-folds and smaller zones of large-scale crenulation cleavage form upstream of the obstructions and the slides emerge from the downstream end of such zones. Few, if any, ramp directly to the surface and most turn to flats within the salt sheet where they may heal as a result of stress-drop annealing.

The competence of the salt mass is assumed to decrease down its length in response to a decrease in grain size and a relative increase in insoluble grains. Such a decrease in competence accounts for the change in character of the internal deformation of the salt sheet when traced downstream. First the slides and then the crenulation cleavage fail to develop upstream of successive obstructions and eventually all internal folding ceases. Instead, the salt glacier moves over the last few bedrock ridges as accommodation folds and another type of slide appears to form by layer parallel slip.

All the slides in the salt glacier originate near irregularities in its bedrock channel. The slide propagation zones remain stationary and the salt tears as it passes through, and beyond them. Instead of eroding its bedrock, the salt glacier effectively smooths irregularities in its channel by shearing over the infilling bodies of static salt.
The Caledonides of northern Norway: relation between preferred orientation of quartz lattice, strain and translation of the nappes
Authors: Boullier A.-M. , Quenardel J.-M.
Published: May 2008

In the area of Birtavarre (eastern Troms) evidence of nappe tectonics exist at all scales. The preferred orientation of the
c
-axis of quartz shows a monoclinic symmetry and consists of a girdle containing
Y
and oblique to
Z
. The obliquity of the girdle-angle ϑ between the
c
-axis girdle and the
YZ
plane-is interpreted in terms of simple shear, and the strain is calculated from these data. The results from this study are: (1) almost all the quartz-bearing samples give a consistent sense of simple shear (movement towards the SE) except for some gneisses supposed to be allochthonous Precambrian. (2) the calculated strain is too low to explain the displacement of the nappes (more than 40 km for a 2 km thick unit):
X/Z
ratio is less than 10 on the average. Possible reasons for this are discussed.
Structure and distribution of fault rocks in the Alpine Fault Zone, New Zealand
Authors: Sibson R. H. , White S. H. , Atkinson B. K.
Published: May 2008

The Alpine Fault Zone, which is the dominant strand in a dextral system of transform faults along the boundary between the Indo-Australian and Pacific plates, forms a pronounced lineament about 1 km in width. Within the fault zone, the cross-strike passage from cataclasite through augen mylonite (both derived from largely granitoid basement to the NW) to mylonites derived from the high-grade Alpine Schists on the upthrown south-eastern side, is thought to reflect the original distribution of fault rocks with depth below near-surface gouge zones. Structural data suggest that the fault rocks had their fabrics impressed during the late Cenozoic phase of oblique compression when the Southern Alps were ‘ploughed-up’ along the fault. Mylonitic foliation indicates a dip approaching 50°SE for the fault zone at depth, while penetrative stretching lineations plunge in a direction sub-parallel to the present-day interplate slip vector, consistent with dextral-reverse-oblique shear across the fault zone. Pseudotachylyte friction-melt is fairly widely distributed and can be found cutting all other fault rock types apart from gouge. Models for the evolution of the fault zone are considered.
Uplift rates and thermal structure in the Alpine Fault Zone and Alpine Schists, Southern Alps, New Zealand
Authors: Adams C. J.
Published: May 2008

Rates of vertical and horizontal movement at the Alpine Fault Zone and within the adjacent Alpine Schists (Haast Schist Group) are deduced from K-Ar age data of metamorphic and mylonitic rocks. The relative contributions of Jurassic-Cretaceous (Rangitata) and Miocene-Recent (Kaikoura) orogenic phases are also estimated.

After Jurassic regional metamorphism, the Haast Schists were uplifted regionally 18–20 km at rates 0.3–0.4 mm/yr until mid-Cretaceous times. No differential vertical movements occurred on the Alpine Fault at this time. During the Miocene-Recent orogeny, a maximum of 10 km uplift, produced mylonitic rocks at the Alpine Fault and brought Alpine Schists (Haast Schist Group) to the surface. Ages of mylonite, blastomylonite and pseudotachylyte indicate intense fault movement was occurring 5–10 Ma ago. Ages as young as 0.7 Ma occur in the adjacent Alpine Schists. From these data, uplift rates at the Alpine Fault for the period late Miocene-Pliocene are in the range 2.5–5 mm/yr and late Pleistocene-Recent, 7–14 mm/yr.

The estimation of horizontal displacements at the Alpine Fault in the Jurassic-Cretaceous and Miocene-Recent orogenic phases depends critically on the reality of a Cretaceous (?) lamprophyre dyke swarm cut by the Alpine Fault. If this is accepted, then about 360 km horizontal movement is pre-Upper Cretaceous and 120 km is younger. If the dyke swarm evidence is dismissed, then a total 480 km displacement on the Alpine Fault is entirely post-Cretaceous and may have occurred during the Miocene-Recent orogenic phase. This is, then, broadly associated with large-scale motions of the Indian and Pacific plates in the late Cenozoic and Quaternary.

Analysis of the age patterns within the Alpine Fault mylonites and the adjacent Alpine Schists support the possibility that a 15 km wide thermal aureole, about the Alpine Fault or at zones of intense shearing within the Alpine Schists, has been created by frictional generation of heat during relatively rapid aseismic shear sustained over the last 5 Ma. This thermal effect has been sufficient to cause complete argon loss from rocks over a 10 km wide zone immediately E of the Alpine Fault.
The internal geometry of nappes: criteria for models of emplacement
Authors: Cooper M. A.
Published: May 2008

The geometric and stratigraphic properties of thrust sheets and fold nappes are controlled by the mechanisms by which they were initiated and emplaced. Thrust sheets that have developed by gravity gliding have their internal stratigraphies truncated by the basal thrusts which cut up-section at the trailing edges. In contrast thrust sheets that have developed by gravitational spreading have their internal stratigraphies truncated by the overlying thrust sheets, and their basal thrusts cut up-section at the leading edges. Fold-nappes have complicated geometries and correlation of stratigraphy between successive nappes is difficult. The basic geometric and stratigraphic properties of thrust sheets and fold-nappes developed by different mechanisms may be used as criteria for testing the applicability of these mechanisms to particular areas of orogenic belts. The Valley and Ridge Province of the Appalachians has geometric and stratigraphic properties consistent with thrust sheet development by gravitational spreading. The western nappes of the Central Scandinavian Caledonides are more complex but have properties suggesting development by buoyant upwelling of the orogen infrastructure. There is, however, a lack of detailed cross-sections of orogenic belts based on geophysical and/or borehole data in addition to the geological data. The interpretation of geological data in isolation is often too dependent on subjective information coloured by the mechanism preferred by the individual concerned.
The Moine Thrust Zone: an overview
Authors: McClay K. R. , Coward M. P.
Published: May 2008

The geometry of structures within the Moine Thrust Zone are similar to those found in classic thin-skinned tectonic regimes such as the southern Canadian Rocky Mountains. The deformation style and strain patterns along the strike of the Moine Thrust Zone are controlled by the position of the floor (or Sole) thrust. The stacking sequence of thrust nappes is from E to W in the direction of transport. Reactivation of some thrusts has caused reversals in the stacking sequence. The mylonites associated with the Moine Thrust have undergone a long and complex history of progressive deformation.

Detailed descriptions of the Moine Thrust Zone at Loch Eriboll are given and the relationships of the Moine Thrust Zone to other major Caledonian tectonic features of the NW Highlands are discussed.
Tectonic slides in the Caledonides
Authors: Hutton D. H. W.
Published: May 2008

Despite the continuing use of the term ‘tectonic slide’ in the Caledonian fold belt of Britain and Ireland for the past fifty years, the concept has found few advocates elsewhere. To many geologists outside the British Isles it remains a subject of some confusion.

A review of the literature suggests that slides are a general term for faults which form in close association with syn-metamorphic regional deformation. They are often related to major folds although this is not a diagnostic feature. Neither is any movement sense implied in the term slide; thus, they may be thrusts, lags, oblique slip etc. Classically, these structures lie along and subparallel to the boundaries of major lithological units but transgress the stratigraphy on a larger scale. Structural analogies with the thrusts of the lower grade or non-metamorphic deformed cover sequences can often be seen in slides. However, they are more commonly and possibly fundamentally the result of differing responses to deformation of contrasting, adjacent lithologies in areas of higher strain and metamorphism.