A study of the relation between jointing and structural evolution

This paper records the results of a regional jointing survey in the country adjacent to a part of the Neath disturbance, South Wales, in an area of Old Red Sandstone and Carboniferous Strata. A well developed joint system exists which is comprised of up to six sets of joints striking at 350°, 330°, 290°, 270°, 240° and 210°. The joint patterns undergo stratigraphical and areal variation, which can be correlated with lithology and tectonic setting. A comparison of the joint trends with the faults and folds shows that a genetic relationship exists between the structures. It is suggested that the joint sets formed early in the tectonic evolution of the area, and that these and the faults can be attributed to a general N-S compressive force, and its related first and second order shear regimes.     

Facies change in upper devonian and lower carboniferous rocks of Southern Ireland

In south-west Ireland 8,000 ft (2,440 m) of marine sandstones and mudstones, the Cork Beds (?Upper Famennian to E Zone, Carboniferous, in age), overlie the Old Red Sandstone. Farther north the Old Red Sandstone is succeeded by thin Lower Limestone Shales overlain by thick Waulsortian bank limestones. A critical section (North Ringabella) west of Cork Harbour, in which 6,500 ft (1,981 m) of Old Red Sandstone and Cork Beds is exposed, is described and divided into ten formations. By comparison with sections to the south the upper beds of the Old Red Sandstone are shown to pass southward into marine sandstones (Cork Beds) of ?Upper Famennian age. The successions in the Cloyne and Cork Synclines are described and reveal the progressive northward change in the Lower Carboniferous from the argillaceous Cork Facies through a zone of isolated bank limestones (Cloyne) to a thick, 4,000 ft (1,219 m), Waulsortian bank complex (Cork). Finally an attempt is made with use of isopachyte maps to reconstruct the palaeogeography of southern Ireland in Upper Devonian and Lower Carboniferous times.     

The ‘old red sandstone’ of county mayo,Northwest Ireland

The distribution and stratigraphic relations of the Old Red Sandstone of the Clew Bay area. County Mayo are described. They comprise two groups both dominated by conglomerates, one Lower Devonian and one Middle Devonian, which have differing clast contents and which are separated by faults. The sediments are described in terms of five facies which are all interpreted as alluvial fan deposits. Provenance data from palaeocurrents and clast composition suggest transport predominantly to the west-northwest but one formation shows an opposite sense of transport. The rocks are folded about northeast-southwest axial traces and are affected by numerous faults. Structures are more complex than those of the Lower Carboniferous rocks which unconformably overlie the Devonian. A similar tectonosedimentary setting to the Old Red Sandstone of parts of the Scottish Caledonides is indicated.     

Sedimentology of the Old Red Sandstone (Siluro-Devonian) in the Clee Hills area,Shropshire, England

The sequence of approximately 1300 m is divided by a major unconformity (Middle Devonian) into the thick Lower Old Red Sandstone (Siluro-Devonian), resting disconformably on Ludlovian (Silurian) marine strata, and the much thinner Upper Old Red Sandstone (Upper Devonian) overlain by the Carboniferous.The Lower Old Red Sandstone commences with littoral sediments (Downton Castle Formation) followed by tidal mud-flat deposits (Temeside Formation) formed after a brief marine transgression. The predominant remainder of the sequence (Ledbury Formation, Ditton Group, Abdon Group, Woodbank Group), characterized by fining-upwards cyclothems, records the establishment during a marine regression of extensive and persistent alluvial plains. Prior to Ditton Group times, detritus came from relatively distant regionally metamorphosed rocks lying to the north or west of the Clee Hills. Subsequently, apparently as the result of river-capture or drainage-reversal consequent on the commencement of the final (mid-Devonian) phase of Caledonian movement, high-level crustal rocks closer at hand (largely Wales) replaced the metamorphics as the sources of sediment, the earlier Lower Old Red Sandstone itself being recycled. To judge from the calcretes preserved in the alluvial formations, the area lay near the Equator and experienced a relatively dry hot climate.The Upper Old Red Sandstone likewise reveals fining-upwards cyclothems. The overlying Carboniferous rocks evidence the renewed marine transgression of the area, after the removal of the effects of the mid-Devonian movements.     

Palaeomagnetic studies of fold development and propagation in the Pembrokeshire old red Sandstone

The geometry of two folds in lower Devonian Old Red Sandstone in Pembrokeshire, deformed in the Hercynian, has been defined at intermediate stages during their evolution by this preliminary palaeomagnetic study. This is possible because remagnetization appears to have occurred in discrete episodes during deformation and the shape can be reconstructed by restoring the fold limbs to an angle at which the remagnetization directions coincide. The plunge of the folds is shown to vary considerably during evolution, one fold having an early easterly plunge which changed to the present westerly plunge, the other fold having an early plunge which was more steeply to the west than its present attitude. Clockwise rotation of the whole of southern Pembrokeshire by 40° towards the end of, or since, the Hercynian deformation, is indicated by the palaeomagnetic data and a structural model for this is proposed.     

Stratigraphical relationships and sedimentary environments of the silurian—early Old Red Sandstone of Pembrokeshire

In Pembrokeshire, northward Hercynian thrusting has brought together Silurian and Old Red Sandstone rocks of widely different stratigraphical and sedimentological successions. Five structurally separate blocks are centred from south to north on Freshwater, Marloes, Winsle, Rosemarket and Haverfordwest.     

Remagnetization and fluid flow in the Old Red Sandstone along the Great Glen Fault, Scotland

The Devonian Old Red Sandstone in the vicinity of the Great Glen Fault (GGF) in Scotland contains two different components residing in hematite: a postfolding Carboniferous CRM1 in the Loch Ness area and a Cretaceous or perhaps Triassic CRM2 near Hilton. The CRM1 could be related to major fluid flow events in the Late Paleozoic which caused hematite authigenesis and remagnetization along other faults in Scotland. The CRM2 near Hilton was also related to a fluid event in the Cretaceous or Triassic which caused hematite authigenesis. The presence of different CRMs residing in hematite along different segments of the GGF is similar to what has been reported for other major faults in Scotland.     

Dimlington Stadial (Late Devensian) ice-wedge casts and involutions in the Severn Estuary,southwest Britain

The largely concealed sub-Flandrian bedrock surface in the Severn Estuary, chiefly known from borings and chart soundings, has the form of a valley within a valley. In the aftermath of an unusual storm in March 1986, which removed from many places the obscuring modern sediments normally present, periglacial structures were recognized on the floor and upper margins of the upper valley at ten widely scattered localities. The structures occur only where the valley is formed on relatively soft Triassic and Jurassic mudrocks; they involve gravels and/or red-brown sands belonging to the Main Terrace sequence, but in some places incorporate local gravels. There are no completely convincing indications of cold conditions where the relatively strong Silurian and Old Red Sandstone rocks and the Carboniferous Limestone form the valley floor. The altitudinal relationships of the involutions and wedges, together with the scale of the outer valley, suggest that this sub-Flandrian rock bench was merely re-excavated during the Late Devensian. Then permafrost developed while the river lay confined within the much narrower inner gorge.     

Pre-Permian uplift and diffuse extensional deformation in the High Zagros Belt (Iran): integration in the geodynamic evolution of the Arabian plate

The High Zagros Belt includes exposures of Lower Palaeozoic rocks in the core of several thrusted anticlines developed during the Cenozoic Zagros orogeny. The structural style and tectonic evolution of this area during the Palaeozoic remain poorly understood due to the complexity of the subsequent deformation. We present the preliminary results of a field study focusing on the structural geology of Palaeozoic rocks in this area. We confirm the existence of an angular unconformity below the Lower Permian Faraghan formation. In the geological literature, this unconformity is reported as the “Hercynian Unconformity” suggesting a relationship with the Hercynian (Variscan) orogeny, which affected Western Europe and westernmost Africa during the Carboniferous. Surprisingly the only observable structures sealed by this unconformity are N to NE trending normal faults and tilted blocks without any evidence of compressional deformation. This pre-Permian extensional deformation, which is general at the scale of the HZB, raises questions about the geodynamic significance of the “Hercynian unconformity” in the study area and, more generally, in the Arabian plate.     

A Large-Scale Palaeozoic Dextral Ductile Strike-Slip Zone:the Aqqikkudug-Weiya Zone along the Northern Margin of the Central Tianshan Belt,Xinjiang,NW China

The nearly E-W-trending Aqqikkudug-Weiya zone, more than 1000 km long and about 30 km wide, is an important segment in the Central Asian tectonic framework. It is distributed along the northern margin of the Central Tianshan belt in Xinjiang, NW China and is composed of mylonitized Early Palaeozoic greywacke, volcanic rocks, ophiolitic blocks as a mélange complex, HP/LT-type bleuschist blocks and mylonitized Neoproterozoic schist, gneiss and orthogneiss. Nearly vertical mylonitic foliation and sub-horizontal stretching lineation define its strike-slip feature; various kinematic indicators, such as asymmetric folds, non-coaxial asymmetric macro- to micro-structures and C-axis fabrics of quartz grains of mylonites, suggest that it is a dextral strike-slip ductile shear zone oriented in a nearly E-W direction characterized by "flower" strusture with thrusting or extruding across the zone toward the two sides and upright folds with gently plunging hinges. The Aqqikkudug-Weiya zone experienced at least two stages of ductile shear tectonic evolution: Early Palaeozoic north vergent thrusting ductile shear and Late Carboniferous-Early Permian strike-slip deformation. The strike-slip ductile shear likely took place during Late Palaeozoic time, dated at 269(5 Ma by the40Ar/39Ar analysis on neo-muscovites. The strike-slip deformation was followed by the Hercynian violent S-type granitic magmatism. Geodynamical analysis suggests that the large-scale dextral strike-slip ductile shearing is likely the result of intracontinental adjustment deformation after the collision of the Siberian continental plate towards the northern margin of the Tarim continental plate during the Late Carboniferous. The Himalayan tectonism locally deformed the zone, marked by final uplift, brittle layer-slip and step-type thrust faults, transcurrent faults and E-W-elongated Mesozoic-Cenozoic basins.     

Strain partitioning and deformation mode analysis of the normal faults at Red Mountain, Birmingham, Alabama

In a low-temperature environment, the thin-section scale rock-deformation mode is primarily a function of confining pressure and total strain at geological strain rates. A deformation mode diagram is constructed from published experimental data by plotting the deformation mode on a graph of total strain versus the confining pressure. Four deformation modes are shown on the diagram: extensional fracturing, mesoscopic faulting, incipient faulting, and uniform flow. By determining the total strain and the deformation mode of a naturally deformed sample, the confining pressure and hence the depth at which the rock was deformed can be evaluated. The method is applied to normal faults exposed on the gently dipping southeast limb of the Birmingham anticlinorium in the Red Mountain expressway cut in Birmingham, Alabama. Samples of the Ordovician Chickamauga Limestone within and adjacent to the faults contain brittle structures, including mesoscopic faults and veins, and ductile deformation features including calcite twins, intergranular and transgranular pressure solution, and deformed burrows. During compaction, a vertical shortening of about 45 to 80% in shale is indicated by deformed burrows and relative compaction of shale to burrows, about 6% in limestone by stylolites. The normal faults formed after the Ordovician rocks were consolidated because the faults and associated veins truncate the deformed burrows and stylolites, which truncate the calcite cement. A total strain of 2.0% was caused by mesoscopic faults during normal faulting. A later homogenous deformation, indicated by the calcite twins in veins, cement and fossil fragments, has its major principal shortening strain in the dip direction at a low angle (about 22°) to bedding. The strain magnitude is about 2.6%. By locating the observed data on the deformation mode diagram, it is found that the normal faulting characterized by brittle deformation occurred under low confining pressure (< 18 MPa) at shallow depth (< 800 m), and the homogenous horizontal compression characterized by uniform flow occurred under higher confining pressure (at least 60 MPa) at greater depth (> 2.5 km).     

Palaeomagnetic re-examination of the basal Caithness Old Red Sandstone; aspects of local and regional tectonics

A palaeomagnetic re-examination of the basal strata of the Caithness Old Red Sandstone has given results that are fully compatible with previous palaeomagnetic findings in this region. After structural correction the dominant remanence component has D = 205°, I = +3°, α₉₅ = 6.4° (N = 27). The existence of this shallow inclined magnetization in the Middle Devonian strata of Caithness invalidates the model, proposed by Van der Voo and Scotese (1981), involving a ca. 2000 km sinistral offset along the Great Glen Fault in the Carboniferous. However, the available data are in favour of a few hundred kilometres sinistral movement along this fracture zone. However, the possibility of there having been a much larger transcurrent shift between Europe and North America in late/post-Devonian times, accumulated along various fracture zones within the Caledonian fold belt, is discussed. On the basis of an inferred overprinted magnetization, it is tentatively concluded that the tectonic deformation of the Old Red Sandstone of Caithness has a mid-Jurassic or younger age.     

Strain partitioning in a pluton during emplacement in transpressional regime: the example of the Néouvielle granite (Pyrenees)

This microstructural and anisotropy of magnetic susceptibility study of the internal structures of the Hercynian Néouvielle granite pluton (100 km²) provides new data indicating that the pluton was emplaced during the main Hercynian tectonic event recognized in the Pyrenees. It also provides new data about the later Alpine deformation localized along narrow mylonitic bands. These bands acted as reverse faults and have not rotated the Hercynian structures that define the main part of the pluton. The pluton is composed of two structural domains: the northern half of the pluton displays a beak shape in map view, with subhorizontal E-W trending lineations of magmatic origin; the southern half is semi-circular and displays rather steeply northward plunging lineations corresponding to magmatic and high temperature (HT) solid-state microstructures. These features are associated with magma deformation during emplacement. Magma deformation corresponds, in the northern half of the pluton, to an E-W strike-slip deformation recognized in the enveloping pelitic metasediments of Carboniferous age and, in the southern half of the pluton, to southward overthrusting recognized in the enveloping quartzites of Devonian age. Juxtaposition in a single granite body of transcurrent and compressive domains is viewed as a strain partitioning in the magma. This strain partitioning is linked to both the transpressive character of the main regional deformation event and the rheological contrast between the pelitic country rocks and quartzose country rocks.     

Stratigraphy and facies development of the pre-Cenozoic sediments in southern Egypt: a geodynamic approach

The study of the geology of southern Egypt, in-between the Red Sea and the Libyan borders, south of latitude 24°30′N reveals a succession of the Precambrian Arabo–Nubian Shield along the Red Sea coast overlain by Paleozoic–Mesozoic sediments. The Paleozoic section in the study area is well developed in three sub-basins, namely, Uweinat–Gilf, South Nile, and Etbai. Paleozoic sediments are well developed in the three sub-basins mostly sandstones of Cambrian overlain by glaciogene sediment conglomerates at base namely Gabgaba Formation and by the Naqus Sandstone at top. The tectonic events during the Early Paleozoic Caledonian Orogeny are marked by several unconformities and tectonic uplift and down faulting expressed in the many faults in the Uweinat–Gilf and South Nile sub-basins. The Carboniferous is well-developed sandstones in the three studied sub-basins. The glaciation at the Permian is reflected in sea-level fall, hence continental sediments are well developed in many parts of Egypt—a phase of the Hercynian Orogeny. Volcanics are very well common in the study areas ranging in age from 48 to 34 Ma at Uweinat and Gebel El-Asr. Vulcanicity continued during the Paleozoic and Mesozoic at the Triassic of Nasab El-Balgum and in the south Western Desert, the south Eastern Desert, and Etbai area. The highly seismic conditions in southern Egypt continued up to very recent times where tremors were noticed in the 80s and 90s of the last century pointing to very unstable area.     

贺西地区板块构造分区及晚古生代盆地形成与演化

贺西地区以阿拉善陆块为主体,太古界裸露地表、花岗岩发育,在南北缝合带之间还有六条形成期早、演化时期长的深大断裂,对早古生代壳块分区,晚古生代盆地形成、分布起着控制和分割作用。晚古生代在前陆盆地率先接受了上泥盆统、下石炭统沉积;步入晚石炭世、沉积范围继续扩大至碰撞裂谷盆地后,又扩大到鄂尔多斯西缘并与华北海(陆表海)连为一体。进入海西旋回中晚期,受蒙兴海槽、秦祁海槽会聚、收缩、对接碰撞构造背景和构造运动的影响,使曾经是石炭纪南海北陆的古构造格架、古地理面貌,被全面海退后二叠纪的陆相沉积所代替。晚海西—早印支运动,使上古生界全面发生挤压变形,形成多种类型断裂、褶皱构造与圈闭,并受到了普遍的隆起剥蚀和缺失。晚印支期断裂活动,构造将晚古生代形成的两类盆地解体为八个断陷和五个隆起区,又使新的盆岭构造格局在此基础上形成、发育和演变。     

Interlobate ice-sheet dynamics during the Last Glacial Maximum at Whitburn Bay, County Durham, England

This research reconstructs ice-sheet processes operating during the Late Devensian in northeast England. The article assesses the lithostratigraphy of the Devensian glacial tills of Whitburn Bay, eastern County Durham, and presents the first detailed analysis of petrological, geochemical and biostratigraphical data to reconstruct lithostratigraphy, provenance and iceflow pathways. Two Devensian tractions tills (the Blackhall and Horden tills) are separated by a boulder pavement, pointing to a switch in ice-bed conditions and the production of a melt-out lag prior to deposition of the upper traction till, the Horden Till. The Blackhall Till contains Magnesian Limestone, Carboniferous Limestone, Whin Sill dolerite and Old Red Sandstone, suggesting a northwesterly source, probably from the Midland Valley and the Southern Uplands. The Horden Till contains erratics and heavy minerals derived from crystalline bedrock sources in the Cheviot Hills and northeast Scotland. Within the Horden Till are numerous sand, clay and gravel-filled canals incised downwards into the diamicton which are attributed to a low-energy, distributed, subglacial canal drainage system. Coupled with hydro-fractures and the boulder pavement, this suggests that a partially decoupled, fast-flowing ice stream deposited the Horden Till. The uphill, landward direction of ice movement indicates that the ice stream was confined in the North Sea Basin, possibly by the presence of Scandinavian Ice.     

Proglacial glaciotectonic deformation and the origin of the Cromer Ridge push moraine complex, North Norfolk, England

The landscape of northeast Norfolk is dominated by a high (>50 m) ridge which has been interpreted as an end moraine (Cromer Ridge). This feature is truncated by coastal erosion at Trimingham. Evidence of large- and small-scale compressive styles of deformation is found throughout the sequence, except at the very top, where late Anglian/early Hoxnian lake sediments are found within an undeformed kettle hole. The deformation consists of open folds (including chevron folds) and listric thrust faults. It is suggested that these are the result of a single compressive event, which was caused by proglacial glaciotectonic deformation. It is inferred that this deformation is due to a combination of frontal pushing and compressive stresses transmitted through a subglacial deforming wedge. It is also shown that strain increases towards the ice sheet margin, as reflected by the deformational styles (from open folding up-glacier to listric thrust faulting down-glacier). The Cromer Ridge is shown to be a push moraine complex related to an actively retreating ice margin.     

The lower carboniferous (dinantian) of the Swords area: Sedimentation and tectonics in the Dublin Basin,Ireland

The litho- and biostratigraphy of the Lower Dinantian succession in a deeper part of the Dublin Basin is described. The sub-Waulsortian Malahide Limestone Formation (emended) is described fully for the first time, and has proved to be very much thicker than was previously suspected, in excess of 1200 m. Succeeding the ‘Lower Limestone Shale’ unit, which is transitional from the underlying Old Red Sandstone facies, the following six new members are recognized: Turvey Micrite Member, Swords Argillaceous Bioclastic Member, St. Margaret's Banded Member, Huntstown Laminated Member, Dunsoghly Massive Crinoidal Member and Barberstown Nodular Member (top). The Malahide Limestone Formation is overlain by ‘Waulsortian’ limestones of the Feltrim Limestone Formation (new name) which form overlapping and isolated mudmounds with complex relationships with their enclosing non-mound facies. Though very much thicker, the Courceyan succession is comparable with that elsewhere on the south side of the Basin, and is part of the Kildare Province (Strogen and Somerville 1984). Isopach maps for the region show that this province and the North Midlands are separated by the deepest part of the Dublin Basin, named the ‘East Midlands Depocentre’, in which a shale-dominant facies is present. The top of the ‘Waulsortian’ is of early Chadian age. Formations younger than this are dominated by basinal calcareous shales (Tober Colleen Formation) and by storm deposits and calciturbidites with appreciable terrigenous input from the east (Rush Formation). The Courceyan main shelf conodont biozones are also greatly thickened in this area. The Pseudopolygnathus multistriatus Biozone (> 300 m thick) is succeeded by a very thick (> 900 m) Polygnathus mehli Biozone. The base of the Chadian is considered to occur below the top of the Feltrim Limestone Formation and, although equivocal, may be diagnosed in the Dublin Basin by the first appearance of the problematic microfossil Sphaerinvia piai and a primitive form of the calcareous alga Koninckopora. In the late Courceyan, the Swords area was part of a gently sloping shelf extending northwards into the basin. During deposition of the Feltrim Limestone Formation there was major deepening and there is evidence of initial break up of the Dublin Basin by faulting into separate blocks. By Chadian time the Basin was definitely subsiding by fault displacements and basinal limestones contain shallow water faunas and littoral sand and pebbles derived by turbidite flows from the margins of the higher blocks. The early subsidence was apparently by pure flexure, but in the Viséan the Dublin Basin was fault-controlled, differing from the adjacent Shannon Basin in having both margins strongly faulted.     

Facies relationships in the Upper Devonian-Lower Carboniferous of southwest Ireland and adjacent regions

In southwest Ireland 2,500 m of Upper Famennian to basal Namurian marine sandstones and mudstones, the Cork Beds, overlie rocks of Old Red Sandstone facies. Coastal exposures of the Cork Beds are interpreted as showing gradual upward change from alluvial strata, through thick subtidal and shelf sediments to pyritic muds. A review of recent palaeontological evidence shows that the thick shallow marine part of the Cork Beds is older than the major development of lime-stones north of the Cork Harbour—Kenmare Une, whose equivalents to the south are in the condensed basinal sediments. The Lower Carboniferous portion of the Cork Facies is shown to be thicker in South Cork than in West Cork. In Lower Carboniferous times a positive area–the Glandore High–separated two sub-basins with different depositional histories. Six palaeogeographic maps are used to demonstrate the progressive shift of facies belts as Lower Carboniferous marine transgression progressed. Finally, brief comparison is made with rocks of the same age in southwest England.