Orogenic Movements Research Articles

High-pressure granulites of East Siberia in terms of Archaean and Proterozoic evolution

The southern borderland of the Aldan Shield comprises high-pressure granulites with dominant mafic components combined into the Kurulta Series. These granulites pertain to the Archaean and Palaeoproterozoic history of the lower crust of the marginal part of the Siberia Craton, while the upper crustal layers are composed of largely amphibolite-facies rocks of the Stanovik Series, which together with the Kurulta Series constitute the Stanovik Fold Area. The evolution of the area is characterized by recurrent tectonic activity of the Aldan, Early Stanovik, Stanovik and Jeltulac cycles. The earliest Aldan events at ∼3.5-3.1 Ga gave rise to primary granulite assemblages formed under geothermal gradients of 32-28°/km, homogeneous over the entire Shield. At ∼2.7-3.0 Ga, the northern, more stable, terrains were separated from the mobile Stanovik Area by the Sutam Belt which both controlled the special magmatism and high-pressure metamorphism. These events were governed by a low geothermal gradient of 22-20°/km. High-pressure granulites resulted from sub-crustal accretion including the Aldan assemblages and Early Stanovik basic and calc-alkaline magmatic rocks, recrystallized at ∼750–950°C and 9–12 kbar. The following Stanovik activity at ∼2.4 Ga resulted in uplift and interstacking of altered granulites together with the Stanovik Series, metamorphosed under almandine-amphibolite facies conditions. After all this, the Jeltulac orogenic movements at ∼2.2-2.1 Ga were accompanied by further retrogression and tectonic ascent of granulite fragments along local tectonic zones. These events anticipated the emplacement of mafic dykes in the Tukuringra Zone at ∼1.8–1.9 Ga when high-grade metamorphism was resumed at deep crustal levels.

Continental rift development in Precambrian and Phanerozoic Europe: EUROPROBE and the Dnieper-Donets Rift and Polish Trough basins

Upper crustal structure and lower crustal and mantle character of the European continental lithosphere differ significantly from west to east. The processes of rifting of Phanerozoic western Europe's hotter, thinner crust compared with the more stable, thicker (45 vs 30 km) Proterozoic crust of the East European Craton is being investigated by EUROPROBE case studies of western and eastern European sedimentary basins. The Dnieper—Donets basin transects the southwestern part of the East European platform in a NW—SE direction, lying between the Ukrainian Shield and the Voronezh Massif. Rifting took place from early Frasnian until perhaps the late Visean and was accompanied by major volcanic activity. The distribution of both was affected by pre-existing basement fault systems. The syn- and post-rift sedimentary succession may be as thick as 20 km in the Donets Trough. Basin evolution and the present crustal geometry have been much affected by “inversion” during the Permian in association with Uralian orogenesis. Tectonic events of Bathonian—Oxfordian (∼ 168−154 Ma) and Turonian—Santonian (∼ 91−84 Ma) age are also recorded in the basin stratigraphy. The Polish Trough lies along the boundary between the Phanerozoic and Proterozoic European crustal domains, coincident with the Trans-European Suture Zone. The presence of this rheological boundary may be paramount in structurally controlling the position of the Trough. Tectonic subsidence analysis indicates an initial (late) Rotliegendes—Early Triassic syn-rift phase of development. Subsequently, episodes of increased tectonic subsidence rate occurred during the Oxfordian—Tithonian (≈ 157−146 Ma) and beginning in the Cenomanian (≈ 95−90 Ma). The Oxfordian—Tithonian episode is likely the signature of a second extensional event (correlated with intensification of rifting within the Arctic—North Atlantic rift system) while the Cenomanian and later is a precursor to mild compressional, intraplate orogenic movements. Present-day crustal structure reflects the “docking” of Phanerozoic crust against the Proterozoic during Palaeozoic orogenies, Permo—Carboniferous wrenching and transtensional modifications leading to formation of the Polish Trough, as well as modifications during the Late Cretaceous—Early Tertiary inversion process.

Paleoglobal change during deposition of cyclothems: calculating the contributions of tectonic subsidence, glacial eustasy and long-term climate influences on Pennsylvanian sea-level change

New sedimentological determinations of the water depth and associated sea-level change of Midcontinent Pennsylvanian cyclothems shows that they accumulated in water depths ranging from as low as 32 m to as high as 160 m. depending on which model is used to establish the deepest water facies. These depth determinations also indicate that regardless of model, depth variations existed for different cyclothems both laterally and in time. Average water depth determinations and sea-level change for models of Heckel and Gerhard are 96.4 and 86.0 m, respectively. Analysis of tectonic subsidence permits calculation of the magnitude of tectonic processes and associated climatic effects, which controlled changes in sea level during deposition of Pennsylvanian cyclothems. Far-field tectonic effects in response to regional orogenic movements partially influenced Pennsylvanian sea-level change in the Midcontinent. Organization of Virgilian and Missourian mid-coninent cyclothems into four- to five-fold bundles suggests that sea-level changes in Midcontinent platform areas were influenced both by Milankovitch orbital parameters and longer-term climate change, whereas Desmoinesian sea-level change apparently was influenced more strongly by tectonic subsidence controlled by foreland-basin tectonism. Sea-level change associated with cyclothems in the Illinois basin, Central Appalachian basin, and the Asturias basin of Spain were controlled mostly by foreland basin subsidence and later strike-slip faulting. Sea-level change in the southeast Netherlands coal field was influenced primarily by strike-slip movements within the Hercynian tectonic orogenic zone. The magnitude of tectonically contributed change in sea level varied laterally. In the Midcontinent, tectonic subsidence accounts for ~ 5–20% of the total sea-level change in platform areas, and perhaps as much as 20% in basin depocenters, whereas in the Illinois basin, tectonic subsidence accounts for 90% of estimated sea-level change. The remaining change in sea level is controlled by both short-term glacial eustasy (Milankovitch orbital forcing; approximately up to 70% of sea-level change in the Midcontinent) and long-term climate change (~ 15% of sea-level change in the Midcontinent). These findings suggest that away from orogenic belts, climatic change is the principal driving mechanism controlling sea level change, whereas within orogenic belts, climate becomes somewhat more subordinate as a driving mechanism for Pennsylvanian sea level change even though indicators of climatic change itself are preserved. Methods developed herein permit estimating magnitudes of both tectonic and glacio-eustatic components of sea-level change influencing Pennsylvanian cyclothem deposition, and may be applicable to other cyclic sequences.

Palaeogeography of the western Pelagonian continental margin in Beotia (Greece) during the Cretaceous—biostratigraphy and isotopic compositions ( δ13C, δ18O) of calcareous deposits

The palaeogeography of Beotia, Greece, during the Cretaceous is outlined on the basis of biostratigraphic evidence derived predominantly from the distribution of rudist bivalves and planktonic foraminifera. Observed facies trends are evaluated together with carbon and oxygen isotopic compositions of limestones and rudist shells. The interaction between global sea level fluctuations and gradual subsidence of the Pelagonian continental margin controlled facies distribution during most of the Cretaceous. The sedimentary record begins with Berriasian flysch-type deposits that accumulated during incipient Eohellenian orogenic movements. Barremian deposits with similar sedimentological characteristics probably belong to the same tectonostratigraphic unit. During Aptian-Cenomanian times, terrigenous clastics with a few intercalated marine limestones accumulated in South Beotia. The fossil content of patch reefs presents important biostratigraphic marks in this terrigenous sequence. Carbon and oxygen isotopic compositions recorded unstable marine environments and subsequent decrease of continental influx. Continuous subsidence and rising sea levels are evident in the Late Turonian onlap of rudist limestones on the Eohellenian basement of the Ptoon Mountains as well as in the interfingering of calcareous slope deposits and basinal Globotruncana-mudstones of South Beotia. During Late Coniacian-Early Santonian, a marked regression finally led to a regional emersion and the development of a karst relief. This regression is predated by “heavy” oxygen isotopic compositions in the Coniacian limestones, which probably resulted from isolation of a larger marine embayment from oceanic circulation. The subsequent Late Santonian-Campanian transgression flooded all of Beotia. Its sediments lie over allochthonous laterites. Extended rudist biostromes developed as soon as continental influx ceased, as indicated both by the stable isotope compositions of rudish shells and embedding limestones. During the Late Campanian, the rather uniform platform-type depositional environments differentiated and pelagic sedimentation recurred in South Beotia, whereas depositional environments to the north remained shallow. Incipient tectonics movements were responsible for a Late Maastrichtian emersion of North Beotia, while basinal conditions persisted in the south. The onset of flysch-type sedimentation occurred simultaneously during the Late Paleocene in North Beotia, as well as in the adjacent Parnassus and Pindos zones. However, this important facies transition has not yet been precisely dated in South Beotia.

Geometry and Evolution of a Syntectonic Alluvial Fan, Southern Pyrenees

Syntectonic alluvial fans formed on the northern margin of the Ebro Foreland Basin along the South Pyrenean thrust front during late orogenic thrust movements in the late Oligocene/early Miocene. The present-day geometry, structural relations and sedimentology of one of these fans, the Aguero fan in the province of Huesca, Spain, were studied. Field observations of the architecture of depositional facies and the geometries of syn-tectonic folds and unconformities indicate that the Aguero fan formed as the result of several phases of sedimentation which were primarily controlled by periods of tectonic activity and quiescence. The syntectonic unconformities and growth folds in the fan deposits provide a detailed record of the evolution of a fan adjacent to an active thrust front. Using a computer program to simulate sedimentation and deformation of an alluvial fan it is possible to constrain rates of both sedimentary and tectonic processes by modeling the evolution of the fan body. A facies model for the fan phases indicates that the facies change from proximal (coarse-grained, amalgamated) to distal (finger grained, stacked fining up cycles) in less than 1 km across a fan of radius estimated to be about 2 km.

MAGKAT - A high resolution marine magnetic survey of the Kattegat area

Spatial distribution of isobases of recent vertical crustal movements displayed by the map indicates certain predominant directions (lineaments). The directions coincide with main regional structural directions of the Alpine and Hercynian structural stage in Poland. The movements are concentrated along deeply seated discontinuities and weakness zones of the crust perpendicular to the main direction of tensional normal stress (NW-SE) or parallel to the direction of maximum shear stresses (WNW—ESE, NNE-SSW). The recent vertical crustal movements of Poland are mainly of taphrogenic or block-wave nature. Local halokinetic and orogenic (tangential) movements are also of some importance. Subsidence taking place in a broad meridional zone of the Polish Lowland peripheral to areas glaciated during the last Pleistocene ice stage seems to be at least partly related to glaciisostasy. The wave nature of the recent vertical movements of the earth's crust is confirmed by results of their harmonic analysis. The movements may be described as the net result of overlapping waves of different length and frequency. There are no data which would indicate that recent crustal movements in Poland are directly related to sea-floor spreading of the northern Atlantic Ocean, but it is admitted that this relationship is suggested by some indirect evidence.

Plate tectonics and intracratonic mountain ranges in Morocco - The mesozoic-cenozoic development of the Central High Atlas and the Middle Atlas

Plate tectonic processes in the Atlantic and western Tethyan realm directed the post-Variscan sedimentary and structural evolution of the High Atlas and Middle Atlas intracontinental mountain ranges of Morocco. Plate movements caused a reactivation of an inherited pan-African or Hercynian fault pattern by the variation of stress regimes through time. This resulted in strike-slip as well as vertical tectonics. During times of relative tectonic quiescence eustatic sea-level changes governed the sedimentary development. The most important, often interacting, global tectonic determinants are: taphrogenesis of the NW-African continental margin lasting until the Early Cretaceous (Triassic rifting and subsequent mid-Atlantic spreading), strike-slip-faulting at the Newfoundland-Gibraltar fault zone (Liassic — earliest Eocene), and continental convergence between Europe (Iberia) and Africa which started in the Late Cretaceous and reached its acme in the Neogene. In the realm of the Central High Atlas and the Middle Atlas the interaction of these processes triggered continental rifting (Triassic) and subsequent marine flooding of the intergrown riftgrabens prograding from the Tethys realm (Early Jurassic — earliest Middle Jurassic). After its abortion, the former Atlas rift was filled up with marine sediments (Bajocian — Bathonian), followed by continental redbeds and final uplift (late Mid Jurassic — late Early Cretaceous). Eustatic sea-level changes mostly governed the sedimentary evolution from Aptian to latest Mid Eocene. After a first weak uplift of the central High Atlas during the Senonian major uplift of the intracontinental chains commenced at the Mid/ Late Eocene transition. Diastrophism of the Atlas ranges during the Miocene and Pliocene coincided with the main orogenic movements of the Betico-Rifean arc.

Pennsylvanian cyclothems: Methods of distinguishing tectonically induced changes in sea level from climatically induced changes

Analysis of tectonic subsidence permits calculation of the magnitude of tectonic processes and associated climatic effects, which controlled changes in sea level during deposition of Pennsylvanian cyclothems. Far-field tectonic effects in response to regional orogenic movements influenced Pennsylvanian sea-level change in the Midcontinent of North America and the Illinois basin. Organization of Virgilian and Missourian Midcontinent cyclothems into fourfold to fivefold bundles shows that sea-level changes in Midcontinent platform areas were influenced partly by Milankovitch orbital parameters, whereas Desmoinesian sea-level change apparently was influenced more strongly by tectonic subsidence. Sea-level change associated with cyclothems in the Central Appalachian basin and the Asturias basin of Spain were controlled mostly by foreland-basin subsidence and later strike-slip faulting. Methods developed herein permit calculation of magnitudes of both tectonic and glacio-eustatic components of sea-level change influencing Pennsylvanian cyclothem deposition and may be applicable to other cyclic sequences.

Distribution and Origin of Regional Coal Fracture (Cleat) Domains in Upper Cretaceous Fruitland Formation Coal: Possible Effects on Coalbed Stimulation and Methane Production

Fracture permeability is of primary importance to producibility of coalbed methane. To evaluate controls on fracture (cleat) patterns in coal beds in the Upper Cretaceous Fruitland Formation in the San Juan basin, the authors studied fractures in the coal and adjacent sandstones in 11 cores and 90 outcrop stations along the basin margin and mapped fractures in selected areas. Cleats are perpendicular to bedding, planar, usually uniform in strike within an outcrop or core, and arranged in closely spaced subparallel sets. Face cleats are the first formed (and generally most prominent) fractures; butt cleats formed later, and most cases strike perpendicular to the face cleat. Timing of cleat development is constrained by the age of cleats relative to dated folds and the burial history of the Fruitland Formation. Where beds ar reoriented by folds, cleats remain orthogonal to bedding, indicating that they formed prior to fold development during the early Tertiary. Cleat crosscuts bedding-parallel coal compaction fabrics, indicating that cleat formed later. Cleat development was contemporaneous with orogenesis in the Cordilleran belt, in progress during late Campanian time. Patterns reflect lateral stresses associated with northeast- and southeast-directed Cordilleran orogenic movements. Domains may represent separate deformation events or contemporaneous paleostress provinces.more » Contrasts in cleat development in individual domains can affect hydraulic fracture treatment or cavity completion. The prevalence of strongly developed face and butt cleat in the domain-boundary region and resulting increased coal friability may increase the success of cavity completions in this areas.« less

Geology and Coal Resources of Zonguldak Basin (Northwest Turkey) as a Potential Source for Coal Bed Methane

The Carboniferous clastic sequence of Zonguldak basin contains several coal seams that have been mined since 1848 by underground methods. Coal seams are located in a Namurian to Westfalian D progradational delta and fluid plain sequence that is approximately 3,500 m thick. These units are affected by Hercynian orogenic movements. Related tectonism and uplift led to a widespread erosion. Consequently, younger units, mainly Cretaceous shallow-marine carbonates, rest unconformably on different sections of the Carboniferous strata. There exist up to 8 coal seams in Namurian, 20 to 26 in Westfalian A, and up to 8 coal seams in Westfalian B, C, and D. The average combined thickness values are 8 m, 34 m, and 7 m, respectively. However, due to the lateral changes in seam thickness and due to the erosion, both the number and combined thickness of coal seams may change remarkably. Majority of the coals in the exploitation area are of highly volatile C to A bituminous rank. Vitrinite reflectance values range from 0.6 to 1.2% (R{sub 0} mean). Methane content of some coal seams is determined by desorption data which indicate a methane content of 5 to 16 m{sup 3} per ton of coal. In addition to classicalmore » methods, data from some deep wells have been used to determine the thermal history by the method of basin modeling. Amount of gas generated in coals is then computed with the help of a kinetic approach. Furthermore, timing of gas generation has also been determined, which enabled consideration of migrational and diffusional gas losses. Data from coal geology, geochemistry, and modeling are combined to evaluate the coal bed methane potential of the basin in an integrated and quantitative manner.« less

Tectonic and Depositional Model of the Arabian and Adjoining Plates During the Silurian-Devonian (1)

During the Late Ordovician and Early Silurian, the western part of the Arabian Peninsula was covered by polar glaciers that advanced from the south pole in African Gondwana. During this period, nondeposition, erosion, or marginal marine conditions prevailed in eastern and northern Arabia. When the glaciers melted in the Early Silurian, sea level rose sharply and the paleo-Tethys Ocean transgressed the Arabian and adjoining plates depositing a thick, organic-rich shale directly over the glaciogenic and periglacial rocks and related unconformities. The post-glacial sequence coarsens upward reflecting the passage of a coastline prograding northward from African and Arabian Gondwana to northern Arabia. A sea level drop in the Late Silurian placed the study area in a terrestrial environment; however, as sea level recovered in the Early Devonian, a carbonate sequence blanketed most of the area. The transgression, however, was interrupted by regional uplift and local orogenic movements in the Middle and Late Devonian. These movements constitute the onset of Hercynian tectonism, which resulted in erosion of the older sequences, depositional hiatuses, and regional facies changes.

The dynamic source of tectonic movement

Many geologists have spent much time researching the dynamic source of tectonic movement and have proposed many hypotheses, such as contract hypothesis, isostasy, tidal force, continental drift, geosyncline, plate-tectonics (Hatayama, 1980), and geomechanics (Liu Haikue, 1983) etc. While explaining the dynamic source of tectonic movements they did not consider the igneous intrusion force which had a main influence on the tectonic movement. It has been suggested in this paper, that the dynamic source was the radiate heat from the radiating matter. The magma generated in the deep parts of the Earth, and intruding into the shallow parts along the fault zone caused the shear strain. The tectonic movement takes place in different ways, such as orogenic movement, epeirogenesis, earthquake, volcanic activity, fold and fault of sedimentary or metamorphic layers, igneous intrusion etc. But the horizontal displacement over a long distance such as continental drift and the plate-tectonics hypothesis is not in agreement with many geologic facts. Thus we cannot believe them.

The orogenic movement of the Main Mobile Fault Zone in China

The orogenic movement of the Main Mobile Fault Zone in China

Evidence for global (glacial-eustatic) control over upper Carboniferous (Pennsylvanian) cyclothems in midcontinent North America

Abstract At least 20 Middle/Upper Pennsylvanian (Westphalian D/Stephanian A, B) major marine cyclothems, each consisting of a thin transgressive limestone, thin offshore dark phosphatic shale, and thick regressive limestone member, extend along outcrop for 600 km from the northern shelf region of Iowa, Missouri and Kansas, to the basinal region of central Oklahoma. Each is correlated lithostratigraphically throughout a gridwork of long cores in the northern region beneath Pleistocene cover, and in good exposures southward. This correlation is confirmed biostratigraphically, along the entire outcrop belt using a combination of conodonts, fusulinids and ammonoids. Most of the cyclothems are separated by thin terrestrial deposits with paleosols, but only rare deltas, for about the northern half of the outcrop distance, which rules out delta shifting as a general control for the vertical alternation of terrestrial and marine deposits across the broad shelf. On the northern shelf, all major cyclothems are traced across the cratonic Forest City basin and over the adjacent Nemaha uplift with little change, which rules out local differential tectonics as a general cause. Presence of Gondwanan glacial deposits at this time, in conjunction with the estimated frequencies of these Pennsylvanian cycles within the Milankovitch band of Earth’s orbital parameters (which control variation in solar heating of the mid-latitudes), indicate that glacial eustasy, rather than distant orogenic movements, was the main control over the cyclothems.

Turbidite-hosted gold mineralization at the Dolaucothi gold mines, Dyfed, Wales, United Kingdom

The Dolaucothi gold deposit occurs in thin bedded turbidites of Upper Ordovician to basal Silurian age deposited near the southern margin of the lower Paleozoic Welsh basin. The host rocks were deformed during the Caledonian orogeny (390-410 Ma) into a series of inclined asymmetrical, commonly overturned, tight southeast-facing folds with associated cleavage. Reverse dip-slip faults accompanied the folding and the whole area is cut by late tensional faults. The mineralization is controlled by structures produced during the Caledonian orogeny, in particular a thrust fault which formed the host to the Roman lode, a flat-lying auriferous quartz reef up to 6 m thick.Gold occurs in a variety of hosts including pyritic shales containing both euhedral and framboidal pyrite, quartz-carbonate stringer veins, in steeply or gently inclined veins associated with shear zones and in planar quartz veins associated with the Roman lode. The mineralogy is simple, with pyrite and arsenopyrite being the dominant sulfides. Ankeritic carbonates, hydromuscovite, and the lithium silicate, cookeite, are also present.It is proposed that, during prograde metamorphism associated with early stages of the Caledonide orogeny, fluids circulated through the basement below the southeast margin of the Welsh basin and leached gold, and associated metals, from rocks of igneous or volcanic origin. These overpressured fluids were released to high crustal levels during later orogenic movements and uplifts, perhaps related to reactivation of such basement faults as the infra-Tywi fault. Geothermometric studies indicate that the host rocks experienced upper anchizone facies metamorphism whereas the temperatures of the mineralizing fluids were in the range of 345 degrees to 450 degrees C. Isotopic studies point to a common origin for the sulfides in both the shales and the veins and also indicate that the host rocks provided the source of the carbon now incorporated in the carbonates.

Timing and Conditions of Permian Rotliegende Sandstone Diagenesis, Southern North Sea: K/Ar and Oxygen Isotopic Data

Illitic clay, ranging from pure illite to highly illitic illite/smectite (I/S), is the most abundant diagenetic phase in the eolian and sabkha facies of the Rotliegende Formation of the southern North Sea and northeastern Netherlands. Most K/Ar ages of diagenetic illite and I/S in the eolian sandstones are between 100 and 175 Ma. Illite formation is related to two major phases of tectonic activity, the Jurassic Kimmerian orogenic movements and Late Cretaceous-early Tertiary inversion movements. The burial depth of a sample at the time of diagenetic illite formation can be derived from the K/Ar age and the appropriate burial-history curve. Illite formed in different localities at different times and at different depths. Generally, diagenetic illite is more abundant in samples more deeply buried at the time of illite formation. The correlation between illite formation and tectonic disturbance and the correlation between illite abundance and burial depth may be useful for predicting reservoir properties. Specifically, we would expect reservoir permeability of the Rotliegende in the study area to be least impaired by illite growth in those samples less deeply buried from the Late Jurassic through the Late Cretaceous. Calculated ^dgr18O values of most illite-forming fluids lie between 0 and 4^pmil (SMOW), indicative of meteoric or marine waters modified by interaction with rock. However, 18O-depleted fluids (as low as -5^pmil), indicative of a major meteoric component, are inferred to have been involved in the formation of illite in the Groningen gas field at a time when nearby sections of the Rotliegende were eroded and exposed at the surface.

Glacial-eustatic sea-level curve for early Late Pennsylvanian sequence in north-central Texas and biostratigraphic correlation with curve for midcontinent North America

At least 30 transgressive-regressive cycles of deposition are recognized from the upper Desmoinesian East Mountain Shale to the mid-Virgilian Wayland Shale in north-central Texas. Maximum regressive deposits are typically paleosol mudstones and fluvial sand- stones; maximum transgressive deposits are typically widespread, ammonoid-bearing, conodont-rich, dark phosphatic shales in more major cycles, and persistent fossiliferous shales or limestones overlying terrestrial deposits in more minor cycles. Delta complexes dominate the regressive sequences of many cycles. Using biostratigraphic criteria of first, last, sole, or acme occurrence of ammonoid, conodont, and fusulinid taxa, we correlate 17 cycles in the Texas sequence directly with 17 glacial-eustatic cycles of similar magnitude in the northern midcontinent. This correlation suggests that glacial eustacy was the basic control over the cyclic sequence in Texas, that tectonic masking of the eustatic signal by nearby orogenic movement in Texas was negligible, and that delta shifting, though conspicuous, was only a secondary control over the cyclicity there. Minor cycles recognized between the correlated cycles also match well enough between Texas and the midcontinent to further discount potential tectonic or deltaic masking of glacial-eustatic cyclicity. This strengthens the likelihood of correlating glacial-eustatic events across larger parts of North America, and perhaps with other parts of the world.

Gondwana coals of Bhutan Himalaya - Occurrence, properties and petrographic characteristics

A narrow belt of highly inclined coal-bearing Gondwana strata occurs in the extreme south-eastern part of Bhutan Himalaya. Recently, a systematic survey was undertaken along this coal belt and coals of three areas were analyzed in detail for the evaluation of their physico-chemical properties and petrographic characteristics. The entire region is in the midst of the Great Himalayan orogenic belt, and the whole stratigraphic sequence underwent several diastrophic movements in the geological past. The massive effects of these orogenies is more pronounced in the coal beds of Gondwana sequence, and due to severe crushing and tectonic shearing these coals became powdery and flaky in nature. Significantly, the coals retained their pre-deformational rank exhibiting typical high-volatile, low-rank, bituminous characters, with mild caking propensities. Also these coals are markedly low in sulphur, phosphorus, chlorine and carbonate content like that of Peninsular Gondwana coals. Petrographic studies of these Bhutan coals revealed a close similarity with the eastern Raniganj coals (Upper Permian) of Peninsular India. The tectonic shearing and crushing of the coals are exhibited by the frequent presence of microfolding, microfaulting, and other compressional structures. However, the coals of all the three areas have shown a consistently low order of reflectance values. This typical retention of pre-deformational low-rank bituminous character is a significant feature of Bhutan coals. It shows that massive orogenic movements were only able to physically crush these coals but could not generate the requisite thermal regime to raise the rank of these coals.

A lithostragraphical and structural study on Gebel El-Zeit area, gulf on Suez, Egypt

Gebel El-Zeit area is situated close to the Arabo-Nubian shield, so that it has been affected by Paleozoic (Caledonian and Hercynian) and Mesozoic (alpine) orogenic movements. This paper deals with the effect of these movements on the facies of the sedimentary rocks. The study has led to two main conclusions: a) the occurrence of four stages of sedimentation each seperated by unconformity; b) the existence of complex inter-relationship between rifting and sedimentation, periodic tilting and fracturing of the horst blocks with erosional beveling of older deposits following each stage of sedimentation.

Evolution of the Iapetus Ocean and its borders in pre-Arenig times: a synthesis

Summary On the margins of the Laurentian continent the interval c. 800-492 Ma was dominated by (1) rifting involving continental crust and (2) rift sedimentation with or without related volcanicity and plutonism, culminating in (3) the formation of the Iapetus Ocean and succeeded by (4) initial phases of compressional tectonics. The latter involved the approach of volcanic island-arc systems and the partial or complete obduction of ophiolites of the oceanic crust cutting through the slope-rise prism and, at least in some parts of the orogen, being thrust onto the continental-shelf deposits in the Cambrian and Tremadocian. These processes led to the closing of the ocean in the mid-Ordovician Taconian and the Silurian Caledonian orogenies. The earliest and most complete pre-Arenig orogenic movements are recorded in the Finnmarkian orogeny, including obduction of ophiolites in Scandinavia (520–480 Ma). Related sedimentation gaps, preceded by deformation and ophiolite obduction and/or uplift, were accompanied by tectonic melange development and/or volcanicity. These features are recorded widely from northern Britain and Ireland (Grampian orogeny), and from the Appalachians of Newfoundland (Burlington orogenic episode), Maine, USA, and Quebec, Canada (Penobscot orogeny). Movements interpreted as relating to the Penobscotian are now recognized as widespread in the US Appalachians. The Sardic orogenic phase in southern Europe ( c. 510 Ma) is associated with plutonism and metamorphism and may be related to late Cadomian III orogenic movements. In the southeastern landmasses of Gondwanaland (Avalonian-Cadomian-Pan-African) essentially the same time interval ( c. 800–492 Ma) was also dominated initially by rifting of continental crust overlain by late Proterozoic shelf deposits and the obduction and folding of ophiolites (Cadomian I orogeny) and underlying rocks, followed by intrusion of late- to post-tectonic plutons (Bleida). Cadomian II resulted in the deposition of coarse clastics, volcanics and glacigenic sediments (Tiddiline), and folding, erosion and deposition of a thick wedge of late Proterozoic carbonate and clastic rocks with alkaline volcanics (Adoudounian) overlain conformably by lower and middle Cambrian that are overlain disconformably by Tremadocian clastic sediments. During the rifting phase continental fragments drifted away and/or became volcanic arcs with subduction zones and back-arc basins constituting the weakly orogenic Avalonian terrane to the W and the strongly orogenic but discontinuous belt forming a terrane characterized by Cadomian fragments between the Avalonian terrane of the British Isles and the W African craton. Stratigraphic columns and schematic time-slice stratigraphic profiles with interpreted tectonic environments from various parts of the orogen provide comparisons and contrasts in stratigraphic, faunal, structural, volcanic, plutonic and metamorphic evolution.