1,720,986 research outputs found
Reorganization of convergent plate boundaries. Geologica Ultraiectina (340)
No full textIt is still unclear where a subduction is initiated and what are the responsible mechanisms involved in subduction initiation process. Understanding of subduction initiation will advance our knowledge of how and when plate tectonics started on Earth. Another issue concerning the subduction process is the response to arrival of buoyant continental crust at the trench. Upon entrance of continental crust into subduction zones (known as continental collision), the approximately steady-state subduction turns into a transient state. One of the responses to continental collision is initiation of a new subduction zone on the former overriding plate (subduction polarity reversal). However, there is ample geological and seismological evidence indicating other deformation patterns following continental collision including continuation of subduction, and delamination. The cause of the different responses to collision is still poorly understood. In this thesis, I aim to address both transient stages: subduction initiation and continental collision. To this purpose I use Finite element method. The general conclusion of this thesis is that several parameters such as convergence in the direction perpendicular to the weakness zones at the edges of a slab or continued convergence in collisional settings can result in reorganization of plate boundaries and formation of new subduction zones
The Gibraltar arc: can various (near)surface observations be explained through lithospheric-scale forces?
No full textPhysical and Space Geodes
The long-term evolution of subduction zones: a modelling study
No full textThe motion of oceanic and continental lithosphere, volcanic activity, earthquakes, and other tectonic activities are expressions of processes in the Earth's mantle. Nowadays, it is widely accepted that these phenomena are related to convective flow in the mantle, which forms the mechanism to turn heat from the Earth's interior into mechanical work. One example of that mechanism is the creation of oceanic lithosphere at the mid-ocean ridges, which are zones of upwelling hot mantle material. At boundaries where two plates converge one of the plates, preferebly of oceanic type, is consumed by subduction into the mantle. Most of the information about the Earth's structure has been obtained by seismology. Seismological observations have demonstrated that there is a great variety in subduction zone geometries. Subducted slabs with dip angles varying from very small (e.g. Peru) to nearly 90° (e.g. Mariana) have been observed. Slabs can either stagnate at the upper-lower mantle boundary (e.g. Izu-Bonin) or can penetrate into the lower mantle (e.g. Sunda arc). Initiation or cessation of subduction are manifestations of the time-dependence of the dynamical processes in the mantle. Images of the seismic velocity distribution of the mantle interior provide an instantaneous view of time-dependent structures which must be interpreted in a dynamical context. Understanding the long-term evolution of the subduction process is the primary motivation for this study. More specifically, the aim is to investigate the sensitivity of subduction zone geometry to various parameters, as, for example, plate velocities and viscosity structure of the mantle. For that purpose we have chosen to use a computational method. Numerical studies have the advantage that parameters can be varied over a great range. Plate motions and mantle flow can be included as boundary conditions easily, particularly compared to laboratory experiments. It must be emphasized that the model used here includes several shortcomings. The major simplification is probably the neglect of the third dimension. Over the last two decades numerous studies concerning subduction zones have been performed. In Chapter 2 a summary of some influential work and recent studies from different fields of geophysical disciplines is presented. Constraints on the structure and evolution of subducting slabs obtained from seismology, geoid data, plate tectonic analysis, and numerical and experimental studies are discussed. This information forms the basis for the research presented in this thesis
Dynamics of lithospheric extension: a modeling study
No full textMuch of the work on continental margins and sedimentary basins has been directed at quantifying parameters of the pure shear extension model (or stretching model) (McKenzie, 1978). This is surprising, since a sound physical basis of this kinematic model is lacking. Alternative kinematic models, like the simple shear model (Wernicke, 1981) have been proposed which, at least in some basins, explain the surface observations equally welL In terms of asymmetry, the pure shear model and the simple shear model represent end-members of a spectrum of kinematic models for continental extension (Figure 1). The central issue of this thesis is to determine -by numerical modeling- whether the pure shear model and the simple shear model are physically plausible and to gain insight in the physical conditions which control the style of extension
Gateway exchange, climatic forcing and circulation of the Mediterranean Sea during the late Miocene: A model perspective
No full textOceanen transporteren - en fungeren als opslag van - grote hoeveelheden warmte, zout, en andere chemische verbindingen, waaronder CO2. Ze zijn daarom een belangrijk onderdeel van het aardse klimaatsysteem. Gezien de huidige bezorgdheid over het veranderende klimaat, is het cruciaal om een goed begrip te hebben van het functioneren van oceaancirculatie. Veranderingen in de circulatie en chemische samenstelling van de oceaan zijn veelal terug te zien in de sedimenten die afgezet worden op de oceaanbodem. Niet alleen het type sediment wordt bepaald door zulke veranderingen; de chemische samenstelling en fossielinhoud van het sediment worden er ook door beïnvloed. Op deze wijze vormen de afgezette sedimenten “het geologische archief” van genoemde veranderingen. Het bestuderen van dit archief is een manier om inzicht te verkrijgen in de processen die de oceaancirculatie aandrijven. De circulatie en sedimentatie in door land ingesloten bekkens zijn bijzonder gevoelig voor veranderingen in het klimaat; dit maakt deze bekkens uitermate geschikt om klimaatverandering te bestuderen. De gevoeligheid voor veranderingen in het klimaat is een gevolg van de beperkte grootte van de bekkens en de gelimiteerde interactie met de open oceaan. Het bekendste voorbeeld van een door land ingesloten bekken is de Middellandse Zee, met zijn ligging tussen het Europese en het Afrikaanse continent. Circulatie in de Middellandse Zee wordt aangedreven door de uitwisseling van water met de Atlantische Oceaan en door de atmosferische condities in het Mediterrane gebied en zijn omgeving, zoals winden, verdamping, neerslag en resulterende zoetwater-instroom van rivieren. Op dit moment is de Straat van Gibraltar de enige zeestraat die de Middellandse Zee en de Atlantische Oceaan met elkaar verbindt; in het verleden is deze verbinding gecompliceerder geweest. De wateruitwisseling door een zeestraat wordt sterk beïnvloed door de zeestraat-bathymetrie; hierdoor was het mogelijk dat tektonische veranderingen in het gebied rond Gibraltar gevolgen hadden voor de grootte van de wateruitwisseling. Genoemde atmosferische condities worden sterk beïnvloed door de intensiteit van de inkomende zonnestraling, die op haar beurt afhangt van de baan van de Aarde en positie van de rotatieas van de Aarde ten opzichte van de zon. Het is het samenspel van klimaat en tektoniek dat leidt tot veranderingen in de circulatie in de Middellandse Zee en dat zijn weerslag vindt in het marien-geologische archief. Het herkennen en onderscheiden van de individuele invloed van de twee drijfveren is lastig maar belangrijk voor het verbeteren van ons begrip van oceaancirculatie. De sedimenten die zijn afgezet in de Middellandse Zee in het laatste (d.w.z. jongste) deel van het Mioceen (het Messinien, van 7,25 tot 5,33 miljoen jaar geleden) vertonen een grote variabiliteit in hun samenstelling. Sedimenten kenmerkend voor afzetting in een open oceaan worden afgewisseld door lagen met een hoge organische inhoud. 5.97 miljoen jaar geleden werd deze afwisseling onderbroken door de afzetting van evaporieten (vooral gips en steenzout). Rondom de gehele Middellandse Zee zijn gipsafzettingen uit deze tijd gevonden en kilometers dikke lagen steenzout zijn verborgen onder de huidige zeebodem. Het uitzonderlijke geologische gebeuren dat geleid heeft tot de afzetting van deze evaporieten staat bekend als de Messinien Zoutcrisis en vond plaats tussen 5,97 en 5,33 miljoen jaar geleden. Deze relatief korte periode van evaporiet-afzetting is, ondanks het onderzoek van generaties van wetenschappers, nog steeds één van de grootste mysteries in de geologische geschiedenis. Voortbouwend op een groot aantal waarnemingen en een uitgebreid kwalitatief inzicht in het laat-Mioceen, heeft mijn promotieonderzoek als doel het verbeteren van het kwantitatieve begrip van de invloed van de grootte van de verbindende zeestraat tussen de Middellandse Zee en de Atlantische oceaan en van het klimaat op circulatie en zoutgehalte in de Middellandse Zee. Dit wordt bereikt door bestaande geologische/geochemische waarnemingen te combineren met een theoretische en model-gedreven aanpak van fysische aard. In dit proefschrift ontwikkel ik (1) een theoretische benadering van de relatie tussen zeestraatgrootte en bekkensaliniteit, (2) een vernieuwende aanpak, gebruik makend van meerdere modellen, om een schatting te maken van het zoetwaterbudget (relatie tussen neerslag, rivier-input en verdamping) van de Middellandse Zee in het Mioceen, (3) een gesimplificeerd model van de oceaancirculatie van de Middellandse Zee, dat de ruimtelijke verdeling van saliniteit in het bekken in eerste orde verklaart. De verkregen resultaten kunnen als volgt kort worden samengevat: Om een zoutgehalte in de Middellandse Zee in stand te kunnen houden dat groter is dan het huidige, moet de verbinding via de zeestraat beperkter (ondieper of smaller, of langer, of een combinatie hiervan) geweest zijn dan de huidige Straat van Gibraltar. Hoewel tijdens het laat-Mioceen de zoetwateraanvoer door de Afrikaanse rivieren mogelijk veel groter was (waarschijnlijk door afwatering in de Middellandse Zee van het destijds zeer grote Chad Meer), was het netto zoetwaterbudget van de Middellandse Zee vergelijkbaar met het huidige. Deze resultaten leiden tot de conclusie dat de Messinien Zoutcrisis voornamelijk op gang gebracht is doordat tektonische processen de verbinding van de Middellandse Zee met de Atlantische Oceaan drastisch verkleind hebben. Mijn onderzoek toont aan dat onder deze omstandigheden betreffende wateruitwisseling en zoetwaterbudget de Mediterrane waterkolom sterk gelaagd geweest kan zijn. Dit resultaat werpt nieuw licht op algemeen aanvaarde percepties van de Messinien Zoutcrisis en levert een nieuwe kwantitatieve basis voor toekomstige studies. De behaalde proces-gerelateerde resultaten zijn niet alleen zinvol voor de Middellandse Zee als case study gebied; tezamen met de geïntegreerde onderzoeksstrategie ontwikkeld in dit proefschrift zullen zij naar verwachting bijdragen tot verdieping van inzicht in oceaancirculatie en sedimentatie in andere vergelijkbare gebieden (tegenwoordige of in het geologische verleden) en in oceaancirculatie, in het algemeen
Model analysis of the role of marine gateways in the palaeoceanography of the Miocene Mediterranean basin and Paratethys
No full textIn the Early Miocene (23–16 Ma) the Mediterranean region was different from that at present. The Mediterranean Sea was more extensive than today and it was opened to the Indo-Pacific Ocean, to the Atlantic Ocean, and to the Paratethys—the predecessor of the Black, Caspian and Aral seas—to the north. The progressive palaeogeographic evolution of the Mediterranean region, driven by plate tectonics, resulted in the modern enclosed physiography of the Mediterranean Sea. This process includes changes in bathymetry and geometry of marine basins and marine gateways. Marine gateways are oceanic passages between neighbouring basins which allow interbasinal exchange of water, heat and salt. Opening and closure of these gateways can therefore affect the ocean circulation on the regional and (or) global-scales, with a concomitant effect on climate. Changes in ocean circulation are recorded in the marine sediments, the study of which is essential to reconstruct palaeoenvironments. However, the examination of the sedimentary record is not sufficient to determine the physical processes responsible for the environmental conditions inferred, nor to assess whether the observed conditions are applicable to the entire basin or represent a local phenomenon. In this thesis I have used several regional-scale numerical models to gain physics-based insight into the role of the marine gateways of the Miocene (23-5 Ma) Mediterranean region in the palaeoceanography of the Mediterranean and Paratethys. I address first-order features such as basin temperature and salinity, basin-scale thermohaline circulation, patterns of exchange in the gateways, and sea level. By comparing model results to geological data I am able to discard previously proposed scenarios and formulate new ones. I focus on the shoaling and closure of the gateway which used to connect the Mediterranean Sea to the Indo-Pacific until the Middle Miocene. Using a regional-scale ocean circulation model, and comparing my model results to proxy data, I determine the exchange patterns between the Mediterranean and the adjacent Atlantic and Indian oceans during the different stages of closure—some of them not considered before. I also examine the interplay of the two marine corridors that connected the Mediterranean and the Atlantic during the Late Miocene. Results obtained with a regional-scale ocean circulation model show that the exchange pattern in these corridors only depends on the relative depths of the corridors. This is useful to reconstruct the sequence of events that culminated in the deposition of evaporites in the Mediterranean during the Messinian Salinity Crisis and allows me propose new model-based scenarios as to the exchange patterns in the corridors. Finally, I perform a quantitative analysis to assess the sensitivity of the Late Miocene Paratethys sea level to hydrologic budgets when the Paratethys is isolated from the Mediterranean due to gateway closure. Comparing the model results with (i) salinity estimates inferred from the geological record and (ii) Late Miocene hydrologic budgets calculated from a global climate model simulation, I exclude a 1000 m sea level drop in the Caspian Sea during the time period investigated. In the Black Sea this cannot be completely discarded
Geodynamic evolution and mantle structure
No full textWith the advent of plate tectonic theory a framework has become available in which many observed features of the structure of the Earth can be understood. The theory can explain the geological processes that have resulted in terranes as diverse as oceans, mid-oceanic ridges, mountain belts, and intracontinental basins. However, despite its explanatory power plate tectonic theory is rarely used for its predictive properties, which should after all be an important aim of any scientific theory. In this research I will address some implications of the plate tectonic concepts, by using plate tectonic theory, to predict thermal and elastic properties of the interior of the Earth which were not used to formulate the theory. This prediction is made by forward numerical modelling of lithosphere scale tectonic processes. The work will focus on the Alpine-Mediterranean region, but the method described is applicable to any part of the Earth where Mesozoic or later plate collision processes can be reconstructed with sufficient detail from geological observations
Bauxite formation on Tertiary sediments and Proterozoic bedrock in Suriname
No full textThe lateritic bauxite deposits in Suriname are traditionally distinguished into Coastal plain bauxites and Plateau bauxites, a subdivision that is primarily based on their topographic and geographic position. The first group is located in the lowlands of the coastal plain, while the second group is located on relatively high plateaus in the country’s interior. The Coastal plain deposits are mostly buried under younger sediments. Bauxite formation in Suriname peaked in Late Cretaceous – Early Tertiary times, when favourable conditions for bauxitisation prevailed worldwide. The parent rock composition is the most influential difference between these two deposit groups as the Coastal plain bauxites originated on Cenozoic sediments, and the Plateau bauxites on a diverse collection of Proterozoic rocks, which range from (ultra)high-temperature metamorphic gneissic and amphibolitic rocks in the Bakhuis Mountains (Granulite Belt in west Suriname) to greenschist-facies metabasalts and other meta-igneous rocks in the Greenstone Belt in east Suriname (Nassau Mountains, Lely Mountains and Brownsberg). The more ferruginous character of the Plateau bauxites is directly correlated to the higher iron contents of their crystalline parent rocks. The major element concentrations are linked to the principal minerals in specific zones of the lateritic profile as gibbsite is the prominent aluminum-bearing phase in the bauxite zone, while kaolinite prevails in the saprolite, and hematite and goethite are dominant in the duricrust that is covering the lateritic profiles. Bauxites are products of several weathering processes such as leaching of soluble elements ( e.g. Si, K, Na, Mg and Ca) from unstable minerals in the precursor rock, relative accumulation of poorly soluble or immobile components ( e.g. Al, Ti, Zr, Nb), and iron enrichment controlled by fluctuating groundwater levels and redox conditions. The enrichment of high field-strength elements (HFSE) and heavy rare earth elements (HREE) can be directly correlated to the resistance of zircon and other mineral hosts against weathering, especially in the Coastal plain bauxites where placer-like accumulations of these particular minerals are present in the terrigenous precursor sediments. Trace element signatures and petrologic observations of the Coastal plain deposits revealed that there is no direct genetic relationship between bauxite and the underlying saprolitic clay. The geochemical signatures, grade and volumes of the Surinamese bauxites are influenced by certain weathering processes (leaching, enrichment, fractionation), the primary compositional differences of parent rocks, nature and content of the accessory minerals, landscape morphology, local hydrological conditions, drainage efficiency, and unequal responses to multiple bauxitization cycles
Dynamics of active continental margins : the Andes and the Aegean region
No full textThe research presented in this thesis is aimed at contributing to our
understanding of the forces that control the state of stress and associated
deformation of overriding continental margins. For reasons outlined below, the
study concentrates on two areas where deformation is currently active: the
western margin of the South American plate, comprising the Andean Cordillera,
and, secondly, the Aegean region, comprising the Aegean Sea, the Greek
mainland, and western Turkey. The main objective of this thesis is not to provide
additional observations regarding the tectonics of the regions under scrutiny, but
rather, to investigate the causes of deformation by using the available data in a
quantitative analysis based on a numerical modelling
Bauxite formation on Tertiary sediments and Proterozoic bedrock in Suriname
No full textThe lateritic bauxite deposits in Suriname are traditionally distinguished into Coastal plain bauxites and Plateau bauxites, a subdivision that is primarily based on their topographic and geographic position. The first group is located in the lowlands of the coastal plain, while the second group is located on relatively high plateaus in the country’s interior. The Coastal plain deposits are mostly buried under younger sediments. Bauxite formation in Suriname peaked in Late Cretaceous – Early Tertiary times, when favourable conditions for bauxitisation prevailed worldwide. The parent rock composition is the most influential difference between these two deposit groups as the Coastal plain bauxites originated on Cenozoic sediments, and the Plateau bauxites on a diverse collection of Proterozoic rocks, which range from (ultra)high-temperature metamorphic gneissic and amphibolitic rocks in the Bakhuis Mountains (Granulite Belt in west Suriname) to greenschist-facies metabasalts and other meta-igneous rocks in the Greenstone Belt in east Suriname (Nassau Mountains, Lely Mountains and Brownsberg). The more ferruginous character of the Plateau bauxites is directly correlated to the higher iron contents of their crystalline parent rocks. The major element concentrations are linked to the principal minerals in specific zones of the lateritic profile as gibbsite is the prominent aluminum-bearing phase in the bauxite zone, while kaolinite prevails in the saprolite, and hematite and goethite are dominant in the duricrust that is covering the lateritic profiles. Bauxites are products of several weathering processes such as leaching of soluble elements ( e.g. Si, K, Na, Mg and Ca) from unstable minerals in the precursor rock, relative accumulation of poorly soluble or immobile components ( e.g. Al, Ti, Zr, Nb), and iron enrichment controlled by fluctuating groundwater levels and redox conditions. The enrichment of high field-strength elements (HFSE) and heavy rare earth elements (HREE) can be directly correlated to the resistance of zircon and other mineral hosts against weathering, especially in the Coastal plain bauxites where placer-like accumulations of these particular minerals are present in the terrigenous precursor sediments. Trace element signatures and petrologic observations of the Coastal plain deposits revealed that there is no direct genetic relationship between bauxite and the underlying saprolitic clay. The geochemical signatures, grade and volumes of the Surinamese bauxites are influenced by certain weathering processes (leaching, enrichment, fractionation), the primary compositional differences of parent rocks, nature and content of the accessory minerals, landscape morphology, local hydrological conditions, drainage efficiency, and unequal responses to multiple bauxitization cycles
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