Ove Stephansson

Diapirism theory and experiment

Abstract The application of fluid dynamics to layered systems with one or several flexible boundaries affected by gravity is reviewed. The theoretical equations are verified from models studied by the finite-element method and by experimental tests with liquid models. The rate of growth and geometry of the models studied by the finite-element technique are shown to be very sensitive to different boundary conditions at the top and bottom surfaces of the model. Diagrams of rate of amplitude growth and distributions of principle and normal stresses are presented. Plexiglass boxes filled with two immiscible fluids of known properties were used to test the spacing between domes and rate of growth of domes. The buoyant rise starts as gentle ridges which later split into individual domes. The velocity of rise increases until the domes reach half of the height of the model box. From then on the rise decreases and the material spreads laterally below the roof.

Polydiapirism of granitic rocks in the Svecofennian of Central Sweden

Abstract A classification of igneous intrusions from orogenic zones is presented. Depending on the tectonic appearance of the intrusions (size, structure, contacts, ect.) these are divided into epi-, meso- and catatectonic diapirs. The Svecofennian of Central Sweden has been divided into tectonic units such as batholiths and subprovinces. The division is supported by the Bouguer anomalies. The geological situation of several of the batholiths in Central Sweden suggests a mechanism of formation which is described as polydiapirism or diapirism within diapirs. This means that younger granite diapirs have forced their way up through diapirically formed granitoid rocks and supracrustal septa. The driving force is the density contrast between the diapir and the surrounding rocks. A positive correlation exists for the different granitoid massives and the Bouguer anomalies. An interpretation of the gravity anomaly for one of the granite domes in a twin dome structure suggests that the late-kinematic granites extend to depths of about 18 km. Finally the theory of polydiapirism is applied to the evolution of the Svecofennian orogeny in Central Sweden.

The finite element method in tectonic processes

Abstract The powerful method of finite elements permits almost all problems of stress analysis and analysis of finite deformations of geological structures to be presented in a mathematical form suitable for solution on a digital computer. A plane strain analysis of a boudinage structure in elastic materials brings out the barrel shape so characteristic of many natural boudins. The fill-in of newly grown minerals in the gap between boudins is afforded a plausible explanation from the mean stress distribution of the structure. Laterally compressed multilayers of rocks tend to develop folds with a characteristic wavelength. Beginning with folds of the dominant wavelength in a five-layer system with a small amplitude/wavelength ratio, the successive changes in geometry and stress distribution were analysed for small increments in nodal velocities. The geometry of folds and the stress distribution was found to be very sensitive to changes in compressibility of the materials involved. An application of the method to isostatic adjustment was tested on a two-layer system consisting of a light layer with sinusoidal deflection at its free surface overlying a dense half space. The flattening of the free surface and the generation of a second wave at the interface as functions of time were considered to agree quite satisfactorily with theory and model experiments. Most of the characteristics of the mid-Atlantic ridge indicate that the mechanism of spreading should be searched for at a shallow depth. Modelling a cross section of the ridge down to 100 km depth gives a surface deflection undulation with sagging and thinning of the basement layer of about 1 cm/y. High values of tensile and shear stresses are concentrated in the basement layer at the crestal zone. The theory of ridge formation caused by the injection of buoyant bodies relatively close to the surface of the ocean bottom and now slowly spreading laterally finds strong support in the results of the finite element analyses.

Stress-induced diffusion during folding

Abstract The application of thermodynamics to folding shows that mechanical disequilibria also correspond to chemical disequilibria and that they can be adjusted by the diffusion of elements down the free-energy gradient. Mechanical disequilibria in the form of heterogeneous stress distribution for the two-dimensional, large-amplitude folding of competent layers in a less competent matrix have been determined by a series of finite-element analyses. Folds with high viscosity contrast ( 100 1 ) have a gradient of the mean stress (pressure) in the competent layer which is directed perpendicularly to the layer. The incompetent layers in a folded multi-layer sequence show a mean stress gradient directed parallel with the layer, and the pressure minima are located at the hinge zone. Folds with low viscosity contrast have the isopachs (lines of constant mean stress) oriented almost parallel to the direction of compression and a minimum mean stress on the outer arc. The variations of mean stress in experimentally produced folds are in qualitative agreement with the variation in: 1. (1) the mineral composition; 2. (2) the bulk chemical composition; 3. (3) the chemical composition of a mineral; and 4. (4) the grain-size variation of natural folds.

Compression of floating elastic and viscous plates affected by gravity, a basis for discussing crustal buckling

Abstract The theory of buckling of floating elastic and viscous plates affected simultaneously by gravity and lateral compression is reviewed. The theoretical equations are verified by experiments with elastic rubber plates and viscous resin plates (viscosity 108–109 poises) floating on mercury or a solution of potassium-iodide. The assumption occasionally expressed by earth scientists that crustal buckling of geosynclinal dimensions is physically impossible is strongly supported by experiments and theory. Taking the known rheological properties of crustal rocks into account, one finds that troughs wide enough to function as geosynclines cannot possibly form by crustal buckling in response to lateral compression. There are at least two ways of showing the fallacy of the buckling model of geosynclinal formations: 1. (1) Introduction of the maximum strength of silicate rocks (4.109 dynes/cm2) and the thickness of the crust (say 30 km) into buckling equations yields a maximum wavelength of 90 km, or 45 km as the maximum width of the troughs. This is too narrow to represent geosynclines. Consideration of the viscosity of the subcrust yields much shorter buckles. 2. (2) There is a certain relationship between the rate of buckling and the rate of uniform compressive strain in a surface layer under lateral compression. Low compressive stress and large layer thickness render the uniform shortening and thickening of the layer more important than the buckling. Thus controlled by the ultimate strength of a rock, there is a maximum thickness which a surface layer cannot exceed without completely masking the buckling by uniform compression. On this basis, the maximum thickness of a granite layer capable of buckling is a little above 1 km and the consequent length of the resulting buckles is less than 15 km. The conclusion is therefore that whereas lateral compression certainly may generate small and moderate buckling folds of layered rocks within a geosyncline, the crust as a whole is not capable of responding to lateral stress by buckling of geosynclinal dimensions.

Granite diapirism in the Rum Jungle area, Northern Australia

Abstract Early studies in the Rum Jungle area suggested an intrusive relationship between the Rum Jungle and Waterhouse “Granites”, and the overlying sediments. It was later shown that the granitic “intrusions” were Archaean basement complexes onto which Lower Proterozoic sediments had been deposited. Polyphase folding was postulated as being responsible for doming of the basement and cover rocks. This paper proposes to show that the domed structures in the Rum Jungle area, and the emplacement of Middle Proterozoic granites in the Pine Creek Geosyncline were related, and caused by diapiric intrusion of granites, in a solid state, into basement complexes and cover rocks. Structural and metamorphic evidence in support of diapiric intrusion in the Rum Jungle area includes: pebble deformation within steeply dipping beds of quartz conglomerate; disappearance of polyphase fold structures away from the basement complexes; bending of folded country-rock strata into concordance with the complex—sediment contact; and metamorphic and metasomatic alteration of sediments in contact with the basement complexes. Gravity data show mass deficiencies in the Archaean complexes which possibly coincide with young granite diapirs.

Seismo-tectonics in Fennoscandia

Abstract Studies of seismic risk in Sweden indicate that there is coherence between the epicentres of earthquakes and the tectonics. On the basis of the existing seismic data and the tectonics, eleven seismo-tectonic zones are suggested for the Fennoscandian Shield. The correlation between the gravity data, the late glacial movements and the seismo-tectonic zones is found to be positive.

The Caledonian Research Project (CRP): A Swedish contribution to the International Geodynamics Project (IGP)

Abstract The Swedish contribution to the International Geodynamics Project (IGP) is concentrated in two major fields of research, the Caledonian Research Project and the Study of Postglacial Earth Movements. Both projects are financed by the Swedish Natural Science Research Council This paper summarizes the research activities of the Caledonian Research Project (CRP). The IGP-CRP is an inter-disciplinary study involving geologists and geophysicists with a wide range of interests and drawn from institutions in Goteborg, Lund, Lulea, Stockholm and Uppsala. Various collaborative projects with foreign institutions are in progress. Research is concentrated geographically in a Geotraverse some 300 km long and 200 km wide, extending from the Caledonian Front in the vicinity of Ostersund to the Norwegian coast west of Trondheim. CRP research is organized in nine subprojects, namely, Basement and Deep Structure, Basement—Cover Relationships, the Sarv Nappe, the Seve—Koli Nappe Complex and Higher Tectonic Units, Pr...

A tectonic model of the Offerdal—Olden area, Jämtland, Sweden

Abstract The geology of a narrow traverse running NW. from Offerdal towards the Swedish—Norwegian border is described. Imbrication structures consisting of slices of basement rocks, quartzites, limestones and graywackes are found in the eastern part. Similar structures, but predominantly of graywackes, appear in the central part and can befollowed to the border of the Olden massif. The author finds no support for the Follinge Nappe and the Olden Nappe. The Olden massif is found to be autochthonous in most parts. The massif, and large portions of the parautochthonous graywackes, are overridden by the Offerdal Nappe, a composite unit of meta-arkoses, schists and mylonites, with Offerdal Schist at the top, then a polymict conglomerate, sometimes with quartzites, dolomites and shales (the Offerdal Conglomerate) and a silty shale at the bottom (the Ronnofors Shale). The rocks of the Offerdal Nappe are of Precambrian age and have been correlated with the Rensjonas Group, east of the Mullfjallet basement window.

Deformation of a jointed rock mass

Abstract The finite-element method and the discrete-block method have been used to study the defor-mational behaviour of a jointed rock mass subjected to simple shear displacement. Blocks in models with large sizes of block make contact at few points which cause large openings of joints and high stress concentrations in the blocks and surrounding joints. The introduction of more joints into the models evens out the forces, reduces the stresses and causes a gentle type of deformation. From this study, we draw the practical conclusion that joints in rock plinths are eligible for the storage of radio-active waste, if we take into account the effect of simple shear displacement from faulting.