Niels Balling

Heat flow and thermal structure of the lithosphere across the Baltic Shield and northern Tornquist Zone

Abstract Terrestrial heat-flow data from Archean and Proterozoic Provinces of the Baltic Shield, its southwestern margin, and the tectonic transition to the younger tectonic units of Central Europe (northern segment of the European Geotraverse) are compiled and examined for local and regional variations. A 2-D numerical lithospheric temperature and heat-flux model is presented covering 1800 km of the deep seismic Fennolora profile across the Baltic Shield and 500 km of Eugeno-s / northern Eugemi seismic lines across the northern Tornquist Zone and the Danish and North German basins. Thermal lithosphere, defined as the outer layer with relatively low temperatures ( T C ) in which heat transfer is dominantly by conduction, is found to vary markedly in thickness from 85–110 km beneath basins and Tornquist Zone to about 150 km in central shield areas, and 200–250 km in areas of low heat flow along the northern shield profile. These results are in very good agreement with seismological information from surface wave dispersion analysis and the refraction / wide-angle seismic interpretation of Fennolora data. The combined thermal and seismologic evidence indicates that the uppermost mantle region stratified in seismic velocity is restricted to the upper, relatively cold thermal layer, which is interpreted to constitute shield lithosphere. Local and to some extent regional variations in surface heat flow are found to be closely related to variations in near-surface radiogenic heat production. For the southern Baltic Shield a heat-flow / heat-production relationship of q 0 = 32.8 + 7.6 H 0 is derived. Regional increase in heat flow from northern and central shield provinces to the shield margin and the Tornquist Zone is interpreted to be related partly to differences in upper-crustal heat generation, partly, and in southernmost areas mainly, to an increased heat flux from the upper mantle. Upper-mantle heat flux is estimated at 20–25 mW m −2 along the northern and central shield transect, increasing to 30–40 mW m −2 beneath the shield margin, basins, and Tornquist Zone. Lithospheric thickness variations in this region of northern Europe are found to be closely associated with variations in heat flux from the uppermost mantle and perhaps to a large extent controlled by geodynamically determined lateral differences in upper-mantle heat flux. Earthquake focal depth data and information on crustal strength show Baltic Shield seismicity to be limited generally to the upper 20–25 km brittle part of the crust at temperatures less than 300–400°C. Maximum source depth of magnetic anomalies at temperatures near 600°C is in the deepest part of the crust near the crust-mantle boundary in the southern region (about 25 km depth along the Tornquist Zone and about 35–40 km in southern shield areas) and in the uppermost mantle (60–70 km depth) in the northern shield areas. Some petrologic implications are discussed. The present deep crust is supposed to be at lower temperature-pressure conditions than at early stages of formation. However, also the present deepest parts of the shield crust (> 35–40 km) are within the stability field of eclogites. The general absence of significant gravity anomalies in areas of marked crustal thickness variations indicate high-density eclogites to be present in significant amounts in the deep shield crust.

MONA LISA : Deep seismic investigations of the lithosphere in the southeastern North Sea

The MONA LISA collaborative project has collected 1112 km of seismic normal-incidence reflection data (recorded to 26 s) and wide-angle data from 26 onshore and 2 offshore locations along 4 profile ...

A numerical dynamic model for the Norwegian–Danish Basin

Abstract A 2D numerical dynamic model for the Late Palaeozoic and Mesozoic formation and evolution of the Norwegian–Danish Basin (NDB) is presented. This basin developed in the eastern part of the Northern Permian Basin within the system of Northwest European sedimentary basins. The basin forming processes are modelled in a lithospheric plane-strain model along a 400-km profile across the basin. Layered rheology is used with elasto-plastic behaviour in the brittle parts and elasto-viscous behaviour at higher temperatures and pressures. The mechanical equations of equilibrium and the heat equation are solved by the finite element technique. The model contains two tectonic events. Basin initiation in Late Carboniferous and Early Permian times is modelled by combined lithospheric heating and lithospheric extension. Additional lithospheric extension, however, less pronounced and taking place over a longer period, is introduced during the Triassic. Isostatic effects associated with lithospheric extension combined with thermal contraction and sediment loading accounts for up to 6 km of sediments of which 2–3 km are of Triassic age. The model shows a generally good agreement with observations in terms of basin geometry, sediment layer thicknesses, stratigraphy, crustal thickness, and heat flow. In the central parts of the basin, the model implies a crustal thinning factor of about 1.5—from 35 km to about 23 km, close to that observed in seismic data.

Seismological evidence for a fossil subduction zone in the East Greenland Caledonides

The postorogenic collapse of the early Paleozoic Caledonian orogeny is well documented; however, several different plate tectonic models exist for the convergent phase involving closure of the Iapetus Ocean and the collision of Laurentia and Baltica. Receiver function analysis of 11 broadband seismometers along a 270 km transect in the East Greenland Caledonides reveals the existence of an east-dipping high velocity slab. Numerical modeling demonstrates that relict subducted and eclogitized crust is a plausible explanation. Thus, eastward subduction preceded subsequent west-dipping subduction during the formation of the East Greenland and Scandinavian Caledonides. This is a key constraint for understanding the Caledonian and continental margin evolution in the North Atlantic realm.

Transient heat flow in a stratified medium

Abstract The equation of heat conduction is solved for a horizontally stratified medium initially at constant temperature and subject to a step-function surface temperature change. The problem is solved by Laplace transformation and by applying a concept of further stratifications of the medium into unitary layers of constant ratio of thickness to the square root of thermal diffusivity. The solution takes the form of an infinite sum of complementary error functions, with coefficients given by recursion relations, is suitable for numerical applications and offers an attractive alternative to harmonic and quasi transient approaches in calculating the penetration of transient surface temperature variations into a layered medium. Use of the theorem of superposition yields a general expression for an arbitrary surface temperature function. The utility of the theory is illustrated by modelling examples of palaeoclimatically induced subsurface temperature and heat flow perturbations.

Post-Permian evolution of the Central North Sea: a numerical model

Abstract A dynamic thermomechanical model is presented for the post-Permian evolution of the Central North Sea including the Central Graben. Permo-Carboniferous and Jurassic thermal events, as well as Triassic and Jurassic extensional phases and Late Cretaceous shortening, are considered. The equations of force balance for a continuum and the transient heat equation are solved using the finite element method. The lithosphere responds to applied forces by elastic deformation at low deviatoric stress levels, fracture at low temperature and pressure conditions and nonlinear viscous flow at high temperature and pressure. A first-order rheological strain softening is incorporated to simulate a transition from strong to soft material behaviour, such as a change from dislocation creep to diffusion creep. Seismic refraction and reflection data indicate crustal thinning below the Central Graben by a factor of ∼2. While the Triassic rifting appears to have been associated with a more distributed deformation pattern, the Upper Jurassic rifting is more localised. Pure shear extensional models have been advocated to explain this. However, dipping seismic reflectors in the upper mantle indicate that simple shear could be important. The strain softening of the present model allows for modelling of strain localisation during the Upper Jurassic rifting phase and detailed modelling of Moho structures. The present model accounts for the main structural and stratigraphic observations in the Central North Sea. Furthermore, the results indicate that in the part of the Central North Sea investigated in this paper, a pure shear regime in Triassic changed into a simple shear-dominated regime with mantle shear zones developing during Late Jurassic extension.

A sub-crustal piercing point for North Atlantic reconstructions and tectonic implications

Plate tectonic reconstructions are usually constrained by the correlation of lineaments of surface geology and crustal structures. This procedure is, however, largely dependent on and complicated by assumptions on crustal structure and thinning and the identification of the continent-ocean transition. We identify two geophysically and geometrically similar upper mantle structures in the North Atlantic and suggest that these represent remnants of the same Caledonian collision event. The identification of this structural lineament provides a sub-crustal piercing point and hence a novel opportunity to tie plate tectonic reconstructions. Further, this structure coincides with the location of some major tectonic events of the North Atlantic post-orogenic evolution such as the occurrence of the Iceland Melt Anomaly and the separation of the Jan Mayen microcontinent. We suggest that this inherited orogenic structure played a major role in the control of North Atlantic tectonic processes.

Accuracy and resolution in continuous temperature logging

Abstract The linear Backus-Gilbert theory is used for the deconvolution problem of continuously logged borehole temperatures. The main elements of the theory and some numerical and experimental results are given. The accuracy and resolution of the temperature and temperature gradient records are determined by the logging velocity, the probe time constant, the sampling period, the statistical accuracy of data, and the characteristics of the inverse filter. By applying the inverse approach the interrelation between parameters is studied, and the trade-off between accuracy and resolution is emphasized. The inverse method yields well defined temperatures and temperature gradients with associated standard deviations and averaging functions. It is suitable for the processing of continuously logged temperature data with reference to application in terrestrial heat flow and stratigraphic studies.

Seismic modelling of the Norwegian-Danish basin along a refraction profile in northern Jutland

Abstract The seismic velocity of the sedimentary structure of the Norwegian-Danish Basin and its underlying Precambrian basement have been modelled by 2D ray tracing interpretation of a 200 km section of a deep refraction seismic profile in northern Jutland (EUGENO-S Project, profile 3). A model is presented which satisfies the quite detailed prior geological information from wells and shallow reflection seismic interpretations for the upper sedimentary layers and the traveltime observations from three shotpoints along the profile. The model shows a basin thickness of about 7 km in the southern part of the profile, thinning to 1.5–2 km in the northern part towards the Fennoscandian Border Zone. In the central part, the model thickness is about 10 km. Sedimentary P-wave velocities vary from 1.9 to 3.8 km/s in the upper layers (Quaternary, Tertiary and Upper Cretaceous units) to about 5.5 km/s in the deepest Palaeozoic unit. The Precambrian basement shows velocities of 5.9–6.5 km/s. There is a clear indication of a basement reflector at depths of 11 km (velocity contrast 6.0 6.4 km ).

Regional Geothermal 3D Modeling in Denmark

We present first results of a 3D model of the subsurface temperature in Denmark. Updated temperatures are contoured for the Gassum formation which is currently utilized for geothermal energy production. The Gassum formation of Upper Triassic/Lower Jurassic age is present in most of Denmark with sufficient net sand thickness and temperatures between 50oC and 80oC in large areas. This makes this formation of particular interest for further geothermal exploration. The regional geothermal model is expected to aid decision makers in future prospecting of geothermal resources in Denmark.