The Lithosphere beneath Northern Europe: Structure and Evolution over Three Billion Years - Contributions from Geophysical Studies

Processes of crustal growth and crustal evolution are revealed for a time interval of more than three billion years from the oldest Archean, 3.5-2.6 Ga old crustal units in the northeastern Baltic Shield, to the youngest Palaeozoic and Mesozoic sedimentary basins in the North Sea region. This long geological time span, with a series of tectonic events, together with the availability of extensive geophysical and geological data, makes the region Northern Europe a fascinating natural laboratory for the study of lithospheric structure and evolution. Although not uniquely defined, a number of geophysical experimental techniques and modelling methods are capable of providing information of “thickness of lithosphere” or depth to “lithosphere-asthenosphere boundary”. Several studies consistently show thick lithosphere, around 200-250 km (and more?), in the older northeastern and central parts of the Baltic Shield, and thinner lithosphere, around 100 km and less, beneath the tectonically younger deep sedimentary basins to the southwest in Danish, North German and North Sea areas. Regional variations in thickness of lithosphere are found to be related to regional differences in heat flow. Surface heat flow increases by a factor of two to three across the study area, from low values around 30-40 mW/m2 in the northeast, to 70-80 mW/m2 in the southwest. Thermal modelling has shown that heat flow from the mantle (estimated at 15-20 and 35-40 mW/m2 in the northeast and the southwest, respectively) plays a dominant role in controlling thickness of lithosphere. High resolution 3-D teleseismic tomography models, covering southern Scandinavia and adjacent areas, show upper mantle with contrasting P-wave velocity. An exceptionally deep (up to more than 300 km) and narrow boundary separates shield areas of high seismic velocity to the east from deep basin areas of low velocity to the southwest, including most of southern Norway. Rather than the Sorgenfrei-Tornquist Zone (as currently defined), this deep “Shield Border Zone”, outlined further to the northeast in the northern Kattegat and eastern Skagerrak, seems to be of major significance in understanding the tectonic transition between shield and deep sedimentary basins, as well as in indicating a northward continuation into, and across, southern Norway. The near-normal incidence seismic reflection technique is a unique tool in providing high-resolution, deep-structural images and a link between surface geological observations and structures at depth. Results, in particular from two marine deep seismic experiments called BABEL (Gulf of Bothnia and Baltic Sea) and MONA LISA (central and southeastern North Sea), demonstrate how structural details are resolved over the entire crustal depth, including deep shield crust, and to great depth into mantle lithosphere. Dipping mantle reflectivity, with associated Moho offsets, found to represent characteristics of fossil subduction zones, provide key observational deep-structural constraints for tectonic reconstruction. Such features are described from Palaeoproterozoic and Mesoproterozoic shield crust and from the North Sea, where the Caledonian collision between Baltica and Avalonia is imaged at a lithosphere depth scale. These observations, together with equivalent results, in particular from the Canadian Shield (including fossil subduction as old as 2.7 Ga), have demonstrated that plate tectonic processes, quite similar to those currently observed, were operating throughout the Proterozoic and at least since the Late Archean. In addition to their tectonic significance, the recognition of old structures preserved to great depth provides accurate information on the (minimum) thickness of the coherent lithosphere plate, a measure which is otherwise very difficult to obtain. Detailed information on crustal structure is available from a large number of mainly controlled-source seismological experiments, and recently also from receiver function analysis. The Baltic Shield has a crustal thickness around 45 km, but spans a wide range from 30-35 km up to 50-60 km. Since marked crustal thickness variations in the shield do not generally result in any significant Bouguer gravity anomalies or marked variations in surface elevation, the lower high-velocity part of shield crust must be very dense, close to that of the uppermost mantle. Thick, dense and refractory lower crust, together with thick, relatively cold, depleted lithosphere, seem to constitute important elements in stabilizing and preserving old shield lithosphere. In clear contrast to shield areas, the Scandes Mountains show marked negative Bouguer gravity anomalies that strongly correlate with surface elevation and show a general correlation with crustal thickness variations. From Norwegian coastal areas to beneath areas of highest topography, crustal thickness typically increases by about 10 km (from c. 30 km to c. 40 km), correlating with a marked decrease in Bouguer gravity of about 100 mgal. Buoyancy from the thickened crust plays a dominant role in the isostatic sustainment of the mountains. Thickened buoyant crust may (largely) date back to the Caledonian orogeny resulting from continental collision processes, suggesting that present-day high topography, is (mainly) a remnant from the Caledonian mountains rather than of much younger Cenozoic age. Evolution of the Danish and central North Sea Basins is modelled by phases of deep background heating, coupled with significant lithospheric stretching. Such models explain the observed marked crustal thinning, major elements of basin subsidence history and associated distribution of sedimentary sequences, and are consistent with present-day heat flow. Lithospheric stretching is found to be the most important mechanism for basin generation. Recent and current dynamic activity in the region of study is displayed by the Fennoscandian land uplift and by modest seismic activity. The glacial isostatic nature of land uplift is now unequivocally confirmed. Updated gravity maps confirm the existence of a significant central Fennoscandian long-wavelength free-air gravity low (about -15 mgal), and much new information is currently becoming available from satellite-based observations. Previous measurements of present (relative) uplift rates are generally confirmed by accurate GPS observations that also provide information on the absolute rate of uplift. Gravity-satellite data display a clear signal of gravity increase, demonstrating the existence of deep inflow of mass into the region of uplift. The Danish area constitutes part of the southwestern peripheral area of uplift that, however, is submerged as a result of regional sea level rise. The Fennoscandian lithospheric stress field, and associated seismicity, seem to be explained mainly by a combination of the regionally dominant ridge push from the North Atlantic, together with locally important contributions from gravitational potential stresses generated by marked lithospheric density variations. Stresses associated with glacial rebound also seem to play a part. High-quality temperature and heat-flow data from deep boreholes (Danish Basin, central Sweden and eastern Finland) display a significant increase of conductive heat flow with depth. Such information is critical for determination of undisturbed surface boundary heat flow and, furthermore yields new independent information on past climate. Information may be extracted concerning a characteristic surface temperature level during the last glacial period and with the magnitude of temperature increase around the termination of glaciation. This increase of temperature seems to be of the order of 10-15 °C in much of the Fennoscandian region.

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