3.5.3. Effects of lithospheric and sublithospheric processes on topography development

The Norwegian margin has been extensively explored for petroleum during the past three decades, rendering it probably the world's best-documented volcanic passive margin. Information obtained from seismic reflection/refraction surveys, potential field studies and tomographic inversions coupled with the results of related studies (e.g. ESF EUROCORES EUROMARGINS) that focused on the deep-crustal architecture offshore Mid-Norway and East-Greenland make Scandinavia's margin an excellent natural laboratory to evaluate the effects of deep lithospheric and sub-lithospheric processes on its syn- and post-rift evolution.

Processes controlling both dynamic and isostatic topography development played an important role in the evolving North-Atlantic margins. To enhance their understanding, onshore and offshore field studies and numerical modelling will be carried out. The structural configuration of offshore basins and their internal sedimentary architecture will be regionally mapped based on the available dense grid of mainly 2D industrial multi-channel reflection-seismic lines, supplemented by deep seismic reflection/refraction and potential field data, plus 3D seismic data where available. Complementary onshore studies will encompass in-depth structural field work in close conjunction with geochronologic and geomorphologic studies to 'set the stage' and place results in a coordinated, well-constrained, tectonically integrated framework. This will be supported by a regional 3D interpretation based on seismic tomography, potential field data and heat flow measurements (Olesen et al., 2005).
Particular emphasis will be given to the construction of a set of comparative, lithospheric-scale regional transects across the Norwegian and Greenland margins, from far offshore to well inshore (Mosar, 2003; Gernigon et al., 2006). Numerical methods will be applied to determine the present-day structure of the lithosphere from a wealth of geophysical data (Pascal, 2006). Stepwise palinspastic restoration of these transects, incorporating sediment decompaction and corrections for water depths and data on uplift and denudation, will permit to assess the extensional strain distribution and rates in time and space, as well as subsidence and uplift patterns.

Of particular interest is the computation of dynamic topography due to large-scale mantle density anomalies and flow, using codes developed by Steinberger (2007). For very long wavelength of >3000km, a reasonably good match, both in the pattern and amplitude, can be obtained in the Arctic-North Atlantic / Scandinavian region between computed and observation-based present-day dynamic topography (Fig. 59; Steinberger, 2007). The effect of varying modelling assumptions (mantle density anomalies, viscosity structure, plate motions etc.) on the computed dynamic topography will be investigated in an effort to define in which range of models a good fit with observations can be achieved. For successful models the advection of mantle density heterogeneities will be modelled backward in time (Steinberger and O'Connell, 1997), together with the resulting change in dynamic topography. Although such a procedure cannot recover the past mantle density structure in regions where diffusion was the main heat transport mechanism (i.e., the lithosphere), computations will be meaningful for assessing how the change of density anomalies and flow in the sub-lithospheric mantle has contributed to topography changes through time.

Surface topography induced by the Iceland plume will be quantified. With large-scale flow models, the position of the Iceland plume through time will be re-computed, as in Steinberger (2000). Based on the computed location of the Iceland plume and the mid-Atlantic ridge through time, and on buoyancy flux and models of plume-ridge and plume-lithosphere interaction (Mihalffy et al., 2007), topography changes will be estimated and compared to observations. This modelling study will explicitly address the questions during what time interval the Iceland plume has been entrained by the Atlantic spreading ridge, and how it may have contributed to the late-stage uplift of Scandinavia. Modelling will use input from, and results will be compared with detailed mantle tomography to be acquired through the EUROARRAY component of TOPO-EUROPE.

Topography is composed of a signal from the mantle, as described above, and contributions from lithosphere isostasy and local crustal processes that are not yet in equilibrium. The surface deflection that can be caused by deep mantle loads is a function of the strength of the lithosphere. Crustal- to lithospheric-scale finite element models will be combined with mantle flow models to constrain the evolution of surface topography. Lithosphere models have a high resolution and sophisticated (visco-elasto-plastic) rheology (Buiter et al., 2004) and can thus 'filter' the dynamic topography obtained from mantle flow models. Finite element models will be used to test if and how inherited structures are reactivated during the evolution of the margin. Vertical surface motions will be quantified for the localization of rifting on (Caledonian) compressional structures, reactivation of normal faults in extension, and reactivation of normal faults in contraction (Buiter and Pfiffner, 2003; Panien et al., 2006). The interaction between lithospheric flexure and sedimentation and erosion will be examined with numerical flexural models to assess the contribution of long-wavelength isostatic processes to the Scandinavian surface topography. In order to test mechanical concepts and assist the interpretation of geological and geophysical data, analogue modelling will be performed in close connection with numerical modelling. Experiments will examine the development of transfer zones which separate domains of contrasting normal fault polarities, and experiments will be carried out to evaluate the role of inherited structures in localizing subsequent deformation.