Improved Resolution of Moho Geometry From Modelled Internal Crustal Load Variations via a Novel Analytical Solution (ASEP) -- Norwegian Barents Sea

Gravity and magnetic data allow one to image the entire crust from the surface down to the Moho. Seismic, geological and borehole information are generally used to constrain the model, but information about the Moho geometry is rather scarce. The interpretation of the Moho is often based on long wavelength undulations of the gravity field, which is not unique due to other deep sources of density contrast. Moho information, independent of the gravity field, is therefore useful to constrain the modelling. The flexural response of the Earth's crust due to loading can be estimated using a 4th order differential equation describing the flexure of a thin elastic plate. Until recently, the solution for this differential equation analytical for a real topography has been approximated by spectral methods with the disadvantage of low lateral resolution. The analytical solution of an elastic plate (ASEP) is a novel approach which overcomes the drawbacks of the spectral method. The ASEP method allows the analytical calculation of a flexural Moho geometry which directly uses the crustal load. Load is defined as product of density, thickness and gravity acceleration; the crustal load is the sum of basement load, sediment load and load of topography/water above a reference depth. The novel approach, to estimate the Moho geometry from these internal crustal load variations via ASEP, was applied to data from the south-western Barents Sea with emphasis on the southern Nordkapp Basin (NKB) (25-29°E and 71.7-72.7°N). An area of about 230.000 km2 was selected to avoid edge effects in the area of interest (14.000 km2). The factor of enlargement for the area to be calculated is defined by a flexural function that depends on the average rigidity of the Earth's crust. The NKB is a Permian rift basin of more than 10 km sedimentary depths with salt diapirism. Reflection seismic investigations for hydrocarbon exploration had considerable problems to define the base of the salt. In order to overcome this problem, the southern NKB was investigated using high resolution gravity, gravity gradiometry, magnetic and electro-magnetic data. The combination of these data types with 2D and 3D seismic allowed the construction of a full sedimentary model. The top basement was difficult to define due to lack of reflection seismic information as well as small density and magnetic susceptibility contrasts. The basement modelling is directly linked to the topography of the Moho. A few old deep seismic lines exist, but the information about the Moho was sparse and rather uncertain. This NKB density model was transferred into load in order to be used as input for the ASEP Moho estimations. This is superior to depth-density-functions as these would lead to overestimated densities in areas with salt diapirism. Outside of the NKB model, the sediment densities were approximated via absolute sediment thickness data (recent mapping) and a depth-density-function. This function was amended in such a way that the regional density variations roughly match the density distribution in the NKB model. On the poster, the determination of crustal load is described, the advantage of using the crustal load and combination of modelled and approximated sediment loads is demonstrated and the impact of the ASEP Moho on the basement modelling in the NKB is shown.