The sampling of fault populations in dolerite sills of Central Sweden and implications for resolution of seismic data

Abstract

Geological mapping of faults affecting dolerite sills was carried out along the Baltic coast in central Sweden and scaling relationships for the faults analysed in order to assess the seismic imaging of the sills. The sills are 250–400 m thick and dip at low angle (< 20°) to a depth of 10–12 km beneath the Baltic Sea where they are observed as sub-horizontal bright seismic reflections in the BABEL data. Characteristics studied in outcrop included the nature of sill contacts, the horizontal continuity and variations in mineralogy of individual sills, and offsets in the sills by faults. We also compiled, from field observations and detailed geological and tectonic maps, independent data sets on the lengths and displacements of faults. Both fault length and fault displacement populations exhibit power-law scaling over more than 4 orders of magnitude, with exponents that are consistent with a linear relationship between maximum displacement and fault length. The power-law exponent for a one-dimensional fault displacement population is 0.69 ± 0.07, whereas the exponent for a two-dimensional fault length population is 1.66 ± 0.1. These results for the surface were used to predict the perturbation of the seismic reflections caused by deeper faulting in the dolerite sills. Predicted fault displacements would need to be an order of magnitude greater in order to account for the observed segmentation of the seismic reflections. The incoherence in the seismic data is therefore attributed, on the larger scale, to primary structural features of the sills, such as 10-km scale cuspate segments linked by transgressive steps and, on the smaller scale, to the scattering effect of heterogeneities in the overlying crust.