The role of brittle shear zones and the associated fractures in the formation and characteristics of bedrock surface depressions in crystalline bedrock (Turku, southwestern Finland)

While the impact of shear zones on the surface morphology of crystalline bedrock is well recognised, the contribution of associated smaller-scale structures is often overlooked. This leads to inaccurate interpretation of the bedrock surface elevation, for example, in studies that require knowledge about the overburden thickness overlying the bedrock surface. In this study, multi-scale analysis and 3D modelling of the bedrock surface in the Turku area, Western Finland, enhance our understanding of how brittle structures contribute to the formation and characteristics of bedrock surface depressions. We measured the prevailing orientation of foliation and fracture sets using standard field mapping on the bedrock outcrops. The orientation of the brittle shear zones and elongated depressions was first detected in remote sensing data and subsequently evaluated using our interpolated digital elevation model of the bedrock surface. The results of these analyses enabled us to examine the relationship between bedrock structures and bedrock surface morphology across different scales. Our findings indicate that the orientations of brittle shear zones, elongated bedrock depressions, and foliation are correlated, as expected. Even parallel outcrop-scale fracture sets are distinguished; prevailing fracture orientations are striking NNE to NNW, which does not correlate with large-scale bedrock structures. Therefore, these outcrop-scale fractures occurring in greater numbers near the brittle shear zones largely determine the dimensions and detailed surface morphology of the bedrock depressions.