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

Abstract. Surface morphology of the crystalline bedrock is controlled by brittle bedrock structures, and while the contribution of the large-scale structures such as shear zones and faults are acknowledged, the contribution of detailed brittle structures associated to large-scale structures is more often overlooked.

In this study, multi-scale analysis of brittle structures and detailed 3D-modelling provide improved knowledge how the detailed brittle structures contribute on the formation and character of the bedrock surface depressions.

While the shear zones, faults and elongated bedrock depressions show parallel trends and overlap, the outcrop-scale fractures largely determine the dimensions and detailed surface morphology of the bedrock depressions. The surface morphology and orientation of bedrock depressions along shear zones and faults are contrasting particularly due to the strong ductile precursor associated with the shear zones and their lack for faults. However, for both the shear zones and faults, localized brittle deformation controls the generation of outcrop-scale fracture, which further controls the detailed surface morphological signatures along the elongated bedrock depressions, and particularly in their damage zones.

This study provides new insights into the detailed bedrock structures, that contribute to the development of complex bedrock surface morphologies within the bedrock surface depressions which are controlled by the brittle deformation and kinematics along the underlying shear zones and faults.