The near-surface structure in the area of the Børglum fault, Sorgenfrei-Tornquist Zone, northern Denmark: Implications for fault kinematics, timing of fault activity and fault control on tunnel valley formation

Abstract

Previous studies showed evidence that the Sorgenfrei-Tornquist Zone (STZ) in northern Denmark is seismically active and fuelled the demand for further analyses of the fault structure and its seismic hazard potential. We focus on the area of the Børglum fault, which represents the northern boundary fault of the STZ and extends at least 250 km from the western Baltic Sea across Jutland into the southeastern North Sea. The Børglum fault has previously been interpreted to represent a glacially-triggered fault. Two shear-wave reflection seismic sections were acquired to image the near-surface structure in the vicinity of the Børglum fault. Detailed insights into the acquisition, processing and interpretation of the seismic sections will be provided. The seismic sections indicate that the Børglum fault in the study area is not an isolated fault, but most likely represents a complex fault system with a strike-slip component. The fault interpretation is based on systematic vertical reflector offsets, the observation of steeply dipping thin transparent zones that separate the offset reflectors, abrupt lateral changes in the reflector pattern and in some cases, the presence of a fault shadow (a diffuse zone on a seismic section below a fault). This interpretation is mainly based on the identification of positive flower structures on the seismic sections, and supported by the presence of elongated sedimentary mini-basins and topographic depressions in the study area, which are partly interpreted to represent strike-slip basins. A WNW-ESE striking tunnel-valley system is imaged on the seismic sections. The spatial relationship between the faults and the tunnel valley suggests a fault-control on tunnel-valley system formation in this area. The seismic sections indicate that faults in the study area were probably active at the end of the Saalian glaciation (MIS 6). In addition, soft-sediment deformation structures and shear-deformation bands developed in Late Pleniglacial to Lateglacial sediments point to repeated phases of fault activity. The reflector pattern on one of the seismic sections indicates that a major fault nearly reaches the Earth's surface. A step in the topography above the fault probably represents a fault-scarp. Because the ongoing glacial-isostatic adjustment will lead to a change in the stress magnitude in this area, future movements along the faults are possible.