Abstract
Pedersen, S.A.S. 2005: Structural analysis of the Rubjerg Knude Glaciotectonic Complex, Vendsyssel, northern Denmark. Geological Survey of Denmark and Greenland Bulletin 8, 192 pp.
 The Rubjerg Knude Glaciotectonic Complex is a thin-skinned thrust-fault complex that was formed during the advance of the Scandinavian Ice Sheet (30 000 – 26 000 B.P.); it is well exposed in a 6 km long coastal profile bordering the North Sea in northern Denmark. The glaciotectonic thrust-fault deformation revealed by this cliff section has been subjected to detailed structural analysis based on photogrammetric measurement and construction of a balanced cross-section. Thirteen sections are differentiated, characterising the distal to proximal structural development of the complex. The deformation affected three stratigraphic units: the Middle Weichselian arctic marine Stortorn Formation, the mainly glaciolacustrine Lønstrup Klint Formation and the dominantly fluvial Rubjerg Knude Formation; these three formations are formally defined herein, together with the Skærumhede Group which includes the Stortorn and Lønstrup Klint Formations. The Rubjerg Knude Formation was deposited on a regional unconformity that caps the Lønstrup Klint Formation and separates pre-tectonic deposits below from syntectonic deposits above.
 In the distal part of the complex, the thrust-fault architecture is characterised by thin flatlying thrust sheets displaced over the footwall flat of the foreland for a distance of more than 500 m. Towards the proximal part of the complex, the dip of the thrust faults increases, and over long stretches they are over-steepened to an upright position. The lowest décollement zone is about 40 m below sea level in the proximal part of the system, and shows a systematic step-wise change to higher levels in a distal (southwards) direction. The structural elements are ramps and flats related to hanging-wall and footwall positions. Above upper ramp-hinges, hanging-wall anticlines developed; footwall synclines are typically related to growth-fault sedimentation in syntectonic piggyback basins, represented by the Rubjerg Knude Formation. Blocks and slump-sheets constituting parts of the Lønstrup Klint Formation were derived from the tips of up-thrusted thrust sheets and slumped into the basins. Mud diapirs are a prominent element in the thrust-fault complex, resulting from mud mobilisation mainly at hanging-wall flats and ramps.
 Shortening during thrust-fault deformation has been calculated as 50%. Only about 11% of the initial stratigraphic units subjected to thrust faulting has been lost due to erosion. The thrust-fault deformation was caused by gravity spreading of an advancing ice sheet. Overpressured mud-fluid played an important role in stress transmission. The average velocity of thrust-fault displacement is estimated at 2 m per year, which led to compression of a 12 km stretch of flat-lying sediments, c. 40 m in thickness, into a thrust-fault complex 6 km in length. The thrust-fault complex is truncated by a glaciotectonic unconformity, formed when the advancing ice sheet finally overrode the complex. When this ice sheet melted away, a hilland- hole pair was formed, and meltwater deposits derived from a new ice-advance (NE-Ice) filled the depression. The NE-Ice overran the complex during its advance to the main stationary line situated in the North Sea. When this ice in turn melted away (c. 19 000 – 15 000 B.P.), the glacial landscape was draped by arctic marine deposits of the Vendsyssel Formation (new formation defined herein).
Highlights
Glaciotectonic studies in Denmark have a long tradition, and an important part of structural geology studies in Denmark concern glacial tectonic deformation resulting from the southward advance of the Scandinavian Ice Sheet in the Pleistocene (Fig. 1)
This study focuses on the structural framework and dynamic development of the glacial tectonic thrustfault complex at Rubjerg Knude, Lønstrup Klint
The construction of the balanced cross-section for the Rubjerg Knude Glaciotectonic Complex was based on the geological cross-section, which displays the geometry of the thrust sheets in sufficient detail to allow calculations of their displacements and cross-sectional areas (Plates 1, 2)
Summary
Glaciotectonic studies in Denmark have a long tradition, and an important part of structural geology studies in Denmark concern glacial tectonic deformation resulting from the southward advance of the Scandinavian Ice Sheet in the Pleistocene (Fig. 1). The construction of the balanced cross-section for the Rubjerg Knude Glaciotectonic Complex was based on the geological cross-section, which displays the geometry of the thrust sheets in sufficient detail to allow calculations of their displacements and cross-sectional areas (Plates 1, 2). In the Rubjerg Knude cross-section (Plate 1), the displacement is measured and estimated mainly from the distance between the intersection of the L/R-unconformity (the unconformity between the Lønstrup Klint and Rubjerg Knude Formations, see below) and the footwall ramp, and the intersection of the L/Runconformity and the hanging-wall ramp (Fig. 11). This estimate is based on a simple geometric construction of the tip-triangle from the dips of the hanging-wall ramp and the L/Runconformity, respectively (Fig. 11) This line balance is subsequently controlled by the width of the piggyback basin more or less corresponding to the upper footwall flat. The marine environment referred to as the Older Yoldia Sea, which extended into the Vendsyssel region, formed in the Late Saalian, and the climatic change from a mild climate in the Eemian to a glacial
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