Piggyback imbrications, duplex stacking and sequential superimposed deformation in the Fanø Bugt Glaciotectonic Complex, Danish North Sea

This study provides a detailed structural analysis of a selected part of the Fanø Bugt Glaciotectonic Complex in the south-eastern part of the Danish North Sea. The 200 km2 study area was mapped in 3D using high-resolution, 2D multichannel seismic data. The interpretation of seismic profiles demonstrates an architecture markedly separated into a lower and an upper thrust fault level, separated by the upper décollement surface. The lower level is characterised by >15 thrust sheets, with crests that form subsurface ridges with reliefs up to 150 m, scattered over c. 15 km. The upper level is characterised by thrust sheets grouped in imbricate complexes with thrust faults connecting to the upper décollement. The structural style changes in the direction of transport, possibly related to the position of the ice-sheet margin responsible for the thrusting and changes in the properties of the basal décollement. The structural style is generally large-scale thrusting and folding, suggesting proglacial deformation. However, the hinterland of the upper thrust fault level displays heavily folded layers or a chaotic reflection pattern associated with subglacial deformation. Special attention is drawn to two exceptional structural frameworks containing a hidden hill-hole pair: SF1, an imbricate thrust fault fan in a 5 km long and 2.5 km wide basin, developed above an extensional normal fault imbricate, and SF2, a frontal ramp uplifting an imbricated fan c. 90 m above the average level of thrusting. Restored cross-sections demonstrate a shortening of the lower thrust fault level between 9–43% and 44–49% of the upper level across SF1 and SF2, respectively. We suggest that the glaciotectonic complex was formed proglacially due to gravity spreading in front of an ice margin. Gravity gliding due to an inclined décollement surface of 0.5° and elevated porewater pressure at the décollement might also have facilitated the deformation.