3D seismic imagery of a buried glacitectonic thrust complex in the North Sea and the dimensions of composite ridge systems in glaciated landscapes

ABSTRACT Sets of sub‐parallel composite ridges in glaciated landscapes have long been regarded as landforms produced by glacitectonism, the brittle and ductile deformation of sediments at and immediately beyond glacier and ice‐sheet margins resulting from stresses produced by ice. The morphology and internal structure of a glacitectonic thrust complex buried beneath up to 130 m of sediment on Egersundbanken, North Sea, is examined using a 3D seismic cube (ABP23001, area 580 km 2 ). Several ridge complexes are identified, totalling 117 km 2 in area and volume about 7 km 3 . The ridges are the tops of individual imbricate thrust sheets that extend below local palaeo‐surfaces. Depressions between ridges relate to thrust faults. Ridge‐system bases are defined by a decollement surface. Ridge orientation implies past ice flow from the ENE, from an ice sheet centred over Scandinavia probably during the Saalian glaciation. The geomorphology of Egersundbanken glacitectonic thrust complex, combined with evidence on the dimensions of other composite ridges from terrestrial and marine settings around the North Sea, provides a geometric baseline against which other types of glacier‐produced ridges orientated transverse to ice‐flow direction can be distinguished (e.g., corrugation, annual, De Geer, crevasse‐squeeze and recessional‐ and terminal‐moraine ridges). There are orders of magnitude differences between ridge types. Glacitectonic complexes are large relative to many other glacial ridges, often 10 2 km 2 in area and with volumes of 10 km 3 . Corrugation and crevasse‐squeeze ridges are much smaller than composite ridges, whereas major ice‐recessional moraine ridges are often more widely spaced and much larger than glacitectonic complexes. Glacitectonic thrust complexes therefore have a relatively distinctive surface morphology and internal architecture when compared with other transverse‐to‐flow ridges in glaciated landscapes.