Structural observations of the basement-cover contact in the Central Fjord region of the East Greenland Caledonides suggest Silurian to Devonian crustal thinning with top-to-the-east displacement of the cover sequence. The east-dipping, low-angle shear zone separating the Late Proterozoic (Eleonore Bay Group) to Lower Ordovician cover sequence from the underlying high-grade gneisses has previously been interpreted as the Caledonian sole thrust in the region, displacing the cover sequence toward the Caledonian foreland in the west. However, emplacement of younger, low greenschist facies rocks on top of Archean to Middle Proterozoic gneissic rocks across the shear zone instead favors an extensional origin. This interpretation is supported by ductile and brittle shear-sense indicators in the footwall formed under progressively lower temperature conditions, consistently showing top-to-the-east displacement. The cover sequence in the hanging wall is cut by numerous extensional faults. The inferred earliest fault set developed contemporaneously with the deposition of the unconformably overlying Devonian deposits, whereas the youngest set crosscut the entire Devonian stratigraphy. Neither of these late brittle extensional faults are found in the basement, suggesting that they merge with the extensional shear zone at depth. Collectively, these observations suggest that the Devonian basin formed as a supradetachment basin during collapse of the Caledonian orogen.

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