Structural evidence for the timing of initial rifting in East Greenland

Rifting in East Greenland is traditionally thought to have commenced during the Late Devonian, following the break-up of the Caledonian orogeny. However, there is a distinct lack of structural or sedimentological evidence for rifting within the Late Devonian to Early Permian stratigraphy. These units comprise up to 3 km of continental clastics and appear to have experienced the same wrench tectonics, albeit to a lesser extent, that began in the Early Devonian due to extensional collapse of over-thickened Caledonian crust and ongoing Caledonian shear. We present field evidence from the Mid-Late Permian unconformity between 71-73°N, which suggests instead that rifting began in the Late Permian. The unconformity outcrops as an angular discordance between ubiquitously folded Carboniferous-Early Permian fluvial red beds (Mesters Vig Fm.) and a Late Permian clast-supported conglomerate unit (Huledal Fm.) that thickens across growth faults and is overlain by a progradational sequence of Late Permian-Early Triassic marine units (Foldvik Creek Group). Structural relationships on either side of the unconformity surface therefore indicate that it followed a phase of compression and preceded a phase of extension that is interpreted to mark the start of protracted rifting in East Greenland. This conclusion is supported by a wealth of apatite fission track data that documents widespread Mid-Late Permian cooling and very few pre-Mid-Permian ages. Our field observations indicate that the Mid-Late Permian unconformity in East Greenland is of regional importance and records a major tectonic regime change. It is likely that other undated unconformities across East Greenland are part of the same surface. As such, an understanding of the Mid-Late Permian unconformity will likely aid the interpretation of seismic sections from the NE Greenland Shelf.