Sill-intrusion, flood basalt emplacement and deep crustal structure of the Scoresby Sund region, East Greenland

Abstract

Abstract The late Palaeozoic-Mesozoic Jameson Land basin in East Greenland was situated during early Tertiary break-up 300–400 km north of a presumed plume centre and 100–200 km landward of the initial line of North East Atlantic opening. The basin continues southward below Scoresby Sund fiord and the Scoresby Sund plateau basalts south of the fiord. By combining surface mapping with marine reflection and refraction seismics from Scoresby Sund and deep (6–12s) reflection seismics from the exposed part of the basin, it is possible to construct a composite image of the entire crustal structure in this region from the details of basalt stratigraphy at the top, through the basin fill, the foundation of the basin, and into the uppermost mantle. At the basin centre there is a very thin crystalline crust (6–8 km?) below as much as 16–18 km of basin fill. Virtually all crustal extension took place prior to mid-Permian time, and the post-rift Upper Permian to Cretaceous section contributes to only one third or less of the total thickness of the succession. During Tertiary break-up the basin was intruded by basaltic sills and dykes, and basaltic flood basalts flowed over the basin with an apparently decreasing thickness to the north. The sills are mainly exposed along the basin margins as rather thin (10–15 m) layers, but apparently increase in intensity and thickness at depth and towards the basin centre. It seems that magmas were intruded as sills up to 300 m thick in the deep (10–15 km) central parts of the basin, from where they ascended towards the basin margins and to much younger stratigraphic levels. Their geometry and possible volume make them potential candidates as mid-crustal magma chambers and crustal magma pathways for the flood basalts which show low-pressure fractionation. However, extension surface geological data and deep crustal reflection seismic data show no faulting or crustal extension associated with this intense break-up volcanism. In Scoresby Sund there is a general rather conformable relationship between the basin stratigraphy and the gross stratigraphy of the flood basalts, suggesting limited or no initial uplift prior to flood basalt volcanism. The present-day high elevation of the basalts is considered part of large regional margin uplift post-dating N Atlantic break-up by 20 Ma or more. The apparent guidance exerted by the basin on the break-up magmatic activity without renewed rifting of the basin itself, the apparent lack of a broad initial uplift during break-up, and the late regional margin uplift, all seem at odds with several current plume models.