Distribution, crustal properties and significance of seawards-dipping sub-basement reflectors off E Greenland

H. C. Larsen and S. Jakobsdóttir
Geological Society, London, Special Publications, 39, 95-114, 1 January 1988, https://doi.org/10.1144/GSL.SP.1988.039.01.10

Summary

We report in this paper the existence of seawards-dipping sub-basement reflectors along the entire E Greenland margin. The study is based on 8000 km of multichannel seismic data and a sonobuoy refraction seismic study providing information on the geographical and stratigraphical extension, internal geometry and crustal structure of the E Greenland dipping reflector sequence. A basaltic, subaerial seafloor-spreading origin of the reflector sequence is concluded from seismic stratigraphic analysis, including well information from the Rockall Plateau and the Vøring Plateau. Formation of the basaltic dipping reflector sequence off E Greenland took place within a period of a few million years along the axis of opening within the NE Atlantic. Duration of spreading above sea-level was relatively short (2 My) in areas of present-day deep basement, as opposed to 5–8 My in areas of present-day more shallow basement. On the highly elevated Iceland-Greenland Ridge, subaerial seafloor spreading continued into the Neogene and most likely into present-day subaerial spreading in Iceland. Following the mid-Tertiary westward shift of spreading towards Greenland, N of Iceland, spreading again took place above sea-level along this part of the Greenland margin until late Miocene, but this development only caused an erratic and shallow development of seawards-dipping reflectors.

Application of the kinematic model for crustal formation in Iceland (Pálmason 1980) onto the E Greenland dipping reflector sequence demonstrates a striking similarity between the two structures. However, volcanic productivity rate within the oldest part of the E Greenland dipping reflector sequence may be as much as three times the volcanic productivity rate recorded in Iceland with an original rift width equal to, or somewhat less than, that in Iceland. The high volcanic productivity rate caused the development of a thick extrusive upper crust (> 5–6 km) dominated by seawards-dipping reflections arising from lava flows or groups of flows which acquired their dip through postdepositional differential subsidence towards the rift zone. Refraction seismology defines a fairly flat-lying velocity zonation of the igneous crust with an anomalously thick layer 2 (3–5.5 km) in areas of well-developed dipping reflectors. The layer 2/3 boundary is seen to cut strongly across the dipping reflectors suggesting a metamorphic origin of this boundary. Initiation of seafloor spreading above sea-level is seen as a result of early upwelling of anomalously hot asthenospheric material that was able to ascend through a relatively mechanically unstretched crust and lithosphere and create a thick extrusive upper crust. Formation of a thick extrusive upper crust above sea-level only continued beyond the early spreading phase in the area of the Icelandic hot-spot (Iceland-Greenland Ridge).

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