EUNAseis:a seismic model for Moho and crustal structure in Europe, Greenland, and the North Atlantic region

AbstractWe present a new digital crustal model for Moho depth and crustal structure in Europe, Greenland, Iceland, Svalbard, European Arctic shelf, and the North Atlantic Ocean (72W–62E, 30N–84N). Our compilation is based on digitization of original seismic profiles and Receiver Functions from ca. 650 publications which provides a dense regional data coverage. Exclusion of non-seismic data allows application of the database to potential field modeling. EUNAseis model includes Vp velocity and thickness of five crustal layers, including the sedimentary cover, and Pn velocity. For each parameter we discuss uncertainties associated with theoretical limitations, regional data quality, and interpolation.By analyzing regional trends in crustal structure and links to tectonic evolution illustrated by a new tectonic map, we conclude that: (1) Each tectonic setting shows significant variation in depth to Moho and crustal structure, essentially controlled by the age of latest tectono-thermal processes; (2) Published global averages of crustal parameters are outside of observed ranges for any tectonic setting in Europe; (3) Variation of Vp with depth in the sedimentary cover does not follow commonly accepted trends; (4) The thickness ratio between upper-middle (Vp<6.8km/s) and lower (Vp>6.8km/s) crystalline crust is indicative of crustal origin: oceanic, transitional, platform, or extended crust; (5) Continental rifting generally thins the upper-middle crust significantly without changing Vp. Lower crust experiences less thinning, also without changing Vp, suggesting a complex interplay of magmatic underplating, gabbro-eclogite phase transition and delamination; (6) Crustal structure of the Barents Sea shelf differs from rifted continental crust; and (7) Most of the North Atlantic Ocean north of 55°N has anomalously shallow bathymetry and anomalously thick oceanic crust. A belt of exceptionally thick crust (ca. 30km) of probable oceanic origin on both sides of southern Greenland includes the Greenland–Iceland–Faeroe Ridge in the east and a similar “Baffin Ridge” feature in the west

Seismic explosion sources on an ice cap – technical considerations

Controlled source seismic investigation of crustal structure below ice covers is an emerging technique. We have recently conducted an explosive refraction/wide-angle reflection seismic experiment on the ice cap in east-central Greenland. The data-quality is high for all shot points and a full crustal model can be modelled. A crucial challenge for applying the technique is to control the sources. Here, we present data that describe the efficiency of explosive sources in the ice cover. Analysis of the data shows, that the ice cap traps a significant amount of energy, which is observed as a strong ice wave. The ice cap leads to low transmission of energy into the crust such that charges need be larger than in conventional onshore experiments to obtain reliable seismic signals. The strong reflection coefficient at the base of the ice generates strong multiples which may mask for secondary phases. This effect may be crucial for acquisition of reflection seismic profiles on ice caps. Our experience shows that it is essential to use optimum depth for the charges and to seal the boreholes carefully

Moho and magmatic underplating in continental lithosphere

AbstractUnderplating was originally proposed as the process of magma ponding at the base of the crust and was inferred from petrologic considerations. This process not only may add high density material to the deep crust, but also may contribute low density material to the upper parts of the crust by magma fractionation during cooling and solidification in the lower crust. Separation of the low density material from the high-density residue may be a main process of formation of continental crust with its characteristic low average density, also during the early evolution of the Earth. Despite the assumed importance of underplating processes and associated fractionation, the available geophysical images of underplated material remain relatively sparse and confined to specific tectonic environments. Direct ponding of magma at the Moho is only observed in very few locations, probably because magma usually interacts with the surrounding crustal rocks which leads to smearing of geophysical signals from the underplated material. In terms of processes, there is no direct discriminator between the traditional concept of underplated material and lower crustal magmatic intrusions in the form of batholiths and sill-like features, and in the current review we consider both these phenomena as underplating. In this broad sense, underplating is observed in a variety of tectonic settings, including island arcs, wide extensional continental areas, rift zones, continental margins and palaeo-suture zones in Precambrian crust. We review the structural styles of magma underplating as observed by seismic imaging and discuss these first order observations in relation to the Moho