Understanding the structure of the ocean-continent transition (OCT) in passive margins is greatly enhanced by comparison with onshore analogues. The North Atlantic margins and the “fossil” system in the Scandinavian Caledonides show variations along strike between magma-rich and magma-poor margins, but are different in terms of exposure and degree of maturity. They both display the early stages of the Wilson cycle. Seismic reflection data from the mid-Norwegian margin combined with results from Ocean Drilling Program Leg 104 drill core 642E allow for improved subbasalt imaging of the OCT. Below the Seaward-Dipping Reflector (SDR) sequences, vertical and inclined reflections are interpreted as dike feeder systems. High-amplitude reflections with abrupt termination and saucer-shaped geometries are interpreted as sill intrusions, implying the presence of sediments in the transition zone beneath the volcanic sequences. The transitional crust located below the SDR of the mid-Norwegian margin has a well-exposed analogue in the Seve Nappe Complex (SNC). At Sarek (Sweden), hornfelsed sediments are truncated by mafic dike swarms with densities of 70%–80% or more. The magmatic domain extends for at least 800 km along the Caledonides, and probably reached the size of a large igneous province. It developed at ca. 600 Ma on the margin of the Iapetus Ocean, and was probably linked to the magma-poor hyperextended segment in the southern Scandinavian Caledonides. These parts of the SNC represent an onshore analogue to the deeper level of the mid-Norwegian margin, permitting direct observation and sampling and providing an improved understanding, particularly of the deeper levels, of present-day magma-rich margins.

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