Structural evolution of an A‐type subduction zone, lofoten‐Rombak Area, northern Scandinavian Caledonides

The climactic phase of the Caledonian orogeny in Scandinavia involved collision of the Baltic and Greenland cratons in late Ordovician (?) to Silurian time. We infer that the collision resulted in large‐scale subduction of the leading edge of the Baltic craton beneath an ‘Andean‐type’ Greenland continental margin. Deep erosional levels in the Lofoten‐Rombak area, Norway‐Sweden (˜69°N) permit direct examination and evaluation of the structural evolution of a continental or A‐type subduction zone. The early stages of collision involved en bloc underthrusting of the continental margin to depths of at least 30 km. During this process, deformational effects in the lower plate were restricted to its upper few hundred meters. Ultimately, en bloc A‐type subduction should be limted by buoyancy effects. As convergence continued, the lower plate became involved in a series of imbricate thrusts with the same vergence as the intial subduction zone. The temporal and spatial relationships between these basement‐involved thrusts are similar to those observed in foreland fold and thrust terrains. Field and geophysical data bearing on lower plate behavior elsewhere in the Caledonldes and in other collisional orogens suggest that the structural features observed in the Lofoten‐Rombak area may be generally characteristic of A‐type subduction in continent‐continent collisional settings.