THE ULTRAHIGH-PRESSURE ROCKS OF WESTERN NORWAY ARE ALLOCHTHONOUS

The two paradigms most commonly invoked for the exhumation of ultrahigh–pressure (UHP) rocks involve large-scale extension: either the rocks are exhumed as a coherent entity in the footwall of a single extensional structure, or they are extruded as buoyant crustal slivers bounded by an extensional fault above and a contractional fault below.

UHP rocks in western Norway formed during the Caledonian collision between the Baltica and Laurentia continental plates in the Late Silurian/Early Devonian (425–400 Ma). Previous tectonic models have assumed that the UHP rocks formed the leading edge of the Baltica margin, were subducted semi-intact to extreme depths of 135 km or more, and exhumed in the footwall of a large-scale extensional detachment, the Nordfjord–Sogn Detachment.

However, our detailed field mapping in the Nordfjord area has revealed that at least the southern part of the UHP province, on the Stadlandet peninsula, lies above this detachment. Therefore, the UHP province is allochthonous with respect to the Baltica basement farther east (Western Gneiss Region). Moreover, if the late-orogenic extensional displacement along the Nordfjord–Sogn Detachment is removed, the UHP rocks are restored to a structurally higher position much farther inland, overlying the lower-pressure basement. Thus, the Nordfjord–Sogn Detachment may have originally operated as a contractional fault during the collision, emplacing a deeper part of the telescoped Baltica margin, with UHP conditions preserved at lower levels, back over the Baltica autochthon.

This new interpretation suggests that extrusion of the subducted edge of Baltica from the mantle, perhaps synchronous with oblique plate divergence and orogenic extension, occurred during the latter period of the Caledonian orogeny. It also implies that a Caledonian extensional fault with >100 km vertical displacement lies offshore.