BASEMENT-INVOLVED CONTRACTIONAL TECTONICS IN THE CALEDONIDES OF NORTH-CENTRAL SCANDINAVIA

The Caledonides of north-central Scandinavia are dominated by a series of crystalline thrust sheets (“nappes”) that structurally overlie autochthonous and internally imbricated crystalline basement and cover rocks. Late stage, ESE-directed translation of the assembled nappe stack occurred across the margin of Baltica along a major greenschist facies basal shear zone in a largely passive, piggy-back style. At least one major late-stage Caledonian thrust (Øse thrust) also causes regional scale repetition and truncation at higher structural levels within the Caledonian tectonostratigraphy. Hanging-wall and footwall geometries indicate that the initiation and progressive development of this thrust closely relates to imbrication of underlying basement rocks, partly through reactivation of pre-Caledonian structures.

Structures recorded in these late Caledonian thrust zones are dominated by complex systems of W- or NW-vergent back-folds. Previously, these have been interpreted as resulting from late-stage extensional backsliding of the orogen. In the basal thrust zone, however, back-folds of differing geometry vary with position relative to major culminations in the underlying basement. Along moderately dipping western margins to the culminations, back-folds are typically open to tight, moderately inclined structures that refold top-to-the-SE shear indicators. A SE-dipping spaced contractional crenulation foliation is often also developed. Eastwards of the monoclinal “hinge” to the basement culminations, back-folds typically become dismembered through superimposed ESE-directed shearing. A further characteristic of the late-stage Caledonian folding in this area is the development of transport parallel (WNW-ESE) “cross-folds”, again with a demonstrable spatial association to basement culminations and pre-Caledonian structures within the basement.

A model is developed to account for these structural geometries during Late Caledonian contractional tectonics. This involves progressive ESE-directed thrust sheet translation over evolving basement ramp-duplex systems. It is concluded that major Caledonian extensional collapse structures are largely absent in north-central Scandinavia. Rather the late orogenic structural geometries are dominated by basement-involved contractional tectonics, probably through to the Late Devonian.