Linkage of crustal deformation between the Archaean basement and the Proterozoic cover in the Peräpohja Area, northern Fennoscandia

Abstract

Reconstructed variations in stratigraphic thicknesses combined with mapped variations in fold-and-thrust belt structural styles and patterns of the entire Palaeoproterozoic Peräpohja Belt in northern Finland show that both the deposition of the 2.4–1.9 Ga supracrustal rocks and the structural evolution during the subsequent Svecofennian Orogeny (1.9–1.8 Ga) have been tightly controlled by (i) the palaeo-topography of the Archaean (>2.5 Ga) basement, and (ii) the reactivation of Archaean basement faults during the multiple stages of rifting and deposition of the supracrustals, as well as during subsequent basin inversion and fold-and-thrust belt development. Folds and thrusts in the supracrustal sequence are found to be largely detached from the underlying basement along the Petäjäskoski Detachment Zone (PDZ) that formed within a mechanically weak stratigraphic unit in the basal part of the Peräpohja Belt stratigraphy. The pattern of contractional structures is complex, but developed during one phase of simple, progressive south-vergent thrusting. Two styles of basement-cover linkage are recognised: The hard-linked style is best illustrated by the reverse Sihtuuna Fault currently defining the northern margin of the E-W to ENE-WSW trending basement horsts and the overlying anticlines, and juxtaposing the stratigraphically lowermost Peräpohja Belt rocks with higher-up stratigraphy, indicating the fault is inherited after a pre-existing basement structure. Based on the distribution and thickness variation of the Palaeoproterozoic supracrustal units, the faults bounding the basement horst were active as extensional faults during deposition. The soft-linked style relationship is evident as abrupt terminations of thrust-related elongate domes and localisation of the compressional overprint into the NW-SE trending central graben. Understanding the basement architecture and basement-cover linkages is highly significant because the majority of the known mineralizations of the belt are located along or close to the basement faults.