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

摘要

Reconstructed variations in stratigraphic thicknesses combined with mapped variations in fold-and-thrust belt structural styles and patterns of the entire Palaeoproterozoic Perapohja 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 Petajaskoski Detachment Zone (PDZ) that formed within a mechanically weak stratigraphic unit in the basal part of the Perapohja 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 Perapohja 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 th