Paleoproterozoic lateritic paleosol–ultra-mature/mature quartzite–meta-arkose successions in southern Fennoscandia—intra-orogenic stage during the Svecofennian orogeny

Abstract

A Paleoproterozoic lateritic paleosol overlain by ultra-mature/mature quartzite is reported for the first time from three localities in the Svecofennian province in southern Finland. These are important indicators for warm paleoclimate and continental paleoenvironment. Ultra-mature quartzites above the lateritic paleosols are overlain by matrix-supported conglomerates followed by mature quartzites. These latter rocks are related to incipient rifting followed by main rift stage starting with increasingly more immature meta-arkose. We present a tectonic model in which exhumation, after 1.88–1.87 Ga contraction, unroofed plutonic rocks. Volcanism and exhumation-related erosion produced deposition of both volcanic and sedimentary material. Weathering of these rocks and deposition of ultra-mature/mature quartzites took place between 1.87 and 1.85/1.84 Ga followed by rifting at 1.84 Ga. Tectonic slicing during basin inversion thrust the pre-rift quartzites and rift-related volcanic and sedimentary rocks over older mid-crustal rocks and produced a low-angle stacking system. To accommodate voluminous magmatism at deeper crustal levels and widespread sedimentation at the surface, we propose that exhumation due to faulting followed by stabilization of the upper crust (0–15/20 km) occurred simultaneously as the detached hot middle and lower crust experienced vertical shortening via ductile flow and extrusion. The ca. 1.85 Ga paleosol-quartzite successions in southern Fennoscandia have many similarities with the widespread ca. 1.7 Ga paleosol-quartzite successions in the United States. Both are intra-orogenic, formed on hot orogens and have simultaneous magmatism at deeper crustal levels and sedimentation at the surface. Accretion-type models presented for southern Fennoscandia at 1.87–1.80 Ga are not compatible with the available data and either far-field setting during outboard subduction or a newly formed continental interior setting are proposed. Anyhow tectonic models proposed for southern Fennoscandia between 1.87 and 1.80 Ga should take into account the occurrence of lateritic paleosols and ultra-mature quartzites.