title: Deposition and deformation at a Lower Proterozoic cratonic margin, south-east Finland
Creators: name: Ward, Peter
Publisher: name: University of Glasgow
Year: 1985
Subjects: subject: Geology, sedimentary geology.

description: In south-eastern Finland, lower Proterozoic sediments were strongly deformed and metamorphosed to amphibolite facies during the Svecokarelian orogeny approximately 1.9 Ga. Three early schistosities and associated folds share an enveloping surface parallel to primary lithological layering whereas the six younger groups of structures recognized have steep attitudes and zonal distribution; only the earliest of these younger groups of structures are regionally significant in the study area. Deformation commenced with northwards translation of thrust nappes onto the adjacent Archaean eraton and continued during prograde metamorphism. Internal deformation of the metasedimentary sequence became progressively more intense, as recorded by recumbent F2 and F3 folds and S1, S2 and S3 schistosities, in particular differentiation layering. S3 development included mylonite formation with local transposition of Archaean structures near the basement-cover interface. Younger deformation is recorded by NW-trending ductile shear zones and minor folds (with rarer antithetic conjugate structures) followed by substantially vertical displacements along N-trending normal faults and shear zones. These are intrepeted dynamically as recording a progressive relative increase in vertical crustal stresses in response to gravitational and thermal disequilibrium induced by burial of Archaean basement beneath the earlier nappe structures and/or ultra-metamorphism and accumulation of buoyant magmas in the lower crust. In spite of this complex structural history, lithofacies investigations were practicable and resulted in the recognition of two distinct stratotectonic provinces that represent the deformed remnants of two separate, though possibly pene-contemporaneous basins. The more westerly Savo province contains chemogenic and mafic volcanogenic lithologies, serpentinites and graphitic metapelites but is dominated by monotonous quartz-plagioclase-biotite-garnet metapsammites. Rarely preserved sedimentary structures suggest that these metapsammites originated as thick-bedded mass flow deposits in a prograding submarine fan setting. Near the present eastern margin of the province, intercalated meta-psephites contain exclusively intraformational detritus consistent with deposition from debris flows and high-density turbidity currents in a submarine canyon or channelized inner fan environment. Abundant Na-plagioclase and biotite in Savo province metapsammites is considered to reflect sediment supply from a relatively K-poor terrane rich in chlorite-montmorillonite but petrographical criteria alone cannot discriminate between a possible Archaean provenance (with derivation from both greenstones and granitoids) or penecontemporaneous intermediate plutonic or bimodal volcanogenic sources. Preliminary Nd-Sm whole rock and U-Pb detrital zircon isotopic data indicate however, the likelihood of a sedimentary mixing of detritus of both Archaean and lower Proterozoic igneous origin. In contrast, the Hoytiainen province to the east contains better preserved and more diverse primary depositional features, with unambiguous evidence for a more local provenance, including Archaean granitoids and gneisses, Jatulian ortho-quartzites and penecontemporaneous metabasites. This diversity of source material is also reflected in a wider scatter in eNd ratios, including a component from penecontemporaneous tholeiitic volcanism derived from a mantle that was REE depleted with respect to CHUR. Near the present western margin of the province, coarse-clastic lithofacies are interpreted as proximal, partly channelized prograding fan sequences with several fining and thinning upward sequences recognized, passing upward and laterally into quartzose metapsammites less diagnostic of a suprafan - middle fan environment. Stratigraphical relationships with associated pelitic and chemogenic lithogies are obscure, due to complex deformation but the coarser lithologies may record a terrigenous clastic influx prograding across finer-grained and more distal deposits, correlating with the upper Jatulian - Kalevian disconformity and facies transition widely recognized in eastern Finland. On the basis of this dual intrabasinal volcanogenic and Archaean basement provenance the Hbytiainen province precursor is thus perceived as having been a linear intracratonic trough within a more extensive system of rifting or fragmentation of the Karelian cratonic margin during the period 2.1 - 1.9 Ga. The Savo province precursor is envisaged as a large submarine fan complex that was evidently flanked to the east by the Archaean craton, which locally acted as both depositional basement and source terrain but the ultimate nature and location of provenance for the bulk of the sediment remains problematical. Such en echelon basin development at the craton margin is independent from, though compatible with extant and conceivable plate-tectonic paradigms; a geometrical analogue for the Finnish Proterozoic is distilled from the Neogene of the Australia Banda arc region, incorporating continent-arc collision, a curvilinear subduction zone trace and transcurrent deformation but it is advocated that alternative mechanisms to crustal growth involving subduction and fore-arc accretion be considered tenable until a more comprehensive understanding of Finnish geology is achieved.