Relative and absolute timing of tectonothermal events in the Pohjanmaa Belt: A structural study of the Paleoproterozoic coupled Bothnian oroclines, Finland.

Date

2018-04-30

Authors

Dawson, Travis

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Abstract

Studies on the formation of oroclines (bends of originally linear mountain belts) are restricted to Phanerozoic examples. Here I provide a structural, metamorphic and geochronological study of the meta-sedimentary Pohjanmaa Belt, which lies on the most northerly limb of one of the few examples of Paleoproterozoic oroclines, the coupled Bothnian oroclines in Finland. My primary goal is to determine if the structures in the Pohjanmaa belt are consistent with the Svecofennian orogenic belt originating as a linear feature that was subsequently deformed into a pair of coupled oroclines. My detailed structural mapping focused on the geometry of F1, F2, and F3 folds within the Pohjanmaa Belt in west-central Ostrobothnia, Finland. Over 170 measurements were collected including interlimb angles, the attitudes of S1 and S2 foliations, and F2 fold axes. Also, 28 oriented core samples were collected to conduct a U-Pb geochronological study of monazites and an analysis of metamorphic textures in thin section. I first provide a review and comparison of similar structures around the coupled Bothnian oroclines. In my structural analysis I address the relative timing of deformation and metamorphism and use new U-Pb monazite geochronological data to constrain the absolute timing of tectonothermal events. My findings suggest that: 1) D1, D2, D3 deformation stem from a protracted event that records progressive deformation and strain partitioning from pure shear (D1 shortening) to simple shear (D2, D3 sinistral shear) as a result of counter clockwise rotation; 2) D2 folding and coeval garnet + staurolite metamorphism are consistent with early Svecofennian deformation, which occurred 80-90 m. y. prior to late-stage isothermal decompression and staurolite breakdown dated at 1.80 Ga by U-Pb monazite analysis; 3) My analysis, in combination with subsidiary data, provides evidence that sinistral shear during D2-3 is the result of counter clockwise rotation of the Pohjanmaa belt possibly in response to buckling of the coupled Bothnian oroclines during the early Svecofennian Fennia event.

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Keywords

Finland, Geology, Structural Geology, Orocline

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