Evidence for gravity subsidence and granite diapirism in the 1.8-1.9 Ga Proterozoic succession of W. Bergslagen, Sweden

Abstract

Western Bergslagen, Central Sweden contains a number of large scale features including long, narrow, synformal, sediment-filled basins, separated by wide intervening areas of felsic metavolcanics in which anticlinal structures are absent. Synvolcanic granites intrude the felsic metavolcanics. Bedding, foliation and mineral lineations are sub-parallel both where bedding is sub-vertical or more rarely sub-horizontal. These features, taken in their geological context, are consistent with a dynamic system in which granite diapirism and gravity tectonic processes operated. Previous models (Oen et al., 1982; Oen, 1987) emphasize the continental rift setting of Bergslagen. We propose the following four stage tectono-magmatic model to account for the structure of the area: Phase 1: A primary crust forming event at about 2.1.Ga. Phase 2: Subsequent attenuation produced rifting accompanied by melting of the lower crust to give large scale felsic volcanism, contained predominantly in sinking grabens of a wider rift structure. Granite diapirism was initiated. Phase 3: Tectonic inversion followed, with an uplift of the graben floors, and a higher emplacement of the granite diapirs. At the same time a second generation of rift basins evolved on the flanks of the updomed areas, to be filled with debris derived from the felsic volcanics. A gravity instability developed as the heavier sediments filled the grabens, contributing to the overall tectonic process. Phase 4: A younger event of granitic magmatism. The development of the sediment filled rift basins is the surficial expression of the deeper gravity tectonic system.