A complex model of Mesoproteozoic sedimentary and igneous suites in a graben setting north of Gotland, Baltic Sea

Marine geological and geophysical investigations have revealed the complex nature of the deep Mesoproterozoic Strombus and Admete basins in graben settings. The Strombus Basin extends in a northwestern direction from northern Gotland to the Landsort Trench. In the northeast a less prominent tectonic basin, named the Admete Basin, flanks it. The Jotnian sandstone layer forms the bulk of the basin infill. In the Strombus Basin the thickness of it increases from 400 to 1600 m towards the southwest, but is stable c. 500 m in the Admete Basin. Regional gravity surveying has revealed distinct gravity lows confined to the basins. According to the geophysical modelling, the Strombus Basin gravity low fits best with the model of the 500 to 1500 m thick Jotnian sandstone layer underlain by an approximately 5 km thick minor rapakivi pluton. The pronounced linear magnetic high, which conforms to the western and southwestern boundaries of the basin, can be interpreted as a large mafic dike or sheet connected to the fault delimiting the basin. A distinct magnetic high of about +800 nT indicates the presence of highly magnetic rocks northeast of the Strombus Basin. In their origin and age of the mafic intrusions may be related to the Subjotnian rapakivi-related gabbro-anorthosite. Alternatively, they may be related to the Jotnian mafic intrusions. The grabens filled with Jotnian have inherited their positions from deep-seated rapakivi intrusions. They are probably derived from the extensional faulting causing the Postjotnian diabase magmatism