Granite studies I. A gravity investigation of two Precambrian granites in South Norway.

Abstract

The Bouguer gravity anomaly map, based on 778 stations, covers the Precambrian rocks along the southeast coast of Norway from the Oslo region to Kristiansand. The principal features of the map are the low gravity gradient over the Telemark rocks and the steep gradient over the Babmle rocks, upon which are superposed the anomalies of smaller geological bodies. The Bamble anomalies, which increase toward the coast, attain a value of +-50 mgal along the coast. Negative gravity anomalies occur over the late- to postkinematic Herefoss and Grimstad granites and the synkinematic Oddersjå granite, and a positive gravity anomaly occurs over the nickeliferous Evje amphibolite. The gravity gradients over the Telemark and Bamble rocks change at the trace of the \"great friction breccia\" that separates the Telemark and the Bamble rocks. A wedge of more dense material that thickens from the breccia to the coast is postulated as the source of the steep Bamble gravity gradient. The Wedge corresponds to the succession of the Bamble supracrustal rocks overlying the Telemark granitic gneisses. The throw at the breccia zone is calculated to be a minimum figure of 0.5 km. A residual gravity anomaly map is presented for the Herefoss and Grimstad granites and vicinity. The Grimstad granite has an anomaly of -13 mgal.