Lithosphere and upper-mantle structure of the southern Baltic Sea estimated from modelling relative sea-level data with glacial isostatic adjustment

摘要

During the last glacial maximum, a large ice sheet covered Scandinavia, whichdepressed the earth's surface by several 100 m. In northern central Europe,mass redistribution in the upper mantle led to the development of aperipheral bulge. It has been subsiding since the begin of deglaciation dueto the viscoelastic behaviour of the mantle.We analyse relative sea-level (RSL) data of southern Sweden, Denmark,Germany, Poland and Lithuania to determine the lithospheric thickness andradial mantle viscosity structure for distinct regional RSL subsets. We loada 1-D Maxwell-viscoelastic earth model with a global ice-loadhistory model of the last glaciation. We test two commonly used icehistories, RSES from the Australian National University and ICE-5G from theUniversity of Toronto.Our results indicate that the lithospheric thickness varies, depending on theice model used, between 60 and 160 km. The lowest values are found in theOslo Graben area and the western German Baltic Sea coast. In between,thickness increases by at least 30 km tracing the Ringk?bing-Fyn High. InPoland and Lithuania, lithospheric thickness reaches up to 160 km. However,the latter values are not well constrained as the confidence regions arelarge. Upper-mantle viscosity is found to bracket[2–7] × 1020 Pa s when using ICE-5G. Employing RSES muchhigher values of 2 × 1021 Pa s are obtained for the southernBaltic Sea. Further investigations should evaluate whether this ice-model versionand/or the RSL data need revision. We confirm that the lower-mantle viscosityin Fennoscandia can only be poorly resolved.The lithospheric structure inferred from RSES partly supports structuralfeatures of regional and global lithosphere models based on thermal orseismological data. While there is agreement in eastern Europe and southwestSweden, the structure in an area from south of Norway to northern Germanyshows large discrepancies for two of the tested lithosphere models. Thelithospheric thickness as determined with ICE-5G does not agree with thelithosphere models. Hence, more investigations have to be undertaken tosufficiently determine structures such as the Ringk?bing-Fyn High as seenwith seismics with the help of glacial isostatic adjustment modelling.