The Fennoscandian uplift and late cenozoic geodynamics: geological evidence |
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| Authors: | N-A Mörner Dr |
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| Institution: | (1) Geological Institute, Stockholm University, Box 6801, S-11386 Stockholm, Sweden |
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| Abstract: | The history of the recording and interpretation of the Fennoscandian uplift illustrates the main history of Earth sciences because the results obtained had (and still have) immediate impact of the interpretation of a large number of fundamental problems in Earth sciences. Thanks to a paper of De Geer in 1888, the glacial isostatic origin was established. Fennoscandia became the classic area of glacial isostasy, and its sea level records were used for geophysical calculations of the properties and dynamics of the mantle and crust. The varve dated sea level curve of Lidén (1938) from the center of uplift provided an exceptionally well dated record. With the radiocarbon method, the records of shorelines and shorelevel displacement curves were drastically improved providing a totally new basis for the understanding of the geodynamics of the Fennoscandian uplift and for the geophysical interpretation of the data obtained. This is especially true in combination with the repeated levelling data obtained during the last decades for Finland and Sweden.The Late Cenozoic long term movements of the Fennoscandian Shield are characterized by a considerable subsidence. The postglacial uplift of Fennoscandia is complex (an exponential and a linear factor) and caused by two different mechanisms. The total absolute movement in relation to the last glaciation is an elliptic uplift cone of 830 m height surrounded by a subsidence through of 170 m height. The mass in the uplift cone and in the subsidence through is as 1:1 with a volume 0.7 × 106km3. The disappearence/appearance of mass give evidence of a mass transfer in a low viscosity asthenosphere. The properties and conditions of the asthenosphere are found to be: 1–10 × 1020 Poises in viscosity, 3 × 10–14 – 3 × 10–16 sec–1 in strain rates, 0.7% of the melting temperature, 3 mm in grain size, and 5–0.4 bar in stress. The main isostatic uplift (the exponential factor) originates from an asthenospheric dislocation glide process which in early-mid Holocene time changed over into a diffusion creep process. The present linear uplift factor (identified through the last 8000 yrs) seems to originate from mesospheric motions under the following approximate conditions: 0.6% of the melting temperature, 2 × 1022 Poises in viscosity, 3 × 10–16 sec–1 in strain rates and 8 bars in stress. Uplift irregularities and neotectonism are frequently established and often reveal an old geodynamic inheritance (e.g. the Pre-Baikalian/Gothian bedrock seam of high geodynamic activity). The peak rates of glacial isostasy are associated with intensive fracturing, faulting and seismic activity. |
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