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Summary:
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The last ice age of Fennoscandinavia continues to have geological repercussions across Finland despite the last ice having retreated almost 10,000 years ago: land uplift, shoreline retreat, and the stress state of the crust continues to evolve. This report focusses on the glacial rebound signals for Finland and the Gulf of Bothnia and explores the consequences of the ongoing deformation. The rebound signals include the geological evidence as well as instrumental observations: the tide gauge and lake-level measurements of the past century, the changes in geodetic levels recorded in the repeat levelling surveys of the region and the direct measurement of crustal deformation (radial and horizontal) using high-precision space-geodesy measurements. These signals provide constraints on the Earth's rheology, its elasticity and viscosity, and the glacial history of the region. Once observationally constrained, the rebound models are used to predict both the ongoing evolution of shorelines and the changing state of stress within the crust.
This report covers:
(i) A review of glacial rebound modelling for Scandinavia (Sections 2 & 3).
(ii) Review of observational evidence relating to sea-level change and crustal rebound (Section 4).
(iii) New earth and ice-sheet model results from the inversion of the geological evidence for sea-level change, including models of shoreline evolution (Sections 5 & 6).
(iv) Earth-model results from the inversion of the geodetic evidence for sea-level change (Section 7).
(v) Development of crustal stress models for past and present stress states (Section 8).
(vi) Conclusions and recommendations (Section 9).
Specific conclusions reached pertain to:
(i) Thickness of ice cover over Scandinavia since the Last Glacial Maximum, particularly for the Lateglacial period.
(ii) Sea-level change and shoreline evolution for the Baltic area since the time the region became ice-free for the last time.
(iii) The predicted rates of present-day crustal rebound and sea-level change within the Baltic Basin.
(iv) The magnitude and orientation of the regional stress field in the crust since the time of the last glaciation, including the present-day residual stress field.
The current models have reached a state where realistic regional stress patterns can be predicted that provide the background data for developing high-resolution local numerical models. This study is confined to the regional stress field only. We use as a measure of the significance of the incremental stress the change in Fault Stability Margin (FSM) resulting from the changes in ice (and ocean) load with time. While ice is present, the crust is stabilized except outside the area of loading, but when the ice retreats the crust becomes less stable and faults that are close to failure may be reactivated. For much of central Finland the deviatoric FSM reach their maximum values at about 10 ka BP but the magnitudes relax with time and at present they represent about 20% of their original values. Thus any failure within the crust triggered by glacial loading and unloading will have occurred preferentially when the region first became ice-free and the potential for reactivating faults today by this process must be considered as negligibly small.
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