**3. Flexure-Related Stress E**ff**ects**

As mentioned above, the flexural stresses that occur during glaciations will be related to the isostatic response of the lithosphere, which is a time-dependent process. The lithosphere is subsiding under the weight of ice, and uplifted back to original state during unloading. During this process, the stresses in the elastic part of the lithosphere will change.

The estimation of stress regime in the lithosphere due to flexural stress during glaciations depends on the timing of crustal stress equilibrium. Two elastic models have been proposed for the estimation of stress changes by flexural stresses during glaciations. Stephansson [42] assumes that the lithosphere is in equilibrium before the ice load is introduced (Figure 4, left). The resulting stress state during ice loading then becomes compressive underneath the ice sheet, and extensional outside the margin of the ice sheet (forebulge area). During ice withdrawal, the horizontal stresses slowly diminish until the uplift ceases and a new equilibrium is reached. Stein et al. [43] present a model where the lithosphere is assumed to have reached equilibrium after some time of ice loading (Figure 4, right). Thus, during melting and uplift the area originally covered by ice experiences extensional stresses, and the area outside the former margin of the ice sheet becomes subject to compressive stresses. These stress changes decrease with depth and are inverted in the lower half of the lithosphere.

If we assume that the duration of ice loading and unloading is long enough for the lithosphere to reach equilibrium during both glaciation and deglaciation, then both models are applicable during one glacial cycle. The average duration of glaciations and interglacials during Quaternary has been 40,000 to 100,000 years. The last ice age ended around 10,000 years ago, and at present the crust is nearly in isostatic equilibrium. The assumption of crustal equilibrium during both glaciations and interglacials is therefore not unrealistic, as the relaxation time for the Fennoscandian uplift is less than 4000 years [34,44].

**Figure 4. Left**: Flexural stress model ([42]) assuming crustal stress equilibrium prior to the ice load. The resulting stresses are large during loading and diminish during unloading. The resulting stress state is compressional beneath the ice sheet and extensional in the hinge zones. **Right:** Flexural stress model ([43]) assuming crustal stress equilibrium after a period of ice loading. From [28].
