Postglacial crustal doming, stresses and fracture formation with application to Norway

Postglacial doming of Fennoscandia is estimated to have reached a maximum uplift value of 850 m. Vertical uplift of this magnitude generates stresses that affect the tectonic activity in the domed area. Using a simple model of a circular, elastic plate subject to deflection, it is shown that the maximum tensile stresses generated by this uplift occur in the centre of the dome, and at the surface reach a numerical value of nearly 30 MPa. This tensile stress is an order of magnitude greater than the typical in-situ tensile strength of the uppermost part of the crust, so that tension fractures are easily generated by this stress. Tension fractures can, under certain conditions, reach crustal depths of several hundred metres, but if they propagate to greater depths they change into normal faults. The tensile stresses at the surface decrease in magnitude with distance from the centre of the dome and become compressive in the marginal parts of the uplifted crustal plate. At the surface of these parts, the maximum shear stress exceeds 10 MPa. Because the inferred driving stresses (stress drops) in earthquakes are commonly only several MPa, it follows that the driving shear stress in these marginal parts is large enough to initiate or reactivate seismogenic faults. These model results are in broad agreement with data on the current seismotectonics of Fennoscandia. The formation and reactivation of tensile and shear fractures in Fennoscandia as a result of postglacial uplift is likely to increase the hydraulic conductivity of the solid rocks that constitute the uppermost part of the crust. This conclusion is supported by reported observations which indicate that the production of groundwater in wells in many areas in Norway varies positively with the maximum postglacial uplift. (C) 1999 Elsevier Science B.V. All rights reserved.