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

Postglacial doming of Fennoscandia is estimated to have reached a maximum u plift value of 850 m. Vertical uplift of this magnitude generates stresses that affect the tectonic activity in the domed area. Using a simple model o f a circular, elastic plate subject to deflection, it is shown that the max imum tensile stresses generated by this uplift occur in the centre of the d ome, and at the surface reach a numerical value of nearly 30 MPa. This tens ile stress is an order of magnitude greater than the typical in-situ tensil e strength of the uppermost part of the crust, so that tension fractures ar e easily generated by this stress. Tension fractures can, under certain con ditions, reach crustal depths of several hundred metres, but if they propag ate to greater depths they change into normal faults. The tensile stresses at the surface decrease in magnitude with distance from the centre of the d ome and become compressive in the marginal parts of the uplifted crustal pl ate. At the surface of these parts, the maximum shear stress exceeds 10 MPa . Because the inferred driving stresses (stress drops) in earthquakes are c ommonly only several MPa, it follows that the driving shear stress in these marginal parts is large enough to initiate or reactivate seismogenic fault s. These model results are in broad agreement with data on the current seis motectonics 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 o bservations which indicate that the production of groundwater in wells in m any areas in Norway varies positively with the maximum postglacial uplift. (C) 1999 Elsevier Science B.V. All rights reserved.