Tectonic significance of present-day stress relief phenomena in formerly glaciated regions

Investigations were conducted in several regions of Norway with the purpose of detecting and measuring stress-relief features and to derive from them valuable information on the shallow-crustal stress state. Stress-relief features are induced by blasting and sudden rock unloading in road construction and quarrying operations, and are common in Norway and probably in other regions of Fennoscandia, as they are in NE America. Stress relief at the Earth's surface is diagnostic of anomalously high stress levels at shallow depths in the crust and characterizes the formerly glaciated Baltic and Canadian Precambrian shields. The mean orientation of the maximum horizontal compressive stress axis, as deduced from the orientation of stress-relief features, was found to be NW-SE, consistent with North Atlantic ridge-push forces. We show that our determined stress orientations are in excellent agreement with other kinds of stress data. As a final step, we estimate ridge-push force magnitudes from gravity data in the North Atlantic region. Our computations suggest that strong ridge-push forces characterize the Baltic and Canadian shields. We conclude from this quantitative analysis and our fieldwork results that present-day stress relief, in Fennoscandia and NE America, is mostly triggered by plate-scale ridge-push forces and not by residual glacial loading stresses.