Active deformation in the eastern Swiss Alps: post-glacial faults, seismicity and surface uplift

Abstract

Post-glacial tectonic faults in the eastern Swiss Alps occur as single lineaments, clusters of faults or extensive fault zones consisting of several individual faults aligned along the same trend. The orientation of the faults reflects the underlying lithology and the pre-existing structures (joints, pervasive foliations) within these lithologies. Most post-glacially formed faults in the area around Chur, which undergoes active surface uplift of 1.6 mm/year, trend E–W and cut across Alpine and glacial features such as active screes and moraines. Additionally, there are NNW and ENE striking faults reactivating pervasive Alpine foliations and shear zones. Based on a comparison with the nodal planes of recent earthquakes, E–W striking faults are interpreted as active faults. Because of very short rupture lengths and mismatches of fault location with earthquake distribution, magnitude and abundance, the faults are considered to be secondary faults due to earthquake shaking, cumulative deformation in post- or interseismic periods or creep, and not primary earthquake-related faults. The maximum of recent surface uplift rates coincides with the youngest cooling of the rocks according to apatite fission-track data and is therefore a long-lived feature that extends well into pre-glacial times. Isostatic rebound owing to overthickened crust or to melting of glacial overburden cannot explain the observed surface uplift pattern. Rather, the faults, earthquakes and surface uplift patterns suggest that the Alps are deforming under active compression and that the Aar massif basement uplift is still active in response to ongoing collision.