Evidence for preservation of crustal root beneath the Proterozoic Lapland-Kola orogen (northern Fennoscandian shield) derived from P and S wave velocity models of POLAR and HUKKA wide-angle reflection and refraction profiles and FIRE4 reflection transect

We present an analysis of the crust and upper mantle in the northern Fennoscandian shield, based on new P and S wave 2-D velocity models of the POLAR and HUKKA wide-angle reflection and refraction profiles and results of a new seismic reflection experiment in Finland (Finnish Reflection Experiment). The profiles are almost collocated and crossed the Proterozoic Lapland-Kola orogen. A substantial difference of the depth of the wide-angle Moho'' (40-42 km) and the reflection Moho'' (47-50 km) was found. In order to explain this difference, we compared the velocity models to published values of Vp and Vp/Vs for the main types of lower crustal and mantle rocks. We found that the main reason for disagreement is that the wide-angle Moho and the reflection Moho correspond to different petrological boundaries. In the southwest and northeast portions of the profiles, the wide-angle Moho marks contact of either anorthositic or granulitic lower crust with a reflective layer in the upper mantle composed of peridotites and pyroxenites. The reflection Moho represents the bottom of this lower layer. In the center of the profile the wide-angle Moho marks the top of a large eclogitic body in the upper mantle, representing a well-preserved crustal root beneath the Lapland-Kola orogen formed because of the collision of three former Archaean crustal blocks (terranes or microcontinents). Lack of postorogenic tectonic collapse suggests another mechanism for stabilization of the lithosphere in the area. Upper mantle reflectors at depth of 65-75 km may mark the upper boundary of the cold and mechanically strong Archaean upper mantle wedge. Alternatively, these reflectors may represent a top of uplifted asthenosphere that can explain preservation of crustal root.