Evidence for preservation of crustal root beneath the ProterozoicLapland-Kola orogen (northern Fennoscandian shield) derivedfrom P and S wave velocity models of POLAR and HUKKAwide-angle reflection and refraction profiles and FIRE4reflection transect

摘要

We present an analysis of the crust and upper mantle in the northern Fennoscandianshield, based on new P and S wave 2-D velocity models of the POLAR and HUKKAwide-angle reflection and refraction profiles and results of a new seismic reflectionexperiment in Finland (Finnish Reflection Experiment). The profiles are almost collocatedand crossed the Proterozoic Lapland-Kola orogen. A substantial difference of the depthof "the wide-angle Moho" (40-42 km) and "the reflection Moho" (47-50 km) wasfound. In order to explain this difference, we compared the velocity models to publishedvalues of Vp and Vp/Vs for the main types of lower crustal and mantle rocks. Wefound that the main reason for disagreement is that the wide-angle Moho and the reflectionMoho correspond to different petrological boundaries. In the southwest and northeastportions of the profiles, the wide-angle Moho marks contact of either anorthositic orgranulitic lower crust with a reflective layer in the upper mantle composed of peridotitesand pyroxenites. The reflection Moho represents the bottom of this lower layer. In thecenter of the profile the wide-angle Moho marks the top of a large eclogitic body in theupper mantle, representing a well-preserved crustal root beneath the Lapland-Kola orogenformed because of the collision of three former Archaean crustal blocks (terranes ormicrocontinents). Lack of postorogenic tectonic collapse suggests another mechanism forstabilization of the lithosphere in the area. Upper mantle reflectors at depth of 65-75 kmmay mark the upper boundary of the cold and mechanically strong Archaean uppermantle wedge. Alternatively, these reflectors may represent a top of uplifted asthenospherethat can explain preservation of crustal root.