Crust‐mantle boundary in the central Fennoscandian shield: Constraints from wide‐angle P and S wave velocity models and new results of reflection profiling in Finland

Abstract

Our paper presents analysis of the crust‐mantle boundary in the central Fennoscandian shield based on new P and S wave two‐dimensional velocity models of the SVEKA'81, SVEKA'91 and FENNIA wide‐angle reflection and refraction profiles and on results of a new seismic reflection experiment in Finland (Finnish Reflection Experiment (FIRE)). In this area, the crust is extremely thick (50–60 km), and the Moho boundary is difficult to detect using methods based on interpretation of P waves only (near vertical profiling and wide‐angle experiments). However, the S wave reflections from the Moho boundary (SmS) are frequently more pronounced in wide‐angle data than the P wave reflections (PmP). In order to infer the crust‐mantle transition, we developed new P and S wave velocity models using reprocessing of the old data and compared them to record sections of collocated reflection profiles and to published values of Vp and Vp/Vs for the main types of lower crustal and upper mantle rocks. On the basis of the lateral variations of Vp, Vp/Vs, and reflectivity in the lower crust and upper mantle, three main types of the crust‐mantle boundary were distinguished. The first type corresponds to eclogitized lower crust that overlies peridotitic upper mantle. In this case, the Moho coincides with the lithological crust‐mantle boundary. The second type corresponds to lower crust composed of mafic garnet granulites overlying the peridotitic upper mantle, for which the Moho and the lithologic crust‐mantle boundary coincide as well. The third type corresponds to the mafic garnet granulites underlain by a layer of eclogitic upper mantle. In the latter case, the lithological crust‐mantle boundary is deeper than the Moho.