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.