3-DIMENSIONAL IMAGE OF THE MOHO UNDULATIONS BENEATH THE GULF OF BOTHNIA USING WIDE-ANGLE SEISMIC DATA

The BABEL marine seismic experiment has been carried out to investigat e the lithospheric structure and antecedent tectonic signatures of the Baltic Shield, including the Archaean-Proterozoic collisional structu re in the northern part of the Gulf of Bothnia. Lithospheric seismic-r eflection streamer data and simultaneously recorded wide-angle reflect ion and refraction data collected in the Gulf of Bothnia as part of th e BABEL project have been used for 3-D modelling. The distribution of land stations around the Gulf provides a good 3-D ray coverage of the PMP reflection data recorded at the eight stations in the area and all ows an estimation of strikes and dips of the Moho boundary in the area . The traveltimes of reflected phases are calculated using a method th at utilizes the finite-difference solution of the eikonal equation. Th e Moho wide-angle-reflection (PMP) traveltimes are modelled using an i nversion method. A 2-D model from the Gulf of Bothnia extended into th e third dimension is used as an initial model. During the inversion th e velocity is kept constant and only the Moho boundary is allowed to v ary. To estimate the strike of the Moho boundary and the stability of the inversion, two initial models with different strikes are examined. \ The results indicate that the Moho depth in the Gulf of Bothnia undu lates and has a maximum depth of 55 km in the south, rising to 42 km i n the north, The Moho depth variations seem to be step-like. This chan ge in the Moho depth coincides with the location of the presumed fossi l subduction zone in the area. The crustal-thickness variations seem t o be well approximated by a nearly 2-D structure striking parallel to a postulated subduction zone immediately to the south of the Skellefte area, The presence of the step at the crust/mantle boundary can be in terpreted as a result of a plate-collision event at about 2 Ga.