PROTEROZOIC SUTURES AND TERRANES IN THE SOUTHEASTERN BALTIC SHIELD INTERPRETED FROM BABEL DEEP SEISMIC DATA

A hitherto unknown terrane and its bounding sutures have been revealed by a combined study of normal-incidence and wide-angle seismic data a long the BABEL profile in the Baltic Sea. This Intermediate Terrane is situated between a Northern Terrane of Svecofennian age and a Southwe stern Terrane of Gothian age. It is delimited upwards by two low-angle and oppositely dipping sutures and occupies mainly middle and lower c rustal levels with a width of similar to 300 km at Moho level. The sim ilar to 1.86 Ga suture against the Northern Terrane is imaged by a pro minent almost continuous NE-dipping crustal reflection from 3.5 to 14 s twt over 175 km. Where it downlaps on the Moho, sub-Moho velocities change from 8.2 to 7.8 km/s (+/-0.2) over less than 25 km. A relativel y strong, NE-dipping normal-incidence and wide-angle reflection at 19- 23 s twt indicates that the suture extends into the upper mantle. The pervasive NE-dipping reflection fabric of the Intermediate Terrane is interpreted as shear zones that developed during collision and possibl y were reactivated by later events. High Poisson's ratios suggest a ma fic composition or high fluid content. The similar to 1.86 Ga collisio n was probably succeeded by continental break-up and removal of an unk nown continent, except for the Intermediate Terrane, Subsequent format ion of an east-dipping subduction zone further to the west led to the emplacement of 1.81-1.77-Ga-old granitoids in the southern part of the Transscandinavian Igneous Belt. The similar to 1.65-1.60 Ga suture ag ainst the Southwestern Terrane is defined by a semi-continuous band of strong SW-dipping reflections between 3 and 8 s twt over 65 km, which are interpreted as a law-angle thrust zone along which Gothian crust overrode the Intermediate Terrane. The identification of three individ ual seismic terranes in the southeastern part of the Baltic Shield pro vides new evidence for Palaeoproterozoic plate tectonic processes.