CONTINENTAL BREAK-UP AND COLLISION IN THE NEOPROTEROZOIC AND PALEOZOIC - A TALE OF BALTICA AND LAURENTIA

During the Neoproterozoic and Palaeozoic the two continents of Baltica and Laurentia witnessed the break-up of one supercontinent, Rodinia, and the formation of another, but less long-lived, Pangea. Baltica and Laurentia played central roles in a tectonic menage a trois that incl uded major orogenic events, a redistribution of palaeogeography and a brief involvement of both with Gondwana. Many of these plate re-organi sations took place over a short time interval and invite a re-evaluati on of earlier geodynamic models which limited the speeds at which larg e continental plates could move to an arbitrarily low value. Baltica a nd Laurentia probably shared a common drift history for the time inter val 750-600 Ma as they rotated clockwise and drifted southward from an equatorial position during the opening of the Proto-Pacific between L aurentia and East Gondwana (initial break-up of Rodinia). On their com bined approach toward the south pole, Baltica and Laurentia were glaci ated during the Varanger glaciations. Although the two continents drif ted toward the south pole during the Late Proterozoic, they began to s eparate at around 600 Ma (rift to drift) to form the Iapetus Ocean thr ough asymmetric rifting and relative rotations of up to 180 degrees. I nitiation of rifting on the Baltic margin is marked by the 650 Ma Eger sund tholeiitic dykes (SW Norway) which contain abundant lower crustal zenoliths, and the tholeiitic magma was probably derived from a mantl e plume. In latest Precambrian time, the final redistribution of Rodin ia is characterised by high plate velocities. In particular, Laurentia began a rapid, up to 20 cm/yr, ascent to equatorial latitudes and ess entially stayed in low latitudes throughout most of the Palaeozoic. Th e high velocities suggest either that Laurentia was pushed off a lower mantle heat anomaly originating from supercontinental mantle insulati on or that Laurentia was pulled toward a subduction-generated cold spo t in the proto-Pacific. Baltica, except for a short and rapid excursio n to lower latitudes in the Late Vendian, remained mostly in intermedi ate to high southerly latitudes and closer to the Gondwana mar in unti l Early Ordovician times. In Early Ordovician times, Arenig-Llanvirn p latform trilobites show a broad distinction between the continents of Laurentia/Siberia/North China Block (Bathyurid), Baltica (Ptychopygine /Megalaspid) and the areas of NW Gondwana/Avalonia/Armorica (Calymenac ean-Dalmanitacean). During the Ordovician, Baltica rotated and moved n orthward, approaching close enough to Laurentia by the late Caradoc fo r trilobite and brachiopod spat to cross the intervening Iapetus Ocean . Ducking appears to have been irregular both in time and manner: the collision between Scotland/Greenland and western Norway resulted in th e early Scandian Orogeny in the Silurian (c. 425 Ma), but further sout h, there is evidence of late Silurian impingement with subduction of A valonian continental crust (in England and Ireland) below the eastern edge of Laurentia until the Emsian. In the northern Appalachians the m ain time of collision appears to have been during the Emsian/Eifellian Acadian Orogeny, Recent analyses invalidates the traditional concept of a sustained orthogonal relationship between Baltica and Laurentia a cross a single Iapetus Ocean throughout the Caledonide evolution. The active margin of Baltica (Scandinavian Caledonides) faced Siberia duri ng the Late Cambrian and Early Ordovician with oceanic separation betw een these landmasses in the order of 1200-1500 km. This may explain th e local occurrences of Siberia-Laurentian type Bathyarid trilobite fau nas in Central Norwegian Caledonian nappes, earlier interpreted as Lau rentia-Baltica trilobite mixing. Subsequent counterclockwise rotation of Baltica transferred the Caledonian margin in the direction of Laure ntia by Silurian times, when the two continents once again started to collide to form Euramerica. This rotation, along with the strongly asy mmetric opening of the Iapetus at around 600 Ma, demonstrates a comple xity in Precambrian-Palaeozoic plate tectonics, i.e. a collage of meta stable plate boundaries which have perhaps too often been simplified t o an orthogonal Wilson cycle tectonic scenario.