Baltica. A synopsis of vendian-permian palaeomagnetic data and their palaeotectonic implications

In light of recent additions to the Palaeozoic palaeo-magnetic data-base, particularly for the Ordovician era, a revised apparent polar wander (APW) path for Baltica has been constructed following a rigorous synthesis of all Late Precambrian-Permian data. The APW path is characterized by two prominent loops. Firstly, a Late Precambrian-Cambrian loop probably relating to a rifting event and secondly, a younger loop relating to a Mid-Silurian (Scandian) collision event. These features imply major change in plate-tectonic reconfiguration.

Baltica probably represented an individual continental unit in Early Palaeozoic times and was positioned in high southerly latitudes in an “inverted” geographic orientation. In such a reconstruction Baltica was separated from the northern margin of Gondwana by the Tornquist Sea and from Laurentia by the Iapetus Ocean. The Tornquist Zone is thus interpreted as a passive or dextral transform margin during the early Palaeozoic.

While undergoing counter-clockwise rotations (up to 1.6°/Ma), Baltica drifted northward through most of the Palaeozoic; except for a short period of southerly movement in Late Silurian-Early Devonian times after collision with Laurentia. Rapid movements in latitude (up to 9 cm/yr) are noted in Late Precambrian/early Palaeozoic times and significant decrease in velocities throughout Palaeozoic time probably reflect the progressive amalgamation of a larger continent by Early-Devonian (Euramerica) and Permian (Pangea) times.

The Tornquist Sea had a principal component of palaeo-east-west orientation. Hence it is difficult to be precise in the timing of when micro-continents such as Eastern Avalonia and the European Massifs ultimately collided along the southwestern margin of Baltica. These micro-continents are considered to have been peripheral to Gondwana (in high southerly latitudes) during the Early Ordovician. Eastern Avalonia clearly had rifted off Gondwana by Llanvirn-Llandeilo times and may have collided with Baltica during Late Ordovician times, although the present available Silurian palaeomagnetic data from Eastern Avalonia may suggest collision in Late Silurian times.

Across the Iapetus facing margin of Baltica, Laurentia was situated in equatorial to southerly latitudes during most of the Lower Palaeozoic. These continents collided in Mid-Silurian times, i.e. a first collision between southwestern Norway and Greenland/Scotland which gave rise to the early Scandian Orogeny (425 Ma) in southwestern Norway possible followed by a later, but less dramatic, Scandian event in northern Norway at around 410 Ma. Since Baltica was geographically inverted in early Palaeozoic times, the collisional margin could not have been a margin that once rifted off Laurentia as assumed in a number of plate-tectonic models.