Palaeozoic palaeogeography of Laurentia and its margins: a reassessment of palaeomagnetic data

Abstract

S U M M A R YThe recognition of wide-spread remagnetization events during the Palaeozoic has greatly added to the problems of reconstructing the Palaeozoic drift history of Laurentia and in recent years increasing emphasis has been placed on evaluating the relative reliability of palaeomagnetic data, in an attempt to overcome some of these difficulties. In the light of these developments a revised apparent polar wander (APW) path has been constructed following a rigorous evaluation of all published Palaeozoic palaeomagnetic data from Laurentia and its Palaeozoic margins (i.e. North America, northern Britain, Northern Ireland and Greenland). The use of data from the Laurentian margins, when corrected for the opening of the North Atlantic, has resulted in a revision of previously published Palaeozoic palaeogeographic reconstructions and these new palaeogeographic reconstructions for Laurentia are in good agreement with faunal and lithological data. A comparison of this new APW path with other APW paths from Avalonia and Baltica reveals a very close agreement between all three paths from mid-Silurian times onwards, indicating that the closure of the Iapetus ocean and the Tornquist sea was essentially complete by this time.Laurentia occupied equatorial latitudes from Cambrian to early Silurian times and its drift history was dominated by anti-clockwise rotation (up to 0.7" Ma-') and low latitudinal drift rates (up to 3 cm yr-l). Late Silurian and early Devonian times were characterized by increased rates of anti-clockwise rotation (up to 1.3" Ma-') and southerly directed latitudinal drift (rates up to 13 cm yr-l) of the continent, which place Laurentia in moderate southerly latitudes (30-40"s) by the early Devonian. These palaeolatitudes are comparable with those from Avalonia and Baltica for this time indicating that the amalgamation of Eurmerica had already taken place. By late Devonian time Laurentia had begun to drift north (rates up to 5 cm yr-') towards equatorial latitudes by mid-Carboniferous time.

Precessional and half-precessional climate forcing of Mid-Devonian monsoon-like dynamics

“…This basin was limited by the Euramerican Old Red Continent to the north and by the Normannian and Mid-German highs to the south (Ziegler, 1990). The apparent polar wander paths of Avalonia and Euramerica are well established (Torsvik et al, 1993;Mac Niocaill and Smethurst, 1994). Torsvik et al (1993) estimated the paleolatitude of Southern Britain at 389 Ma to be 6.9 • S. During the Middle-Devonian, the studied section and Southern Britain were only a few hundred kilometres apart (Cocks et al, 1997) and consequently this value serves as a good estimate for the paleolatitude of the section.…”

Tethyan oceans

“…• palaeomagnetics - Embleton (1984), Klitgord & Schouten (1986), Rowley & Lottes (1988), Van der Voo (1993) and Powell & Li (1994);• palaeoreconstructions -Zonenshain et al (1985), Ziegler (1988b), Hutchison (1989), Ziegler (1990), Zonenshain et al (1990), Baillie et al (1994, Khain (1994a), Niocaill & Smethurst (1994), Seng0r & Natal'in (1996), Stampfli (1996 Krahl et al 1983Krahl et al , 1986Krahl 1992) are not affected by Variscan metamorphism.…”

Lower Palaeozoic evolution of the northeast German Basin/Baltica borderland

“…Harper (1992) suggests that faunal provinciality in brachiopods between Eastern Avalonia and Baltica was still discernable in late Ordovician times, thus implying a degree of separation. By the mid-Silurian times, faunal evidence suggests that Eastern Avalonia and Baltica were separated by < 1000 km (McKerrow & Cocks, 1986), while palaeomagnetic data suggest a distance of c. 1000-1500 km (Torsvik et al 1993), largely longitudinal (Torsvik et al 1993;Mac Niocaill & Smethurst, 1994). Thus, differences in the sedimentological successions of Baltica and Avalonia may be interpreted in terms of longitudinal variations together with the variable tectonic environments that prevailed in the individual regions.…”

“…Cambrian to Llandovery palaeogeographic reconstructions for Baltica and Eastern Avalonia (afterPickering, Bassett & Siveter, 1988;Torsvik et al 1993;Mac Niocaill & Smethurst, 1994; present study).…”