Neoproterozoic Laurentia-Baltica fits

Opening of the Palaeozoic Iapetus ocean, marking the final breakup of Rodinia, is generally modelled as involving three major continental blocks – Laurentia (including Greenland, Rockall plateau, and blocks of present-day Scotland), Baltica, and Amazonia (e.g. Cawood et al., 2001). However, the reconstruction of the pre-Iapetus configuration of these blocks is debated. In particular, a variety of late Neoproterozoic Laurentia-Baltica reconstructions have been proposed (Figure 1). These include western Scandinavia against the Rockall-Scotland-SE Greenland segment of Laurentia (Figure 1a; e.g., Winchester, 1988; Hoffman, 1991; Park, 1992; Starmer, 1996; Weil et al., 1998; Grelling and Smith, 2000; Pisarevsky et al., 2003), as well as progressively more northern positions (in present-day coordinates) for western Scandinavia with respect to Laurentia (Figs. 1b, 1c). In Figure 1b the position of Baltica was constrained by the northern position of Amazonia with respect to Laurentia, which was suggested on the basis of apparent geological similarities between Neoproterozoic strata of Scotland and the Arequipa massif in South America (Dalziel, 1994, 1997). This suggestion, however, contradicts Palaeozoic palaeomagnetic data (Forsythe et al., 1993). Recently this reconstruction has been dismissed on the basis of comparative isotopic studies between Laurentian and Amazonia (including the Arequipa-Antofalla basement; Loewy et al., 2003, and references their in). Baltica lay in an even more northerly position in the reconstruction of Dalziel (1992; Fig. 1c) but he subsequently (Dalziel, 1997) dismissed this in favour of the inferred geologically constrained reconstruction in Figure 1b. Some Neoproterozoic Laurentia-Baltica reconstructions (e.g., Karlstrom et al., 1999) use the pre-Grenvillian (roughly pre-1000 Ma) fit (Fig. 1d) based on pre-Neoproterozoic palaeomagnetic data (Poorter, 1981; Stearn and Piper, 1984; Buchan et al., 2000; Pesonen et al., 2003) and on the comparison of pre-Neoproterozoic crustal blocks and orogenic belts in both cratons (e.g., Gaal and Gorbatschev, 1987; Winchester, 1988; Gower et al., 1990). In particular, this reconstruction enables an overall linear trend of the Mesoproterozoic Sveconorwegian belt of Baltica with the similarly aged Grenville belt of east Laurentia (Gower et al., 1990). The relevance of this reconstruction to the Neoproterozoic, pre-Iapetian opening continental configurations has been criticised by Larson and Stigh (2000; see also Åhall and Karlstrom, 2000) as it ignores the inferred Mesoproterozoic break-up of Baltica from Laurentia proposed by Park (1992) and Starmer (1996). Hartz and Torsvik (2002) proposed a “Baltica upside down” reconstruction (Fig. 1e) based mainly on the palaeomagnetic data from the Finnmark province of Norway and northern parts of the Kola Peninsula in Russia (see also Torsvik, and Rehnström, 2001). This fit is poorly justified. It contradicts the correlation of the Precambrian crustal blocks and orogens between northeastern Laurentia and western Baltica (e.g., Winchester, 1988; Hoffman, 1989; Gorbatschev and Bogdanova, 1993; Park et al., 1994; Åhall and Connelly, 1998; Gower, 1996; Karlstrom et al., 1999). Furthermore, the absence of any traces of the Grenvillian orogeny along the eastern margin of Baltica, and data from lithostratigraphic and provenance studies in the Urals (e.g. Willner et al., 2003) are also inconsistent with this model. The Uralain margin of Baltica contains evidence for a thick passive margin sedimentary succession of Mesoproterozoic to mid-Neoproterozoic (Riphean) age which changed to an active margin setting in the late Neoproterozoic (ca. 620 Ma, early Vendian; e.g. Nikishin et al., 1996; Willner et al., 2001, 2003; Maslov and Isherskaya, 2002; Puchkov, 2003 and references therein). This data completely refutes the “upside down” model (Fig. 1e) which predicts an intracontinental rift basin setting for the Urals until Iapetian opening at ca. 550 Ma followed by a passive margin setting until at least 500 Ma. In particular, it requires a thick Cambrian passive margin succession, yet, Cambrian deposits are virtually absent in Urals (Maslov et al. 1997 and references therein).