Reply to comment by J. G. Meert and R. Van der Voo on ‘New palaeomagnetic result from Vendian red sediments in Cisbaikalia and the problem of the relationship of Siberia and Laurentia in the Vendian’

We thank J. Meert and R. Van der Voo for their attention to our paper. We are also glad that it has reopened a discussion about latitudinal position of Laurentia in Late Vendian, which was one of the aims of our publication. Unfortunately, Meert and Van der Voo partly misinterpreted our conclusions. Our aims were to present a working hypothesis that explained the most reliable palaeomagnetic data. The principle of multiple working hypotheses permits us to present a new model without compelling us to restate all of the arguments used to support previous models. We have decided not to accept somepalaeomagnetic results that are central to the existing model of a polarVendian position of Laurentia (Meert et al. 1994; Torsvik et al. 1996). On the other hand, this widely accepted model dismisses important data that support our new interpretation. In particular, many of our differences are based on theacceptance or rejection of palaeomagnetic baked-contact and fold tests. The Sept-Iles ‘A’ pole (Tanczyk et al. 1987) is the only one among coeval Laurentian poles that has a full contact test, unlike Catoctin ‘A’ (Meert et al. 1994) or the Callander pole (Symons & Chaisson 1991). It is very hard to explain in thiscase why the magnetization of the main intrusion is younger than the magnetization of the cutting dykes (pole ‘B’), which was suggested by Meert and Van der Voo in this forum and earlier publications. The result of the contact test in the Callander Complex described by Symons & Chaisson (1991) can be also explained by a regional bipolar remagnetization; the remanence direction of the country rocks is almost antiparallel to that of the studied dyke and close to the negatively inclined remanence direction of some other sampled sites of the Complex (sites 5, 6,10 in Table 1 of Symons & Chaisson 1991). Of course, a primary magnetization is possible too. So, our conclusion was that at present it is very hard to propose a model that will fit both Callander and Sept-Iles ‘A’ results, and that the palaeomagnetic community should equally consider accepting either one of those results. Meert and Van der Voo also insist that their Catoctin ‘A’ pole is primary, and Catoctin ‘B’ is secondary (Meert et al. 1994). We think that Meert's and Van der Voo's claim that the fold test for the A component is positive at the 92 percentconfidence level is less than rigorous. Although somewhat arbitrary, the 95 per cent confidence level has been universally accepted asa fair benchmark of statistical significance. Although we agree with Meert and Van der Voo that unfolding leads to better grouping, in all fairness they must report their fold test asstatistically inconclusive. Their discussion has, nonetheless, prompted us to re-examine their original data more carefully (Table 2 and Fig. 7 in Meert et al. 1994), and we note that the increase of Catoctin ‘A’ grouping after the tilt correction is due to just one site (site 3). Without this site the grouping before and after the tilt correction are statistically the same. Taking the data at face value, then there are only two high-quality poles for Laurentia between 580 Ma and 560 Ma: 565 Ma Sept-Iles ‘A’ pole and 575 Ma Callander pole. Meert and Van der Voo suggest that Sept-Iles ‘A’ is a result of laterremagnetization, but it is also possible that the Callander pole is a result of remagnetization: the bakedcontacttest fortheSept-Iles is more rigorous than for the Callander or Catoctin ‘A’. Meert and Van der Voo agreed with us about a 615 Ma agefor the Long Range pole (Murthy et al. 1992; Kamo & Gower 1994). Any palaeogeographic model incorporating all of these three poles must include a low-latitude position at 615 Ma, high-latitude for 575 Ma and again low-latitude for 565 Ma. This requires unusually rapid continental motions. So, we prefer to consider one of two alternative models based either on the Callander pole, or on the Sept-Iles‘A’ pole. Although weprefer the palaeogeographically and geodynamically simpler low-latitude model, we agree that there is not enough evidence yet to reject the other model. Meert's and Van der Voo's argument that Callander and Catoctin A poles match no known younger poles from Laurentia is strong. However, the time interval between 560 Ma and 530 Ma for Laurentia is poorly constrained by palaeomagnetism. Torsvik et al. (1996) reported about four poles with assigned ages of 550 Ma: Buckingham Flows (Dankers & LaPointe 1981), Long Range Dykes A (Murthy et al. 1992), Johnnie Rainstorm Formation (Van Alstine & Gillet 1979) and Double Mer Formation (Murthy et al. 1992). All these results show thelow palaeolatitude. However, the only age constraint forthe Buckingham Flows is the K–Ar determination of 573±32 Ma. Recent correlation of the Johnnie Formation with terminal Neoproterozoic strata in Australia suggest an age significantly older than 550 Ma (Wernicke & Hagadorn 2000; Abolins et al. 1999). The age of Long Range Dykes A pole is well established by Kamo & Gower (1994) as 615 Ma. Finally, the only age constraint for the Double Mer Formation is that it post-dates Grenvillian deformation (Murthy et al. 1992). The ages of all these poles may thus be greater than 550 Myr. The lack of high-quality Laurentian poles for the Proterozoic–Cambrian boundary interval permits a high-latitude position ofLaurentia, and correspondingly a steep-inclined magnetic overprint during that time (see Park 1992; Evans & Kirschvink 1999). Of course, this suggestion is yet to be proved or disproved. Two possible palaeoreconstructions for 570 Ma: (a) high-latitude position of Laurentia; (b) low-latitude position of Laurentia. Am—Amazonia, An—East Antarctica, Au—Australia, Co—Congo, DML—Dronning Maud Land, Fl—Florida block, Gr—Greenland, In—India, Ka—Kalahari, La—Laurentia, MBL—Marie Byrd Land, Md—Madagascar, R—Rockall, SF—Sao Francisco, Si—Siberia, WA—West Africa. Plotted with assistance of the Plates program of the University of Texas in Austin. We agree with Meert and Van der Voo about the variety ofpossible Siberia–Laurentia fits. Our result matches one of them—the one that seems to have better geological evidence (Hoffman 1991; Pelechaty 1996). However, these geologicalarguments and our palaeomagnetic result are merely permissible, but not conclusive for this model, so there are not enough reasons to exclude other models. It is also possible that there were no connection at all. However, the Laurentia–Siberia fit is a separate issue from the analysis of the Laurentian poles, and it was not our intention to ‘rescue’ a particular reconstruction, as mentioned by Meert and Van der Voo. Meert and Van der Voo proposed two global reconstructions based on high-latitude and low-latitude positions of Laurentia. They correctly refer to the suggestion of Trompette (1997) about the collision of Congo–Sao Francisco, Kalahari and Rio deLaPlata cratons by c.600 Ma, but they forget to mention the Amazonia craton. Trompette (1997) also suggested that West Africa, Amazonia and Rio de La Plata combined a single mega-continent. Fig. 1(c) of Meert and Van der Voo is in somedisagreement with their own claim about the mentioned c. 600 Ma collision—they show the Braziliano Ocean still open at 565–580 Ma. As Meert and Van der Voo correctly mentioned, the longitudinal distances between cratons are arbitrary, so weshow here two other possible reconstructions for 570 Ma for the high-latitude and low-latitude models correspondingly (Figs 1a and b). These reconstructions show, that the Braziliano and Adamastor Oceans were already closed by this time, but the Mozambique Ocean could still be open (e.g. Kroner et al. 2000; Meert & Van der Voo 1997). In conclusion, we cannot agree that our paper is overinterpretation.On the contrary, probably we were too cautious.We have tried to show both in the original paper and in thisforum that at present the objective use of available palaeomagneticinformation does not yet permit us to choose betweena high-latitude and a low-latitude position of Laurentia at ~580–560 Ma.

Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.