Here we report Precambrian zircons recovered from basaltic beach sands on Mauritius, 900 km distant from the nearest continental crust (Madagascar). Some twenty zircon grains were recovered from two basaltic sand samples from the northwest (Sample E04-1) and southeast (Sample MBS1) coast of Mauritius. The use of sand samples avoids potential contamination from rock crushing apparatus. The zircons are generally subhedral to anhedral, show diversity in shape and presence of inclusions, and range in size from 50 to 300 µm. The zircons were analysed for U and Pb isotopes by TIMS (Fig. 2, Supplementary Table S1). Sample E04-1 from the Intermediate Series yielded fifteen zircon grains; six were selected for analysis. Sample MBS1 from the Older Series had fewer zircons and two were used for age determination. Most results are somewhat discordant (Fig. 2) Table S1). Their presence in exclusively basaltic detritus suggests that they were brought up by mafic magmas that assimilated underlying sialic crust, likely at relatively shallow levels. There is no clear cut geochemical or isotopic signature of continental crust in the Mauritian basalts, although some of their variability in εNd values (3.9-6.1; refs 8,9) could indicate variable crustal contamination. We suggest that a crustal signature need not be detectable in basaltic lavas that carry xenocrystic zircons.Although small amounts of zircon have been found as crystallization products in young oceanic 2 mafic volcanics and intrusives 11,12 , older xenocrystic zircons have been reliably documented only from oceanic gabbros drilled at the mid-Atlantic ridge 13 . The young Mid-Atlantic ridge gabbros that contain old xenocrystic zircons have lower Zr concentrations 13 (mean ~20 ppm) than Mauritian basalts 8 (mean ~145 ppm), and also lack geochemical indicators of continental crust assimilation.To identify regions in the northwest Indian Ocean that may be underlain by continental crust, we determined crustal thicknesses by gravity anomaly inversion incorporating a lithosphere thermal gravity anomaly correction 14 . The gravity inversion predicts contiguous crust of thickness >25-30 km beneath the Seychelles and northern Mascarenes, which extends southwards towards Mauritius (Fig. 1). Sensitivity tests ( Supplementary Fig. S1) show that predicted crustal thicknesses from gravity inversion under the Seychelles, Mascarenes, Mauritius, Laccadives, Maldives and Chagos are not significantly dependent on breakup and ocean age isochrons used to determine the lithosphere thermal gravity anomaly correction. Crustal thickness determined from gravity inversion for the Seychelles is consistent with wide-angle seismic studies 15 where crustal thicknesses of 32 km and velocity structure are interpreted as continental. On the conjugate Indian margin, the Laccadives, Maldives and Chagos also appear to be underlain by contiguous crust of thickness >25-30 km.Seismic Moho depths (~24 km) beneath the Laccadives 16 and crustal thicknesses from Chagos (up to 27 km) obtained from gravity modell...

We propose that Baltica was geographically inverted throughout the Neoproterozoic and therefore suggest reassessment of the classic Wilson Cycle paleoreconstruction depicting an Atlantic-type early Paleozoic (Iapetus) ocean between western Norway and East Greenland. Our new reconstruction dismisses the need for 180؇ rotation of Baltica after breakup of Iapetus and presents more plausible geologic correlations between Baltica, Laurentia, and Gondwana than those accompanying previous fits. In our reconstruction, the breakup that led to the formation of the Iapetus Ocean was initiated at a junction between a rift (Laurentia-Gondwana), a right-lateral fault (Laurentia-Baltica), and a trench (inverted Baltica-Gondwana), thereby linking the late Precambrian Iapetus opening to the Timanian and Avalonian orogenies between Gondwana and an inverted Baltica.

We present tectonic models of progressive basin formation in the south‐west Barents Sea derived as part of the PETROBAR project (Petroleum‐related studies of the Barents Sea region). The basin architecture developed as a multi‐stage rift preceding the creation of the sheared/transtensional margin conjugate to NE Greenland. N‐ to NNE‐striking basins, with sediment thicknesses in places exceeding 15 km, are separated by basement highs. We use two basin analysis approaches, BMT™ backstripping and TecMod™time‐forward modelling, to determine stretching factors through time along the profile PETROBAR‐07. This 550 km‐long profile derived from wide‐angle reflection/refraction seismic data acquired in 2007, coincident with deep multichannel seismic reflection data. Detailed stratigraphic analysis of the reflection profile, in concert with a dense grid of 2D profiles tied to wells, provides timing and water depth constraint for the models. Velocity analysis of the wide‐angle data provides constraint on the cumulative crustal stretching. The north‐west trending cross‐section extends from continental craton, at the Varanger Peninsula, to within 16 km of the interpreted continent–ocean boundary. Rifting along the profile was episodic, with four distinct phases of basin formation during the Carboniferous, the Late Permian–Triassic, the Late Jurassic–Early Cretaceous and the Late Cretaceous–Eocene. Collectively, the basins exhibit a general trend of younging, narrowing, and deepening oceanward, suggesting a gradual focusing of rifting prior to final breakup. Cumulative stretching factors derived from BMT and TecMod correlate well with observed crustal thinning, and the two models provide uncertainty bounds for stretching factors for the separate rift phases. In contrast to orthogonally rifted margins, stretching is relatively minor immediately prior to transform breakup, with greater stretching occurring during earlier rift phases.

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Abstract.Tectonic models for the North Atlantic Caledonides typically show Baltica being subducted below Laurentia prior to late orogenic collapse and exhumation of deep crustal rocks, conveniently explaining the high-and ultrahighpressure provinces in the Scandinavian Caledonides. However, these models offer no tectonic explanation for the deformation and overthickening of the overriding Laurentian plate.