Continental Rifted Margins 2. Gwenn Peron-Pinvidic

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СКАЧАТЬ granulites, all interpreted as lower crustal rocks, while Site 1068 drilled serpentinized peridotites, which were interpreted as serpentinized mantle (Whitmarsh et al. 1998). Underlying rotated blocks between Sites 1065 and 1067, the H detachment is warped up and separates Site 900 from Site 1068 (Figure 1.4), so similarly to S, H likely represents a serpentine detachment delineating the CMB (Chian et al. 1999). Cutting across and rotating the H detachment, the fault bounding the western side of Site 1068 has been interpreted as marking the development of a newly formed detachment, which has been variously termed F detachment (Krawczyk et al. 1996) and HHD detachment (Manatschal et al. 2001, pp. 405–428). F/HHD defines the top of seismic basement from this high down to a fault block drilled at Site 1069. This rider block appears to be an allochthonous continental slice (based on the recovered sediment) resting on the same peridotitic basement as the one sampled at Site 1068. Site 899 recovered serpentinite breccias intercalated with sediments, indicating the presence of a nearby serpentinized high (Whitmarsh et al. 1998), and Site 897 recovered highly sheared, partially serpentinized mantle peridotites (Sawyer et al. 1994). Finally, Site 1070, located 15 km east of the M0 magnetic anomaly (Figure 1.1), recovered serpentinized mantle rocks and gabbro.

      1.2.3. Conjugated West Iberian and Newfoundland Margins

      The DGM faces the southeast flank of the Flemish Cap at the Newfoundland Margin (Figure 1.1a and sections SCREECH 1/ISE1 on Figure 1.2), whereas the SIAP faces the North Newfoundland Basin (Figure 1.1a and sections SCREECH 2/LG12-TGS on Figure 1.2). Palaeographic reconstructions have proposed that both the Western Iberian and the Newfoundland margins represented a pair of conjugated margins before continental breakup (e.g. Sutra et al. 2013). The final root of S observed at the DGM is incidentally believed to be currently located on the Flemish Cap (Reston et al. 2007).

      A key finding of the drilling at the SIAP was the demonstration of the presence of a wide-zone of unroofed mantle (from Site 900 to 1070 and oceanward, Figure 1.4) that extends between the hyper-thinned continental crust and the first clear magnetic polarity reversals (Figure 1.1, M0), outlining the areas where seafloor spreading occurred. This observation west of Iberia has underpinned the concept of mantle unroofing on a large-scale at other rifted margins (Whitmarsh et al. 1998; Manatschal et al. 2001). It has also led to the concept of margin pair asymmetry (e.g. Pérez-Gussinyé and Reston 2001), as comparison of sections across both margin segments (Figure 1.2) clearly shows the asymmetry of the unroofed mantle (e.g. Reston 2009). The mantle extends across ~250 km within the SIAP (sections LG12-TSG, IAM-9, Figure 1.2), versus ~50 km west of the COT offshore the Northern Newfoundland Basin (section SCREECH2, Figure 1.2). Asymmetry between the margin pair also exists when considering the thickness of the continental crust: comparison between the two margins still shows a “normal” crustal thickness of ~30 km at the Flemish Cap, whereas at the Galicia Margin, the GIB and the GB are characterized by relatively thin crust, of about ~10 km (e.g. Druet et al. 2018). This asymmetry between the WIM and its Newfoundland conjugate margin has been interpreted as having developed once the crust had become completely brittle (e.g. Pérez-Gussinyé and Reston 2001; Reston 2009).

1.3. Synrift tectono-stratigraphy and age and evolution of extension across the West Iberian Margin

The general evolution of the WIM has been derived from an extensive network of 2D seismic reflection profiles, calibrated by scientific drilling acquired during ODP Legs 103 at the DGM (Boillot and Winterer 1988), and Legs 149 and 173 at the SIAP (Sawyer et al. 1994; Whitmarsh et al. 1998) (Figures 1.3 and 1.4). The recovered cores and geophysical logging data provided constraints on the lithology, biostratigraphy, palaeomagnetics, geochemistry, bulk density, thermal conductivity, and compressional seismic velocities on the rocks forming the basement and the synrift sediments. ODP results have also provided sparse information on the ages of the synrift sequences, mainly restricted to the top of some tilted blocks and basement highs.

Figure 1.5. The extension discrepancy across the Iberian margin. a) The Flemish Cap/Galicia transect. Here the amount of measurable extension b) across the Galicia Margin is too slight to explain the observed thinning. c) The same is true, but to a lesser extent, across the SIAP (composite profile LG12/TGS, interpretation modified from Mohn et al. 2015): there is still an extension discrepancy at the deep margin despite the very large amount of extension associated with different generations of detachments. d) Cross-plot of crustal thinning versus thinning from faulting for these two margins showing the relative magnitude of the extension discrepancy

The rifting west of Iberia affected Variscan crystalline continental basement that has been sampled from two tilted blocks at ODP Site 639 and at GAL11 diving site (Boillot and Winterer 1988; Boillot et al. 1988; Figure 1.3), and interpreted more widely from seismic velocities (Bayrakci et al. 2016; Davy et al. 2018). Based on a 3D fault analysis, Lymer et al. (2019) subdivided the synrift stratigraphy at the DGM in terms of age relative to the local faulting (e.g. the synrift of block 3 is divided into the packages 3A, 3B, 3C, Figure 1.3). Thus, the lowest unit A is likely prerift or early synrift, unit B is considered to be synfaulting, and unit C to be synrift but post-local faulting (see Figure 1.3 for details). Following the Variscan Orogeny, multiple phases of extension affected the WIM over 100 Myr, from the Triassic until the mid-Cretaceous, when eventual northward propagating breakup led to diachronic opening of the North Atlantic (e.g. Srivastava et al. 1990; Tucholke et al. 2007). Late Triassic to Early Jurassic early rifting affected a wide zone (Tucholke et al. 2007), producing large intracontinental basins (Figure 1.1 – LP, PB; Wilson et al. 1989, pp. 341–361; Murillas et al. 1990). Subsequent Jurassic extension thinned the crust to ≥10 km and led to the development of the GIB (e.g. Pérez-Gussinyé et al. 2003). ODP Site 639 recovered shallow water Tithonian carbonates at the western edge of the GB (Boillot and Winterer 1988) (Figures 1.1 and 1.3), that could correspond on seismic sections to relatively thin, poorly reflective packages (unit A on Figure 1.3). At the Early Cretaceous, the DGM developed due to extreme crustal thinning and mantle exhumation processes (Boillot et al. 1987), with hyper-thinning of the crust from ~10 km at the western edge of the Galicia Bank to zero in areas of exhumed serpentinized mantle (Figure 1.3). Syn-tectonic sediments (unit B) recovered at the top of a fault block at ODP Sites 638 and 639 have been СКАЧАТЬ