Читать книгу Continental Rifted Margins 2 - Gwenn Peron-Pinvidic - Страница 12

The WIM is sediment-starved and magma-poor (Boillot and Winterer 1988), allowing for optimal geophysical imaging of the margin’s structures, and have thus been studied since the early stages of deep offshore scientific exploration.

First geophysical and drilling data: early reflection data across the GM were the Galice-Portugal profiles, collected in 1975 and 1980 by the Institut Français du Pétrole (Mauffret and Montadert 1987). These data were used as the basis for the first dedicated drilling leg on the margin, ODP Leg 103 (Boillot and Winterer 1988). In the late 1980s, these data were supplemented by the Lusigal profiles, which also covered the SIAP (Krawczyk et al. 1996). The widely spaced Iberian Atlantic Margin (IAM) profiles, acquired in 1993, included the IAM-9 across the SIAP, and IAM-5 across the TAP (Figures 1.1 and 1.2) and wide-angle/refraction profiles. In 1997, the Iberia Seismic Experiment collected reflection profiles on both the GM and the SIAP during the UK’s Discovery 215 cruise (Whitmarsh et al. 2000), which also collected wide-angle data using a fleet of ocean bottom seismometers (OBSs). The Discovery and Lusigal reflection profiles supported two ODP legs on the SIAP margin (149 and 173 – Sawyer et al. 1994; Whitmarsh et al. 1998; Whitmarsh et al. 2000).

The Galicia 3D volume: enhancing the data available at the GM, a 3D volume of seismic reflection data was acquired in 2013 and provides spectacular new observations of the 3D structure of the DGM (Figures 1.1 and 1.3). These 3D data provide exceptional images of the edge of the hyper-extended crust down to 14 s TWT around the ODP Sites 638, 639 and 641, as well as images of the exhumed mantle around Site 637, at a resolution level never achieved before along this margin. Wide-angle data within the 3D volume were recorded by 72 OBS and hydrophones along a 2D profile (Davy et al. 2018), which were subsequently distributed into four irregular lines of 18 instruments for 3D inversion (Bayrakci et al. 2016). Together with the 2D data and the ODP drilling results, the 3D volume provides one of the best existing databases at rifted margins worldwide.

Figure 1.2. Cross-sections through the Iberian-Newfoundland conjugated rifted margins

Figure 1.3. Structure of the Deep Galicia Margin. Top: summary of drilling results from ODP Leg 103 (source: adapted from Reston 2005)

CONTINUATION OF CAPTION FOR FIGURE 1.2.– Adapted from Reston (2009 and 2010) (original sources: SE Flemish Cap: Funck et al. (2003); Galicia: Zelt et al. (2003); South Iberia Abyssal Plain: Chian et al. (1999); North Newfoundland Basin: Van Avendonk et al. (2006); South Iberia Abyssal Plain, IAM9: Dean et al. (2000); Newfoundland Basin (SCREECH3): Lau et al. (2006); South Newfoundland Basin: Reid (1994); Tagus Abyssal Plain: Neves et al. (2009). All noted velocities are based on Ocean Bottom Seismometers and Multi-Channel Seismic data. Note the close match between the predicted landward limit of embrittlement during rifting and of the reduced velocity mantle, interpreted as undercrusting serpentinites. The volumes of water required to generate serpentinization can only come from above, by passage through an entirely brittle crustal section. Thick synrift, especially salt, provides a barrier to these fluids and prevents mantle serpentinization.

CONTINUATION OF CAPTION FOR FIGURE 1.3.– Aptian-Barremian age of postfaulting units on block 2 (Site 641) is potentially the same age as syn-faulting units on block 4 (Site 640) suggesting that faulting is diachronous. Thickness of the stratigraphic units are not shown as ODP drills only partially sampled each unit; b) Seismic sections across the Galicia 3D volume with location of the ODP Sites. The location of the seismic sections is shown on a bathymetric map (inset) of the DGM generated within the 3D volume (location shown in Figure 1.1c). Color code for the faults is the same as for Figure 1.6. Block numbering from Ranero and Perez-Gussinyé (2010). Synrift labels and fault numbering from Lymer et al. (2019). The lowest unit A is cut and rotated by the faulting, but does not thicken into the block bounding faults and so is likely prerift or early synrift; Unit B thickens towards the faults within the half-grabens and is considered to be syn-faulting; Unit C is tilted with the top of the faulted blocks, but onlaps unit B and infills the fault-generated topography, thus being interpreted as synrift, but post-local faulting.