they reached apparent continental basement (black circle), peridotite basement (green circle), or no basement (white circle); Sites with good evidence that peridotite basement was faulted and uplifted 3–14 Myr after it was emplaced have black dots at circle centers. Pink circles show the locations of dredge sampling (Boillot et al. 1988). Blue rectangle indicates the location the 68.5 km x 20 km Galicia 3D volume and of the map of the seafloor shown in c); c) Bathymetric map of the DGM generated within the Galicia 3D volume. The color bar shows the depth scale in meters. Black lines indicate the locations of the sections shown in Figure 1.2 across the Western Iberian and Newfoundland margins (Datasources: SCREECH1, Funck et al. 2003; Hopper et al. 2004; SCREECH2, Van Avendonk et al. 2006; Shillington et al. 2006; IAM9, Dean et al. 2000; Pickup et al. 1996). AF: Aveiro Fault; DGM: Deep Galicia Margin; ES: Estramadura Spur; GB: Galicia Bank; GM: Galicia Margin; GIB: Galicia Interior Basin; LB: Lusitanian Basin; NF: Nazare Fault; OB: Orphan Basin; PR: Peridotite Ridge; Porc. B: Porcupine Basin; PB: Porto Basin; SIAP: South Iberian Abyssal Plain; TAP: Tagus Abyssal Plain; TF: Tagus Fault; TS: Tore Seamounts. Bathymetric data for a) and b) are shown in meters and are from https://www.ngdc.noaa.gov/.
Crustal hyper-thinning, extensional discrepancy, crustal embrittlement, development of detachment faulting, mantle serpentinization and exhumation are all defining characteristics of individual magma-poor margins observed at other margins worldwide (e.g. Reston 2009; Turner and Wilson 2009; Autin et al. 2010; Zalán et al. 2011; Ball et al. 2013; Gillard et al. 2015; Osmundsen et al. 2016) that were first observed and described at either the WIM or at the neighboring Biscay margin. de Charpal et al. (1978) recognized tilted fault blocks above the S detachment, and similar detachment faults at the crust-mantle boundary (CMB) were subsequently identified at the SIAP (see section 1.2.2, “H” detachment, Figure 1.4; Hoffmann and Reston 1992; Krawczyk et al. 1996). Exhumed mantle was first recovered, by dredging then by drilling, at the DGM (Boillot et al. 1980; Boillot and Winterer 1988), and first shown to be an expanse ~100 km wide at the SIAP (Pickup et al. 1996). Le Pichon and Barbier (1987) and subsequently Sibuet (1992) pointed out extensional discrepancy in the Biscay and Galicia margins (Figure 1.5). Serpentine undercrust was proposed (Boillot et al. 1989) and modeled for the first time within the Galicia Margin (Pérez-Gussinyé and Reston 2001). It was also there where seismic velocities were first of sufficient resolution to both map out the degree of serpentinization and relate the causal fluid ingress to slip along the overlying crustal faults (Bayrakci et al. 2016). Furthermore, the most current models of continental breakup (see section 1.4) have been designed from observation at the Galicia Margin, namely: crustal DDS (Davis and Kusznir 2004, pp. 92–136), polyphase faulting (Reston et al. 2007) and sequential faulting (Ranero and Pérez-Gussinyé 2010).
The WIM is therefore considered a “classic” example of a magma-poor margin. In this chapter, we present an overview of the structure of the WIM and summarize the main tectono-stratigraphic models proposed to explain its evolution. We then highlight the key remaining questions concerning the geodynamics of that margin, and also other magma-poor margins, thus exploring why the WIM has been and still remains at the forefront of research into continental breakup.
1.2. Structures of the West Iberian Margin
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
