These key questions are global in scope, apply to the breakup of all continents and center around the fourth dimension: time. Despite the recent progress of the scientific community in understanding the processes of development of rifted margins, mostly favored by international cooperation (e.g. the “International Ocean Drilling Program”, see also Peron-Pinvidic et al. (2019)), the timing of the key steps at rifted margins remains mostly unconstrained. Observations from seismic data have revealed the geometry of rifted margins, but not the absolute timing and rate of the key steps of rifting, thus the ages of the syn-tectonic units remain interpretive (Figure 1.7). Onshore, observations from analogues of margins cannot fully constrain the processes of rifting because they are discontinuous and overprinted by later, often strong, deformations. Other approaches, such as numerical models, are dependent on time input parameters, for example the velocity of plate divergence, that are often too poorly constrained. The time aspect of the geological events at rifted margins can only be addressed by scientific drilling. Given the amount of previous work, the presence of a 3D dataset and the key role the GM has played in framing these questions, the logical place to carry out such an experiment would be the Galicia Margin.
1.6. Conclusions and outlook
The WIM has historically been, and remains, at the forefront of fundamental research on the breakup and development of oceanic basins. Three ODP legs (103, 149 and 173), many geophysical campaigns, and a recent 3D seismic volume place the WIM among the best-documented continental margins. These data allowed the definition of the earliest concepts of oceanic basin development, and still today provide the foundation to the definition of the most recent models of continental breakup. They have provided a glimpse of how extension of the continental lithosphere occurs at rifted margins, demonstrating that rifting is a diachronous process across margins, and suggesting that extension focused/migrated towards the future oceanic spreading center during the rifting history. The latest concepts can be summarized through three main models (section 1.4): depth dependent stretching, cross-cutting polyphase faulting and sequential faulting.
These models imply fundamental differences in terms of the timing and mechanisms of faulting and thinning of the continental crust, of the number of fault phases involved in rifting, of the location of the locus of extension through time, of the rate, temperature and amount of magma involved during lithospheric separation. However, because of the technical, scientific and human implications in recovering direct data from deep present-day rifted margins, these models remain fundamentally unconstrained. Indeed, while the vast amount of data available West of Iberia has provided precise images of the structures of the margin, they currently miss the temporal control to understand the kinematics and the evolution of rifting processes. To provide further time constraints during rifting of the WIM, we need to sample the pre-, syn- and post-rift sediments, as well as the basement, at successive basins across the margins to determine the detailed rift history by:
1 1) characterizing the nature of the basement and the oldest synrift/prerift unit;
2 2) dating the sequences deposited during slip on the block-bounding faults above extensional detachment faults, thus recording time constraints on the slip of individual faults across the distal margin;
3 3) constraining the age and the nature of the youngest synrift units deposited after local faulting to define the mechanisms of post-faulting uplift and faulted-blocks rotation;
4 4) defining the later subsidence of the margin from discrete palaeoenvironmental data (e.g. the local CCD and depth of the bathyal sediments) obtained through the post-rift sequence.
FURTHER READING.– The above descriptions are abbreviated and often simplified. If interested in reading and learning further, the reader is referred to the following list of publications and references.
1 – General: (Biari et al. 2021; Cresswell 2018; Epin and Manatschal 2018; Gómez-Romeu 2019; King et al. 2020; Reston and McDermott 2014; Somoza et al. 2019; Sutra et al. 2013; Whiting et al. 2021).
1.7. References
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Autin, J., Leroy, S., Beslier, M.O., d’Acremont, E., Razin, P., Ribodetti, A., Bellahsen, N., Robin, C., Al Toubi, K. (2010). Continental break-up history of a deep magma-poor margin based on seismic reflection data (northeastern Gulf of Aden margin, offshore Oman). Geophysical Journal International, 180(2), 501–519.
Ball, P., Eagles, G., Ebinger, C., McClay, K., Totterdell, J. (2013). The spatial and temporal evolution of strain during the separation of Australia and Antarctica. Geochemistry, Geophysics, Geosystems, 14(8), 2771–2799.
Bayrakci, G., Minshull, T.A., Sawyer, D.S., Reston, T.J., Klaeschen, D., Papenberg, C., Ranero, C., Bull, J.M., Davy, R.G., Shillington, D.J., Pérez-Gussinyé, M., Morgan, J.K. (2016). Fault-controlled hydration of the upper mantle during continental rifting. Nature Geoscience, 9, 384–388.
Boillot, G. and Malod, J. (1988). The north and north-west Spanish continental margin. Revista de la Sociedad Geológica de España, 1(3–4), 295–316.
Boillot, G. and Winterer, E. (1988). Drilling on the Galicia margin: Retrospect and prospect. Proceedings of the Ocean Drilling Program, Scientific Results, 103, 809–828.
Boillot, G., Grimaud, S., Mauffret, A., Mougenot, D., Kornprobst, J., Mergoil-Daniel, J., Torrent, G. (1980). Ocean-continent boundary of the Iberian margin: A serpentinite diapir west of the Galicia Bank. Earth and Planetary Science Letters, 48, 23–24.
Boillot, G., Recq, M., Winterer, E.L., Applegate, J., Baltuck, M., Bergen, J.A., Comas, M.C., Davies, T.A., Dunham, K., Evans, C.A., Girardeau, J., Goldberg, G., Haggerty, J., Jansa, L.F., Johnson, J.A., Kasahara, J., Loreau, J.P., Luna-Sierra, E., Moullade, M., Ogg, J., Sarti, M., Thurow, J., Williamson, M. (1987). Tectonic denudation of the upper mantle along passive margin: A model based on drilling results (Ocean Drilling Program leg 103, western Galicia margin, Spain). Tectonophysics, 132(4), 335–342.
Boillot, G., Comas, M.C., Girardeau, J., Kornprobst, J., Loreau, J.-P., Malod, J., Mougenot, D., Moullade, M. (1988). Preliminary results of the Galinaute Cruise: Dives of the submersible nautile on the western Galicia margin. Proceedings of the Ocean Drilling Program, Scientific Results, 103, 37–51.
Boillot, G., Feraud, G., Recq, M., Girardeau, J. (1989). “Undercrusting” by serpentinite beneath rifted margins: The example of the west Galicia margin (Spain). Nature, 431, 523–525.
Brune, S., Heine, C., Pérez-Gussinyé, M., Sobolev, S.V. (2014). Rift migration explains continental margin asymmetry and crustal hyper-extension. Nature Communications, 5, 4014.
Brune, S., Heine, C., Clift, P., Pérez-Gussinyé, M. (2017). Rifted margin architecture and crustal rheology: Reviewing Iberia-Newfoundland, Central South Atlantic, and South China Sea. Marine and Petroleum Geology, 79, 257–281.
Buck, W.R. (1988). Flexural rotation of normal faults. Tectonics, 7, 959–973.
de Charpal, O., Guennoc, P., Montadert, L., Roberts, D.G. (1978). Rifting, crustal attenuation and subsidence in the Bay of Biscay. Nature, 275, 706–711.
Chian, C., Louden, K.E., Minshull, T.A., Whitmarsh, R.B. (1999). Deep structure of the ocean-continent transition in the southern Iberia Abyssal Plain from seismic refraction profiles: Ocean Drilling Program (Legs 149 and 173) transect. Journal of Geophysical Research,