Abstract
The Galicia margin, which is characterised by the presence of a series of tilted fault blocks and underlying seismic reflectors well imaged by multichannel seismic (MCS) reflection profiles, is a classic example of a rifted continental margin. Since the early 1980’s, several models have been proposed for the formation of this margin. All these models were based mainly on seismic imagery obtained from MCS data with age control provided by data collected during DSDP leg 47B and ODP leg 103. To understand fully the evolution of this margin, pertinent geophysical data are needed, especially to constrain seismic reflection interpretations. We discuss a new preliminary seismic refraction model and deep-tow magnetic data collected across the Galicia continental margin and show how they constrain the deep structure in this region and the precise location of the ocean-continent transition.
West of the peridotite ridge, the oceanic crust is reduced to a thickness of 5 km which increases to normal oceanic thickness some 10 -15 km further west. The base of the crust consists of 7.4 - 7.5 km/s material inteipreted as serpentinized peridotite. East of the peridotite ridge, the thinned continental crust overlies a 7.4 km/s transitional layer with a limited landward extent of 25 km. The 7.4 km/s material is interpreted as upper mantle peridotite serpentinized by sea-water percolating through the extremely thin crust disrupted by normal faults created during rifting. At the ocean-continent boundary a peridotite ridge has been drilled. Near-bottom magnetic anomalies recorded just above the sea-bottom show that oceanic crust abuts the peridotite ridge on its west side and that the magnetization of the peridotite ridge is similar to that of the adjacent thinned continental crust, that is about ten times smaller than the oceanic crust. Seismic velocities are consistent with the whole ridge being a serpentinized peridotite body. Drilling data, as well as the presence of a normal fault along its eastern flank, suggest that the ridge has been uplifted at the end of the rifting phase. As the extensional rate during rifting and the half spreading rate during the emplacement of early oceanic crust are both lower than 10 mm/yr, theoretical modelling supports the presence of serpentinized peridotite at the base of both the thinned continental and the oldest oceanic crusts, with only a limited generation of melt.
The S reflector is a detachment fault located east of the peridotite ridge which follows the top of the peridotite body over 20 km (which explains its highly reflective character on seismic reflection profiles), cuts through much of the lower continental crust and then merges with the top of the lower continental crust, which may represent the brittle-plastic interface at the time of rifting.
We favour a composite pure and simple shear model of formation of the Galicia margin and its conjugate margin in which a detachment surface (the S reflector), active since the beginning of rifting, crops out on the Canadian side and merges beneath the Galicia margin with the brittle- plastic interface. The peridotite ridge is emplaced during the late stages of rifting by mantle upwelling through the highly fractured, thin veil of continental crust. After the onset of spreading, peridotite ridges are left on both sides of the ocean. The lower portion of the thin oceanic crust immediately adjacent to the peridotite ridges consist of serpentinized peridotite and is the result of poor magma supply for several M.y. immediately after continental breakup.
This is a preview of subscription content, log in via an institution to check access.
Access this chapter
Tax calculation will be finalised at checkout
Purchases are for personal use only
Preview
Unable to display preview. Download preview PDF.
References
Boillot, G., Beslier, M.-O., and Girardeau, J., 1994. Nature, structure and evolution of the ocean- continent boundary: the lesson of the West Iberia margin. In Banda, E., Talwani, M. and Tome, M., eds., Rifted ocean-continent boundaries, Kluwer, this volume.
Boillot, G., et al., 1987a. Tectonic denudation of the upper mantle along passive margins: a model based on drilling results (ODP Leg 103, western Galicia margin, Spain). Tectonophysics, 132, p. 335–342.
Boillot, G., Winterer, E.L., Meyer, A.W., and Shipboard Scientific Party, 1987b. Introduction, objectives and principal results: Ocean Drilling Program Leg 103, West Galicia margin. In Boillot, G., Winterer, E.L., Meyer, A.W. and Shipboard Scientific Party, eds., Initial Reports of Ocean Drilling Project, U.S. Government Printing Office, Washington, Leg 103, (Part A), p. 3–17.
Boillot, G., Winterer, E.L., Meyer, A.W., and Shipboard Scientific party, 1988. Proceedings ODP, Leg 103 Scientific Results. Ocean Drilling Program, Government Printing Office, Washington D.C., College Station, Texas.
Bougault, H., et al., 1993. Fast and slow spreading ridges: structure and hydrothermal activity, ultramafic topographic highs, and CH4 output. Journal of Geophysical Research, 98, p. 9643–9651.
Bown, J.W., and White, R.S., 1994. Variation with spreading rate of oceanic crustal thickness and geochemistry. Earth and Planetary Science Letters, 121, p. 435–449.
Bown, J.W., and White, R.S., 1995a. The effect of finite extension rate on melt generation at continental rifts. Journal of Geophysical Research, in press.
Bown, J.W., and White, R.S., 1995b. Finite duration rifting, melting and subsidence at continental margins. Proceedings of the NATO-ARW workshop on rifted ocean-continent boundaries, 11–14 May 1994, Mallorca, Spain, this volume.
Chapman, C.H., and Drummond, R., 1982. Body wave seismograms in inhomogeneous media using Maslov asymptotic theory. Bulletin of the seismological Society of America, 72, p. 277–317.
Charlou, J.-L., and Donval, J.-P, 1993. Hydrothermal methane venting between 12°N and 26°N along the mid-Atlantic ridge. Journal of Geophysical Research, 98, p. 9625–9642.
Dunlop, D.J., and Prévot, M., 1982. Magnetic properties and opaque mineralogy of drilled submarine intrusive rocks. Geophysical Journal of the Royal Astronomical Society of London, 69, p. 763–802.
Etheridge, M.A., Symonds, P.A., and Lister, G.S., 1989. Application of the detachment model to reconstruction of conjugate passive margins. In Tankard, A.J. and Balkwill, H.R., eds., Extensional tectonic and stratigraphy of the north Atlantic margins, Tulsa, AAPG, AAPG memoir 46, p. 23–40.
Evans, C.A., and Girardeau, J., 1988. Galicia margin peridotites: undepleted abyssal peridotites from the north Atlantic. In Boillot, G., Winterer, E.L., Meyer, A.W. and Shipboard Scientific party, eds., Ocean Drilling Program, College Station, Texas, Government Printing Office, Washington D.C., Proceedings ODP, Leg 103 Scientific Results, p. 195–207.
Féraud, D., Girardeau, J., Beslier, M.-O., and Boillot, G., 1988. Datation 39Ar/40Ar de la mise en place des péridotites bordant la marge de Galice (Espagne). Comptes Rendus de l’Académie des Sciences de Paris, 307, p. 49–55.
Girardeau, J., Evans, C.A., and Beslier, M.-O., 1988. Structural analysis of plagioclase-bearing peridotites emplaced at the end of continental rifting: hole 637, ODP leg 103 on the Galicia margin. In Boillot, G., Winterer, E.L., Meyer, A.W. and Shipboard Scientific party, eds., Ocean Drilling Program, College Station, Texas, Government Printing Office, Washington D.C., Proceedings ODP, Leg 103 Scientific Results, p. 209–223.
Horsefield, S.J., 1992. Crustal structure across the continent-ocean boundary. University of Cambridge, 215 p.
Horsefield, S.J., Whitmarsh, R.B., White, R.S., and Sibuet, J.-C., 1994. Crustal structure of the Goban Spur rifted continental margin, NE Atlantic. Geophysical Journal International, 119, p. 1–19.
Keen, C.E., Peddy, C., de Voogd, B., and Matthews, D., 1989. Conjugate margins of Canada and Europe: Results from deep reflection profiling. Geology, 17, p. 173–176.
Kirk, R.E., Langford, J.J., and Whitmarsh, R.B., 1982. A three component ocean bottom seismogram for controlled source and earthquake seismology. Marine Geophysical Researches, 5, p. 327–341.
Kuznir, N.J., and Egan, N.N., 1989. Simple-shear and pure-shear models of extensional sedimentary basin formation: application to the Jeanne dArc basin, Grand Banks of Newfoundland. In Tankard, A.J. and Balkwill, H.R., eds., Extensional tectonic and stratigraphy of the north Atlantic margins, Tulsa, AAPG, AAPG memoir 46, p. 305–322.
Lallemand, S., and Sibuet, J.-C., 1986. Tectonic implications of canyon directions over the Northeast Atlantic continental margin. Tectonics, 5, p. 1125–1143.
Le Pichon, X., Angelier, J., and Sibuet, J.-C., 1983. Subsidence and stretching. In Watkins, J.S. and Clarke, C.L., eds., Studies in continental margin geology, Tulsa, AAPG Memoir 34, p. 731–741.
Le Pichon, X., and Sibuet, J.-C., 1981. Passive margins: a model of formation. Journal of Geophysical Research, 86, p. 3708–3710.
Louden, K.E., Sibuet, J.-C., and Foucher, J.-P, 1991. Variations in heat flow across the Goban Spur and Galicia Bank continental margins. Journal of Geophysical Research, 96, p. 16131–16150.
Louvel, V., Dyment, J., and Sibuet, J.-C., 1995. Thinning of the Goban Spur continental margin and formation of the early oceanic crust: constraints from forward modelling and inversion of marine magnetic anomalies. Geophysical Journal International, in press.
Mauffret, A., and Montadert, L., 1987. Rift tectonics on the passive continental margin off Galicia (Spain). Marine and Petroleum Geology, 40, p. 49–70.
Nazarova, K.A., 1994. Serpentinized peridotites as a possible source for oceanic magnetic anomalies. Marine Geophysical Researches, 16, p. 455–462.
ODP Leg 149 Shipboard Scientific Party, 1993. ODP drills the west Iberia rifted margin. EOS, 74, p. 454–455.
Parson, L.M., Masson, D.G., Pelton, C.D., and Grant, A.C., 1985. Seismic stratigraphy and structure of the east Canadian continental margin between 41 and 52°N. Canadian Journal of Earth Sciences, 22, p. 686–703.
Radel, G., and Melosh, H.J., 1987. A mechanical basis for low-angle normal faulting in the Basin Range. EOS, Trans. AGU, 68, p. 1449.
Recq, M., Whitmarsh, R.B., and Sibuet, J.-C., 1991. Anatomy of a lherzolite ridge, Galicia margin. Terra abstracts, 3, p. 122.
Reston, T.I., Krawczyk, C.M., and Wilkens, D., 1994. Detachment faulting and continental breakup: the S reflector offshore Galicia. Annales Geophysicae, Supplement 1 to vol. 1, p. C43.
Shipboard Scientific Party, 1994. Site 897. In Sawyer, D.S., Whitmarsh, R.B., Klaus, R.B. and Shipboard Scientific Party, eds., Proc. ODP, Init Repts., College Station, TX (Ocean Drilling Program), 149, p. 41–113.
Shive, P.N., Frost, B.R., and Peretti, A., 1988. The magnetic properties of metaperidotitic rocks as a function of metamorphic grade: implications for crustal magnetic anomalies. Journal of Geophysical Research, 93, p. 12187–12195.
Sibuet, J.-G, 1987. Contribution à l’étude des mécanismes de formation des marges continentales passives. Thèse Doctorat d’Etat, Université de Bretagne Occidentale, Brest, 351 p.
Sibuet, J.-C., 1992a. Formation of passive margins: a composite model applies to the conjugate Galicia and southeastern Flemish Cap margins. Geophysical Research Letters, 19, p. 769–772.
Sibuet, J.-C., 1992b. New constraints on the formation of non-volcanic continental Galicia- Flemish Cap conjugate margins. Journal of the Geological Society of London, 149, p. 829–840.
Sibuet, J.-C., and Collette, B., 1991. Triple junctions of Bay of Biscay and North Atlantic: new constraints on the kinematic evolution. Geology, 19, p. 522–525.
Sibuet, J.-C., and Ryan, W.B.F., 1979. Initial Reports of the Deep Sea Drilling Project. U.S. Government Printing Office, Washington, 47B, part 2.
Sibuet, J.-C., Thomas, Y., Marsset, B., Louvel, V., Savoye, B., and Le Formal, J.-P, 1995. Detailed relationship between tectonics and sedimentation from Pasisar deep-tow seismic data acquired at ODP Site 897 in the Iberian Abyssal Plain. In Sawyer, D.S., Whitmarsh, R.B., Klaus, R.B. and Shipboard Scientific Party, eds., Proc. ODP, Scientific Results., College Station, TX (Ocean Drilling Program), 149, in press.
Srivastava, S.P., Roest, W.R., Kovacs, L.C., Schouten, H., and Klitgord, K., 1990. Iberian plate kinematics: a jumping plate boundary between Eurasia and Africa. Nature, 344, p. 756–759.
Stokking, L.B., Merill, D.L., Haston, R.B., Ali, J.R., and Saboda, K.L., 1992. Rock magnetic studies of serpentinite seamounts in the Mariana and Izu-Bonin regions. In Fryer, P., Pearce, J.A. and Stokking, L.B., eds., Proceedings of the Ocean drilling Program, Scientific Results, College Station, Texas, Government Printing Office, Washington D.C., 125, p. 561–579.
Wernicke, B., 1985. Uniform-sense normal simple shear of the continental lithosphère. Canadian Journal of Earth Sciences, 22, p. 108–125.
White, R., and McKenzie, D., 1989. Magmatism at rifted zones: the generation of volcanic continental margins and flood basalts. Journal of Geophysical Research, 94, p. 7685–7729.
White, R.S., McKenzie, D., and O’Nions, K., 1992. Oceanic crustal thickness from seismic measurements and rare earth element inversions. Journal of Geophysical Research, 97, p. 19683–19715.
Whitmarsh, R.B., Miles, P., Sibuet, J.-C., and Louvel, V., 1995a. Geological and geophysical implications of deep-tow magnetometer observations near ODP Sites 897, 898, 899, 900 and 901 on the west Iberia continental margin. In Sawyer, D.S., Whitmarsh, R.B., Klaus, R.B. and Shipboard Scientific Party, eds., Proc. ODP, Scientific Results, College Station, TX (Ocean Drilling Program), 149, in press.
Whitmarsh, R.B., and Miles, P.R., 1995. Models of the development of the West Iberia continental margin at 40°30’N deduced from surface and deep-tow magnetic anomalies. Journal of Geophysical Research, in press.
Whitmarsh, R.B., Pinheiro, L.M., Miles, PR., Recq, M., and Sibuet, J.-C., 1993. Thin crust at the ocean-continent transition off Iberia and ophiolites. Tectonics, 12, p. 1230–1239.
Whitmarsh, R.B., White, R.S., Sibuet, J.-C., Horsefield, S.J., and Recq, M., 1995b. The ocean- continent boundary off the western continental margin of Iberia - III. Crustal structure across the Galicia Bank margin. Geophysical Journal International, in prep.
Zhao, X., 1995. Magnetic signatures of serpentinites and peridotites from ODP Leg 149 and their implications. In Sawyer, D.S., Whitmarsh, R.B., Klaus, R.B., and Shipboard Scientific Party, eds., Proc. ODP, Scientific Results., College Station, TX (Ocean Drilling Program), 149, in press.
Author information
Authors and Affiliations
Editor information
Editors and Affiliations
Rights and permissions
Copyright information
© 1995 Springer Science+Business Media Dordrecht
About this chapter
Cite this chapter
Sibuet, JC. et al. (1995). Constraints on Rifting Processes from Refraction and Deep-Tow Magnetic Data: The Example of the Galicia Continental Margin (West Iberia). In: Banda, E., Torné, M., Talwani, M. (eds) Rifted Ocean-Continent Boundaries. NATO ASI Series, vol 463. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-0043-4_11
Download citation
DOI: https://doi.org/10.1007/978-94-011-0043-4_11
Publisher Name: Springer, Dordrecht
Print ISBN: 978-94-010-4024-2
Online ISBN: 978-94-011-0043-4
eBook Packages: Springer Book Archive