Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021 Non-volcanic rifted margins, continental break-up and the onset of sea-floor spreading: some outstanding questions G. BOILI/3T 1 & N. FROITZHEIM 2 l Observatoire Ocdanologique, Gdosciences Azur, B.P. 48, 06235 Villefranche-sur-Mer, France (e-mail: [email protected]) 2Geologisches Institut, Nussallee 8, D-53115 Bonn, Germany Abstract: During the last 20 years, regional studies on the West Iberia margin and on the former margins of the Tethys have considerably advanced the understanding of processes related to continental break-up and the onset of sea-floor spreading. However, some ques- tions remain outstanding. To tentatively answer these, a coherent interpretation of available data is proposed, based on the detachment fault concept applied to the continental as well as the oceanic lithosphere, and on the hypothesis of a multi-staged rifting process. The interpretation addresses the nature of the lower crust beneath non-volcanic passive margins, the origin of ophicalcites, the probable time gap between syn- or post-rift crystallization of gabbros and extrusion of basalts on the sea floor, and the significance of dipping reflectors within oceanic lithosphere adjacent to non-volcanic passive margins. The interpretation also considers the symmetry v. asymmetry of continental rifting and break-up, the location of the ocean-continent boundary, and the possible association of magnetic quiet zones with ultramafic sea floor (serpentinized peridotite) bordering non-volcanic passive margins. Twenty years after the discovery by dredging of not) occurring in slow-spreading oceanic litho- the peridotite ridge bounding the Galicia sphere, especially in the vicinity of passive mar- margin (Boillot et al. 1980; Sibuet et al. 1987), gins? Are they seismic images of detachments? and after three Ocean Drilling Program (ODP) (5) Are the fault systems accommodating rifting Legs (103, 149, 173; Fig. 1), three French and continental break-up symmetric or asym- diving cruises, and several British, German, US metric (with respect to the rift axis) on a litho- and French geophysical surveys on the West spheric scale? What are the evolutionary stages Iberia margin, the advance in the understanding of the rift from the initial lithosphere stretching of processes controlling continental break-up to continental break-up? (6) Where is the actual and the onset of sea-floor spreading appears to ocean-continent boundary (OCB)? Is it located be spectacular, and many of the major problems at the lithospheric or at the crustal OCB? (7) seem to be solved. It is of interest, however, to What is the significance of magnetic quiet consider also some outstanding questions, zones adjacent to rifted margins, in so far as which necessitate both further research and they do not correspond to periods of stability of acquisition of new data: (1) What are the the global magnetic field? Do they represent respective components of serpentinized perido- serpentinitic sea floor resulting from tectonic tite, syn-rift gabbro and pre-rift lower continen- unroofing of mantle rocks and their hydrother- tal crust in the lower crust of passive margins? real alteration? (8) Where are the best study (2) Does a time gap exist between crystallization areas, at sea and on land, for answering these of gabbros and extrusion of basalts at slow- questions? spreading oceanic ridges, as predicted by the This paper does not attempt a review of the model of unroofing of peridotite and gabbros by scientific results recorded by the international detachment faulting? (3) What is the geody- community from the regional study of the West namic significance of ophicalcites covering Iberia margin or of Tethyan margins in the ultramafic sea floor in the present ocean and in Alps, nor does it present new data. The data Alpine ophiolites? What are the respective roles mentioned in the text and their interpretations of, among other possible processes, hydraulic are, or will soon be, published and accessible. fracturing and extensional detachment faulting Instead, we will focus on some remaining scien- in the genesis of these rocks? (4) What is the tific problems, to point out possible new origin of dipping reflectors (continentward or research objectives for the near future. In From: WILSON, R.C.L., WHITMARSH,R.B., TAYLOR,B. & FROITZHEIM,N. 2001. Non-Volcanic Rifting of Continental Margins: A Comparison of Evidence from Land and Sea. Geological Society, London, Special Publications, 187, 9-30. 0305-8719/01/515.00 © The Geological Society of London 2001. Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021 10 G. B OILLOT & N. FROITZHEIM 14 ° 13 ° 12 ° 11 o 10 ° 9 ° 8°W ~ , ~ 45°N ~o / .... 43 ° ÷ ( ÷ Galicia 43 ° J |• ( i Portug al ,~ 41 ° 41° ~m + 897 ( : ~o~o ~( I ) # # , Iberia Abyssal Plain 40 ° R4 ÷ 40 ° | 39 ° ~ ~ 39 ° 14 ° 1 .':1° 19o 11 o 1N ° _¢1° R ° Fig. 1. The West Iberia margin to the north of 39°N. Numbers refer to drilling sites of ODP Legs 103, 149 and 173. Drilling data were completed by diving data on the Galicia margin and adjacent sea floor (about 300 samples recovered from pre-rift sediments and oceanic or continental basement; Boillot et al. 1988a, 1995b). R1 -R4, segments of the peridotite ridge (after Beslier et al. 1993). A-B, location of the cross-section depicted in Figure 2. Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021 OUTSTANDING QUESTIONS 11 addition, we will discuss the process of slow (4) Beneath the most distal part of the passive sea-floor spreading, which immediately follows margin, the detachments imaged by seismic continental break-up. It is only at fast-spreading reflectors result in the tectonic contact between ridges, where the lithosphere is extremely thin, the thinned continental crust of the margin and that the partial melting of peridotite as a result the underlying serpentinized peridotites. Conse- of adiabatic decompression is efficient enough quently, the oceanward boundary of the conti- to completely accommodate plate divergence. nental crust coincides with the trace of the At slow-spreading ridges, as in continental rifts, detachment on the sea floor (Etheridge et al. extensional tectonics play a very important role, 1989). exclusive in some cases, in balancing plate (5) The serpentinization of peridotites and divergence. The data collected from the West the retrometamorphism of gabbros in the Iberia margin and adjacent oceanic area strongly greenschist facies result from syn-, and possibly constrain the models accounting for these tec- post-rift hydrothermal activity at shallow levels tonic processes. of the lithosphere (Agrinier et al. 1988, 1996; In this paper, we consider as firmly estab- Sch~irer et al. 1995). The seismic Moho is the lished the following results of previous studies. boundary at depth between fresh and serpenti- (1) The passive margin of Iberia, and prob- nized peridotite, i.e. the hydrothermal front ably many other non-volcanic passive margins (lower limit of hydrothermal circulation, or, in the world, is bordered by a continuous belt of more probably, thermal barrier for serpentiniza- serpentinized peridotite and, locally, gabbro. tion; see P6rez-Gussiny6 et al. 2001). Moreover, This ultramafic sea floor culminates along the the serpentinite layer extends continentward peridotite ridge, a structural high of the base- beneath the most thinned continental crust of ment, partly buried by post-rift sediment on its the margin and oceanward beneath a thin, post- eastern side, 10-12km wide and 2-3km high rift basalt layer (Boillot et al. 1989b, 1991; Recq et al. 1996; Whitmarsh et aL 1996c; Dean (Mauffret & Montadert 1987; Boillot et al. et al. 2000). 1987b, 1988a,b,c; Beslier et al. 1990, 1993). (6) Oceanic basalts locally cover serpenti- However, the ultramafic sea floor is wider than nized peridotite and gabbro to the west of the the peridotite ridge (>100km wide in the peridotite ridge (Kornprobst et al. 1988; Malod Iberia Abyssal Plain; Beslier et al. 1996; Whit- et al. 1993; Charpentier et al. 1998) and thicken marsh & Sawyer 1996; see also Manatschal oceanward (Sibuet et al. 1995; Whitmarsh et al. et al. 2001; P6rez-Gussiny6 et al. 2001). 1996c). Gabbros, however, appear to be older (2) The ultramafic sea floor results from syn- than the continental break-up (and thus, older rift tectonic unroofing of subcontinental perido- than post-rift oceanic basalts) when radiometric tite intruded by gabbro. Simple shear along ages are available (F~raud et al. 1988, 1996; lithospheric detachments may have been the Sch/irer et al. 1995, 2001). main process in stretching and thinning of the Figures 2 and 3 are schematic depictions of lithosphere and in the unroofing of subcrustal the crustal structure of the OCB off the Galicia mantle (Boillot et al. 1987a, 1989a, 1995a; margin (Fig. 2) and of the African (Apulian) Beslier & Brun 1991; Brun & Beslier 1996). margin of the Mesozoic Tethys (Fig. 3) as (3) The geophysical signatures of the main reconstructed from field data in the Swiss-Ita- lithospheric faults and shear zones responsible lian Alps by Trommsdorff et al. (1993). Figure for mantle unroofing are strong seismic reflec- 4a and b shows simplified diagrams of conti- tors, the most famous of them being the S nental break-up inspired by the analogue model reflector of the Armorican and Galicia margins experiments of Brun & Beslier (1996). (De Charpal et al. 1978; Le Pichon & Barbier 1987; Hoffmann & Reston 1992; Sibuet 1992; Krawczyk & Reston 1995; Reston et al. 1996), The lower crust in non-volcanic named H in the Iberia Abyssal Plain (Beslier et al. 1995; Krawczyk et al. 1996; Manatschal passive margins et al. 2001). The geological signature of the tec- To correctly address the problem of the nature, tonic shearing and exhumation of subcrustal behaviour and fate of the pre-rift continental mantle is the foliation and brecciation of the crust during rifting and continental break-up, it peridotite and intruded gabbro, caused by defor- is necessary to distinguish between three kinds mation at falling temperature and decreasing of lower crust beneath passive margins, as fol- pressure (Girardeau et al.