Non-Volcanic Rifted Margins, Continental Break-Up and the Onset of Sea-Floor Spreading: Some Outstanding Questions

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 questions 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 °

|• ( 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. 1988, 1998, 1999; lows. Kornprobst & Tabit 1988; Beslier et al. 1990, (1) The seismic lower crust made of serpenfi- 1996; Cornen et al. 1996a,b). nized peridotite, of syn-rift (and possibly post- Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

12 G. BOILLOT & N. FROITZHEIM W E OCEAN CONTINENT CRUSTAL BOUNDARY PERIDOTITE(Ori,ing ~i.e .RIDGE 63Z) I Dive Site 11.. SEDIMENTS ~- _ B~SALT ,~ =,, + + + + 10- km

ii ii t ÷ ÷ + ÷ A B Fig. 2. Synthetic cross-section of the Galicia passive margin; location shown in Figure 1. Beneath the peridotite ridge, the Moho is the boundary between 'fresh' and partly serpentinized peridotite, i.e. a limit of mineralogical phase change corresponding to a palaeo-hydrothermal front. Oceanward, to the west of the peridotite ridge, the sea floor is made also of serpentinized peridotite, covered by a thin basaltic layer emplaced after the continental break-up (see Fig. 5). On the eastern side of the ridge, the deepest blocks of the margin, made of upper continental crust, rest directly on serpentinized peridotite through a tectonic contact (a detachment), the image of which is the S seismic reflector. Normal faults bounding the crustal blocks of the margin are steep (60- 70°), and root at depth in the subhorizontal detachment located at the bottom of the blocks (after Boillot et al. 1995c). W E FORNO MALENCO MARGNA MORB OC JB JB

CONTINENTAL CRUST \" ======I\/\ ""

LITHOSPHERIC MANTLE ~s~= MANTLE

Fig. 3. Ocean-continent transition in the Tethyan Ocean (Ligurian basin) from which the Forno-Malenco and Margna nappes are derived. G, gabbro; T, pieces of lower continental crust, metamorphosed in the granulite facies; SE, serpentinite; OC, ophicalcite resting on the serpentinite sea floor; JB, Jurassic breccias; MORB, mid-ocean ridge basalts (after Trommsdorff et al. 1993). rift) origin. This is necessarily located in the (2) The lower crust made of underplated most distal part of the margin, where hydrother- gabbro, of syn-rifl origin. Non-volcanic passive real circulation and metamorphism of mantle margins are not entirely anaagmatic. For rocks are possible, owing to the thinning and instance, peridotites sampled along the West fracturing of the upper continental crust (Boillot Iberia margin underwent about 8-10% of et al. 1989b). In this case, the definition as magma extraction by syn-rift partial melting lower crust is based on the seismic character (Girardeau et al. 1988; Cornen et al. 1996a,b). The resulting tholeiific magma, of which no (P-wave velocities ranging between 6.5 and superficial volcanic equivalent is known on the 7.9kin s-l; see Recq et al. 1996; Whitmarsh margin, was necessarily underplated as gabbro, et al. 1996c; Chian et al. 1999; Dean et al. at least partly, beneath the thinned crust of the 2000) and not on its geological nature. Actually, rift (Fig. 4a and b). On the Galicia margin, 'undercrusting' (crustal thickening by addition such gabbros, initially crystallized at high tem- of serpenfinite at the bottom of the crust) results perature, were sheared at falling temperature and in this case from hydrothermal transformation finally retrometamorphosed in the greenschist of fresh peridotite into serpentinized peridotite facies. They were recovered at the top of the of lower density and seismic velocity. 'lower plate', close to the tectonic contact sep- Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

OUTSTANDING QUESTIONS 13 RIFT t7;'+'7"Vz*t'l CC Km (a)

p+g P 50 km

0 (b) ¢= M ~^^',: :-,:"'*"" t,:~"~" ~"['[~;" u~.~.t,^,:'~"['?'L, ^*^ ^ ^ 1

MARGIN 2 .I SLOW RIDGE MARGIN 1

o ~ (c) M=~ 1

l l

(o)

...... ,., ,,, ,,, ,,, ^ ^ ~" ^ ,.,...... ^ ^ ^ ^ ^ ,*. ,., ^ * ^ .,, ,., ^ ,,,, ^ M

FOLIATION (p+g) OPHICALCITE SEDIMENT , 10 km , (e) ~;~::?::;~ ~:~~ ~; i:;~!;/;;:;~:~ ~:~:j~ ;;%~ ;/;t:~iW-;~~'~? ~i';:; ~ ~; i: ~:~;~:~,,;~

15 ...... ~"><'-~ ~ ' 4 ...... Fig. 4. Conceptual model of continental break-up and onset of sea-floor spreading at non-volcanic passive margins, cc, continental crust; b, basalt; g, gabbro; p, peridotite (serpentinized near the surface). The asthenosphere-lithosphere boundary is not represented (after Boillot & Coulon 1998). Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

14 G. BOILLOT & N. FROITZHEIM arating the upper continental crust from serpen- 2001). However, U/Pb SHRIMP analysis of zir- tinized peridotite (chlorite-bearing schist mylo- cons from the metagabbm yielded 270Ma nite in Fig. 5). The U-Pb age of zircons (Rubenach, pers. comm.). These results show included in the rock (122.1 + 0.3 Ma; Sch~irer that the Leg 173 metagabbros-amphibolites are et al. 1995, 2001) clearly indicates that the gab- not syn-rift, but belong to Variscan (pre-rift) bros crystallized during the rifting stage, which lower continental crust captured in the OCB. lasted from 140 to 115 Ma in this part of the A similar but clearer situation is recorded in margin (Boillot et al. 1987a, 1988b). On the former Tethys margins (Malenco and Tasna other hand, the current position of metamor- areas; Fig. 3) where sheared gabbms intercalated phosed gabbros at the top of mantle rocks between upper continental crust of the margin suggests that they were underplated beneath the and exhumed mantle peridotite have yielded continental crust of the rift, before they under- pre-rift, Permian U/Pb zircon ages (Mtintener & went shearing, uplift and tectonic denudation in Hermann 1996; Hermann et al. 1997; Froitz- the latest stage of the rifting (Sch/irer et al. heim & Rubatto 1998). These gabbros demon- 1995, 2001). strate a widespread thermal and tectonic event (3) The pre-rift lower continental crust. Close that affected the Hercynian belt at c. 270Ma. to the OCB in the Iberia Abyssal Plain, metagabbro was cored during ODP Leg 149 (Sawyer Their exhumation is due to Mesozoic rifting, et al. et al. 1994; Cornen et al. 1996b; Whitmarsh and not their formation (see Desmurs et al. 1996b). The rock was tentatively inter- 1999; Mtintener & Hermann 2001). preted as a syn-rift intrusion, based on a plagio- These findings indicate that pre-rift lower clase 4°Ar/39Ar cooling age of 136Ma (F~raud crust exists along the OCB of passive margins, et al. 1996). During Leg 173 (Ocean Drilling although it is rare and scattered. In fact, it Program Leg 173 Shipboard Scientific Party seems from available sampling that the pre-rift 1998; Whitmarsh et al. 1998), metagabbro and lower crust is thin and dismembered at the rift amphibolite were cored nearby, and yielded a axis when continental break-up occurs, and that similar plagioclase 40 Ar/-39 Ar age of 137Ma it can apparently be completely missing at the and a hornblende 4°Ar/39Ar age of 161 Ma OCB (this is the case for the Galicia margin; (Turrin, pers. comm.; see Manatschal et al. see Fuegenschuh et al. 1998.

UPPER PLATE (CONTINENTAL CRUST)

RR.

...... ~?i:~':!!~¸ S.R.

...... SAN DSTON " ~.';. :~4--~- CATAC;LAST'ITE BASALT • ~~',~-&~& ~>~ ~<~ ~-~ -¢ ~j~_~ ~_;, ~ ~ ~ ,~ ~ C,~o' ~ PERIDOTITEAND GRANITE BRECCIA I ~ S ~ CHLORITE BEARING SCHIST MYLONITE I ~ ~ ULTRAMYLONITE : LOW TEMPERATURE MAIN DEFORMED PERIDOTITE DETACHMENT ~ ~

~ ~ "4--'- INTERMEDIATE TO HIGH TEMPERATURE DEFORMED PERIDOTITE AND GABBRO ~ ~ IN LOCALIZED SHEAR ZONES LOWER PLATE (MANTLE) Fig. 5. Schematic cross-section showing the tectonic contact between the upper continental crust and serpentinized peridotites at the edge of the Galicia passive margin, from diving data (after Boillot et al. 1995b). ER., post-rift sediments; S.R., syn-rift sediments. Not to scale. Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

OUTSTANDING QUESTIONS 15

Time gap between gabbro crystallization close to the age of the first sediment (radiolarite) deposited on the mafic basement, generally and superficial basalt flooding of Callovian-Oxfordian age (e.g. Ohnenstetter In the previous section, we mentioned some et al. 1981; Bill et al. 1997; Rubatto 1998; Late Palaeozoic ages of gabbros recovered close Rubatto et al. 1998). However, Pinet et aL to the OCB in the Iberia Abyssal Plain or (1989); Caby (1995); Costa & Caby (1997), associated with Alpine ophiolites. In contrast, from their studies of the Queyras and Mont- gabbros at the OCB of the Galicia margin, Genbvre ophiolites, concluded that intrusion and where continental break-up occurred at 115 Ma, intra-oceanic deformation of gabbros clearly are 122.1 _ 0.3Ma old (Schgrer et al. 1995) predate extrusion of the overlying pillow basalts. whereas the oldest oceanic basalt sampled sev- The occurrence of radiolarite beneath basalt eral kilometres off the margin is c. 100 Ma old (Bortolotti et al. 1991) also confrms that (F6raud, in Malod et al. 1993). Therefore, it is basalts postdate gabbros in many cases. necessary to consider, for oceanic crust of the The question posed above concerning the current oceans as well as for ophiolites, the time taken for gabbro formation and subsequent probability of significant time gaps between the unroofing remains open because of the lack of crystallization of gabbros and the emplacement high-precision radiometric ages for gabbros and basalts from the same locality, in the current of overlying basalts. This point is of importance ocean as well as in ophiolites. However, deter- as it concerns the assumed unroofing process of mination of the actual time gap, if any, between gabbro, first crystallized at depth, then sheared gabbro crystallization and basalt extrusion is a and brought up by extensional detachment fault- major objective in constraining the tectonic pro- ing to the sea floor. How much time does this cesses at slow-spreading, or very slow-spreading process take, from the formation of the gabbro oceanic ridges (for further discussion see Des- to its final exposure on the sea floor? murs et al. 2001). Published data that would allow comparison of the ages of gabbro and basalt at the same locality in present oceanic areas formed by slow or ultra-slow spreading have not been found. Ophicalcite and lithospheric shear zones With the notable exception of Permian gabbros Ophicalcites (Figs 6 and 7) are serpentinite and (see above), modem radiometric ages for gab- gabbro breccias covering ultramafic-gabbroic bros in Alpine ophiolites from the Ligurian basement in many ophiolite sheets from the basin range between 185 and 158Ma, very Tethyan ocean (e.g. Bernoulli & Weissert 1985;

~- =~ "--__ -__- _ _ ._ - _-~.=__-_ Low. Cret. Up. Jur. Rad. Chert (Mid. Jur.)

B BASALTS 11O0 m G GABBROS 500 m S SERPENTINITES I I OC OPHICALCITES

11 I I. 1 II~1 1 1 I I I I I I l I I I I I II i . , . _. . . . • . . . .~ - , , . , , - . • , ~,----~ -- . • • . . . . .

:.OCz.• . . :-... . '.. .~.,. . .'..:. . ~ ,~ • . . .:.. ~ - • . ... -: ^,,~-:...'• . • . .. .:. . . ' . ..~..tZ.Z~.~,,~<,~,-,-~,,x,~, • ~ ~ ~'-. ~ ~

~,-,,,,-,,;,;,,,,,,-,-,, -,,, ~, ,;,/-',-,',;, ;,-,-,-,,;,/,,-,--,;, ;,,,,,,,,-,;,;,-,,--,,;, ;,-,,,--,;, ~,-,-,-,,;` ~,-,,,,-,

Fig. 6. Sea floor of the Ligurian ocean in late Jurassic time. Post-rift sediments directly overlie serpentinized peridotites, gabbros or basalts, depending on location. Ophicalcite 1 (OC1) is a tectonic breccia, whereas ophicalcite 2 (OC2) is a sedimentary formation resulting from the reworking of OC1 on the sea floor. Rad. Chert: Radiolarian chert (after Lagabrielle & Lemoine 1997). Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

16 G. BOILI.DT & N. FROITZHEIM

Fig. 7. Photograph of ophicalcite in the French Queyras ophiolite (Western Alps). (Compare with Fig. 8)

Treves & Harper 1994; Lagabrielle & Lemoine Ophicalcites 1 and 2 are locally covered by 1997). The matrix of the breccia is either car- oceanic basalt (pillow basalt and pillow brec- bonate (mainly calcite) or second-generation cia), which formed the actual sea floor for sedi- serpentinite. The 'clasts' are variable in size ments (Lagabrielle et al. 1984; Lagabrielle & and shape (Fig. 7). According to the fabric of Lemoine 1997; Manatschal & Nievergelt 1997). the rock, two kinds of ophicalcites can be This stratigraphic setting implies that sedimen- defined (Tricart & Lemoine 1989). Ophicalcites tation (OC2) occurred before basalt extrusion, at 1 (OC1) are tectonic breccias resting directly on least in some places. This conclusion is con- deeply fractured basement. Consisting of ser- firmed by the local occurrence of radiolarite pentinite and gabbro cross-cut by calcite, dolo- between the ophicalcites and basalt (Bortolotti mite or serpentinite veins, OC1 show a gradual et al. 1991), which implies a time gap between transition into fractured ultramafic basement the crystallization of gabbro and the emplace- with which they share the same petrological ment of oceanic basalts as mentioned in the composition. Ophicalcites 2 (OC2) are detrital previous section. sediments generally devoid of fossils. OC2 Serpentinite and gabbro breccias cemented result from submarine reworking and short-dis- by calcite, similar to ophicalcite, have been tance transport of underlying OC1. found at the top of ultramafic basement in the Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

OUTSTANDING QUESTIONS 17

Iberia Abyssal Plain (Whitmarsh et aL 1996b, both. Alpine ophicalcite OC1 may then be 1998; Wilson et al. 2001). Cataclasis, at least in interpreted as a brittle fault rock formed along some cases, appears to have occurred relatively extensional detachment faults (Florineth & late, possibly after continental break-up, as the Froitzheim 1994). In the case of the Galicia calcite cement crystallized at low temperatures margin, the detachment was clearly syn-rift and from sea water (<80°C; Agrinier et al. 1988). associated with the continental break-up pro- Another example was discovered on the Galicia cess; in the Alps, part of the ophicalcites are margin in the form of a tectonic breccia located syn-rift (e.g. in the Tasna nappe of the Eastern at the top (the brittle level) of the lithospheric Alps; Florineth & Froitzheim 1994); others may shear zone separating the peridotite ridge and be post-rift and formed at slow-spreading ocea- the upper continental crust of the margin (Boil- nic ridges (e.g. in the Western Alps, Lagabrielle lot et al. 1995b; Figs 5 and 8). This tectonic & Lemoine 1997; see below for further discus- breccia includes clasts from both the upper and sion). The possible setting of ophicalcites lower plates, i.e. of continental and mantle formed during slow spreading is indicated in origin (granite and metamorphic sandstone, ser- Figure 4e. pentinite and gabbro, respectively). This mix of The problem of the origin of ophicalcite con- fragments originating from rocks in tectonic cerns directly the process of sea-floor formation. contact is strong evidence in support of the Is the fracturing of serpentinite a hydraulic pro- relationship of the breccia to the shear zone sep- cess related to the oceanic hydrothermal circula- arating the two kinds of rocks. Continental and tion (Frtih-Green et al. 1990; Treves & Harper oceanic clasts have also been found interbedded 1994), a result of brittle deformation along in some Alpine ophicalcites, but are of detrital transform faults (Lemoine 1980; Weissert & character (e.g. Polino & Lemoine 1984; Ber- Bernoulli 1985), or a result of cataclasis along noulli & Weissert 1985). extensional faults or detachments, as proposed The resemblance in lithological character and in this paper? In the case of the Galicia margin, structural position between the Galicia margin the detachment fault interpretation (which does breccia (Figs 5 and 8) and Alpine ophicalcite not exclude the hydraulic fracturing hypothesis, (Figs 6 and 7) suggests a similar origin for as fluids are active in faulted zones) is sup-

Fig. 8. Submarine photograph showing the tectonic breccia ('peridotite and granite breccia' in Fig. 5) located at the top of the mantle terrane, on the northwestern slope of the Galicia Bank. (Compare with Fig. 7.) Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

18 G. BOILI_DT & N. FROITZHEIM ported by the structural setting of the breccia. In hand, the common occurrence of flaser gabbros the case of Alpine ophicalcites, more field stu- and foliated serpentinized peridotites in ophio- dies are necessary to reconstruct the geometry lites and in the oceanic lithosphere indicates of ophicalcite-bearing faults and to distinguish deformation under elevated temperature and between rift-related, transform-related, and pressure, i.e. in shear zones at depth, before slow-spreading-related ophicalcites. The chal- uplift and unroofing on the sea floor. Moreover, lenge is important: if it can be established that extensional detachment faults and related core ophicalcites are the geological signature of complexes have also been identified along the uppermost parts of extensional shear zones, present Mid-Atlantic Ridge (e.g. Tucholke et al. then ultramafic sea floors covered by the breccia 1997, 1998; Blackman et al. 1998). have to be considered as structural surfaces It is concluded that shear zones probably play (unroofed detachment fault planes). Beyond, the an important role in the sea-floor spreading pro- question concerns the process of tectonic sea- cess following continental break-up, and, more floor spreading at slow ridges, and the possible generally, at slow ridges, where the magmatism role of dipping reflectors in the sea-floor empla- is episodic. The next section of the paper is an cement of ultramafic rocks and foliated gabbros attempt to visualize this process. (see the next two sections).

A conceptual model for the onset of sea- Dipping reflectors and shear zones in floor spreading oceanic lithosphere The detachment hypothesis, first applied to con- Dipping reflectors are not restricted to volcanic tinental rifts, has also been tested by analogue margins. Sloping mostly continentward, some- modelling of tectonic accretion at slow ridges times oceanward, they appear also on many (Schemenda & Grocholsky 1994). The experi- seismic lines recorded on oceanic lithosphere ments showed that sea-floor spreading may be adjacent to non-volcanic passive margins. For accommodated by upward motion of mantle example, clear images of such reflectors were rocks along detachments, dipping continentward recorded in the Iberia Abyssal Plain off the on each side of the ridge. The following con- Portugal passive margin (Pickup et al. 1996; ceptual model (Fig. 4) accounts both for these Fig. 9), and in the Biscay Abyssal Plain bound- experiments and for the available seismic and ing the Goban Spur (Masson et al. 1985). Both geological data from passive margins and the these areas comprise oceanic lithosphere adjacent oceanic lithosphere. It addresses the emplaced immediately after the rifting of the process of ultra-slow sea-floor spreading margin, when one can assume that sea-floor immediately following rifting and continental spreading was particularly slow. Similar reflec- break-up. The model is a simplification in that tors have also been observed in the oceanic it does not account for the 3D structure of slow lithosphere bordering the Eastern Canada ridges, where detachments seem to be confined margin (Keen & De Voogd 1988). These fea- to the extremities of ridge segments or to non- tures cannot be interpreted as images of volca- transform offset zones of the ridges (e.g. nic piles, as there is no regional evidence of Tucholke & Lin 1994; Tucholke et al. 1997, extensive volcanism. Moreover, they dip prefer- 1998; Blackman et al. 1998; Ranero & Reston entially toward the continent, whereas along 1999; Gracia et al. 2000). volcanic margins the reflectors dip toward the In stage A, the lithosphere is thinned as a ocean. result of motion along conjugate shear zones The continentward dipping reflectors imaged (Brun & Beslier 1996), and gabbros are in the oceanic lithosphere bordering passive intruded and underplated beneath the rift at the margins resemble the S reflector of the Galicia top of the mantle (see Scharer et al. 1995, and Armorican margin, or the H reflector in the 2001). Iberia Abyssal Plain. It is probable that they In stage B, the continental crust is broken up also represent palaeo-shear zones within the at the rift axis. Mantle peridotite and syn-rift crust and mantle terranes. The frequent associ- intruded or underplated gabbros are unroofed ation of reflectors dipping both toward the con- and metamorphosed as a result of hydrothermal tinent and toward the ocean suggests the processes (including serpentinization). Simul- occurrence of conjugate shear zones that have taneously, unroofed serpentinized peridotite acted simultaneously during tectonic extension experiences ultimate tectonic extension, indi- of the lithosphere at slow ridges (Cannat et al. cated by normal faulting of the serpentinite at 1997; Lagabrielle et al. 1998). On the other every scale, especially on both sides of the peri- Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

OUTSTANDING QUESTIONS 19

W E

lV 13 (a) km

.,9 ¸ ,,,

(b) Fig. 9. Continentward dipping reflectors (indicated by bold lines) in the Iberia Abyssal Plain basement (off Portugal). (a) Vertical exaggeration x2; (b) another section of the same line, without vertical exaggeration (after Pickup et al. 1996). dotite ridge (Fig. 2). As plate divergence accel- the two conjugate margins, erupts onto the sea erates, the asthenosphere rises more rapidly and floor and covers the previously tectonically undergoes more partial melting because of denuded peridotite, gabbro and ophicalcite. As faster decompression. New basaltic magma a result, there is a time gap between gabbro and intrudes the thin lithosphere remaining between basalt formation. Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

20 G. BOILLOT & N. FROITZHEIM

In stage C, new sheets of peridotite and passive margins and within the oceanic litho- gabbro are emplaced. The plate divergence is sphere resulting from slow spreading. These not rapid enough to allow the asthenosphere to reflectors can be interpreted as seismic signa- reach the surface. As beneath continental rifts tures of low-angle normal faults (detachments). (stage A), mantle rocks are exhumed by low- However, considerable uncertainties remain angle normal shear zones or faults now imaged about the transition in space and time between as continentward dipping reflectors. Successive continental rifting and sea-floor spreading sensu shear zones are localized, or initiated, at the stricto. In particular, the lithospheric shear level of superficial magma chambers, where the zones, depicted with a uniform, continentward strength of the lithosphere is lowered by the pre- dip in Figure 4, may form conjugate sets sence of magma. For example, at Gorringe accounting for the subordinate occurrence of Bank, off South Portugal, a gabbro laccolith is oceanward dipping reflectors in addition to the included within serpentinized peridotite. The continentward dipping ones. top and bottom boundaries of the laccolith were intensely deformed under high temperatures along normal shear zones dipping shallowly Rift evolution and the geometry of towards the continent, before peridotite and gabbro were exposed on the sea floor at the extensional fault systems onset of sea-floor spreading (Girardeau et al. A major contribution towards resolution of 1998, 1999). The kinematic evolution and struc- whether the fault and shear zone systems ture are in agreement with the above interpret- accommodating rifting are symmetric or asym- ation of continentward dipping reflectors metric, with respect to the axis of the rift, on a beneath the Iberia Abyssal Plain. lithospheric scale has been made by analogue Finally, as a result of increasing rate of plate experiments of continental rifting (Beslier & divergence and a correlative increase in partial Brun 1991; Brun & Beslier 1996). These exper- melting within the mantle, the basaltic layer iments, which used sand as analogue for brittle covering the mantle rocks becomes thicker and layers and silicone for ductile layers of the thicker oceanward and the crust thus gradually lithosphere, resulted in boudinage of brittle becomes typically oceanic. layers, representing the upper crust and the According to the model, the role of tectonics uppermost mantle, accommodated by shear is greater than that of magmatism in the initial zones located (1) in the lower crust and (2) stages of sea-floor spreading, and, more gener- below the base of the uppermost mantle. The ally, in the case of slow- or ultra-slow-spreading model developed from these experiments is fun- ridges. The model accounts in a coherent way damentally symmetric with respect to the rift for field data, recovered at sea and in ophiolites axis, and envisages the continuous activity of on land: (1) in many cases, peridotites and gab- the shear zones mentioned above, from the bros are foliated as a result of deformation in early stages of rifting to the continental break- shear zones at depth, under high temperatures. up. Although this model represents a good first- Overlying basalts are undeformed, except for order approximation of the rifting process, three local superficial faulting. (2) Where age con- sets of observations necessitate some modifi- straints are available, gabbros often appear to be cations: (1) the model does not take into older than the overlying basalts. The time gap account the rheological and thermal changes in is variable, from several tens (to hundreds) of the lithosphere during the rifting and continen- million years at the foot of passive margins tal break-up processes, and especially the pre- where pre-rift gabbros crop out in some places, sence of magma chambers at the top of the to a few million years and probably less at slow mantle. (2) Field observations in the Alpine oceanic ridges. The delay between gabbro crys- former margins (Froitzheim & Manatschal tallization and basalt flooding is also supported 1996; Hermann & Mfintener 1996; see also by the frequent intercalation of a sedimentary Handy et al. 1999) suggest a discontinuous, event (OC2 and/or radiolarite) between intrusive two-stage evolution of rifting, with a fundamen- and extrusive igneous rocks. (3) A progressive tal reorganization of fault patterns between the transition appears to exist between the areas two stages. In the Alps, the second stage where the peridotites exposed at the sea floor appears to be governed by asymmetric, unidirec- are of subcontinental (lithospheric) origin, and tionally dipping detachment faults (Fig. 10). A areas where such rocks are of oceanic (astheno- similar two-stage evolution is also proposed for spheric) origin. (4) Continentward dipping the Iberia-Newfoundland conjugate margins reflectors occur, at least in some places, within (Manatschal & Bernoulli 1999). (3) Drilling in the ultramafic-mafic basement bordering the the Iberia Abyssal Plain during Leg 173 Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

OUTSTANDING QUESTIONS 21 revealed the presence of probable pre-rift lower on the common inference that the rheology of crust (Rubenach et al., pers. comm.; see the first the lower crust is generally governed by quartz section of this paper). The lowerocrustal terrane and therefore that the lower crust behaves as a is made up of metagabbro to amphibolite. The weak layer during rifting. If the drilled rocks are scarcity of quartz within these rocks casts doubt representative of the pre-rift lower crust of the

W FUTURE DETACHMENT FAULTS A I PLIENSBACHIAN AFTER RIFTING PHASE 1 I L_~# 2

t • • • ..1

CRUST

LITHOSPHERIC 100 km MANTLE

100 km I i~l~w lithosphere

B MIDDLE JURASSIC (END OF RIFTING PHASE 2) UNROOFED MANTLE (Platta) UNROOFED MANTLE (Malenco)

C IJURASsIC-CRETACEOUS l BOUNDARY (DRIFTING STAGE)I Lithospheric Crustal Oceanic crust ocean-continentboundary ocean-continentboundary Passive

Fig. 10. Hypothetical kinematic evolution of the Tethyan ocean (Ligurian basin) and its passive margins in Mesozoic times. Pre-, syn- and post-rift sedimentary series are neglected. 'Stretched lithosphere' is pre-rift continental lithosphere; 'new lithosphere' is syn- or post-rift lithosphere derived from cooling asthenosphere. Be, Bernina; Br, Brian~onnais; M, Margna; S, Sella (after Froitzheim & Manatschal 1996). Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

22 G. BOILLOT & N. FROITZHEIM margin, the rheology of the lower crust is gov- question remains open, mainly because of the erned by plagioclase rather than quartz, and a difficulty of obtaining comparable datasets from six-layer rheological profile needs to be envi- conjugate margins (see below). saged instead of the four-layer profile assumed Thus, we suggest that rifting starts as an over- by Brun & Beslier (1996). The same conclusion all symmetric boudinage process, and continues may be drawn from the field study of a lower- with an asymmetric or symmetric detachment crustal terrane in the Malenco area (Italian fault stage. It is difficult, if not impossible, to Alps) where quartz is a minor constituent and test this hypothesis using data from the Iberia did not control the theology of the rocks margin alone. New numerical or analogue mod- (O. Mtintener, pets. comm.) elling is necessary. Kinematic balancing of Accordingly, it is proposed that the initial whole-crust profiles of conjugate margins (e.g. lithosphere configuration comprises six layers: a the Iberia and Newfoundland margins) is also brittle upper crust, ductile middle crust (hot necessary. Another promising target is to restu- enough for viscous behaviour of quartz), brittle dy the (unfortunately strongly metamorphosed lower crust, ductile lowermost crust (hot enough and deformed) former margins that once were for viscous behaviour of plagioclase), brittle conjugate with the well-preserved Tethys mar- uppermost mantle, and ductile lithospheric gins NW of Apulia (Froitzheim & Manatschal mantle overlying asthenosphere. 1996; Hermann & Mtintener 1996; Manatschal From this initial rheological structure, it is & Nievergelt 1997; Desmurs et al. 1999, 2001) anticipated that the rifting evolves in two stages: and NW of the Brianqonnais terrane (Florineth (1) the boudinage stage, controlled by boudi- & Froitzheim 1994). nage of brittle layers, including the uppermost crust, part of the lower crust and the uppermost layer of the mantle; (2) the detachment fault Location of the ocean-continent stage, preceding and allowing continental break- boundary, and significance of some up and tectonic unroofing of lower crust and magnetic quiet zones bordering mantle rocks. In a sense, the first stage exhibits an overall pure-shear geometry, whereas the passive margins second stage is typically governed by simple If the lithosphere is considered as a whole, the shear. peridotite ridge, derived from terranes initially The proposed scenario accounts for the fact located beneath a continental rift, belongs to the that detachment faults are generally late struc- continent and not to the ocean. The limit tures in the evolution of the margin (e.g. Mana- between the continental lithosphere and the tschal & Nievergelt 1997), and neither coincide 'true' oceanic lithosphere derived from astheno- nor are necessarily linked kinematically with sphere (convective mantle) is farther offshore. the earlier high-angle faults bounding tilted The OCB is then located between the 'stretched blocks. It is also supported by thermo-mechan- lithosphere' delineated in Figure 10 (i.e. the ical modelling (see P6rez-Gussiny~ et al. 2001). unroofed continental mantle) and the 'new Another question is whether (or why) the lithosphere' (i.e. the oceanic lithosphere derived continental break-up is a symmetrical or asym- from cooling asthenosphere). In other words, metrical process. From their analogue exper- the 'transition zone' (also referred to as the iment, Brun & Beslier (1996) came to the ocean-continent transition (OCT) by many conclusion that conjugate margins are probably workers) composed of subcontinental mantle is symmetrical at the lithospheric scale, whereas inseparable from the margin. Moreover, the seis- other workers have proposed they are fundamen- mic velocity structure in the OCT is different tally asymmetric (Wernicke 1985; Boillot et al. from that in typical oceanic crust (Dean et al. 1987a, 1989a; Lemoine et al. 1987; Etheridge 2OOO). et al. 1989; Wernicke & Tilke 1989; Froitzheim It is, however, difficult to accurately locate & Manatschal 1996, Fig. 10). In fact, both pos- the actual OCB according to this definition. sibilities may exist, depending on the partition How can an unambiguous distinction be made of motion between the two possible sets of between mantle rocks derived from subconti- faults during the detachment fault stage of the nental lithosphere and those derived from the rifting. The final stretching of the lithosphere asthenosphere rising beneath a slow-spreading can be accommodated either by subequal distri- ridge at the beginning of sea-floor spreading? bution of motion between conjugate structures To do so would require many rock samples to (subsymmetrical margins; Fig. 4), or by motion be recovered from the basement of the margin of one of the two sets that becomes dominant and the adjacent ocean and analysed geochemi- (asymmetric margins; Fig. 10). However, this cally. Both conditions are rarely fulfilled. More- Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

OUTSTANDING QUESTIONS 23 over, because of ubiquitous serpentinization, the the boundaries of continental crust on both geochemical signature of the original peridotite sides of the ocean together, and not the bound- is difficult to determine, and the subcontinental aries of exhumed subcontinental mantle. Before v. suboceanic origin of the rocks remains some- fitting the continents together, the amount of what ambiguous and controversial (e.g. Evans syn-rift stretching of the margins has to be & Girardeau 1988; Kornprobst & Tabit 1988; accounted for, which is possible for continental Cornen et al. 1996a). The origin of mafic rocks crust but would be very difficult, if not imposs- (gabbro, basalt, etc.) is more easily determined ible, for subcontinental mantle. using isotope data (e.g. Scharer et al. 1995; Another linked question concerns the origin Cornen et al. 1996b; Seifert et al. 1997; Char- of some magnetic quiet zones bordering passive pentier et aL 1998). However, the genesis of margins, when these quiet zones do not result these mafic rocks, as previously discussed, is from sea-floor spreading during periods of stab- not necessarily related to the emplacement of ility of the global magnetic field. Do these cor- the surrounding peridotite at the sea floor. respond, at least in some cases, to a zone of Wide-angle seismic data can be a criterion for ultramafic sea floor? separating the transition zone from the oceanic With few local exceptions, serpentinized peri- crust (Dean et al. 2000). Unfortunately, the dotites and associated gabbros bordering the method is difficult to apply everywhere in the West Iberia margin do not give rise to signifi- global ocean, and does not permit a precise cant magnetic anomalies (Sibuet et al. 1995; location of the oceanic crust boundary. Whitmarsh & Miles 1995; Whitmarsh et al. For these reasons it is probably more realistic 1996a) and so they constitute a magnetic quiet to locate the OCB with reference to the conti- zone. Typical oceanic anomalies are recognized nental crust boundary and not to the transition only to the west of the peridotite ridge where zone-oceanic crust boundary. The continental basalts occur. Then, the question is: are mag- crust of the margin can be identified without netic anomalies missing there because of the ambiguity using its geophysical and geological lack or scarcity of basalt on the serpentinized characters. On the other hand, oceanic domains peridotite, or because tectonic unroofing and can be defined as areas where no continental serpentinization of mantle rocks occurred crust occurs, the oceanic crust being made of during the Cretaceous magnetic quiet period? mafic or ultramafic rocks derived either from To answer the question unambiguously it is subcontinental lithosphere or from astheno- necessary to study passive margin and magnetic spheric mantle. In the case where mantle win- quiet zones younger than the West Iberia ones. dows and extensional allochthons of continental (The reason why serpentinized peridotite does crust exist along the margin (Figs 2 and 10), the not give rise to significant magnetic anomalies edge of the most oceanward located continental is the subject of another debate.) terrane can represent the OCB. According to this pragmatic definition, the Summary and conclusions peridotite ridge bordering the West Iberia margin is part of the ocean, like the 'normal' (1) The detachment-shear-zone models for oceanic crust and lithosphere, even if it orig- evolved rifts, as depicted in Figure 4, partly inates from subcontinental mantle. The OCB is derived from the analogue experiments of Sche- defined as the trace on the sea floor of the tec- menda & Grocholsky (1994) and of Brun & tonic contact (the detachment fault) separating Beslier (1996), account for the available geo- the continental crust of the margin from serpen- logical and geophysical data and processes tinized peridotites and gabbros exhumed in the recorded and interpreted on the West Iberia final stage of the rifting (Fig. 2). The boundary margin, especially: (a) the syn-rift tectonic corresponds to a major tectonic structure active unroofing of gabbro and mantle peridotite during extensional tectonics and passive later. It (transformed into serpentinite as a result of is not a more or less blurred transition zone but hydrothermal activity) in the uppermost level of a linear boundary that is relatively easy to delin- the lithosphere; (b) the age discrepancy between eate. And finally, the definition proposed here is pie- or syn-rift gabbros and overlying post-rift in agreement with the continental break-up con- basalts cropping out at the edge of the margin; cept, which concerns the continental crust rather (c) the structural contrast between undeformed than the continental lithosphere. basalts and peridotite or gabbro, both sheared at This discussion is not only semantic, as it depth and at falling temperature before unroof- concerns the validity of plate kinematic recon- ing; (d) the rare occurrence of pre-rift, lower structions. To reconstruct the pre-break-up continental crust at the OCB; (e) the presence locations of continents, it is appropriate to fit of strong reflectors (S on the Galicia and Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

24 G. BOILLOT & N. FROITZHEIM

Armorican margins; H in the Iberia Abyssal unambiguous distinction between these different Plain), which mark the tectonic contact between kinds of lower crust, which have practically the thinned continental crust and underlying syn-rift same seismic velocities and densities. (e) lower seismic crust (serpentinized peridotite and Where are the best places on land to test the gabbro); (f) the location of the seismic Moho proposed model of tectonic sea-floor spreading at the boundary between fresh and serpentinized by direct observations of lithospheric shear peridotite, i.e. at the hydrothermal front. zones in ophiolites? (2) To account for the geological and geo- (4) The last question introduces a final physical characters of the oceanic lithosphere remark on the synergy between offshore and bordering the current margins and of some inland studies of passive margins and oceanic Alpine ophiolites as well, the detachment model lithosphere. In the late 1970s, marine geologists can be extended to slow or ultra-slow oceanic imaged the first tilted crustal blocks of the ridges (Fig. 4). Specifically, the model explains: Armorican passive margin (e.g. Montadert et al. (a) the frequent age discrepancy and unconfor- 1979). Soon after, field geologists recognized mity between gabbros and basalts within ocea- similar tilted blocks, in some cases inverted nic lithosphere resulting from slow spreading; into thrust nappes by Cenozoic tectonics, for (b) the ubiquitous occurrence of tectonic brec- instance in the French Alpine crystalline massifs cia (ophicalcite) at the top of serpentinite base- (e.g. Lemoine et al. 1981; Lombardo & Pog- ment, interpreted as a result of brittle ( possibly nante 1982). The approach was then typically hydraulic) deformation in the uppermost level uniformitarian, with the model of a modern of former shear zones (detachments); (c) the rifted margin (Armorican margin) being used to seismic images of the upper level of the litho- interpret palaeo-rifted margins (e.g. French sphere, where strong low-angle reflectors dip Western Alps). Later, the knowledge of the continentward (most frequently) or oceanward structure of the tilted blocks as imaged by seis- (less frequently). Although they are located mic data in passive margins or in extensional within oceanic lithosphere, it is suggested that basins profited considerably from the field these reflectors are the seismic signatures of observations, allowing interpretation of the seis- former lithospheric, conjugate shear zones, mic images recorded at sea in a realistic way. similar to the S or H reflectors imaged beneath This is a clear example of the synergy between the deep margin. More generally, the model research work conducted at sea and on land. accounts for the nature of sea-floor spreading In the 1980s, the discovery of the serpenti- along slow ocean ridges, where thin lithosphere, nized peridotite ridge bounding the Galicia continuously collapsed and extended, covers the margin provided a new uniformitarian model to asthenosphere at the boundary between the two interpret the ultramafic ophiolites of the internal divergent plates. Franco-Italian Alps (e.g. Lemoine et al. 1987). (3) However, several questions remain open, As a result of the field-based observations of including the following: (a) Is the evolution of the ophiolites, the relations between peridotite, the rift actually a two-stage process as proposed gabbro and ophicalcite in the oceanic crust and in this paper (pure shear in the boudinage ultramafic sea floor near the OCB were clari- stage, simple shear in the detachment fault fied. This is a second example of fruitful trans- stage)? (b) Where is the actual OCB? Pragmati- fer of knowledge and concept from ocean to cally, a location is proposed at the limit of the mountain belt, and vice versa. continental crust (the margin sensu stricto). The The detachment model for unroofing of meta- definition implies considering the new litho- morphic core complexes was based on studies spheric surface created by unroofing of subcon- of the Basin and Range province (e.g. Lister & tinental mantle terranes (the peridotite ridge Davis 1989). Wernicke (1985) applied the and adjacent areas) as part of the oceanic model to the whole lithosphere, to account for domain. (c) What is the actual time gap between the unroofing of lower continental crust and the formation of gabbro and basalt now crop- even the uppermost mantle. Later the concept ping out together on the sea floor, and formed was used to explain: (1) the occurrence of ultra- by slow spreading, and what are the relations mafic sea floor at the foot of the Iberia margin, between that time gap and the spreading rate at and the shearing of mantle rocks at falling tem- slow ridges? (d) Where and how does the pre- perature and decreasing pressure; (2) the poss- rift, old lower crust of the continent give way to ible age discrepancy between gabbro and basalt the syn-rift, new seismic lower crust of the dee- ophiolites derived from the Ligurian ocean; (3) pest part of the margin formed by undercrusted the occurrence of detachment faults marked by serpentinite or underplated gabbro? So far, the S reflector within the seismic crust of the available geophysical data do not permit an deepest part of the Armorican and Galicia mar- Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

OUTSTANDING QUESTIONS 25 gins. In this case, the insight came first from BESLIER, M.O. & BRUN, J.-E 1991. Boudinage de la field studies on the continent, to later illuminate lithosphbre et formation des marges passives. the geodynamics of the passive margin and of Comptes Rendus de l'Acaddmie des Sciences, the ocean. More recently the interpretation of Sdrie II, 313, 951-958. BESLIER, M.O., ASK, M. & BOILLOT, G. 1993. the West Iberia OCB as a result of low-angle Ocean-continent boundary in the Iberia Abyssal normal shearing of the lithosphere was success- Plain from multichannel seismic data. Tectono- fully applied to the palaeo-OCB of the Valais physics, 218, 383-393. and Ligurian oceans preserved in the Alpine BESLIER, M.O., BITRI, A. & BOILLOT, G. 1995. Struc- belt (Florineth & Froitzheim 1994; Froitzheim ture de la transition continent-ocran d'une & Manatschal 1996; Hermann & Mtintener marge passive: sismique rrflexion multitrace 1996; Manatschal & Nievergelt 1997; Mana- dans la plaine abyssale ibrrique (Portugal). tschal & Bernoulli 1999). In turn, Alpine stu- Comptes Rendus de l'Acaddmie des Sciences, dies have aided interpretation of the Iberia 320, 969-976. margin, mainly because of the relatively easy BESLIER, M.O., CORNEN, G., GIRARDEAU, J. 1996. availability of structural kinematic data (shear- Tectono-metamorphic evolution of peridotites sense determination, overprinting relations from the ocean/continent transition of the Iberia Abyssal Plain margin. In: WHITMARSH, R.B., between faults and shear zones, etc.). SAWYER, D.S., KLAUS, A. et al. (eds) Proceed- These three examples illustrate the benefits of ings of the Ocean Drilling Program, Scientific integrated marine and onshore studies for the Results, 149. Ocean Drilling Program, College advance of geosciences. The first success of Station, TX, 397-412. this strategy was the understanding of the ocea- BESLIER, M.O., GIRARDEAU, J. & BOILLOr, G. 1990. nic crust by comparison with ophiolitic com- Kinematics of peritotite emplacement during plexes. The next and not less important success North Atlantic continental rifting, Galicia, NW was the transfer of experience and concepts con- Spain. Tectonophysics, 184, 321-343. cerning passive margins and adjacent ultramafic BILL, M., BUSSY, E, COSCA, M., MASSON, N. & sea floor. The two communities (onshore and HUNZIKER, J. 1997. High-precision U-Pb and marine geologists) do not work at the same 4°mr]39Ar dating of an Alpine ophiolite (Gets scale nor with the same tools, but they have the nappe, French Alps). Eclogae Geologicae Hel- vetiae, 90, 43-54. same scientific objectives. BLACKMAN, D.K., CANN, J.R., JANSSEN, B. & SMITH, D.K. 1998. Origin of extensional core com- We thank M. O. Beslier for useful scientific discus- plexes: evidence from the Mid-Atlantic Ridge at sion, and D. Bernoulli, J. Girardeau, Y. Lagabrielle, Atlantic fracture zone. Journal of Geophysical M. Lemoine and anonymous reviewers for comments Research, B9, 103, 21315-21333. and suggestions on the first version of this paper. BOILLOT, G. & COULON, C. Za Ddchirure continen- This paper is Contribution 325 of the UMR Gros- tale et l'Ouverture ocdanique: Gdologie des ciences Azur (CNRS, UPMC, UNSA, IRD). Marges passives. Gordon and Breach, Paris. BOILLOT, G., AGRINIER, P., BESLIER, M.O. & 9 OTHERS 1995b. A lithospheric syn-rift shear zone at the ocean-continent transition: prelimi- References nary results of the GALINAUTE II cruise (Nau- AGRINIER, P., CORNEN, G. & BESLIER, M.O. 1996. tile dives on the Galicia Bank, Spain). Comptes Mineralogical and oxygen isotopic features of Rendus de l'Acaddmie des Sciences, Sdrie IIa, serpentinites recovered from the ocean-conti- 322, 1171-1178. nent transition in the Iberia Abyssal Plain. In: BOILLOT, G., BESLIER, M.O. & COMAS, M.C. 1991. WH1TMARSH, R.B., SAWYER, D.S., KLAUS, A. Seismic image of undercrusted serpentinite et al. (eds) Proceedings of the Ocean Drilling beneath a rifted margin. Terra Nova, 4, 25-33. Program, Scientific Results, 149. Ocean Drilling BOILLOT, G., BESLIER, M.O. & GIRARDEAU, J. 1995c. Program, College Station, TX, 541-551. Nature, structure and evolution of the ocean- AGRINIER, P., MEVEL, C. & GIRARDEAU, J. 1988. continent boundary: the lesson of the West Gali- Hydrothermal alternation of the peridotites cia Margin (Spain). In: BANDA, E., TORN~, M. cored at the ocean/continent boundary of the & TALWANI, M. (eds) Rifted Ocean-Continent Iberian margin: petrologic and stable isotope Boundaries. Kluwer, Dordrecht, 219-229. evidences. In: BOILLOT, G., W1NTERER, E.L., BOILLOT, G., BESLIER, M.O., KRAWCZYK, C.M., MEYER, A.W. et al. (eds) Proceedings of the RAPPIN, D. & RESTON, T.J. 1995a. The for- Ocean Drilling Program, Scientific Results (Part mation of passive margins: constraints from the B), 103. Ocean Drilling Program, College crustal structure and segmentation of the deep Station, TX, 225-234. Galicia margin, Spain. In: SCRUTTON, R.A., BERNOULLI, D. & WEISSERT, H. 1985. Sedimentary STOKER, M., SCHIMMIELD, G.B. & TUDHOPE, fabrics in Alpine ophicalcites, South Pennine A.W. (eds) The Tectonics, Sedimentation and Arosa zone, Switzerland. Geology, 13, 755-758. Palaeooceanography of the North Atlantic Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

26 G. BOILLOT & N. FROITZHEIM

Region. Geological Society, London, Special exposures at the Mid-Atlantic Ridge: geological Publications, 90, 71-91. mapping in the 15°N region. Tectonophysics, BOILLOT, G., COMAS, M.C., GIRARDEAU, J. & 5 279, 193-213. OTHERS 1988a. Preliminary results of the Gali- CHARPENTIER, S., KORNPROBST, J., CHAZOT, G., naute cruise (dives of the submersible 'Nautile' CORNEN, G. & BOILLOT, G. 1998. Interaction on the West Galicia margin, Spain). In: BOILIXYr, entre lithosphere et asthrnosph~re au cours de G., WINTERER, E.L. & MEYER, A.W. et al. (eds) l'ouverture oc~anique: donn~es isotopiques pl~- Proceedings of the Ocean Drilling Program, liminaires sur la marge passive de Galice Scientific Results, 103. Ocean Drilling Program, (Atlantique-Nord). Comptes Rendus de l'Acadd- College Station, TX, 733-740. mie des Sciences, Sciences de la Terre et des BOILLOT, G., Ft~RAUD, G., RECQ, M. & GIRARDEAU, Planktes, 326, 757-762. J. 1989b. 'Undercrusting' by serpentinite CHIAN, D., LOUDEN, K., MINSHULL, T.A. & WHIT- beneath rifted margins. Nature, 341, 523-525. MARSH, R.B. 1999. Deep structure of the BOILLOT, G., GIRARDEAU, J. & KORNPROBST, J. ocean-continent transition in the southern 1988b. Rifting of the Galicia margin: crustal Iberia Abyssal Plain from seismic refraction pro- thinning and emplacement of mantle rocks on files: Ocean Drilling Program (Legs 149 and the seafloor. In: BOILLOT, G., WINTERER, E.L., 173) transect. Journal of Geophysical Research, MEYER, A.W. et al. (eds) Proceedings of the 104, 7443-7462. Ocean Drilling Program, Scientific Results, 103. CORNEN, G., BESLIER, M.O. & GIRARDEAU, J. Ocean Drilling Program, College Station, TX, 1996a. Petrologic characteristics of the ultrama- 741-756. fic rocks from the ocean-continent transition in BOILLOT, G., GRIMAUD, S., MAUFFRET, A., MOUGEN- the Iberia Abyssal Plain. In: WHITMARSH, R.B., OT, D., KORNPROBST, J., MERGOIL-DANIEL, J. & SAWYER, D.S., KLAUS, A. et al. (eds) Proceed- TORRENT, G. 1980. Ocean-continent boundary ings of the Ocean Drilling Program, Scientific of the Iberian margin: a serpentinite diapir west Results, 149. Ocean Drilling Program, College of the Galicia Bank. Earth and Planetary Station, TX, 377-395. Science Letters, 48, 23-34. CORNEN, G., BESL1ER, M.O. & GIRARDEAU, J. BOILLOT, G., MOUGENOT, D., GIRARDEAU, J. & WIN- 1996b. Petrology of the mafic rocks cored in the TERER, E.L. 1989a. Rifting processes on the Iberia Abyssal Plain. In: WHITMARSH, R.B., West Galicia Margin, Spain. In: TANKARD, A.J. SAWYER, D.S., KLAUS, A. et al. (eds) Proceed- & BALKWILL, H.R. (eds) Extensional Tectonics ings of the Ocean Drilling Program, Scientific and Stratigraphy of the North Atlantic Margins. Results, 149. Ocean Drilling Program, College American Association of Petroleum Geologists, Station, TX, 449-470. Memoirs, 46, 363-377. COSTA, S. & CABY, R. 1997. Evolution of the Ligur- BOILLOT, G., RECQ, M., WINTERER, E.L. & 21 ian Tethys in the Western Alps: Sm-Nd and OTHERS 1987a. Tectonic denudation of the upper mantle along passive margin: a mode based on U-Pb geochronology and REE study of the drilling results (Ocean Drilling Program Leg Mont-Gen~vre ophiolite (Southeast of France). 103, Western Galicia Margin, Spain). Tectono- In: LUG 9th Meeting Abstracts. Cambridge Pub- physics, 132, 335-342. lications, Strasbourg, 510. BOILLOT, G., WINTERER, E.L., MEYER, m.w. et al. DEAN, S.M., MINSHULL, T.A., WHITMARSH, R.B. & (eds) 1987b. Proceedings of the Ocean Drilling LOUDEN, K.E. 2000. Deep structure of the Program, Initial Reports (Part A), 103. Ocean ocean-continent transition in the southern Drilling Program, College Station, TX. Iberia Abyssal Plain from seismic refraction pro- BOILLOT, G., WINTERER, E.L., MEYER, A.W. et al. files: the IAM-9 transect at 40°20rN. Journal of (eds) 1988c. Proceedings of the Ocean Drilling Geophysical Research, 105, 5859-5885. Program, Scientific Results (Part B), 103. Ocean DE CHARPAL, O., GUENNOC, P., MONTADERT, L. & Drilling Program, College Station, TX. ROBERTS, D.C. 1978. Rifting, crustal attenuation BORTOLOTTI, W., CELLAI, D., VAGGELLI, G. & VILLA, and subsidence in the Bay of Biscay. Nature, I.M. 1991.4°mr/39Ar dating of Apenninic ophio- 5682, 275, 706-711. lites: 2. Basalts from the Aiola sequence, DESMURS, L., SCHALTEGGER, U., MANATSCHAL, G. Southern Tuscany, Italy. Ofiolitti, 1, 16, 37-42. & BERNOULLI, D. 1999. Geodynamic signifi- BRUN, J.P. & BESLIER, M.O. 1996. Mantle exhuma- cance of gabbros along ancient ocean-continent tion at passive margin. Earth and Planetary transitions: Tasna and Platta nappes, Eastern Science Letters, 142, 161-173. Alps. Journal of Conference Abstracts 4, CABY, R. 1995. Plastic deformation of gabbros in a 1(LUG), 10, 379. slow-spreading Mesozoic ridge: example of the DESMURS, L., MANATSCHAL, G. & BERNOULLI, D. Montgen~vre ophiolite, Western Alps. In: VIS- 2001. The Steinmann Trinity revisited: mantle SERS, R.L.M. & NICOLAS, A. (eds) Mantle and exhumation and magmatism along an ocean- Lower Crust Exposed in Oceanic Ridges and in continent transition: the Platta nappe, eastern Ophiolites. 123-145. Kluwer, Dordrecht. Switzerland. In: WILSON, R.C.L., WHITMARSH, CANNAT, M., LAGABRIELLE, Y., BOUGAULT, T.H., R.B., TAYLOR, B. & FROITZHEIM, N. (eds) Non- CASEY, J., DE COUTURE, N., DIMITRIEV, L. & volcanic Rifting of Continental Margins: a Com- FOUQUET, Y. 1997. Ultramafic and gabroic parison of Evidence from Land and Sea. Geo- Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

OUTSTANDING QUESTIONS 27

logical Society, London, Special Publications, dence of an oceanic ultra-slow mantellic 187, 235-266. accreting centre. Terra Nova, 10, 330-336. ETHERIDGE, M.A., SYMONDS, P.A. & LISTER, G.S. GIRARDEAU, J., EVANS, C.A., BESLIER, M.O. et al. 1989. Application of the detachment model to 1988. Structural analysis of plagioclase-bearing reconstruction of conjuguate passive margins. peridotite emplaced at the end of continental In: TANKARD, A.J. & BALKWlLL, H.R. (eds) rifting: Hole 637A, Ocean Drilling Program Leg Extensional Tectonics and Stratigraphy of the 103 on the Galicia margin. In: BOILLOT, G., North Atlantic Margins. American Association WINTERER, E.L., MEYER, A.W. et al. (eds) Pro- of Petroleum Geologists, Memoirs, 46, 23-40. ceedings of the Ocean Drilling Program, Scien- EVANS, C.A. & GIRARDEAU, J. 1988. Galicia margin tific Results, 103. Ocean Drilling Program, peridofites: underplated abyssal peridotites from College Station, TX, 209-223. the North Atlantic. In: BOILLOT, G., WINTERER, GRACIA, E., CHARLOU, J.L., RADFORD-KNOERY, J. & E.L., MEYER, A.W. et al. (eds) Proceedings of PARSON, L.M. 2000. Non-transform offsets the Ocean Drilling Program, Scientific Results, along the Mid-Atlantic Ridge south of the 103. Ocean Drilling Program, College Station, Azores (38°N-34°N): ultramafic exposures and TX, 195-208. hosting of hydrothermal vents. Earth and Plane- FI~RAUD, G., BESLIER, M.O. & CORNEN, G. 1996. tary Science Letters, 177, 89-103. 4°mr/39Ar dating of gabbros from the ocean/con- HANDY, M.R., FRANZ, L., HELLER, E, JANOTT, B. & tinent transition of the western Iberia Margin: ZURBRIGGEN, R. 1999. Multistage accretion and preliminary results. In: WHITMARSH, R.B., exhumation of the continental crust (Ivrea crustal SAWYER, D.S., KLAUS, A. et al. (eds) Proceed- section, Italy and Switzerland). Tectonics, 18(6), ings of the Ocean Drilling Program, Scientific 1154-1177. Results, 149. Ocean Drilling Program, College HERMANN, J. & MONTENER, O. 1996. Extension- Station, TX, 489-498. related structures in the Malenco-Margna FI~RAUD, G., GIRARDEAU, J., BESLIER, M.O. & BOIL- System: implications for paleogeography and LOT, G. 1988. Datation 39Ar-4°mr de la mise en consequences for tiffing and Alpine tectonics. place des prtidotites bordant la marge de Galice Schweizerische Mineralogische und Petrogra- (Espagne). Comptes Rendus des Sdances de phisches Mitteilungen, 76, 501-519. l'Acaddmie des Sciences, Sdrie III, 307, 49-55. HERMANN, J., MUNTENER, O., TROMMSDORFF, W., FLORINETH, D. & FROITZHEIM, N. 1994. Transition HANSMANN, W. & PICCARDO, G.B. 1997. Fossil from continental to oceanic basement in the crust-to-mantle transition, Val Malenco (Italian Tasna nappe (Engadine window, Graubtinden, Alps). Journal of Geophysical Research, B9, 102, 20123-20132. Switzerland): evidence for early Cretaceous HOFFMANN, H.J. & RESTON, T.J. 1992. Nature of the opening of the Valais ocean. Schweizerische S reflector beneath the Galicia Banks rifted Mineralogische und Petrographische Mitteilun- margin: preliminary results from pre-stack depth gen, 74, 437-448. migration. Geology, 20, 1091-1094. FROITZHEIM, N. & MANATSCHAL, G. 1996. Kin- KEEN, C.E. • DE VOOGD, B. 1988. The continent- ematics of Jurassic rifting, mantle exhumation, ocean boundary at the rifted margin off Eastern and passive-margin formation in the Austroal- Canada: new results from deep seismic reflection pine and Penninic nappes (eastem Switzerland). studies. Tectonics, 7, 107-124. Geological Society of America Bulletin, 108, KORNPROBST, J. & TABIT, A. 1988. Plagioclase bear- 1120-1133. ing ultramafic tectonites from the Galicia FROITZHEIM, N. & RUBATIO, D. 1998. Continental margin (Leg 103, Site 637): comparison of their break-up by detachment faulting: field evidence origin and evolution with low pressure ultrama- and geochronological constraints (Tasna nappe, fic bodies in Western Europe. In: BOILI.OT, G., Switzerland). Terra Nova, 10, 171-176. WINTERER, E.L., MEYER, A.W. et al. (eds) Pro- FROH-GREEN, G.L., WEISSERT, H. & BERNOUILLI, D. ceedings of the Ocean Drilling Program, Scien- 1990. A multiple fluid history recorded in tific Results, 103. Ocean Drilling Program, Alpine ophiolites. Journal of the Geological College Station, TX, 253-268. Society, London, 147, 959-970. KORNPROBST, J., VIDAL, Ph. & MALOD, J. 1988. Les FUEGENSCHUH, B., FROITZHEIM, N. & BOILLDT, G. basaltes de la marge de Galice (NO de la Prnin- 1998. Cooling history of granulite samples from sule ibrrique): h&rrogrnrit6 des spectres de the ocean-continent transition of the Galicia terres rares h la transition continent-ocran. Don- margin: implications for rifting. Terra Nova, 10, nres grochimiques prrliminaires. Comptes 96-100. Rendus de l'Acaddmie des Sciences, Sdrie II, GIRARDEAU, J., CORNEN, G., AGRINIER, P. & 7 306, 1359-1364. OTHERS 1998. Preliminary results of nautile KRAWCZYK, C.M. & RESTON, T.J. 1995. Detachment dives on the Gorringe Bank (West Portugal). faulting and continental break-up: the S reflector Comptes Rendus de l'Acaddmie des Sciences, offshore Galicia. In: BANDA, E., TORNi~, M. & 326, 247-254. TALWANI, M. (eds) Rifted Ocean-Continent GIRARDEAU, J., CORNEN, G., BESLIER, M.O. & 7 Boundaries. Kluwer, Dordrecht, 231-246. OTHERS 1999. Extensional tectonics in the Got- KRAWCZYK, C.M., RESTON, T.J., BESLIER, M.O. & tinge Bank rocks, Eastern Atlantic ocean: evi- BOILI_DT, G. 1996. Evidence for detachment tec- Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

28 G. BOILLOT & N. FROITZHEIM

tonics on the Iberia Abyssal Plain rifted margin. MANATSCHAL, G. & NIEVERGELT, P. 1997. A conti- In" WHITMARSH, R.B., SAWYER, D.S., KLAUS, nent-ocean transition recorded in the Err and A. et al. (eds) Proceedings of the Ocean Dril- Platta nappes (Eastem Switzerland). Eclogae ling Program, Scientific Results, 149. Ocean Geologicae Helvetiae, 90, 3-27. Drilling Program, College Station, TX, 603- MANATSCHAL, G., FROITZHEIM, N., RUBENACH, M. & 615. TURRIN, B.D. 2001. The role of detachment LAGABRIELLE, Y. & LEMOINE, M. 1997. Alpine, Cor- faulting in the formation of an ocean-continent sican and Apennine ophiolites: the slow spread- transition: insights from the Iberia Abyssal ing ridge model. Comptes Rendus de Plain. In: WILSON, R.C.L., WHITMARSH, R.B., l'Acadimie des Sciences, Sciences de la Terre et TAYLOR, B. & FROITZHEIM, N. (eds) Non-volca- des Planbtes, 325, 909-920. nic Rifting of Continental Margins: a Compari- LAGABRIELLE, Y., POLINO, R., AUZENDE, J.-M. & 8 son of Evidence from Land and Sea. Geological OTHERS 1984. Les trmoins d'une tectonique Society, London, Special Publications, 187, intra-ocranique dans le domaine t~thysien: 405-428. analyse des rapports entre les ophiolites et leur MASSON, D.G., MONTADERT, L. & SCRUTTON, R.A. couverture m&a-srdimentaire dans la zone pir- 1985. Regional geology of the Goban Spur con- montaise des Alpes franco-italiennes. OFIOLITTI, tinental margin. In: DE GRACIANSKY, P.C., 9(1), 67--88. POAG, C.W. et al. (eds) Initial Reports, Deep LAGABRIELLE, Y., BIDEAU, D., CANNAT, M., Sea Drilling Project, 80. US Government Print- KARSON, J.A. & MEVEL, C. 1998. Ultramafic- ing Office, Washington, DC, 1115-1139. mafic plutonic rocks suites exposed along the MAUFFRET, A. & MONTADERT, L. 1987. Rift tectonics mid-Atlantic ridge (10°N-30°N). Symmetrical- on the passive continental margin off Galicia asymmetrical distribution and implications for (Spain). Marine and Petroleum Geology, 4, 49- seafloor spreading processes. Faulting and 70. Magmatism at Mid-Ocean Ridges. Geophysical MONTADERT, L., DE CHARPAL, O., ROBERTS, D., Monograph, American Geophysical Union, 106, GUENNOC, P. & SIBUET, J.C. 1979. Deep drilling 153-176. results in the Atlantic Ocean: continental mar- LEMOINE, M. 1980. Serpeninites, gabbros and ophi- gins and palaeoenvironment. In: TALWANI, M., calcite in the Piemont-Ligurian domain of the HAY, W. & RYAN, B.E (eds) Northeast Atlantic Western Alps: possible indicators of oceanic Passive Continental Margins: Rifting and Subsi- fracture zones and of associated serpentinite pro- dence Processes. American Geophysical Union, trusions in the Jurassic-Cretaceous Tethys. Maurice Ewing Series, 3, 154-186. Archives des Sciences, Genkve, 33, 103-115. MONTENER, O. & HERMANN, J. 1996. The Val Mal- LEMOINE, M., GIDON, M. & BARFETY, J.C. 1981. Les enco lower crust-upper mantle complex and its massifs cristallins externes des Alpes occiden- field relations (Italian Alps). Schweizerische tales: d'anciens blocs basculrs n~s au Lias lors Mineralogisches und Petrographishes Mitteilun- du rifting trthysien. Comptes Rendus de l'Aca- gen, 76, 475-500. dimie des Sciences, Sirie II, 292, 917-920. MONTENER, O. & HERMANN, J. 2001. The role of LEMOINE, M., TRICART, E & BOILLOT, G. 1987. lower crust and continental upper mantle during Ultramafic and gabbroic ocean floor of the formation of non-volcanic passive margins: evi- Ligurian Tethys (Alps, Corsica, Apennines): in dence from the Alps. In: WILSON, R.C.L., search of a genetic model. Geology, 15, 622- WHITMARSH, R.B., TAYLOR, B. & FROITZHEIM, 625. N. (eds) Non-volcanic Rifting of Continental LE PICHON, X. & BARBIER, F. 1987. Passive margin Margins: a Comparison of Evidence from Land formation by low-angle faulting within the and Sea. Geological Society, London, Special upper crust: the northern Bay of Biscay margin. Publications, 187, 267-288. Tectonics, 2, 6, 133-150. Ocean Drilling Program Leg 173 Shipboard Scientific LISTER, G.S. & DAVIS, G.A. 1989. The origin of Party 1998. Drilling reveals transition from con- metamorphic core-complexes and detachment tinental break-up to early magmatic crust. EOS faults formed during Tertiary continental exten- Transactions, American Geophysical Union, 79, sion in the northern Colorado Riover region, 180-181. USA. Journal of Structural Geology, 11, 65-94. OHNENSTETTER, M., OHNENSTETTER, D., VIDAL, Ph., I_£)MBARDO, B. & POGNANTE, W. 1982. Tectonic CORNICHET, J., HERMITTE, D. & MACE, J. 1981. implication in the evolution of the western Alps Crystallization and age of zircon from Corsican ophiolite metagabbros. Ofiolitti, 7, 371-395. ophiolitic albitite: consequences for oceanic MALOD, J.A., MURILLAS, J., KORNPROBST, J. & BOIL- expansion in Jurassic times. Earth and Plane- LOT, G. 1993. Oceanic lithosphere at the edge of tarT Science Letters, 54, 397-408. a Cenozoic active continental margin (north- PI~REZ-GUSSINYI~, M., RESTON, T.J. & PHIPPS west slope of the Galicia Bank, Spain). Tectono- MORGAN, J. 2001. Serpentinization and magma- physics, 221, 195-206. tism during extension at non-volcanic margins: MANATSCHAL, G. & BERNOULLI, D. 1999. Architec- the effect of initial lithospheric structure. In: ture and tectonic evolution of non-volcanic mar- WILSON, R.C.L., WHITMARSH, R.B., TAYLOR, B. gins: present-day Galicia and ancient Adria. & FROITZHEIM, N. (eds) Non-volcanic Rifting of Tectonics, 18, 1099-1119. Continental Margins: a Comparison of Evidence Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

OUTSTANDING QUESTIONS 29

from Land and Sea. Geological Society, SEIFERT, K.E., CHENG-WEN, C. & BRUNOTTE, D.A. London, Special Publications, 187, 551-576. 1997. Evidence from Ocean Drilling Program PICKUP, S.L.B., WHITMARSH, R.B., FOWER, C.M.R. Leg 149 mafic igneous rocks for oceanic crust & RESTON, T.J. 1996. Insight into the nature of in the Iberia Abyssal Plain ocean-continent the ocean-continent transition off the West transition zone. Journal of Geophysical Iberia from a deep multichannel seismic reflex- Research, B4, 102, 7915-7928. ion profile. Geology, 24, 1079-1082. SIBUET, J.C. 1992. New constraints on the formation PINET, N., LAGABRIELLE, Y. & WHITECHURCH, H. of non-volcanic continental Galicia-Flemish 1989. Le complexe du Pic des Lauzes (Haut Cap conjugate margins. Journal of the Geologi- Queyras, Mpes occidentales, France): structure cal Society, London, 149, 829-840. alpines et octaniques dans un massif ophioli- SIBUET, J.C., LOUVEL, V., WHITMARSH, R.B. & 5 tique de type Liguro-Pitmontais. Bulletin de la OTHERS 1995. Constraints on rifting processes SociFtF GFologique de France, V, 317-328. from refraction and deep-tow magnetic data: the POLINO, R. & LEMOINE, M. 1984. Dttritisme mixte example of the Galicia Continental margin. In: d'origine continentale et octanique dans les BANDA, E., TORNE, M. & TALWANI, M. (eds) stdiments jurassico-crttac6 supra ophiolitiques Rifted Ocean-Continent Boundaries. Kluwer, de la T&hys Ligure: la serre du Lago Nero Dordrecht, 197-217. (Alpes occidentales franco-italiennes). Comptes SIBUET, J.C., MAZE, J.P., AMORTILLA, Ph. & LE Rendus de l'AcadFmie des Sciences, SFrie II, PICHON, X. 1987. Physiography and structure of 298, 359-364. the western Iberian continental margin off Gali- RANERO, C.R. & RESTON, T.J. 1999. Detachment cia from seabeam and seismic data. In: BOILLOT, faulting at ocean core complexes. Geology, 27, G., WINTERER, E.L., MEYER, A.W. et al. (eds) 983-986. Proceedings of the Ocean Drilling Program, RECQ, M., WHITMARSH, R.B., SIBUET, J.C., WH1TE, Initial Reports (Part A), 103. Ocean Drilling R.S. & LYNESS, D. 1996. Structure sismique de Program, College Station, TX, 77-97. la ride de ptridotite ~ l'ouest du Banc de Galice TREVES, B.E. & HARPER, G.D. 1994. Exposure of (Ouest-Ibtrie). Comptes Rendus de l'AcadFmie serpentinite on the ocean floor: sequence of des Sciences, SFrie IIa, 322, 571-578. faulting and hydrofractufing in the northern RESTON, T.J., KRAWCZYK, C.M. & KLAESCHEN, D. Apennine ophicalcites. Ofiolitti, 196, 435-466. 1996. The S reflector west of Galicia (Spain): TRICART, P. & LEMOINE, M. 1989. The Queyras evidence from pre-stack depth migration for ophiolite west of Monte Viso (Western Alps): indicator of a peculiar ocean floor in the Meso- detachment faulting during continental break-up. zoic Tethys. Journal Geodynamics, 13, 163- Journal of Geophysical Research, B4, 101, 181. 8075-8091. TROMMSDORFF, W., PICCARDO, G.P. & MONTRASIO, RUBATTO, D. 1998. Dating of pre-Alpine magmatism, A. 1993. From magmatism through metamorph- Jurassic ophiolites and Alpine subductions in ism to seafloor emplacement of subcontinental PhD thesis, ETH Zurich. the Western Alps. Adria lithosphere during pre-Alpine rifting RUBATTO, D., GEBAUER, D. & FANNING, M. 1998. (Malenco, Italy. Schweizerische Mineralogisches Jurassic formation and Eocene subduction of the und Petrographishes Mitteilungen, 73, 191-203. Zermatt-Saas-Fee ophiolites: implications for TUCHOLKE, B.E. & LIN, J. 1994. A geological model the geodynamic evolution of the Central and for the structure of ridge segments in slow Western Alps. Contributions to Mineralogy and spreading ocean crust. Journal of Geophysical Petrology, 132, 269-287. Research, 99, 11937-11958. SAWYER, D.S., WHITMARSH, R.B., KLAUS, A. et al. TUCHOLKE, B.E., LIN, J. & KLEINROCK, M.C. 1998. (eds) 1994. Proceedings of the Ocean Drilling Megamullions and mullion structure defining Program, Initial Reports, 149. Ocean Drilling oceanic metamorphic core complexes on the Program, College Station, TX. Mid-Atlantic Ridge. Journal of Geophysical SCHARER, U., GIRARDEAU, J., CORNEN, G. & BOIL- Research, 103, 9857-9866. LOT, G. 2001. 138-121 Ma asthenospheric mag- TUCHOLKE, B.E., LIN, J., KLEINROCK, M.C., TIVEY, matism prior to continental break-up in the M.A., REED, T.B., GOFF, J. & JAROSLOW, G. North Atlantic and geodynamic implications. 1997. Segmentation and crustal structure of the Earth and Planetary Science Letters, 181, 555- western Mid-Atlantic Ridge flank, 25°25t- 572. 27°10'N and 0-29m.y. Journal of Geophysical SCHARER, U., KORNPROBST, J., BESLIER, M.O., BOIL- Research, B5, 102, 10203-10223. LOT, G. & GIRARDEAU, J. 1995. Gabbro and WEISSERT, H.J. & BERNOULLI, D. 1985. A transform related rock emplacement beneath rifting conti- margin in the Mesozoic Tethys: evidence from nental crust: U-Pb geochronological and geo- the Swiss Alps. Geologische Rundschau, 74, chemical constraints for the Galicia passive 665-679. margin (Spain). Earth and Planetary Science WERNICKE, B. 1985. Uniform-sense normal simple Letters, 130, 187-200. shear of the continental lithosphere. Canadian SCHEMENDA, A.I. & GROCHOLSKY, A.L. 1994. Physi- Journal of Earth Sciences, 22, 108-125. cal modeling of slow seafloor spreading. Journal WERNICKE, B. & TILKE, EG. 1989. Extensional tec- of Geophysical Research, B5,990, 9137-9153. tonic framework of the U.S. Central Atlantic Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2021

30 G. BOILLOT & N. FROITZHEIM

passive margin. In: TANKARD, A.J. & BALK- 901 on the West Iberia Continental margin. In: WILL, H.R. (eds) Extensional Tectonics and WHITMARSH, R.B., SAWYER, D.S., KLAUS, A. Stratigraphy of the North Atlantic Margins. et al. (eds) Proceedings of the Ocean Drilling American Association of Petroleum Geologists, Program, Scientific Results, 149. Ocean Drilling Memoirs, 46, 7-21. Program, College Station, TX, 665-674. WHITMARSH, R.B. & MILES, P.R. 1995. Models of WHITMARSH, R.B., SAWYER, D.S., KLAUS, m. et al. the development of the West Iberia rifted conti- (eds) 1996b. Proceedings of the Ocean Drilling nental margin at 40°31YN deduced from surface Program, Scientific Results, 149. Ocean Drilling and deep-tow magnetic anomalies. Journal of Program, College Station, TX. Geophysical Research, B3, 100, 3789-3806. WHITMARSH, R.B., WHITE, R.S., HORSEFIELD, S.J., WHITMARSH, R.B. & SAWYER, D.S. 1996. The SIBUET, J.C., RECQ, M. & LOUVEL, V. 1996c. ocean/continent transition beneath the Iberia The ocean-continent boundary off the western Abyssal Plain and continental rifting to seafloor- continental margin of Iberia: crustal structure spreading processes. In: WHITMARSH, R.B., west of Galicia Bank. Journal of Geophysical SAWYER, D.S., KLAUS, A. et al. (eds) Proceed- Research, 101(B), 12, 28291-28314. ings of the Ocean Drilling Program, Scientific Results, 149. Ocean Drilling Program, College WILSON, R.C.L., MANATSCHAL, G. & WISE, S. 2001. Station, TX, 713-733. Rifting along non-volcanic passive margins: WHITMARSH, R.B., BESLIER, M.O., WALLACE, P.J. stratigraphic and seismic evidence from the et al. (eds) 1998. Proceedings of the Ocean Mesozoic successions of the Alps and western Drilling Program, Initial Reports, 173. Ocean Iberia. In: WILSON, R.C.L., WHITMARSH, R.B., Drilling Program, College Station, TX. TAYLOR, B. & FROITZHEIM, N. (eds) Non-volca- WHITMARSH, R.B., MILES, P.R., SIBUET, J.C., nic Rifling of Continental Margins: a Compari- LOUVEL, W. et al. 1996a. Geological and geo- son of Evidence from Land and Sea. Geological physical implications of deep-tow magnetometer Society, London, Special Publications, 187, observations near Sites 897, 898, 899, 900 and 429-452.