Estado Absorbente

Kim m erldglan-Tlthonian

Fig. 3.7. The Late Jurassic tectonic framework of the North Atlantic region, illustrating the Tethys-Central Atlantic-Carribean Megashear, running E-W across the lower part of the map. Dark stipple indicates oceanic basins; light stipple indicates deep marine basins (after Ziegler, 1990).

Co.

Ca

Ab

'Ga

Fig. 3.8. Atlantic rifting and failed rifts. C; Cape; Ar: Argentina; L: Luanda; Ga: Gabon; B Benue Trough; SL: Sao Luis; A: Amazon; Ab: Abidjan; T: Taka tu; M: Maracaibo; Ca Casamance; N: Newark; Co: Connecticut; Ns: Nova Scotia; H: Fiaha; Bi: Bay of Biscay; V Viking Graben; Ba: Barentz; N: Nares; Bf: Baffin (after Burke, 1977).

Cretaceous w hen the continents diverged at rates of 4.0 cm y l . The present rate of divergence is approxim ately 2.0 cm y l (Pitm an and Talwani, 1972; Foulger et al., 1992).

3.3. Plate kinematics

The fit of the plates and m icroplates (North America, Eurasia and Iberia) aro u n d the N o rth A tlantic region p rio r to and d u rin g seafloor spreading, has been constrained w ith palaeom agnetic data. The fit m ade by Bullard et al. (1965) although one of the earliest attem pts, still rem ains one of th e m o st a c c ep tab le in te r p r e ta tio n s . M ore re c e n t w o rk on palaeom agnetism and continental fit, incorporating the im portance of basin form ation and evolution has been carried out by Srivastava and V erhoef (1992).

The original uncertainty w ith the Bullard et al. (1965) fit is that there is a considerable am ount of continental overlap at all stages p rio r to seafloor sp read in g . V erhoef and Srivastava (1989) an d Srivastava and Verhoef (1992) have show n that at chron M25 (156 Ma, K im m eridgian, the start of earliest breakup of Iberia from N orth America) there w as substantial overlap centred a ro u n d Flem ish C ap and Galicia Bank (Fig. 3.9). This overlap h ad decreased by the tim e of separation in this region (chron MO, 118 Ma, A ptian) (Fig. 3.10). An explanation for this phenom ena involves th e h y p o th e sis th a t se p a ra tio n b e tw ee n p la te s is n o t geo lo g ically in stan tan eo u s as assum ed in m ost m odels. This theory is discussed by Srivastava and Verhoef (1992) w ho propose that the plates w ere stretched p rio r to their separation at the onset of seafloor spreading. Therefore there is a diachroneity betw een rifting at various points in the N orth Atlantic. For exam ple, at chron MO there is no overlap in the Galicia Bank-Flemish Cap region, an d seafloor spreading h ad already been initiated in this region. H ow ever further north in the Bay of Biscay and to the w est of the Porcupine Bank there is still considerable overlap. It is assum ed that at this tim e these regions w ere being stretched p rio r to breakup. The am o u n t of overlap describes the am ount by which the plates w ere extended. "Destretching' of these p lates to their original geom etry w as m odelled by Srivastava and V erhoef (1992) u sin g the sim p lest a ssu m p tio n th a t th e p lates w ere destretched uniform ly in directions obtained from the rotation poles of the plates. D eform ations of continental fragm ents (i. e. Flemish Cap and Galicia Bank) w ere also om itted. H ow ever, the hypothesis proposes th at the rift basins w ere extended betw een the K im m eridgian and the A ptian, and that there w as extension and dislocation betw een Flemish Cap and Galicia Banks an d th eir associated continental plates. Evidence for th is are arcuate

Fig. 3.9. Continental fit overlap at M25 (Srivastava and Verhoef, 1992). 60 5 0° 4 0° 3 0° 5 0° FC GB GrB 4 0°

Fig. 3.10. Continental fit overlap at MO (Verhoef and Srivastava, 1989). 82

m agnetic anom alies in the eastern G rand Banks region at MO w hich are subparallel to the Ibero-A rm orican Arc. These do not exist in Srivastava and V erh o ef s (1992) reconstruction at the initiation of basin form ation in the Triassic. This suggest that they are a feature caused by a later rotation and m ovem ent of Flemish Cap (Figs. 3.11 and 3.12).

A rgand (1924) w as the first to suggest th at n o rth ern Spain and Brittany w ere once connected and w ere then separated by the opening of the Bay of Biscay. The region w as subsequently som ew hat neglected until Carey (1958) p roposed that the Bay opened by the anticlockwise rotation of the Iberian peninsula by some 40°, involving a scissor-like m ovem ent causing sea-floor spreading in the Bay, and com pression in the Pyrenees (Fig. 3.13). How ever, subsequent geophysical and geological research (reviewed by Ries, 1978 and Garw in, 1985) im plies that the Bay of Biscay opened by an anti­ clockwise rotation of the Iberian plate aw ay from the stable Eurasian plate, follow ed by a translational m ovem ent culm inating in the form ation of the Pyrenees. The Iberian plate itself m oved independently to a certain extent, b u t w as also driven by the African plate for m uch of the time (Srivastava et al., 1990b). The rifting w as initiated by the opening of the N o rth A tlantic Ocean, in w hich sea-floor spreading took place in a series of steps, m oving progressively northw ards (Malod and M auffret, 1990). The onset of oceanic spreading betw een N orth America and Eurasia is generally assum ed to be contem poraneous w ith the end of rifting and extension on the continental m argins, occurring from Late Jurassic to the Early Cretaceous (M asson and Miles, 1984). Two triple junctions on the N orth A tlantic Rift allow ed the m ovem ent of the Iberian plate as a separate entity. Two m odels exist as to the positioning of these junctions. It is accepted that the m ost northerly, the Biscay Triple Junction, a ridge-ridge-ridge arrangem ent betw een the Galicia Rift, Biscay Rift and the Celtic Rift, changed to three spreading centres after continental breakup occurred (Fig. 3.14). M alod and M auffret (1990) believe th at the Tethys Triple Junction lies to the south, a ridge-ridge-ridge or ridge- ridge-transform system active from the late Jurassic to early Cretaceous. The differential rotation of Iberia against Africa caused the form ation of pull- a p a rt basins along the T ethyan rift th ro u g h strike-slip m ovem ent. This ridge became inactive after 118 Ma, after w hich time the Iberian Plate can no longer be considered as separate from the African Plate. Sibuet and Collette (1991) believe th at a ridge-ridge-fault triple junction existed n o rth of the proposed Tethys Triple Junction of M alod and M auffret (1990). This 'dead' triple junction lay just to the south of the Biscay Triple Junction, w ith a NW -SE tre n d in g fau lt m o ving into the T ethyan tra n ste n sio n a l zone

C HR ON MO