173 Scientific Prospectus


Rifted margins contain the principal record of continental rifting and the onset of seafloor spreading, both of which are first-order plate tectonic processes. Such margins exhibit a wide spectrum of characteristics, probably in response to different combinations of asthenospheric temperature, lithospheric rheology, strain rate, stress, and pre-existing heterogeneities. The rifting process, through the indirect effects of concurrent subaerial volcanism as well as greater sedimentation and heat flow, can also have important environmental and resource implications. Drilling commonly affords the only means of directly characterizing the nature, age, and emplacement conditions of igneous, metamorphic, and/or sedimentary rocks formed, deposited, or tectonically exposed during margin formation. Nonvolcanic margins provide opportunities to investigate and understand the tectonic aspects of rifting for two reasons. First, normal faults and shear zones that penetrate deep into the crust and uppermost mantle are sometimes evident on seismic profiles and, as has been demonstrated on the west Iberia margin, allow rocks from deeper lithospheric levels to be exposed at the top of acoustic basement. Second, voluminous intrusives/extrusives, which can obscure crustal tectonics, are limited in volume and commonly appear to be absent. Pairs of conjugate rifted margins often exhibit some asymmetry in structural style. This asymmetry may be related to the mode of lithospheric rifting, e.g., pure or simple shear.

The west Iberia margin is an excellent example of a nonvolcanic rifted margin. The Galicia Bank and Iberia Abyssal Plain segments of the margin were cored during ODP Legs 103 and 149, and have been studied extensively by geophysical methods.

Iberia separated from the Newfoundland margin of the Grand Banks in the Early Cretaceous, after prolonged rifting that began in the late Triassic and is well documented on both sides of the Atlantic (Wilson et al., 1989; Welsink et al., 1989). The subsequent plate tectonic and seafloor-spreading history of this part of the North Atlantic is mostly well constrained by seafloor spreading magnetic anomalies. It demonstrates that Iberia and North America moved apart along roughly east-west fracture zones (e.g., Klitgord and Schouten, 1986).

Throughout its post-rift history the west Iberia margin has remained an essentially undisturbed rifted margin that has experienced only minor compression in the north in Eocene time (Pyrenean phase, short lived subduction of Bay of Biscay crust under northern Spain) and in the south and center in the middle Miocene (Rif-Betic phase, gentle folding of abyssal plain sediments).

Offshore, the west Iberia continental margin has been studied extensively by geophysical techniques and, to a lesser extent by geological sampling (e.g., Beslier et al., 1993; Boillot, Winterer, Meyer, et al., 1987, 1988; Hoffman and Reston, 1992; Sawyer, Whitmarsh, Klaus, et al., 1994; Whitmarsh et al., 1990, 1993; Whitmarsh and Miles, 1995; Whitmarsh et al., 1996; Whitmarsh, Sawyer, Klaus, and Masson, 1996). The margin exhibits tilted continental fault blocks that often, but not always (Reston, 1996), seem to lack a wedge of synrift sediments (Wilson et al., 1996). There is an apparent lack of synrift volcanism, and synrift volcanism is equally absent on shore. Tilted fault blocks and a lack of volcanism are both characteristics of a nonvolcanic rifted margin.

The first drilling of the OCT off the west Iberia margin was carried out by ODP Leg 103 in 1985 (Boillot, Winterer, Meyer, et al., 1988); this leg drilled a short transect of holes west of Galicia Bank (Sites 637-641, Fig. 1). In 1991 the recommendations of the North Atlantic Rifted Margin Detailed Planning Group were accepted by JOIDES Planning Committee, which programmed two drilling legs in the North Atlantic during 1993. One of these, Leg 149, drilled a transect of holes into acoustic basement across the OCT in the southern Iberia Abyssal Plain (Sawyer, Whitmarsh, Klaus, et al., 1994; Whitmarsh, Sawyer, Klaus, and Masson, 1996; Sites 897-901, Fig. 1).

Results of Leg 149

Leg 149 drilled a west-to-east transect of five sites. Three sites (Sites 897, 899, and 900) reached acoustic basement (Figs. 1 and 2). A fourth site (Site 901) enabled a firm prediction to be made that the underlying basement is continental crust. The sites were chosen in the context of a conceptual model of the location of the OCT previously defined by gravity, magnetic, and seismic velocity modeling and by seismic reflection profiles. The results obtained during this leg broadly confirmed this model but also produced some surprises.

The results of Leg 149 proved the existence of a peridotite ridge at the inferred landward edge of the oceanic crust formed by seafloor spreading. They also showed that between this ridge and Site 901, which is situated on a fault block of almost unequivocal continental crust, there exists a 130-km-wide region that is probably underlain mostly by a heterogeneous transitional crust. One indication of the transitional nature of this crust may be the MORB-to-transition like gabbro at Site 900 (Seifert et al., 1996; Cornen et al., 1996). Other indications are the transitional to alkaline mafic clasts in mass wasting deposits at Sites 897 and 899 (Cornen et al., 1996; Seifert and Brunotte, 1996). The magnetic and seismic reflection character and velocity structure of the crust provide additional evidence. Whether the Site 900 gabbro formed by pre- or synrift partial melting, the original metamorphic grade and the 40Ar/39Ar age of the dynamically recrystallized plagioclase in the cores (Cornen et al., 1996; Feraud et al., 1996) imply that the gabbro was exhumed by important synrift shearing that accompanied lithospheric extension. Although Site 899 sampled a serpentinite breccia and an underlying serpentinized peridotite mass-flow deposit, magnetic evidence suggests that the basement of this site may be atypical of the rest of the Iberia Abyssal Plain OCT. Therefore, it may not be correct to infer continuity of peridotite basement between Sites 897 and 899. Leg 149 succeeded in defining the western (oceanward) boundary of the OCT and also limited the eastern boundary, but only sampled a single site between these limits. Models that explain these observations are presented below. To test these models, drilling is planned to focus on the nature and evolution of the basement itself within this zone and on the significance of intrabasement seismic reflectors identified there.

Review of the Galicia Bank and Iberia Abyssal Plain Transects

The transects of Legs 103 and 149 and their associated research contributed to the understanding of a number of features of the rifted west Iberia margin. These are lithospheric detachment faults, block faulting of the crust, the emplacement and exposure of mantle rocks, minor synrift magmatism, and the characterization of the OCT. The two margin segments, about 200 km apart, exhibit both similarities and differences.

The Galicia Bank margin has

The southern Iberia Abyssal Plain (IAP) margin has

In the southern IAP, there is apparently no obvious deep and extensive sub-horizontal reflector like S, although several shorter intrabasement reflectors, onto which higher-angle normal faults sole out, have been recognized. The results of Leg 149 highlighted the need for more basement drilling, principally within the OCT, in order to better constrain the modes of lithospheric breakup and to understand the rift-to-drift tectonic and magmatic processes at this excellent example of a non-volcanic rifted margin. Further, independent geophysical work since Leg 149 led to revised tectonic and magmatic models for the rifting and initial seafloor spreading at this margin which can now be tested by further drilling to basement.

Problems that need further investigation and can be resolved by drilling along the southern IAP transect include:

Southern Iberia Abyssal Plain OCT Models

Several preliminary tectonic and magmatic models for lithospheric rifting of the west Iberia margin have been produced (Brun and Beslier, 1996; Whitmarsh and Miles, 1995; Krawczyk et al., 1996; Pickup et al., in press; Whitmarsh and Sawyer, 1996), based on (1) earlier geophysical observations, (2) the Leg 149 results, (3) an interpretation of a new magnetic anomaly chart ( Fig. 3), (4) the latest time-migrated seismic reflection profiles in the southern Iberia Abyssal Plain (only one is included, Fig. 4) and, (5) analogical models. The models differ in the degree of extrapolation from the boreholes that they employ, in the significance attributed to the peridotite ridge and, in whether the east-west distribution of different basement rocks within the OCT is considered to be systematic or just random. Moreover, some models deal both with modes of lithospheric rifting and OCT formation while others relate to the formation of the OCT alone. Such variety of approaches was valid after Leg 149 because of the small number of drill sites that reached basement.

The above data provide evidence for the ascent of mantle material and the probable underplating of gabbro under the rift zone, possibly just after continental breakup, and for detachment tectonics west of Site 901, at least as far as Site 900.

Two tectonic models propose that extension of the lithosphere occurred along large scale shear zones/detachment faults along which the granulite facies gabbro found at Site 900, and even peridotite (at least to the west in the broad northwest to southeast basement low, east of Site 898), was exhumed. Subsequent coeval asthenospheric upwelling led to adiabatic decompression melting and intrusion/underplating of gabbro near the base of the crust. Partial melting and intrusion occurred during extensional shear deformation in the lithosphere and the subsequent block faulting which led to final breakup. Both models predict the exposure of progressively deeper lithospheric levels to the west of Site 901 and possible synrift intrusion into the lower crust or uppermost mantle west of Site 901. They also recognize that the geometry of tectonic features, interpreted to exist on seismic profiles, includes the result of intense late stage deformation.

The models differ in the geometry of the shear zones/detachment faults. One model proposes a single detachment fault crosscutting the whole lithosphere, on which brittle faults in the upper crust sole out at depth, which is later dissected by further block faulting (Fig.5; Krawczyk et al., 1996). The other proposes the development of two conjugate and coeval normal shear zones, one in the lower continental crust and one in the mantle, which led to extreme thinning or even the disappearance of the lower crust (Brum and Beslier, 1996; Fig. 6).

Two other models have also been proposed (Whitmarsh and Sawyer, 1996). One hypothesis (Sawyer, 1994) is that the OCT between Site 901 and the peridotite ridge was produced by ultra slow seafloor spreading. This can explain the basement exposures of gabbro and peridotite, and the MORB-like character of the gabbro. It is difficult for this model to explain the lack of basalts in the basement cores and the presence of linear margin-parallel magnetic anomolies in the OCT. The other hypothesis (Whitmarsh and Miles, 1995) envisages the progressive tectonic and magnetic disruption of continental crust. It can explain the presence of intruded/underplated MORB-like gabbro but the lack of continental crust in the Leg 149 cores is unexpected.

Additional drilling during this leg will determine whether the above models are correct by testing the detachment hypothesis, by investigating the nature and evolution of the basement in the transition zone, and by seeking evidence of synrift magmatism.

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