Leg 173 Scientific Report


Rifted margins contain the principal record of the breakup that follows 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 in particular 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 are commonly absent. Pairs of conjugate rifted margins often exhibit some asymmetry in structural style that may be related to the mode of lithospheric rifting (e.g., pure or simple shear).

The west Iberia margin (Fig. 1) is an excellent example of a nonvolcanic rifted margin. The Galicia Bank and Iberia Abyssal Plain segments of the margin were cored during Ocean Drilling Program (ODP) Legs 103 and 149, respectively, and have been sampled by submersible and 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 (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 and demonstrates that Iberia drifted away from North America and moved apart along roughly east-west fracture zones (e.g., Klitgord and Schouten, 1986). During the short-lived Late Cretaceous opening of the Bay of Biscay, a ridge-ridge-ridge triple-point existed off northwest Spain (Sibuet and Collette, 1991). However, 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 during the Eocene (Pyrenean phase, short-lived subduction of Bay of Biscay crust under northern Spain) and in the south and center during 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., 1990, 1993; Boillot et al., 1987, 1988, 1995; Girardeau et al., 1988; Hoffman and Reston, 1992; Sawyer et al., 1994; Whitmarsh et al., 1990, 1993; Whitmarsh and Miles, 1995; Whitmarsh et al., 1996). It exhibits tilted continental fault blocks that often, but not always, seem to lack a wedge of synrift sediments (Reston, 1996; Wilson et al., 1996). There is an apparent lack of synrift volcanism offshore and significant synrift volcanism is equally absent onshore. 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). During this leg, a short transect of holes was drilled west of Galicia Bank (Sites 637-641). In 1991, the recommendations of the North Atlantic Rifted Margin Detailed Planning Group were accepted by the JOIDES Planning Committee, which programmed two drilling legs in the North Atlantic during 1993. During one of these (Leg 149), a transect of holes was drilled into acoustic basement across the ocean-continent transition (OCT) in the southern Iberia Abyssal Plain (Sawyer, Whitmarsh, Klaus, et al., 1994).

In the Iberia Abyssal Plain, results of ODP Leg 149 defined landward and oceanward limits to the OCT where the transition zone is defined as the region between a margin-parallel peridotite ridge, marking the landward edge of oceanic crust, and the most seaward tilted fault block of continental crust (Fig. 2). However, only one hole penetrated basement in the 130-km-wide region between these limits. The main objectives of Leg 173 were as follows:

1. Sample acoustic basement, principally within the OCT, to characterize the tectonic and magmatic processes that dominate the transition from continental to oceanic crust in space and time.

2. Determine the role of detachment tectonics in the evolution of the margin by drilling through a seismic reflector which has been interpreted as a major tectonic contact or detachment on the east side of the basement high where Site 900 has already been drilled.

3. Determine the role and extent of synrift magmatism in the OCT basement, which is inferred to exist from the new magnetic anomaly chart and other data.

4. Sample acoustic basement beneath Site 901 or Site Iberia-8A/8B to confirm the predicted existence of continental crust there and to determine from which approximate original crustal level it came, thereby setting an unequivocal landward limit to the OCT and defining geometrical relationships between deep and shallow lithospheric levels.

5. Sample the early formed oceanic crust to complete the whole transect from continental to oceanic crust.

6. Investigate the early sedimentary history of the rifted margin.

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