The multichannel reflection seismic profile LG12 covers part of the drilled transect across the ocean/continent transition in the Iberia Abyssal Plain (see Fig. 1 for location). The extreme ends of the analyzed section (Fig. 3) are marked by two basement highs corresponding to Sites 898 and 901, whereas the central high is at Site 900, all drilled during Leg 149 (Sawyer, Whitmarsh, Klaus, et al., 1994). Drilling results (Fig. 2) proved Site 901 to consist of prerift sediment of Tithonian age dominated by mud and clay, whereas Site 900 sampled highly sheared and fractured gabbro, which probably represents lower continental crust (Reston et al., proposal to ODP, 1994) or material underplated during rifting (see Cornen et al., this volume). Site 898 did not reach basement, but at Site 897 farther to the west, samples of serpentinized peridotite proved the existence of a mantle ridge marking the ocean/continent transition as inferred by Beslier et al. (1993; this volume); Site 899 also cored serpentinized material within three distinct breccia units, indicating the presence of peridotitic basement in the vicinity (see Comas et al., this volume; Cornen et al., this volume). Thus, drilling points to a general deepening of lithospheric level from east to west along the drilled transect and analyzed reflection seismic data.
Between Sites 898 and 900, the postrift sequence on the western part of profile LG12 exhibits a pattern of low-angle, apparently westward-inclined reflections, attributable to a progradational sequence (Fig. 3). These are unconformably overlain by a sequence of mainly terrigeneous turbidites (Shipboard Scientific Party, 1994a). This unconformity (at Site 898 at 7.5-s two-way traveltime depth and 750 mbsf) is correlated with a 10 m.y. hiatus from middle Miocene to late Pliocene, and may be related to gentle regional deformation during the northwest-southeast compressional phase of the Rif-Betic Mountains in southern Spain and North Africa (see Wilson et al., this volume). East of Site 900, the westerly-inclined reflections are no longer imaged, and the postrift sequence shows, except for two places, a regular and subparallel bedding; thickness varies from 750 to 2500 m, and interval velocities are not higher than 3.1 km/s (Fig. 5). Two mounded units east of Site 900 may represent contourites: one is a large sediment ridge extending 14 km between shots 4600 and 4875, the other is a minor one between shots 4950 and 5025 (see Comas et al., this volume; Fig. 3). The lower boundary of a seismic more transparent sedimentary sequence is marked by a major unconformity, which we consider to represent the breakup unconformity, separating postrift from synrift sedimentary units or basement. Confirmation of this interpretation will have to await sampling of the oldest sedimentary units above and of the sediments underneath this unconformity in the deep basins between basement highs.
In the eastern part of the profile (shots 4000-4450, directly to the east of Site 900), thin sedimentary sequences of late synrift age overlie wedge-shaped layers of early synrift/prerift sediments (Vp = 4.1 km/s). Further synrift deposits may be interpreted on top of the basement high between Site 901 and 900 (Fig. 6) as well as east of it (Fig. 7), all characterized by interval velocities between 3.2 and 4 km/s (Fig. 6, Fig. 7) and a maximum thickness of approximately 1750 m (Fig. 8).
The only place where presumed prerift sediments are identified, and also sampled by drilling (Shipboard Scientific Party, 1994c), is Site 901 at the eastern end of the profile. Here, the seismic section displays a more transparent layer of Tithonian strata tilted after deposition (Fig. 9).
Within the basement, the profile images the most striking and relevant features for discussing the transition between continental and oceanic crust and the mechanism of lithospheric extension on this Iberia Abyssal Plain segment of the Iberian passive margin. Of particular importance are strong basement reflections imaged between Site 900 and Site 901 (Krawczyk et al., 1994; Beslier et al., in press). Other basement reflections east of Site 900 may also be important, but are less easily interpreted.
The easternmost high on LG12 appears to be a tilted fault block, capped by a 450- to 750-m-thick seismically transparent layer of prerift or pretilting sediment (Fig. 9). The basement itself exhibits velocities of 5.5 to 6 km/s, whereas within the pretilting layer the velocity values between 2.8 and 3.4 km/s are relatively low compared to approximately 4.1 km/s close to Site 900 (Fig. 6, Fig. 7). One reason may be the higher degree of probable basement fragments mixed with the directly overlying sediment at Site 900 during continued extension, and the different lithology, described here by mud dominated clay and silty clay (Shipboard Scientific Party, 1994c). It is however also possible that this sequence is less compacted than the more deeply buried units to the west.
500 m west of Site 901, the seafloor is marked by a 100-m-high fault scarp (Fig. 8). This is the surface expression of a large fault structure forming the western side of the large fault block drilled at Site 901. The fault can be followed to depth as a bright reflection (L) between shotpoints 4400 and 4900; this is a distance of almost 25 km. L appears on the time section (Fig. 3) to be a listric structure, flattening beneath the fault block (labeled here FB) immediately to the west of Site 901. Thus, L is clearly a major structure, and probably controlled extension leading to breakup for this part of the transect. West of shotpoint 4400, L becomes hard to follow.
The east flank of the central high (fault block FB) does not show many structures, but its central upper part appears faulted and may contain a small basin with up to 750 m of probably synrift sediment (Fig. 8). More pronounced are the reflections between Site 900 and the basement high FB. West-dipping to subhorizontal reflections (shotpoints 4075-4325) bound landward thickening wedge-shaped blocks, overlain by oceanward-thickening wedges of sediments, interpreted as synrift. These reflections end at depth at the level of another bright reflection (H), which extends between shots 4025 and 4325 in a depth ranging between 6 and 10 km. H cuts down from the central high with an oceanward dip, but flattens, turns and cuts up towards the top of the Site 900 high as a band of strong, landward-dipping reflections.
An alternation of small highs and basins within the basement topography appears west of Site 900 between shotpoints 2825 and 3900, which also images some intracrustal reflections with various dips (Fig. 3). Between shotpoints 3300 and 3800, the seismic section exhibits three irregular lens-shaped features, each of them extending approximately 10 km along the profile. These are tentatively identified as basement, perhaps highly fractured, with a consistent interval velocity of 4.7 km/s, and are clearly separated from the overlying sediment sequences with a maximum velocity of 3.5 km/s (Fig. 5).
In this western part of line LG12, deep reflections of various dip directions are also apparent (Fig. 8). Cutting down from Site 900 towards the west, a reflection (labeled here F) deepens between shot-points 3700 and 4000 from 6 km at Site 900 to at least 9 km depth, but cannot be followed clearly farther oceanward below the next basement block. The basement high next to Site 900 marks the proposed drill location IAP-7 (shotpoint 3775; Reston et al., proposal to ODP, 1994). The major reflector below the IAP-7 basement block to the west of Site 900 may be possibly the same structure as the H reflector east of it, as will be discussed further below.
Three basement reflections with eastward dip are imaged approximately 2-10 km east of Site 898 (shotpoints 3150-3300, 3250-3350 and 3300-3500), extending landward from the top of the basement down to 10 km depth (Fig. 8). These may be interpreted as extensional structures accommodating top-to-the-east shear, and perhaps postdate structure H.
Drilling Site 898 did not reach basement, so this location is mainly described by geophysical investigations (e.g., Pinheiro et al., this volume). The seismic image here shows an irregular surface and a series of small, oceanward-dipping fault structures cutting into the upper 750 m of this basement high (Fig. 8). From here to the western end of the investigated seismic section of profile LG12, the basement deepens in three steps farther oceanward to 9 km depth, and again, the velocity function runs from 1.9 to 3.5 km/s within the sediment sequences to 5.5 km/s within the basement (Fig. 5).