Site 900 lies about 20 km east of Site 899, above a basement high at the eastern edge of the Iberia Abyssal Plain. The basement was encountered at 748.9 mbsf, under a sedimentary sequence of Pleistocene to Paleocene age. Basement rocks were expected to be of continental origin, potentially reflecting both pre-rift and synrift deformation histories. It was also expected that the overlying sediments might display evidence for post-rift deformation along the western Iberia margin. However, few deformation structures were observed in the sedimentary sequence. Basement had been ductily deformed at high temperature and then sheared and fractured at low temperature.
Sediments at Site 900 display a few discrete deformation structures, primarily in concentrated zones containing slumped bedding and microfaults. However, a systematic increase is seen in bedding dips through the deepest part of the sediment column.
Few structural features were observed in Hole 900A above 200 mbsf, where sediments were too soft and disturbed by drilling to preserve structures. The first evidence for structural disruption was observed between 234.1 and 240.0 mbsf (Cores 149-900A-26R and -27R), within an approximately 6-m-thick zone containing faulted and folded beds (Fig. 26) and locally high bedding dips (up to 28°). In places, microfaults offset color-banding in a normal sense; one offsets a Zoophycus of 0.8 cm (Fig. 27). The consolidation state of the sediments involved in this deformation is relatively uniform, suggesting that these features developed prior to sediment burial.
Slight differences in lithology above and below this zone of structural disturbance provide the basis for the subdivision of lithostratigraphic Unit II, posing the possibility that this zone of deformation marks a structural discontinuity (see "Lithostratigraphy" section, this chapter). Such a feature is not evident in seismic profiles that cross the site. Moreover, the excellent recovery through this zone (85%-90 %) suggests that significant deformation features associated with a post-depositional structural discontinuity are not present. The observed deformation, therefore, is most likely to have occurred near the seafloor.
Sediments between 240 and 400 mbsf are relatively flat-lying and undisturbed. Below about 412 mbsf (Core 149-900A-45R), bedding dips steadily increase with depth to about 30° at 731 mbsf (Core 149-900A-78R) almost immediately above the basement. Although at that depth logging data indicate hole deviations of about 8°. The apparent dips are larger and thus must be real. Drilling biscuits from the deepest sediments break preferentially along the dipping bedding planes. Rare microfaults occur in Intervals 149-900A-48R-1, 47-52 cm, to -48R-3, 130-136 cm, and display normal offsets of less than 1 cm and dips of about 47° (Fig. 28). As a result of the low magnetization of these sediments, it was not possible to orient these structures with respect to true north.
Seismic profiles that cross Site 900 do not show the high bedding dips observed in the deeper cores. These dips may reflect local structures associated with the basement high, but this cannot be established at this time.
The basement at Site 900A is made of fine- to locally coarse-grained metamorphic mafic rocks. Two main events of deformation can be distinguished in these rocks: (1) a high-temperature ductile deformation characterized by a well-marked foliation and (2) a later, low-temperature event expressed as narrow shear zones, with intense fracturing that evolves locally into brecciation.
High-temperature Ductile Deformation
This deformation is characterized macroscopically by a clear foliation marked by alternating narrow dark green layers and gray to pale orange elongated lenses. A thin section of a fresh, fine-grained facies (Sample 149-900A-85R-1, 22 cm) shows that the dark layers are made of plagioclase feldspar and the gray ones of pyroxene crystals. The rock texture is porphyroclastic to granoblastic. The plagioclase has been recrystallized in 0.5-mm-sized crystals, some having undulatory extinctions, which locally surround some relict-strained porphyroclasts. A few strained pyroxene porphyroclasts have been preserved in clusters, where they are more or less recrystallized. Such a deformation is classically described in flaser-gabbros. In some places, the rock displays a larger grain size, with porphyroclasts of 5 mm that have been isolated in elongated bands of smaller pyroxene crystals that alternate with bands of fine-grained recrystallized plagioclase (e.g., Interval 149-900A-83R-2, 65-100 cm; Fig. 29). However, the heterogeneity of the grain-size distribution is also illustrated in such intervals, where layers of fine-grained facies are parallel to the foliation. This foliation is present in the overall basement section, even though it is hardly visible in some extremely fine-grained facies. It generally dips from 15° to 45°, but is locally higher and even becomes subvertical in Interval 149-900A-81R-2, 0-28 cm. However, the latter orientation might be a consequence of extensive fracturing and brecciation, which locally can be seen to deviate the foliation clearly.
The presence of amphibole and chlorite, both mainly derived from pyroxene, demonstrates a retrograde metamorphism and re-equilibration in the low amphibolite to greenschist facies (see "Igneous and Metamorphic Petrology and Geochemistry" section, this chapter). Textural evidence suggests that the high-temperature deformation occurred prior to the amphibole development.
Low-temperature Deformation
The low-temperature deformation clearly overprints the high-temperature deformation. It is expressed by three types of structural features:
The first tectonic event recognized in the rocks is an intense ductile deformation that developed a well-marked foliation. Textural evidence suggests that this deformation occurred at high temperature under dry conditions, likely during a shear event. Some intriguing thin fractures appear to have diverted or even bent the foliation. Such brittle features associated with "soft" deformation features might indicate either that (1) the conditions of deformation were close to the brittle/ductile transition during the formation of these fractures or (2) the foliation is progressively diverted or bent by closely spaced microfractures.
As the amphibole derived from primary pyroxene does not appear to be synkinematic, the retrograde metamorphism in low-amphibolite to greenschist facies seems to post-date the high-temperature ductile deformation event.
Next, a complicated structural evolution at low temperature affected the basement rocks. The most striking features associated with this late deformation are the successive types of vein and narrow shear zones unevenly distributed throughout the section. Most of the fractures, and particularly sets of conjugate 45°-dipping veins and brecciated zones having a 60°-dip, are in accordance with a horizontal extensional regime. The presence of shear zones in microbreccias and epidote-chlorite veins demonstrates that a shear event occurred during or after the formation of these veins and microbreccias. The calcite veining and fracturing clearly is the latest event to have occurred. The brecciation thus took place during or after the epidote veining and ended during the calcite veining.
Further petrostructural, geochemical, and geochronological studies may help to define the origin of these rocks, the sequence of the different tectonic and metamorphic events, and the kinematics of the deformations, and thus to constrain the evolution and emplacement mechanism of this mafic body. Although the magnetization of these rocks is very low, possible reorientation of the structures in the geographic reference frame may be obtained from shore-based paleomagnetic data.
Site 900 is located between mantle rock outcrops (Site 897) and rocks having mantle affinities (Site 899) to the west and the continental crust of the passive margin farther to the east. The most likely hypotheses for the origin of these metamorphic basic rocks might be either to associate them with the Mesozoic rifting of the margin (underplated gabbros or atypical oceanic crust), or to consider them as older Hercynian units that have been incorporated in the passive margin. A preliminary comparison with the mantle rocks recovered at Sites 897 and 899 suggests that the tectonometamorphic evolution of the ultramafic and mafic rocks may be comparable at the three sites (high-temperature ductile deformation in the peridotite and the mafic rocks, low-grade retro-metamorphism, late shear deformation and fracturing, similarities in the texture of some breccias, traces of fluid circulation; see "Structural Geology" sections, "Site 898" and "Site 899" chapters, this volume). As regards the synrift emplacement hypothesis, the Site 900 results show that the spatial transition between continental and oceanic crusts may occupy a large area made of transitional crust and serpentinized mantle. However, at present any conclusion would be premature. Further petrostructural studies and geochronological studies of the freshest basement samples will help to understand the presence of such rocks beneath the Iberia Abyssal Plain.