Site 897 is located on the top of a basement ridge of serpentinized peridotite bounding the western Iberia passive margin (see "Igneous and Metamorphic Petrology and Geochemistry" section, this chapter) that is buried under a 677- to 694-m-thick sedimentary sequence of Pleistocene to Hauterivian age. Holes 897C and 897D were drilled 100 m apart and penetrated 67.4 and 143.4 m, respectively, of serpentinized peridotites. Sediments and basement rocks display evidence for several episodes of deformation associated with pre-rift, synrift, or post-rift events. Deformational structures identified in the recovered sedimentary section included large- and small-scale folds, microfaults, and sheared clays. Basement ultramafic rocks mainly exhibit evidence of late heterogeneous deformation of already serpentinized material, with shearing and fracturing that leads locally to complete brecciation of the rocks.
The sediments overlying the basement high at Site 897 can be divided into two domains based on their deformational histories. Domain I consists of lithologic Units I, II, and III (49.9-648.7 mbsf in Hole 897C, 596.0-655.2 mbsf in Hole 897D; see "Lithostratigraphy" section, this chapter), in which the sediments are generally undisturbed except for an ambiguous record of stratigraphic dip. Domain II corresponds to lithologic Unit IV (648.7-677.5 mbsf in Hole 897C, 655.2-693.8 mbsf in Hole 897D), which displays abundant evidence for soft sediment folding, microfaulting, and shearing.
Deformation structures in Domain I include soft sediment features, such as dewatering structures and small slump folds at the centimeter scale. An irregular downhole increase in bedding dip in Hole 897C between 320 (Core 149-897C-29R) and 640 (Core 149-897C-62R) mbsf is the only observation that hints at pervasive, possibly tectonic, deformation within structural Domain I.
Bedding dips were determined from core photographs by measuring the apparent dips within rotated drilling biscuits. We assumed that over a given interval of core, a range of apparent bedding dips would be observed within the randomly rotated and split drill biscuits and that the maximum apparent dip should approximate the true dip angle. The inferred true dip angles for Hole 897C, plotted in Figure 39, reach a maximum of about 25° between 552.1 (Core 149-897C-53R) and 600.5 (Core 149-897C-58R) mbsf. The maximum dips observed between 596.0 (Core 149-897D-1R) and 655.2 (Core 149-897D-6R) mbsf were close to 8° (Fig. 39). Without reliable paleomagnetic measurements from the rotated biscuits, we were unable to determine the dip direction. Bedding within cores from Hole 897D did not display such a range of dips; however, the core records a much shorter interval. If these measured dips are real, this indicates some structural disruption (i.e., faulting or folding) of the strata within the 100 m distance between Holes 897C and 897D.
It is possible that the apparent dips are artifacts of drilling and recovery, either due to tilting of drill biscuits within the core liner or to the deflection of the drill hole from vertical. The lack of correspondence between bedding dips and biscuit thickness, however, discounts drilling-induced rotation of the biscuits. Downhole measurement of hole deflection was not possible at Site 897, but hole deviations of up to 8° at Site 900 (see "Site 900" chapter, this volume) suggest that some of this apparent dip may be an artifact.
Structural Domain II, lithostratigraphic Unit IV (9.3 m in Hole 897C and 10.0 m in Hole 897D), contains many different sedimentary lithologies, as well as large blocks of serpentinized peridotite. Nearly all of the sediments exhibit evidence of deformation, except those clearly identified as sedimentary clasts. Deformation structures identified in Unit IV at Site 897 included folded and dipping strata, microfaults, sheared clays, and structural contacts between sediments and serpentinized peridotites. The expression and distribution of deformation are variable, particularly between the two holes. The modes of deformation in the sediment appear to correlate well with both sediment composition and lithification state, which may provide a first-order explanation for their distribution.
Steeply dipping beds (up to 50°, Interval 149-897C-63R, 80-91 cm) were observed within structural Domain II (Fig. 39); these are commonly associated with folded strata or allochthonous sedimentary clasts. No systematic trend was observed in the distribution of dips in bedding in either hole.
Folded strata were observed within Unit IV in Hole 897D within fairly uniform sediment packages. The clearest examples were observed in Section 149-897D-7R-3, and Sample 149-897D-7R-CC, and Core 149-897D-8R, which were predominantly composed of relatively unconsolidated, carbonate-poor (15%-20%), black clayey siltstones, with fine light-colored, silty laminations. These defined, broad folds extending beyond the diameter of the core, which provide important evidence for soft sediment deformation, possibly resulting from slumping. This unit is still soft. The fine silt laminations are commonly offset by normal microfaults, which often truncate against the more uniform black siltstone (Fig. 40); the largest example occurs at Interval 149-897D-7R-3, 11-24 cm (Fig. 20, "Lithostratigraphy" section, this chapter). This brittle response within the ductily deformed unit points to variations in rheology associated with composition or porosity.
Smaller and more isolated folding occurs at 687.4 mbsf (Interval 149-897D-10R-4, 99-107 cm), within a yellow-orange calcareous claystone. Dips in bedding reach 45° near the top of this unit (Interval 149-897D-10R-3, 3-10 cm), but folds here are poorly defined. This calcareous claystone is bounded by clay-rich units that exhibit high shear (Fig. 41), but it is not clear if these folds predated or accompanied shearing.
No examples of soft-sediment folding were identified in Unit IV in Hole 897C. Although this may be an artifact of the core recovery in the two holes, it may indicate that sediments at the two locations deformed through different mechanisms, possibly controlled by variations in the stress regimes.
Microfaults were identified within several different lithologies having relatively high contents of carbonate (50%-65%) in Holes 897C and 897D; the best example was found in limestones within Sections 149-879C-64R-1 and -64R-2. These faults are evident as fine dark lines, apparently caused by concentrations of clay minerals along the slip surfaces (Fig. 42). The rock tends to break preferentially along fault planes to reveal slickensides (Fig. 43). Several fault sets are evident; however, evidence for the relative timing of these sets is ambiguous. At 659.1 mbsf (Interval 149-897C-64R-1, 65-75 cm), the dominant set has a strike of 180° and dip of 47°, with respect to the archive-half reference frame (see "Explanatory Notes" chapter, this volume). Slickenlines suggest a slip direction that trends 223° and plunges 37°, with apparently normal displacements of millimeters (Fig. 42). Preliminary paleomagnetic determinations suggest a true northwest strike for these features.
Minor microfaults also were observed within sheared clay fragments at 687.3 mbsf (Interval 149-897D-10R-3, 100-150 cm), suggesting that microfaulting and shearing may occur on a continuum of scales.
Zones of distributed shear were recognized in the form of fragmented and highly sheared lenticular fragments of claystone bounded by slickensided surfaces. Claystones associated with the folded and tilted calcareous claystone mentioned above (Sections 149-897D-10R-3 and -10R-4) are pervasively sheared (Fig. 41), as are many other clay-rich units in Domain II. These features resemble the scaly clays that distinguish fault zones in submarine accretionary prisms and thus may mark zones that have accommodated significant displacement (Lundberg and Moore, 1986). The ubiquitous presence of these sheared claystones throughout this domain is the strongest evidence for syn- or post-emplacement, in-situ deformation within Unit IV.
Several unique claystones displayed extreme shear deformation. Interval 149-897D-7R-2, 20-66 cm, at the top of Unit IV, contained very disturbed and folded fissile claystone having highly polished surfaces indicative of slip that grades into moderately deformed claystones below. A similar, highly disturbed, organic-rich clay unit at Interval 149-897C-65R-2, 90-110 cm (late early Hauterivian age) displayed polished and striated surfaces that define a folded mesoscopic fabric (Fig. 44). This unit also contains several centimeter-scale clasts of serpentinized peridotite that had been incorporated into the fabric of the clay unit, demonstrating a significant association between the clasts and sediments during deformation (see below).
The occurrence of serpentinized peridotite within sediments in structural Domain II (i.e., Unit IV) might arise from either tectonic or sedimentary emplacement. Contacts between various sedimentary lithologies and the common serpentinized peridotite units provide contradictory evidence regarding their relationships. Several limestone cobbles found in Interval 149-897D-10R-1, 10-50 cm, contain clasts of serpentinized peridotite up to 4 cm wide. These contacts are loci for calcite veins that penetrate both lithologies, apparently overprinting earlier generations of calcite in the serpentinized peridotite (Fig. 45). This suggests that the two lithologies suffered a common history of burial, lithification, and diagenesis prior to the deposition of Unit IV. By contrast, a dolomitic limestone cobble at the base of Sample 149-897D-9R, 75-77 cm, within highly weathered serpentinite, displays slickensides at the contact, which suggests a deformational contact.
Most of the serpentinite clasts within Unit IV exhibit little evidence for penetrative deformation, such as was observed in basement rocks at Site 897, suggesting that they were passive elements during syndepositional or post-depositional deformation. One clast, at Interval 149-897D-10R-1, 45-52 cm, although highly sheared, also displays a very sharp contact with the surrounding sediment, which was interpreted to be depositional.
In contrast, intensely sheared clays at Interval 149-897C-65R-2, 90-100 cm, are in direct contact with highly altered serpentinite and have entrained rounded, elongate, serpentinized peridotite clasts during shearing. The uppermost contact is relatively sharp and oblique to the foliation of the serpentinite (Fig. 44); granular fragments of serpentinite in the claystone near the boundary decrease in size away from the contact, suggesting a weathered depositional contact. Within the fissile claystone, the elongate clasts are aligned with the tightly folded clay. In places, the serpentinized peridotite clasts appear to have been sheared with the claystone, although the foliation within the clasts is locally discordant with the clay fabric. Elsewhere, nearly equant serpentinized peridotite fragments appear to be relatively undisturbed. These observations appear to document the involvement of serpentinite in the deformation of Unit IV, but possibly only as small clasts.
The general absence of structural features within Domain I is consistent with the accumulation of sediments in a postrift margin setting, although locally high dips in bedding (documented in Hole 897C) may indicate some structural disruption between the two holes. By contrast, evidence does exist for pervasive sediment deformation in Domain II, although the modes and distributions are heterogeneous. Differences between holes also are striking. Hole 897C contains a much lower proportion of sediment than Hole 897D in Unit IV, and soft sediment deformation is volumetrically more important within Hole 897D.
First-order explanations for the different types of deformation feature are provided by contrasts in lithology, consolidation state, and carbonate content, which will control the mechanism of deformation. Most of the soft-sediment deformation was observed in relatively unconsolidated, carbonate-poor, clayey siltstones. Discrete brittle fractures can be detected most clearly in relatively lithified carbonate-rich claystones and limestones. Zones that exhibit distributed shear contain slickensided fragments having high contents of clay and generally low contents of carbonate.
One model favored for the origin of Unit IV is debris-flow emplacement (see "Lithostratigraphy" section, this chapter), which may account for some of the structures observed in structural Domain II (e.g., soft-sediment folds and faults). The uncemented sheared clays and microfaulted limestones are less easily explained by this model as they are likely to break up during transport. This suggests that most of the brittle features formed in situ. It is possible that stress concentrations within the load-bearing clasts resulted in localized brittle deformation. Alternatively, all of the deformation features recognized in Unit IV may document deformation involving basement during the last stages of rifting, which may have trapped sediments within zones of shear. The differences between the two holes may result from lateral structural variations.
Two main types of structure can be distinguished in the serpentinized peridotite basement, although a continuum may exist. These are (1)textures acquired by the mantle rocks at depth at high temperature prior to serpentinization and (2) later structures developed at low temperatures during or after the main serpentinization event of the peridotite, at or near the surface.
Primary high-temperature textures in the peridotites have been tentatively identified in several of the less altered samples. However, the primary fabric has been extensively overprinted by later serpentinization, calcite alteration, and late deformation. Undeformed, coarse-grained equant fabrics, deformed porphyroclastic fabrics, and, locally, mylonitic fabrics were recognized. Most of the recovered peridotites display coarse-grained equant textures with relict, 5- to 10-mm-diameter pyroxenes and no macroscopic or microscopic evidence of deformation.
Porphyroclastic textures were observed only in some well-preserved plagioclase-bearing lherzolitic or pyroxenitic samples (Samples 149-897C-64R-5, 63 cm; -65R-1, 45 cm; -66R-4, 56 cm; -66R-4, 68 cm; -67R-3, 32 cm; -67R-3, 63 cm). Dynamic recrystallization is suggested by strained porphyroclasts (50%-90%) of pyroxenes, olivine, plagioclase, and spinel surrounded by recrystallized grains of reduced size (0.1-0.2 mm). A more intense deformation can be observed in Sample 149-897C-64R-5, 86 cm, and, locally, in Samples 149-897C-67R-3, 63 cm, and -64R-5, 60 cm, which display a mylonitic texture that contains very fine recrystallized grains within thin anastomosing bands.
Deformation of the serpentinized peridotite is heterogeneous, both spatially and in style. Three main types of structural features can be recognized: (1) filled fractures associated with brittle deformation, (2)narrow shear zones characterized by foliation planes and shear bands (C-S fabric), and (3) breccias.
The most obvious structural features are fractures that are observed throughout the serpentinized peridotites. Four types of fractures can be distinguished on the basis of the composition, color, and habit of the infilling material:
In the upper part of Holes 897C and 897D, shear deformation can be observed only in friable breccias (see below). In the deeper part of both holes (below 780 mbsf in Hole 897C and below 800 mbsf in Hole 897D), narrow shear zones are unevenly distributed and usually are characterized by a C-S fabric. The material involved in these zones is highly friable, and their relative scarcity may result from poor core recovery in these intervals. The shear deformation features are marked either by a foliation derived from an intense fracture cleavage of white serpentine veins (Type 2; Interval 149-897C-71R-2, 115-125 cm; Fig. 49), or by preferential orientation of numerous, anastomosing, serpentine veins of Types 2 and 3 (e.g., Intervals 149-897D-21R-2, 105-114 cm; -22R-1, 50-55 cm, and -18R-1, 95-110 cm; Fig. 52). The occurrence of a C-S fabric implies a rotational regime for this deformation (Fig. 52 and Fig. 53; Interval 149-897C-70R-1, 105-110 cm; Sections 149-897D-19R-1 and -19R-2; Intervals 149-897D-21R-2, 105-114 cm, and -22R-1, 49-54 cm). Figure 52 (Interval 149-897D-22R-1, 49-54 cm) shows that this shear deformation develops significant brecciation into small rounded blocks. The deformation generally is more developed in the brecciated rocks having a serpentine matrix. In Sections 149-897D-17R-4 to -17R-5, the foliation is marked in the matrix by (1) alternating layers of variously colored serpentine and thin white calcite layers, (2) the alignment and/or flattening of blocks, (3) a concordant layer of creamy yellow serpentine (see below; Fig. 53). The occurrence of rolling structures (Van den Driessche and Brun, 1987) around some of the blocks and of shear bands clearly indicates that this foliation developed in a rotational shear regime.
Three major types of breccia can be distinguished on the basis of block shape, matrix type, and the relative proportion of blocks to matrix:
Ductile high-temperature deformation of the peridotites was observed rarely at Site 897 and is unevenly distributed. This high-temperature deformation is mainly characterized by thin bands containing very fine recrystallized grains, which indicates that recrystallization occurred under high deviatoric stress and relatively low temperatures (<1000°C). Similar thin zones of extremely reduced grains have been identified as shear bands in the Galicia Bank margin peridotites (Girardeau et al., 1988; Beslier et al., 1990).
The intense fracturing, shearing, and brecciation that overprint the primary textures indicate extensive late-stage and low-temperature deformation. These structures are unevenly distributed throughout the cores. The dominant vein-filling mineral progressively changes downward from calcite to serpentine.
The fractures tend to be localized around preexisting heterogeneities that are at a variety of scales (e.g., around isolated pyroxene crystals and pyroxene-rich dikes, bands, or diffuse pyroxene-rich areas). With increasing deformation intensity, the fracturing evolves into brecciation. This brecciation exhibits little, if any, displacement of the angular blocks. The attitude and geometry of tension gashes is compatible with a horizontal principal extensional stress direction.
Shear zones are characterized by a C-S fabric. Foliation is marked by an intense fracture cleavage, or it is developed in thick or parallel thin serpentine veins associated with a penetrative deformation of the serpentine mesh. This attests to a late deformation, when the peridotite was already at least partly serpentinized. Because serpentine usually is a ductile material, shear zones occur preferentially in serpentine-rich levels and, in particular, in breccias containing a serpentine matrix. The presence of veins of iowaite and brucite, as well as the contorted shape of some veins, suggest that hydrothermal fluids also may have aided deformation. Textural evidence and relationships between the different structural features suggest (1) that the high- and low-temperature deformations may represent stages along a continuum during the uplifting of the mantle rocks, (2) that the lowtemperature shear deformation occurred during and/or after the main serpentinization event, and (3) that the different features associated with the late-stage deformation represent a continuum, with late calcite veining.
Further structural, paleomagnetic, and petrologic studies may help define the conditions during the deformation events, the sequence of the different types of late-stage structures, the kinematics of the deformation (sense of shear), and the attitude of the foliation in the geographic reference frame.
The petrostructural evolution of the Galicia Bank margin peridotites is summarized in the "Igneous and Metamorphic Petrology and Geochemistry" section (this chapter). At this time, the preliminary studies of the Site 897 peridotites allow us to tentatively compare the peridotites sampled at the two sites. Some striking differences in the texture and tectonic evolution of the mantle rocks arise from previous observations: