Lithologies, structures, and geologic setting at Site 1277 are comparable to those drilled on the Iberia margin, including the Galicia Bank (ODP Leg 103) and the Iberia Abyssal Plain (ODP Legs 149 and 173), as well as Gorringe Bank farther south (Fig. F13). In particular, evidence of normal faulting that exhumed mantle lithosphere was previously documented on the Iberia margin at Sites 897, 899 (Sawyer, Whitmarsh, Klaus, et al., 1994), and 1070 (Whitmarsh, Beslier, Wallace, et al., 1998). These sites are located within the inferred ocean–continent transition zone (within the M-series magnetic anomalies). In addition, basalts from earliest formed oceanic crust have also been recovered farther north, on Goban Spur, in the United Kingdom southwestern approaches (Leg 80) (Kempton et al., 2000). Similarities and differences with the recovery on the Newfoundland margin are highlighted below.
In contrast to the strongly depleted serpentinized harzburgite recovered from Site 1277, which indicates a highly depleted mantle source (Müntener and Manatschal, 2006), the Iberian examples include relatively enriched lherzolite, which implies a relatively undepleted mantle source (Cornen et al., 1999). This difference may reflect different types of basement beneath both margins, which could include much older ophiolitic rocks beneath the Newfoundland margin (Müntener and Manatschal, 2006).
The presence of spinel peridotites from the Iberia margin might be explained in three ways. First, they could record the products of melt extraction from juvenile (primary) mantle in an ocean ridge–like setting. This seems unlikely given their position within the ocean–continent transition zone. Second, they could represent inherited mantle that remained relatively undepleted, which would require major, but plausible, differences of basement type across the conjugate margins. Third, they could represent "refertilization" of previously depleted mantle that had a similar composition to mantle at Site 1277 (Müntener and Manatschal, 2006). However, this would require regional differences in a process ("refertilization") that remains poorly understood.
Serpentinite breccias were also drilled on the Iberia margin at Site 1070 during ODP Leg 173 (Whitmarsh, Beslier, Wallace, et al., 1998; Whitmarsh and Wallace, 2001). The "basement" recovered there begins with relatively massive, partially serpentinized peridotite that is intruded at the top by a relatively unaltered gabbro pegmatite dyke (Beard et al., 2002). This is overlain by calcite-cemented peridotite breccias, with clasts of serpentinized peridotite, variable amounts of pyroxene grains, and minor weakly deformed coarse-grained gabbro. The breccia shows a downward transition to noncohesive fault gouge and an interlocking jigsaw texture of clasts and was therefore interpreted as mainly, or entirely, of tectonic origin. In this case the fault gouge can be interpreted as the effect of deformation along an extensional detachment fault. The overlying breccias are likely to reflect brittle fracturing related to the inferred faulting, although some degree of local gravity reworking may also have taken place. Similarly, at Site 1277 the uppermost levels of the serpentinized ultramafic basement show evidence of intense tectonic brecciation along an inferred extensional detachment fault. Breccia clasts in the mass flows above also indicate intense brittle fragmentation of ultramafic and gabbroic rocks (Fig. F3A, F3B).
Carbonate veins within serpentinized basement and individual serpentinite clasts at Sites 897 and 899 exhibit a complex history of fracturing and carbonate precipitation in an extensional setting (Morgan and Milliken, 1996). Some of the veins exhibit successive phases of filling of open fractures. These veins were interpreted as neptunian fractures that were filled with fine-grained carbonate, oxide grains, and serpentinite fragments. This material was possibly derived by brecciation of the wall rocks. The fractures were inferred to relate to stages in the exhumation of serpentinized peridotite to the seafloor. By contrast, later-stage fracturing and the formation of micrite-filled veins were interpreted to be the result of gravity collapse of uplifted basement, although they could also have resulted from continuing extension, as inferred at Site 1277. Stable isotopic and trace element analyses of the vein-filling carbonate at Site 897 and within serpentinite breccia at Site 899 suggest a low-temperature origin (<20°C) from seawater (Morgan and Milliken, 1996). Similar neptunian fissures and geopetal structures occur at Site 1277, as discussed earlier.
Fabric studies of the serpentinized peridotite breccias at Site 899 indicate an origin as cataclasite (Comas et al., 1996; Beslier et al., 1996). Calcite veining is most abundant in the upper part of the breccias and is most intense in clasts at the base of the reworked sequence. Fabric studies also indicate that two cataclastic events affected the breccias. Fabrics of the first event are associated with mylonitic textures, shear foliation, and calcite-free serpentinite veining. A later multiphase event involved serpentinization and penetrative cataclastic brecciation associated with abundant calcite and rare serpentinite veining.
These results are consistent with the existence of an extensional detachment separating an exhumed serpentinized ultramafic basement from overlying mass flows. They also suggest a continuing history of faulting after mass flow emplacement, as inferred for Site 1277.
Serpentinized peridotite breccias, comparable to those recovered at Site 1277, were cored in Holes 897C, 897D, and 899B (Comas et al., 1996) (Fig. F13). At Site 897, up to 143 m of basement rocks were recovered, composed of relatively undepleted peridotites. The peridotite basement in Hole 897C is covered by friable serpentinite breccia. Brecciated peridotite also occurs deeper in the peridotite basement in Hole 897D. The basement of both holes is overlain by heterogeneous deposits ("olistostromes"), including clasts and boulders of serpentinized peridotite and serpentinite breccia in sediments of early Aptian age.
At Site 899 (Hole 899B) no basement was encountered but a 188-m-thick lowermost sedimentary succession of early Aptian age includes heterogeneous serpentinite breccias, boulders, fault gouge, and fragments of weathered basalt, diabase, micrograbbo, and sheared amphibolite. During the Aptian, approximately coeval gravity deposits reached Sites 897 and 899 (located ~20 km apart), implying widespread sediment dispersal from highs to basinal areas (Comas et al., 1996).
On the Iberia-Galicia margin, the drilled intervals of serpentinized peridotite breccia, as thick as 5 m and separated by fine-grained sediment, were interpreted as relatively cohesive mass flows (olistostromes) (Comas et al., 1996). The clasts within the mass flows include reworked synrift sediments similar to those previously reported from Galicia Bank (Boillot et al., 1987; Kornprobst et al., 1988). Comas et al. (1996) proposed a relatively distal source for the serpentinite mass flows, from a remote transform-fault setting, although these authors also considered a more local origin related to normal faulting near or within the ocean–continent transition zone. Comas et al. (1996) also envisioned that the serpentinized peridotite breccias were deposited in a deep basinal setting, followed by block faulting, uplift, and covering by pelagic facies during latest Cretaceous–Paleogene time. In their view the peridotite ridges developed in response to normal faulting that followed continental breakup, possibly associated with a regional change in plate motion. On the other hand Gibson et al. (1996) interpreted the serpentinite breccias from Hole 899B as giant submarine landslips generated by slope failure on a large serpentinite fault scarp affected by extension, and they suggested that this fault was located within a few kilometers of Site 899.
In contrast to the above interpretations, the Site 1277 mass flows are interpreted as relatively proximal deposits that were derived from locally exhumed mantle and an inferred seafloor detachment fault. The writer believes a similar mechanism and tectonic setting of formation is also applicable to the Iberia-Galicia margin mass flows, although some of the material in these deposits is likely to have been transported farther and mixed from several different sources.
Only subordinate clasts or fragments of mafic rocks have been recovered from the Iberia-Galicia margin, consisting of basalts and microgabbros that are relatively undeformed (Cornen et al., 1996). The basalts exhibit porphyritic, intergranular, and variolitic textures. Most samples are strongly altered by low-temperature seafloor metamorphism. Petrographic results, including the discovery of alkaline pyroxene, suggest that some of the basalts may be alkaline in composition, whereas porphyritic basalts containing pseudomorphed olivine phenocrysts and Ca-rich plagioclase are likely to be tholeiitic.
Chemical analysis of the basalt and diabase clasts from the Iberia Abyssal Plain (Holes 899B and 900A) and from Galicia Bank exhibit REE values and normalized spidergram patterns that indicate a wide range of E-MORB, T-MORB, and N-MORB (Kornprobst et al., 1988; Seifert and Brunotte, 1996; Seifert et al., 1997; Cornen et al., 1996) (see Fig. F12). Depleted tholeiites characterized by a marked negative Nb anomaly were recovered from Gorringe Bank, farther south (Cornen et al., 1999). Rare examples of metabasite clasts and boulders from the Iberia margin have been interpreted as Fe-Ti gabbros that were sheared and metamorphosed under greenschist-facies conditions (Schärer et al., 1995). Localized fragments of foliated amphibolites, compositionally similar to differentiated and chemically enriched flaser gabbro recovered at Site 900, may relate to alkaline synrift magmatism (Schärer et al., 1995; Gardien et al., 2001).
It can therefore be inferred that basic magmatic rocks of a wide compositional range were erupted on the Iberia-Galicia margin as a whole, potentially influenced by different magma sources (e.g., synrift vs. earliest seafloor spreading) and differences in the degree of partial melting. By contrast, only basalts of a narrow MORB-type composition are known from the Newfoundland margin.
The serpentinized peridotite breccias recovered from holes drilled on the Iberia-Galicia and Newfoundland margins show many similarities. First, exhumed peridotites are present on both margins, although the composition of the Iberia-Galicia margin peridotite (Müntener and Herman, 2001; Abe, 2001) differs considerably from the peridotite recovered at Site 1277, as the latter is much more depleted (Müntener and Manatschal, 2006). Second, there is structural evidence of an extensional detachment on both margins separating exhumed serpentinized peridotite from mass flow deposits. Third, serpentinite breccias are interpreted as mass flows on both margins, although some of the Iberia mass flows may have traveled farther than those at Site 1277. Fourth, some of the clasts in the mass flows on both margins were derived from inferred extensional detachment surfaces. Fifth, several generations of extensional fracturing, neptunian dike infill, and low-temperature calcite veining followed initial high-temperature ductile deformation, cataclasis, and exhumation on both margins.
There are also several apparent differences between the two margins. First, the serpentinite mass flows of the Iberia margin are more heterogeneous and include clasts of margin-derived terrigenous sediment not seen at Site 1277. Second, MORB was recovered from intact flows at Site 1277, whereas only clasts of igneous rocks have been recovered from the Iberia-Galicia margin. Third, the Site 1277 basalts of MORB composition do not show the highly variable compositions (mainly N-MORB to E-MORB) of the basalt clasts on the Iberia-Galicia margin. These differences may, however, reflect the chance recovery of drilling in minute sections of complex terrain. All of the Iberia and Newfoundland sites can be interpreted to sample different parts of a wide zone (~150–200 km) of crustal exhumation within an ocean–continent transition.