Next Section | Table of Contents

SITE SUMMARIES (continued)

Site 1270

Site 1270 was located along the track of Shinkai 6500 submersible Dive 425 from 4 July 1998, which recovered samples of peridotite and gabbro from nearly planar, striated outcrops along the eastern flank of the Mid-Atlantic Ridge. The slope of the outcrop surfaces was nearly constant throughout most of the dive, leading to the inference that the slope of the rift valley along the dive track represented a single, kilometer-scale fault surface. All holes at Site 1270 were begun in exposed outcrop surfaces. Recovery averaged ~20%, with 37% recovery over 46 m in Hole 1270B.

Hole 1270A was begun near the site of Shinkai 6500 sample 425-R007, a mylonitic peridotite taken from a planar fault surface. Poor drilling conditions, especially the intersection of a zone of unconsolidated fault gouge at 17 mbsf, caused us to abandon this hole. Hole 1270B was situated ~100 m upslope to the east, near another striated, planar outcrop observed during the Shinkai 6500 dive. This hole had better recovery but became unstable after 46 m of penetration and had to be abandoned. Hole 1270C was sited ~200 m upslope from Hole 1270B, in an outcrop ~20 m upslope from the top of a steep, 10- to 20-m-tall cliff, near the site of Shinkai 6500 sample 425-R008, a weathered harzburgite. The outcrop on and above the cliff is more weathered than those at Holes 1270A and 1270B. Poor drilling conditions caused us to abandon this hole after ~19 m. We inferred that a slope-parallel, low-angle fracture along the base of the nearby cliff might have interfered with circulation in the hole. Thus, we sited Hole 1270D in outcrop ~30 m farther upslope. Again, however, poor drilling conditions forced us to abandon this Hole after 57 m of penetration. Overall, fault gouge, altered shear zones, poor recovery, and/or anomalously rapid progress in drilling were encountered at 15–20 mbsf in each hole at Site 1270. Our impression is that the planar fault surfaces exposed on the seafloor are underlain by parallel brittle fault zones over the entire slope sampled by the Shinkai 6500 and our drilling program at this site.

Proportions of Igneous Rocks

The igneous and residual mantle protoliths of recovered core were harzburgite, dunite, gabbro, gabbronorite, and minor pyroxenite (Fig. F20). Gabbro, gabbronorite, and pyroxenite are intrusive into the peridotite in Holes 1270A, 1270C, and 1270D. Hole 1270B includes only a few small fragments of completely altered peridotite and no intrusive contacts, so it is not certain that the gabbroic rocks intrude peridotite. However, by analogy to relationships in Holes 1270A, 1270C, and 1270D and at Sites 1268, 1271, 1272, and 1274, we infer that the gabbros and gabbronorites in Hole 1270B were also intrusive into peridotite.

About 89% of the core from Hole 1270A (total depth = 27 mbsf) is composed of harzburgite, 5% is dunite, and gabbroic rocks compose ~4%, with the remainder being serpentinite and fault breccias with unknown protoliths. Core from Hole 1270B (total depth = 46 mbsf) is 98% gabbro and gabbronorite, with only 2% completely altered peridotite. Core from Hole 1270C (total depth = 19 mbsf) is 81% harzburgite, 17% dunite, 1% gabbroic dikes or veins, and 1% pyroxenite dikes or veins. Core from Hole 1270D (total depth = 57 mbsf) is composed of 91% harzburgite, 7% dunite, and 2% gabbroic dikes or veins.

The overall proportion of rock types in all the holes at Site 1270, weighted by length of recovered core, is ~41% harzburgite, 4% dunite, and 55% gabbro. Weighted by depth of penetration in each hole and using the proportions of rock types recovered in core in each hole, the proportion of lithologies is ~62% harzburgite, 6% dunite, and 32% gabbro and gabbronorite, similar to the proportion of gabbro to peridotite in Hole 1268A (63% harzburgite, 11% dunite, and 26% gabbronorite and gabbro), the proportion of lithologies (2 gabbros/6 peridotites) collected during Shinkai Dive 427, and the proportion of peridotite to gabbro in all dredge and dive samples along the Mid-Atlantic Ridge south of the 15°20' Fracture Zone to 14°40'N (~77 peridotites/~25 gabbros, based on the compilation presented in the Leg 209 Scientific Prospectus, and references cited therein).

As discussed in "Site 1268" above, these data suggest that much of the seismic crust and even parts of the uppermost mantle may be composed of ~75% peridotite and 25% gabbroic rocks, with crack density and alteration in both lithologies decreasing with depth. However, shipboard measurements of seismic P-wave velocities for the fresh gabbroic rocks from Hole 1270B range from 4.6 to 5.8 km/s. These velocities are too high for these rocks to compose a substantial proportion of the seismic crust in this region at depths from <2 to 3 km. Instead, if gabbroic rocks compose a significant proportion of the shallow crust, they must be more altered or there must be a relatively large proportion of cracks compared to the samples from Hole 1270B whose P-wave velocities were measured on board.

Highly Depleted Mantle Peridotites

With some notable exceptions, peridotites from Site 1270 are 100% altered, mainly to serpentine. However, geochemical analyses of these peridotites show exceptionally low concentrations of nominally immobile incompatible elements such as Al, Sc, and V. For example, with the exception of a peridotite sample including a gabbroic dike or vein, Al2O3 concentrations in Site 1270 peridotites are <1 wt% (average = 0.6 wt%), whereas Al2O3 in peridotites from Site 920 along the Mid-Atlantic Ridge at 23°20'N ranges from 1 to 2 wt% (Casey, 1997). The average Al2O3 concentration in 7 peridotites from Site 1270 is the same as that determined for 20 peridotites from Site 1268. If these values have not been modified by hydrothermal metasomatism, then the peridotites from Sites 1268 and 1270 are among the most depleted residual mantle peridotites yet obtained from the mid-ocean ridges.

Molar Mg# of the peridotites from Site 1270 varies from 90.0% to 91.5%. This is a narrower range of Mg# than in Site 1268 peridotites (88%–94%) and lies within the normal range of whole-rock Mg# in residual mantle peridotites from mid-ocean ridges (~89%–92%; average = ~90%–91%) (e.g., Dick, 1989). By comparison, we infer that Mg#s <89% and >92% in Site 1268 peridotites are due to metasomatic changes in Fe/Mg during the extensive hydrothermal metasomatism at that site.

Petrogenesis of Intrusive Gabbronorites

Most of the gabbroic rocks in Hole 1270B are Fe-Ti oxide–bearing gabbronorites (Fig. F21) with substantial proportions of igneous plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxide minerals preserved. The question arises, how much of the oxide is igneous? It has been proposed that some gabbronorites in the Haylayn massif of the Oman ophiolite were originally oxide-free olivine gabbros that were oxidized at near-solidus conditions via the reaction

6 (Mg, Fe)2SiO4 (olivine) + O2 = 3 (Mg, Fe)2Si2O6 (pyroxene) + 2 Fe3O4 (magnetite),

consuming olivine and forming orthopyroxene + magnetite (Boudier et al., 2000). The compositions of Hole 1270B gabbronorites lie within the volume olivine + plagioclase + pyroxene on most major element projections (e.g., Fig. F22). However, the gabbronorites are rich in TiO2. Even if they are assumed to contain no ferric iron, and thus no magnetite, CIPW norms for these compositions yield 2–12 wt% Fe-Ti oxide in the form of normative ilmenite (FeTiO3). In addition, the freshest, least deformed samples of oxide gabbronorite contain coarse discrete oxide and pyroxene crystals separated from each other by plagioclase, suggesting that both oxide and pyroxene crystallized directly from a melt. Thus, we infer that the magma(s) that crystallized the Hole 1270B gabbronorites were saturated in Fe-Ti oxide minerals.

Fe-Ti oxides also form interstitial crystals, even within deformed and recrystallized plagioclase. This suggests that the gabbronorites may have been partially molten and igneous Fe-Ti oxides may have continued to crystallize during high-temperature crystal-plastic deformation. However, interstitial textures can form during metamorphism as well as igneous crystallization. Thus, we view the hypothesis that melt was present during deformation of the gabbronorites as intriguing but uncertain.

Analyzed samples have Mg#s from 39% to 68%, similar to the range observed in gabbronorites from ODP Sites 735, 894, and 923 (e.g., Cannat et al., 1997a; Casey, 1997; Dick et al., 1991, 2002; Natland and Dick, 1996). Evolved crustal gabbronorites formed at pressures <0.3 GPa must form by nearly complete crystallization of orthopyroxene-normative basalts because MORB lavas very rarely contain orthopyroxene crystals and phase equilibria confirm that primitive and normal MORB are not saturated in orthopyroxene at 0.3 GPa or less. However, the coarse igneous textures and scarcity of optical zoning in plagioclase and pyroxene suggest that the rocks are "cumulates" formed by partial crystallization of a melt followed by extraction of the remaining liquid.

The inference that evolved melts were extracted after crystallization of Hole 1270B gabbronorites presents an interesting puzzle because no lavas with appropriate compositions have been found along the Mid-Atlantic Ridge from 14° to 16°N. We do not know the igneous Mg# of the pyroxenes in Hole 1270B gabbronorites because the samples contain substantial amounts of magnetite. Thus, the whole-rock Mg# must be substantially lower than the pyroxene Mg#. Also, postcrystallization Fe/Mg exchange between pyroxenes and oxides—plus possible oxidation reactions—may have modified the original igneous Mg#s. However, we can estimate that the clinopyroxenes have Mg#s of 70% or 75% in the Hole 1270B gabbronorites with the highest Mg# and lowest proportion of oxide minerals. Using an Fe/Mg pyroxene/liquid Kd of 0.23 (Sisson and Grove, 1993a, 1993b) and FeO/(FeO + Fe2O3) of 100–70 wt%, we can infer that liquids coexisting with these clinopyroxenes must have had an Mg# less than ~41%. In contrast, the lowest reported Mg# in glass from lavas dredged along the Mid-Atlantic Ridge from 14° to 16°N is 50%. Indeed, Mg#s <50% are rare along the entire Mid-Atlantic Ridge (Fig. F23). Thus, the final destination of evolved liquids beneath mid-ocean ridges, once they are extracted after crystallizing evolved gabbronorites, is not yet known.

Gabbroic veins intruding peridotite in Holes 1270A, 1270C, and 1270D are heavily altered and have been recrystallized during extensive ductile deformation. They include plagioclase and clinopyroxene, interpreted as igneous phases. Clinopyroxene is replaced by high-temperature pleochroic amphibole, which could be igneous or could be a high-temperature alteration product. Locally, the veins contain oxide minerals, zircon, and apatite (Fig. F24). Natural gamma emissions recorded on the multisensor track show high radioactivity in the intervals containing these veins. As might be expected from the mineral assemblages in the veins and their gamma ray emissions, the whole-rock composition of a peridotite containing one of these veins (interval 209-1270D-3R-1, 63–66 cm) is enriched in incompatible elements; for example, this sample contains 96 ppm zirconium. Thus, we infer that the melt parental to the veins was enriched in highly incompatible trace elements. Perhaps these veins crystallized from liquids extracted from evolved gabbronorites similar to those recovered from Hole 1270B.

Hydrothermal Alteration, Metamorphism, and Metasomatism

Most peridotites from Site 1270 are completely altered to serpentine with minor talc. As is typical for hydrated peridotites worldwide, most of the talc alteration was in early replacement of pyroxene, followed by serpentinization of olivine and remaining pyroxene. Serpentinization occurred at temperatures <400°C under static conditions in most of the core.

Although talc alteration is subdued in Site 1270 peridotites compared to Site 1268 peridotites, samples from both sites show a similar kind of metasomatism with enrichment in SiO2 and/or loss of MgO + FeO. The normative orthopyroxene contents of all but two of the altered peridotites from Sites 1268 and 1270 are >30 wt% (Fig. F25), in contrast to the 20–25 wt% generally observed in fresher residual mantle harzburgites from mid-ocean ridges (e.g., Cannat et al., 1995; Dick, 1989; Dick et al., 1984). It is unlikely that MgO and FeO are geochemically similar in the hydrothermal alteration environment; variable proportions of FeO and MgO loss during hydrothermal alteration would probably modify the Mg#. In view of the fact that most peridotites from both Sites 1268 and 1270 have Mg#s within the normal range for abyssal peridotites (see above and the "Site 1268" summary), we infer that MgO and FeO were comparatively immobile during metasomatism and the high SiO2 contents of peridotite samples from both sites are due mainly to SiO2 enrichment.

An unexplained but consistent observation is that peridotites within a few centimeters of gabbroic veins in Holes 1270C and 1270D and peridotite inclusions within the veins are much less altered than peridotites elsewhere in this hole.

Gabbronorites and gabbros in Hole 1270B were extensively recrystallized during deformation but are relatively free of hydrous alteration phases. Given the extensive recrystallization of pyroxene and oxide phases during subsolidus or near-solidus high-temperature deformation, it is likely that these minerals approached chemical equilibrium. Both quartz and olivine are absent from these rocks, whereas ilmenite and Ti-bearing magnetite are common. Thus, metamorphic conditions during deformation must be bracketed by the reactions

2 (Mg, Fe)2SiO4 (olivine) + 2 FeTiO3 (ilmenite) =
2 Fe2TiO4 (ulvospinel in magnetite) + (Mg, Fe)2Si2O6 (pyroxene)


Fe2Si2O6 (ferrosilite in pyroxene) + 2 FeTiO3 = 2 Fe2TiO4 + SiO2 (quartz).

Exsolution of ilmenite from titanomagnetite also constrains the temperature and oxygen fugacity. And in addition, two pyroxene thermometry will provide independent constraints on temperature. Thus, we are optimistic that the pressure and temperature conditions of high-temperature recrystallization and reequilibration can be quantified in these rocks.

Hydrothermal alteration of gabbros and gabbronorites in Hole 1270B is limited mainly to formation of chlorite-amphibole assemblages along pyroxene/plagioclase contacts. Brown hornblende of high-temperature metamorphic or low-temperature magmatic origin is associated with Fe-Ti oxides and locally replaces pyroxene along grain boundaries.

Gabbroic veins in Holes 1270C and 1270D are extensively altered, though they include fresh recrystallized plagioclase, pyroxene, and even olivine in mylonitic zones. Altered portions of veins include talc + serpentine + albite + tremolite ± chlorite ± brown amphibole. Brown amphibole replaces pyroxene within veins and along contacts between peridotite boudins and veins, but not within the boudins, suggesting that circulation of high-temperature fluids and/or hydrous melts was restricted almost entirely to the veins. Locally, a rodingite assemblage (Ca silicates including prehnite and hydrogrossular) replaces the veins.

Structural Geology

Intrusion of gabbroic rocks into peridotite occurred while the peridotite was at high temperature, >600°C and probably close to 900°C. This inference is based on the presence of coarse-grained relict igneous pyroxene in the veins and tiny anastomosing branches from igneous veins, the association of veins with high-temperature hornblende replacing pyroxene, and high-temperature deformation structures in mylonite zones within gabbroic veins.

Ductile deformation of the peridotites away from gabbroic veins was relatively subdued, with most samples preserving protogranular textures. Hole 1270A peridotites are an end-member in this regard, with protogranular to weakly porphyroclastic textures, very few magmatic veins, and little evidence anywhere in the core for ductile deformation under subsolidus conditions. This is particularly striking given the peridotite mylonite (Shinkai 6500 sample 425-R007) (Fig. F26) recovered from a planar outcrop surface just tens of meters away from the site of Hole 1270A.

In contrast, all of the gabbroic veins in Holes 1270C and 1270D were the locus of high-temperature mylonitic deformation. Additional deformation of peridotites surrounding these mylonitized veins is evident in the form of olivines with well-developed subgrain boundaries and bent pyroxene crystals, but the entire gradient from protogranular to mylonitic textures is present over a few millimeters and true mylonites were not developed in peridotites. This indicates that the gabbroic veins were much weaker than surrounding peridotites during deformation, either because they were partially molten during the onset of deformation or because they were hydrous while surrounding peridotites were dry or simply because gabbroic mineral assemblages had a lower viscosity than peridotites at the conditions of deformation.

High-temperature ductile deformation of the gabbroic rocks in Hole 1270B is extensive and irregularly distributed. The top of the hole had relatively poor recovery, perhaps due in part to ductile and brittle deformation features associated with the fault that formed the planar outcrop surface on which we started the hole. In any case, crystal-plastic deformation intensity in the top 10–15 m of the hole is relatively high. Below this depth, crystal-plastic deformation is concentrated in a second zone at 40–45 mbsf, underlain by less deformed gabbronorites. The intensity of ductile deformation of the gabbroic rocks appears to be correlated with the abundance of oxide minerals (Fig. F27). This is consistent with the tentative hypothesis, based on preliminary data, that there may be a correlation between deformation intensity and the proportion of oxide minerals in oceanic crustal gabbros from Holes 735B and 1105A from Atlantis Bank on the Southwest Indian Ridge, as has been cautiously proposed on a provisional basis (e.g., Dick et al., 1991, 2000, 2002; Natland and Dick, 2002; Natland et al., 1991; Niu et al., 2002; Ozawa et al., 1991; Pettigrew, Casey, Miller, et al., 1999).

The correlation between deformation intensity and magnetite proportion in Hole 1270B gabbronorites is poorly quantified and uncertain because of incomplete recovery, a lack of continuous measurements of oxide proportion (magnetic susceptibility is heavily weighted to ferromagnetic phases) in the recovered core [N4], a "nugget effect" resulting from coarse-grained, irregularly distributed oxide crystals, and the presence of anastomosing ductile shear zones, including less deformed lenses, leading to irregular variations in deformation intensity on the centimeter to decimeter scale. The apparent relationships in Figure F27 should provide an excellent starting point for postcruise research to test and fully quantify the hypothesis that there is a correlation between oxide proportion (measured in detail by image analysis of thin sections) and deformation intensity (quantified by, for example, grain size analysis of recrystallized plagioclase).

Brittle deformation features are relatively uncommon in the core, compared to Site 1268. However, there is a fault gouge zone—probably several meters thick—at ~17 mbsf in Hole 1270A and we encountered poor recovery and difficult drilling at this depth in the other holes at Site 1270 as well. We believe that this indicates the presence of large faults parallel to planar outcrop surfaces and to the overall slope that forms the eastern wall of the rift valley in this region.

Crosscutting relationships define the following sequence of events for the core from Site 1270:

  1. Partial melting of mantle peridotite, together with formation of dunite bands and protogranular textures in residual harzburgite;
  2. Cooling of peridotite at the base of the thermal boundary layer;
  3. Intrusion of gabbroic veins and gabbronorite bodies while peridotites were still at high temperature;
  4. Formation of high-temperature mylonite zones in gabbroic veins within peridotite in Holes 1270C and 1270D, perhaps with synchronous ductile deformation in gabbroic rocks in Hole 1270B, while temperatures were >600°C and probably >900°C;
  5. Cooling of peridotites and gabbroic rocks to <400°C;
  6. Static, pervasive, nearly complete serpentinization of peridotites; and
  7. Continued brittle deformation in large fault zones, probably parallel to exposed fault surfaces in outcrops observed during Shinkai 6500 Dive 425 and to the entire slope of the eastern flank of the Mid-Atlantic rift valley in this region.

This is similar to the sequence of events inferred for Site 1268, except that mylonitic deformation at Site 1270 took place entirely within gabbroic rocks (within massive gabbros in Hole 1270B and within gabbroic veins in peridotite in Holes 1270C and 1270D), whereas at Site 1268 mylonitic deformation was concentrated in zones of mixed peridotite and gabbroic veins and affected both lithologies. Also, replacement of serpentine by talc was common at Site 1268 but rare or absent at Site 1270 and small brittle deformation features are relatively uncommon at Site 1270 compared to Site 1268.

Dips of crystal-plastic foliation planes in gabbros from Hole 1270B and in mylonitized gabbroic veins in Holes 1270C and 1270D dip at 30°–60° (average = ~45°). Thus, foliations in the core are significantly steeper than the striated planar outcrop surfaces with slopes of ~20° observed on the seafloor during Shinkai 6500 Dive 425 and during the camera surveys on this cruise. Also, we did not recover peridotite mylonites (matrix grain size < 10 Shinkai 6500 sample 425-R007 is a peridotite mylonite recovered from a striated outcrop surface just a few tens of meters from the site of Hole 1270A (Fig. F26). Thus, it is apparent that the faults represented by the striated outcrop surfaces are not related in a simple way to the coarser-grained, higher-temperature mylonitic shear zones observed in core from Site 1270. Instead, the fault surface that forms seafloor outcrops dipping ~20° to the west must cut the higher-temperature fabrics at an average angle >20°. In the following section, we argue that the high-temperature foliation in the core probably dips ~45° to the east and thus intersects the west-dipping fault(s) represented by outcrop surfaces at an angle of ~65°.

Paleomagnetic Data and Tectonics

Paleomagnetic data were collected on half cores and individual discrete samples. Some samples retain a stable remanent magnetization. Using these data, we rotated the measured orientations of foliations, faults, veins, and dikes in individual core pieces around a vertical axis, thereby restoring core pieces to an orientation with a common azimuth for the remanent magnetization vector.

In the case of Site 1270, this procedure was complicated because of the low inclinations of the remanent magnetization. Mainly negative stable inclinations in gabbroic rocks from Hole 1270B (average = –14°; see below for details) suggest that these may have acquired their magnetization during a period of reversed polarity. Hole 1270B is 13–18 km from the rift axis, indicative of 1.0–1.4 m.y. of spreading at a half-rate of 12.8 km/m.y. (Fujiwara et al., 2003), so it could have formed and been magnetized during the Matuyama Reversed Chron. This is consistent with—but not required by—the sea-surface magnetic data presented by Fujiwara et al. (2003). Thus, the negative inclinations in Hole 1270B may indicate that the rocks were reversely magnetized. Based on this interpretation, rotation of structural features around a vertical axis into a common magnetic orientation—together with the assumption that tectonic rotations have not substantially modified the azimuth of the remanent magnetization vector—yields the result that high-temperature foliations in the gabbroic rocks from Hole 1270B dipped ~45° to the east when the remanent magnetization vector pointed south.

Some peridotites in Holes 1270C and 1270D retain a stable remanent magnetization. Inclinations of this magnetization are nearly horizontal (average = –3°; see below). Thus, the polarity of the Earth's magnetic field when these rocks acquired their magnetization is highly uncertain. Assuming that the peridotites were reversely magnetized like the gabbroic rocks in Hole 1270B yields the result that mylonite zones in gabbroic veins dipped ~45° to the west when the remanent magnetization vector pointed south, assuming that tectonic rotations have not substantially modified the azimuth of the remanent magnetization vector (Fig. F28A). Alternatively, assuming that both peridotites and gabbroic rocks were normally magnetized implies that the crystal-plastic foliation in the gabbroic rocks dipped to the west and the mylonite zones in the peridotites dipped to the east (Fig. F28B) when the remanent magnetization vector pointed south, again with the assumption that the azimuth of the remanent magnetization vector has not been modified by tectonic rotations.

Holes 1270C and 1270D are only a few hundred meters from Hole 1270B, and the deformation features in all three holes record similar conditions (localized deformation at ~900°C, possibly in partially molten rocks). Thus, the result that the fabrics have dips that differ by 90° seems problematic. An alternative is that the peridotites were magnetized during the Brunhes Normal Chron (or the Jaramillo, or even some older normal polarity chron). In this case, rotation of the high-temperature foliations in Holes 1270C and 1270D yields ~45° dips to the east, parallel to the high-temperature foliation in Hole 1270B (Fig. F28C). This scenario is similar to our interpretation of remanent magnetization in Hole 1268A, in which we inferred that gabbroic rocks acquired their magnetization during cooling of igneous magnetite through 500°–600°C, whereas peridotites acquired their magnetization significantly later as a result of metamorphic growth of magnetite during serpentinization at ~300°C.

The average remanent inclination for 10 discrete samples from the gabbroic rocks in Hole 1270B is –14° (95% CI = +10°/–10°), much shallower than the expected inclination of ±28°. The average inclination of 10 discrete samples of peridotite from Holes 1270C and 1270D is –3° (95% CI = +13°/–13°), which is also significantly shallower than the expected value of 28° but not significantly different from the inclinations in the gabbroic rocks of Hole 1270B.

One possible explanation for the statistically significant difference between the expected and observed inclinations in both gabbroic rocks and peridotites is that tilting occurred after cooling and blocking of the remanence. As discussed in "Paleomagnetic Data and Tectonics" in "Site 1268" above, with additional assumptions paleomagnetic inclination data may be used to infer the direction and magnitude of tectonic rotations that may have occurred between the time the sample recorded remanent magnetization and the present. The stepwise alternation of steep rift-facing slopes and nearly horizontal benches along the flanks of the Mid-Atlantic Ridge is generally interpreted to result from the presence of tilted normal fault blocks. Back-tilting of fault blocks on the east side of the rift valley in the region around Site 1270 is probably top-to-the-east (clockwise around a northeast-striking axis). In this region, the west-facing slopes strike ~010°–020°, parallel to the strike of the rift valley. We infer that tilting may have been produced by clockwise rotation of normal fault blocks along an approximately rift-parallel axis. For a horizontal rotation axis striking 020° and assuming that the gabbroic rocks are reversely magnetized, ~35° of clockwise rotation is required to change an original inclination of –28° to the observed average inclination of –14° in reversely magnetized gabbros from Hole 1270B (Fig. F29). Similarly, if we adopt the interpretation that the peridotites are normally magnetized (Fig. F28C), the peridotites must have undergone ~60° of clockwise rotation to change an original inclination of +28° to the observed average inclination of –3°. Given the uncertainties in the data, both gabbro and peridotite inclinations can be explained, within error, using a model in which there was ~45°–50° of clockwise rotation around a horizontal axis striking 020° after both lithologies acquired their magnetization.

Alternatively, the differences between magnetic inclination in the gabbros (approximately –14°) and in the peridotites (approximately –3°) suggest that the two lithologies might record different amounts of rotation. If we adopt the interpretation in Figure F28B, in which both peridotites and gabbroic rocks are normally magnetized, we can explain this difference in the same manner as for Site 1268. For a horizontal rotation axis striking 020°, ~85° of clockwise rotation is required to change an original inclination of +28° to the observed average inclination of –14°. And, as already stated above, the peridotites must have undergone ~60° of clockwise rotation. In this scenario, clockwise rotation of the Site 1270 section around a horizontal axis striking 020° began before magnetization of the peridotites and continued after magnetization of the peridotites [N5].

An additional uncertainty in inferring the amount of tilting is introduced by the unknown plunge of the rotation axis. Figure F30 illustrates the effect of gently plunging rotation axes. For a rotation axis with a fixed azimuth of 020°, if the plunge were 10° along 020° and gabbroic rocks are normally magnetized, ~115° of clockwise rotation would be required to change an original inclination of 28° to the observed average inclination of the gabbroic rocks in Site 1270B of –14°. Conversely, if the plunge were 10° along 200°, ~75° of counterclockwise rotation would be required for the same change in inclination [N6].

In summary, if the hypothesis outlined here is correct, the gabbroic rocks in Hole 1270B underwent a large amount of clockwise rotation (~45°–100°) around a nearly horizontal, rift-parallel axis after acquisition of the magnetic remanence. The peridotites record only ~50°–65° of this rotation.

These results and hypotheses are similar to our interpretation of paleomagnetic and structural data from Site 1268. At Sites 1270 and 1268, the magnetic inclination data, together with the assumption that tectonic rotations occurred around rift-parallel, nearly horizontal rotation axes, require tectonic rotations of 35°–120°. Also, at both sites the peridotites may record smaller tectonic rotations than the gabbroic rocks, perhaps due to early magnetization of the gabbroic rocks compared to the peridotites.

It must be emphasized again that our tectonic interpretations of structural and magnetic data are nonunique, forward models, chosen from among many other models that could potentially account for the data. However, the low paleomagnetic inclinations at Sites 1268 and 1270 require large tectonic rotations and the presence of faults that do not consistently dip toward the rift valley.

Next Section | Table of Contents