Wherever possible, observations on the Leg 176 cores were combined with results from the upper 500 m of the hole drilled during Leg 118, both through published results, and by combining the Leg 176 observations with data logs for the intensity and orientation of deformation and metamorphic veins for the 118 section provided by H. Dick (pers. comm., 1977). The team also relogged the lowermost 50 m of the Leg 118 portion of the hole to provide a basis for comparison of the Leg 176 observations to those for the upper 500 m of the hole.
The structural geologists marked all the Leg 176 cores for splitting orthogonal to the foliation. The cores were placed into the archive and working halves of split core liners in the same orientation to provide a consistent framework for description and measurement within the reference frame of the core liner. Thus the strong local concentration of poles to foliation shown in the stereo plots in Figure 18 with respect to strike (with reference to the coordinates of the core liner) are a direct artifact of the method in which they were split. They are not geographically referenced. However, shipboard paleomagnetic declinations for these cores show a similar cluster around 250° in the core reference frame throughout the entire cored interval. This unexpected result demonstrates that, overall, the foliations have a consistent orientation and thus can be reoriented into the geographical reference frame by making the assumption that the declination of the remnant magnetic vector dips toward the south. The resulting reorientation suggests that both the crystal-plastic and the late-magmatic foliations predominantly dip to the north, and hence toward the paleo-ridge axis.
Macroscopic Observations
A majority of the rocks from Hole 735B (78%) have coarse- to
medium-grained hypidiomorphic granular, intergranular, and subophitic
textures with no preferred mineral alignment due to late magmatic
deformation. The remainder contain a variably developed magmatic
foliation defined by the preferred orientation of elongate plagioclase
laths, and locally by pyroxene crystals with weak magmatic foliations
predominating and strong fabrics developed only sporadically (Fig. 18).
Magmatic foliations vary in strike and dip, with no systematic variation
with depth. The majority have dips between 20° and 50°.
Crystal-plastic deformation in Hole 735B, as shown in Figure 18, is highly localized, with the most intense deformation observed in the intervals 0 50 and 450 600 mbsf, with intervals of little or no crystal-plastic deformation generally increasing in number and length downhole. Overall, 77% of the rocks recovered during Leg 176 have no crystal-plastic fabric, and only 7% had more than a weak foliation, whereas 71% of the Leg 118 gabbros had no crystal-plastic fabric and 14% had more than a weak foliation. Again, there was no systematic variation of the dip of the foliation with depth, although there is with a strong concentration at about 30°.
A striking feature of the crystal-plastic deformation fabrics from Hole 735B is the number of shear zones with reverse sense of shear concentrated within and below the 20-m-wide shear zone between 945 and 964 mbsf and also above the fault located at 690 mbsf. From the top of the core to 680 mbsf, the majority of the shear zones display normal shear. An example of a reverse shear zone is shown in Figure 19. These are as yet unexplained; however, the high temperature of their formation, and the origin of the Atlantis Bank at the inside-corner high, dictate that shear zones formed in the lower crust beneath the paleo-rift valley of the Southwest Indian Ridge.
A major feature of the Leg 176 cores is the strong positive correlation between magmatic and crystal plastic fabrics (Fig. 18). Obviously, no magmatic fabric is preserved in regions with a very strong crystal-plastic fabric (e.g., 710 730 mbsf), but in general there is a good positive correlation between the intensities of the two fabrics, which is also reflected in a similar strike and dip. There are several interesting exceptions to this correlation, particularly in the lower portion of the hole, demonstrating that the relationship between the two may be complex in detail. The question naturally arises as to whether the "magmatic foliation" is real. This was addressed by a careful, independent assessment of 240 thin sections for the presence of crystal-plastic deformation and magmatic fabrics. Although weak magmatic fabrics are hard to see in thin section, undeformed samples with magmatic fabrics and the relative intensity of crystal-plastic fabric correlated remarkably well to the macroscopic observations of the core.
Retrograde shear zones at less than granulite facies conditions are evidenced by brittle-ductile deformation and the formation of amphibole in the plane of shear and in crosscutting veins and microcracks. Two principal retrograde shear zones are observed in Hole 735B, both of which coincide with zones of earlier late-magmatic deformation and crystal-plastic deformation. The most intense of these is in the upper 100 m of the core, where pyroxene is often entirely replaced by amphibole and true amphibolites are found. The less intense zone occurs around 500 mbsf at the bottom of the Leg 118 section and the top of the Leg 176 section.
Lower temperature cataclastic fabrics and faults are found at several
levels within the Leg 176 section and at the base of the Unit IV Massive
Oxide Gabbro at 274 mbsf (Dick et al., 1991a). Relatively major faults
with the potential for significant displacement (e.g., hundreds of meters
to kilometers) exist at 560 and at 690 700 mbsf, in addition to several
minor ones at different locations. Smaller discrete planar faults with
associated gouge, breccia, cataclasite, and ultracataclasite cut all rock
types. These were logged at 600 locations in the core, but are
concentrated in the upper half of the hole, and are virtually absent below
about 1400 mbsf. The most common of these features are small-offset
microfaults filled with calcite, amphibole, and/or smectite (below 1050
mbsf). Mineral assemblages associated with the brittle tectonic
structures include amorphous silica, prehnite, chlorite, epidote,
actinolite, and secondary plagioclase. These reflect a range of conditions
for cataclastic deformation. Downhole dip shows no consistent pattern,
though there is a maximum of poles to foliation near a plunge of 90° in
stereo plots indicating a preferred subhorizontal fault orientation in the
lower 1000 m of Hole 735B. Of the faults, 8.8% exhibit oblique slip, 2.8%
pure dip slip, and 2.3% strike-slip.
Summary of Structural Evolution of Hole 735B
The cores from Hole 735B contain many late brittle deformation features
with evidence of cataclasis. These are associated with alteration
assemblages extending from the lower greenschist facies to ambient
temperature. These features are almost certainly associated with block
uplift and the later stages of unroofing of the massif at the inside-corner
high of the Southwest Indian Ridge. This in large part reflects the bimodal
metamorphic history of the massif wherein low-temperature alteration is
concentrated near the bottom of the hole, whereas high temperature
amphibolite facies is concentrated at the top.
Gabbroic rocks cored during Leg 176 at Hole 735B display magmatic, crystal-plastic, and brittle deformation features, together with associated overprinting relations consistent with synkinematic cooling and extension in a mid-ocean-ridge environment. The following observations provide a basis for interpreting the conditions of deformation during evolution of this block of lower oceanic crust.
1.Thick intervals of the core (up to ~150 m) are comparatively free of deformation and are either isotropic or contain local intervals with weak to moderate magmatic foliation; these intervals are most prevalent at the bottom of the hole. Magmatic foliations are often overprinted by a weak, parallel crystal-plastic fabric that may record the transition from magmatic to crystal-plastic deformation.
2.Numerous high-temperature reverse-sense shear zones occur in the interval between 900 and 1100 mbsf, including a 30-m-thick shear zone, and these are cut by lower temperature crystal plastic or semi-brittle shear-zones (~1-10 cm thick).
3.Felsic magmatic breccias and veins are abundant throughout the upper 1100 m, and decrease in abundance toward the bottom of the hole.
4.The transition from crystal-plastic to lower temperature brittle deformation is associated with hydrothermal alteration at amphibolite to transitional greenschist-facies conditions.
5.Intense cataclasis is extremely localized downhole into zones of variable thickness up to centimeters thick.
6.Metamorphic veins show a wide variation in abundance and a general decrease in dip downhole; steeply dipping amphibole veins are common to 800 mbsf; moderately dipping, smectite veins dominate between 800 and 1500 mbsf.
7.There is a strong correlation between regions rich in Fe-Ti oxides and regions with strong crystal- plastic deformation. The relationship between crystal-plastic deformation and the concentration of oxide-rich zones, however, is not unique. Macro- and microstructural observations indicate (a) oxide-rich zones occur as late-crystallizing interstitial material,( b) oxide-rich zones are frequently spatially associated with faults and crystal-plastic shear zones, (c) lower temperature crystal plastic and cataclastic deformation locally overprints some oxide zones, (d) oxides locally cut high-temperature crystal-plastic fabrics along shear zones, and (e) many oxide-bearing shear zones cut through oxide-poor undeformed gabbros.
8.Based on thickness and fracture intensity of recovered cataclastic rocks, there are two major zones (560 and 690 700 mbsf) and several minor zones of cataclasis (including 490, 1076, and 1100 1120 mbsf). The sense and magnitude of displacement on these faults is unknown; however, they likely are associated with uplift of this block.
These observations indicate that the processes that control crustal accretion at slow-spreading ridges are strongly influenced by localized deformation at conditions ranging from magmatic to low temperature cataclastic. The correlation between structural domains and igneous intervals suggests that this segment of the Southwest Indian Ridge is not supplied by a steady-state magma source. Rather, intrusion and deformation are episodic phenomena that may occur separately or synchronously but at different rates. In many cases, zones of localized deformation remain active over a wide range of conditions, for example, cataclastic overprint of oxide-rich crystal-plastic shear zones that were initially active under partially molten conditions.