Figure 2. Bathymetric map of the Atlantis II Fracture Zone modified from Dick et al. (1991c). Locations of Hole 735B and the conjugate site 735B' (SWIR 6) are shown. SWIR 6 is located on the counter lithospheric flow-line on crust of the same age and position relative to the paleo transform as Hole 735B. Active southern and northern rift valleys are at 33°40'S and 31°350'S, respectively.
Figure 3. A. Hand-contoured 100-m-contour bathymetric map of Site 735 showing the location of Hole 735B modified from Dick et al. (1991b). SeaBeam tracks hand-shifted by eye to eliminate conflicts in the data. Solid lines indicate actual data, whereas hatched lines show inferred contours. Contour interval is 250 m. Small solid dots and arrows indicate the starting point and approximate track of dredge hauls. Large solid dots show the location of Sites 735 and 732 (just north of the contoured area on the crest of the median tectonic ridge). Filled circles indicate the approximate proportions of rock types recovered in each dredge: + = gabbro; white = basalt and diabase, light stipples = greenstone, and heavy stipples = serpentinized peridotite. B. Hand-contoured bathymetric map of the eastern rift mountains north of the Southwest Indian Ridge axis showing crust of the same age as that at Site 735 and the conjugate position of Hole 735B (735B') on the counter-lithospheric flow line, based on magnetic anomalies and plate reconstruction. This conjugate site is the location of Southwest Indian Ridge 6, the final backup site for Leg 176, where the volcanic carapace originally overlying Hole 735B is preserved intact.
Figure 4. Magnetic anomalies over Site 735 based on the survey of Dick et al. (1991b). Bathymetry contoured at 200-m intervals. Crustal magnetization is shown shaded, with normal polarity crustal magnetization shown as gray and reverse polarity shown as white. Dark gray areas have crustal magnetization greater than 1 A/m. Polarity identifications and numbering modified from Dick et al. (1991b) by M. Tivey (pers. comm., 1997) based on the time scale of Cande and Kent (1995).
Figure 5. Outcrop map in vicinity of Hole 735B constructed from Leg 118 video survey. Swath field of view is ~8 m. The ratio of outcrop to sediment is proportional to the distribution of patterns along the swath. Time stamp and depths from video image and voice-over are noted along swath pattern. Textured sediment refers to a coating of sediment so thin that the texture of the outcrop underneath is discernable. Locations of Holes 735A and 735B along survey lines are also indicated.
Figure 6. Temporal cross sections across the Southwest Indian Ridge rift valley drawn parallel to the spreading direction (not across the fracture zone, but parallel to it), showing the postulated tectonic evolution of the transverse ridge and Hole 735B (Dick et al., 1991b). The sequential sections are drawn at about 18 km from the transform fault. Crust spreading to the right passes into the transverse ridge and spreads parallel to the transform valley. Crust spreading to the left spreads into the rift mountains of the Southwest Indian Ridge parallel to the inactive extension of the Atlantis II Fracture Zone. A. Initial symmetric spreading, possibly at the end of a magmatic pulse. Late magmatic brittle-ductile deformation occurs because of lithospheric necking above (and in the vicinity of whatever passes for a magma chamber at these spreading rates). Hydrothermal alteration at high temperatures accompanies necking and ductile flow in subsolidus regions. B. At some point, the shallow crust is welded to the old, cold lithosphere to which the ridge axis abuts, causing formation of a detachment fault, and nodal basin, initiation of low-angle faulting, continued brittle-ductile faulting, and amphibolite-facies alteration of rocks drilled at Hole 735B. C, D. Block uplift of the rift mountains at the ridge-transform corner forms a transverse ridge enhanced by regional isostatic compensation of the local negative mass anomaly at the nodal basin. Initiation of the block uplift terminates the extension driving cracking, and drastically reduces permeability in the Hole 735B rocks, effectively terminating most circulation of seawater and alteration. Greenschist-facies retrograde alteration continues along the faults on which the block is uplifted to account for the greenschist-facies alteration that predominates in dredged gabbros.
Figure 7. Seismic velocity structure from Muller et al. (1997). A. P-wave seismic velocity model on the north south seismic Line CAM101 of Muller et al. (1997). The velocity contour interval is 0.3 km/s. Numbered OBH positions are shown on the seafloor. The position of ODP Hole 735B has been projected from 1 km west of the line. The Moho is indicated as a thicker line where its depth is constrained by wide-angle reflections. B. Resolution contours for the seismic model. The resolution of each velocity node is given by the diagonal of the inversion resolution matrix a number between 0.0 and 1.0, affected by the ray coverage sampling each node. Values of greater than 0.5 are considered well resolved and reliable.
Figure 8. Lithostratigraphic column showing the change in proportions of igneous intervals through Hole 735B from the seafloor to 1508 mbsf and the locations of pegmatitic gabbros, microgabbros, and igneous layering.
Figure 9. Representative core close-up photographs. 9A. Typical coarse-grained olivine gabbro (interval 176-735B-171R-2, 32 44 cm). 9B. Olivine microgabbro crosscutting olivine gabbro (interval 176-735B-189R-3, 90 120 cm. 9C. Oxide olivine gabbro vein (interval 176-735B-171R 2, 2 19 cm). 9D. Foliated gabbro (interval 176-735B-93R-3, 76 93 cm). 9E. Pegmatoidal olivine gabbro (pieces from Sections 176-735B-177R-3 and 177R-4). 9F. Varitextured olivine gabbro (interval 176-735B-196R-5, 23 97 cm).
Figure 10. Representative layered sections of the Leg 176 core. A. Size-graded and modal layering (interval 176-735B-171R-4, 35 90 cm). B. Troctolitic layer in olivine gabbro (interval 176-735B 186R-4, 35 55 cm). C. Mafic layers in olivine gabbro (interval 176-735B-190R-2, 0 95 cm).
Figure 11. Downhole distribution of secondary phases in Hole 735B, related to the variation in total rock alteration with depth. Data are from thin section observations.
Figure 12. A. Distribution of total veins by percentage of core in gabbroic rocks recovered during Legs 118 and 176. B. Distribution of felsic, plagioclase, and amphibole + plagioclase veins by percentage of core. Below 1250 mbsf, felsic veins are rare. C. Distribution of plagioclase + diopside and diopside veins by percentage of core. Below 750 mbsf, diopside and plagioclase + diopside veins do not occur. D. Distribution of amphibole veins by percentage of core. Below 600 mbsf, amphibole veins are rare. E. Distribution of carbonate and smectite veins downsection. F. Distribution of chlorite and zeolite veins downsection.
Figure 13. Plagioclase and diopside veins, and combinations thereof, on splays from an apparently igneous felsic vein (interval 176-735B-110R-4, 0 38 cm).
Figure 14. Subvertical amphibole vein crosscutting foliated olivine gabbro (interval 176-735B 92R- 1, 10 15 cm).
Figure 15. Vein of pale-green smectite in Sample 176-735B-133R-7 (Piece 1, 126 137 cm). Olivine, plagioclase, and clinopyroxene are highly altered to smectite in the alteration halo. Altered plagioclase appears pale-green, olivine dark, and altered clinopyroxene brown or brilliant at greater distance to the vein (incipient alteration).
Figure 16. Mg number (calculated as Mg2+/Fe2+ + Mg2+; Fe2+ = 0.85 Fe3+) vs. depth for Hole 735B. Samples are subdivided mainly on the basis of TiO2. Filled diamonds = troctolite and olivine gabbro having less than 0.4 wt% TiO2; half-filled diamonds = gabbro, gabbonorite, and disseminated-oxide gabbro having between 0.4 and 1.0 wt% TiO2; open diamonds = Fe-Ti oxide gabbro with more than 1.0 wt% TiO2; half-filled squares = felsic samples or hybrid samples with a significant felsic component. Filled triangle is a sample from a basaltic dike.
Figure 17. Weight percent TiO2 vs. depth in Hole 735B. Symbols as in Figure 16.
Figure 18. A. Late magmatic and crystal-plastic foliation intensities plotted with total vein intensities downhole for Hole 735B. No data exist for the upper 500 m of Hole 735B for magmatic foliation. B. Stereoplots for structures in Hole 735B cores. Poles to foliation are plotted in the reference frame of the core liner, with the strong azimuthal orientation reflecting careful cutting of cores orthogonal to foliation and consistent placement in the core liners for description with respect to foliation dip. C. Representative photomicrographs of magmatic foliation, high temperature (granulite grade) crystal-plastic foliation, and low-temperature cataclastic deformation.
Figure 19. Close-up photograph of reverse fault (interval 176-735B-149R-3, 48 94 cm).
Figure 20. Stable thermal remanent magnetism, magnetic susceptibility, and inclination for Hole 735B after demagnetization of minicores.
Figure 21. Multisensor track log of magnetic susceptibility for all cores from the Leg 176 section of Hole 735B. Measurements are filtered to eliminate empty section of core liner, leaving in excess of 22,000 measurement points. Inset shows correlations between individual peaks and lithologies in a section of core from 1072 to 1084 mbsf.
Figure 22. Comparison of discrete sample bulk density and velocity with percent recovery downhole.
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