Return to Hole 735B [book]

H. Dick, J. Natland, J. Miller
1998 ODP Preliminary Report   unpublished
Leg 176 was devoted to the deepening and logging of Ocean Drilling Program Hole 735B, atop a shallow bank along a transverse ridge of the Atlantis II Fracture Zone, Southwest Indian Ridge. During this leg, a hole initially cored to 500 mbsf on Leg 118 was deepened more than 1 km to a total depth of 1508 m. More igneous rock was cored on this leg than on any other but one, and more igneous rock was recovered by far-some 866 m, representing 86.3% recovery-than on any previous leg. The combined
more » ... ults of drilling during Legs 118 and 176 make Hole 735B one of the most significant accomplishments in the study of the solid Earth. For the first time, a significant proportion of one of the major layers of the Earth's crust, the all but inaccessible Layer 3 in the ocean basins, has been sampled and examined. We are now able to describe the architecture and outline the magmatic, structural, and metamorphic history of a block of the lower ocean crust that formed at this very slowly spreading ridge 11 Ma. The gabbroic ocean crust we have drilled consists of four main blocks of relatively primitive olivine gabbro and troctolite, from 200-m to 700-m thick, each with its own internal chemical and petrological coherence. Overall, each of these bodies is composed of many smaller magma bodies, which probably represent small intrusive masses that penetrated fairly cold rock, in which case contacts are sharp, or crystal mushes, in which case contacts are diffuse or sutured. Each of the composite bodies of rock has more fractionated gabbros toward their top and more primitive rocks toward their base. Gabbroic rocks recovered during Leg 176 display numerous magmatic crystal-plastic and brittle deformation features. The associated overprinting relationships are consistent with synkinematic cooling and extension in this mid-ocean ridge environment. However, thick intervals of the core (up to 150 m) are comparatively free of deformation and are either texturally isotropic, or contain weak to moderate magmatic foliation. Magmatic foliation is commonly overprinted by a weak parallel crystalplastic fabric that may record the transition from magmatic to crystal-plastic deformation. Locally, however, the foliation is fairly well developed, and in one interval ~200 m thick there are many hightemperature reverse shear zones. Many of the deformed rocks show a continuum between crystalplastic and brittle behavior, which was associated with hydrothermal alteration from amphibolite-to Leg 176 Preliminary Report Page 6 greenschist-facies conditions. There are some narrow zones of intense cataclasis, and several fault zones were identified. The gabbros of Hole 735B preserve a complex record of high-temperature metamorphism, brittle failure, and hydrothermal alteration that began at near-solidus temperatures and continued down to very low-temperature conditions. At one extreme, the high-temperature metamorphic effects are transitional to magmatic processes. Some rocks were deformed and recrystallized while they were still partly molten. Faults represented by zones of both high-and low-angle shear penetrated a crystal mush deep beneath the axial rift. Some of the most striking zones of crystal-plastic deformation probably formed under the equivalent of granulite facies metamorphic conditions (>800°-1000°C), when there was little or no melt present. Extensive intervals (>300 m), however, have less than 10% background alteration. Rock magnetic measurements demonstrate that the entire body of rock cored during Legs 118 and 176 has a consistent average stable inclination of ~71° and no observable downhole variation. The rocks have been tilted about 20°. A preferred orientation of declinations around 260° in core coordinates suggests that gross reorientation of structural features of the core may be possible. Physical properties were routinely measured on all cores and magnetic susceptibility measurements proved to be of particular value in outlining the occurrence and distribution of even very small intervals rich in iron-titanium oxides. The intervals are prevalent in the upper 500 m of core examined during Leg 176 (500-1000 mbsf) and correlate remarkably well with both the intensity of deformation and the average oxide percent in the gabbros, as established by visual observation of the core. In summary, the sequence of rocks observed in Hole 735B is unlike that found in well-studied ophiolites. A full on-land counterpart to these rocks has yet to be described. Still less does this sequence of rocks resemble a layered igneous intrusion. Hole 735B, therefore, provides a first assessment of synkinematic igneous differentiation in which the upper levels of the gabbroic crust were enriched in late differentiated melts through tectonic processes, rather than simple gravitationally-driven crystallization differentiation.
doi:10.2973/ fatcat:3bmv43r2rfcavb5qnqjxjpr7gu