Barbara E. John

A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle^ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective

Tectonic structure, evolution, and the nature of oceanic core complexes and their detachment fault zones (13°20'N and 13°30'N, Mid-Atlantic Ridge)

Microbathymetry data, in situ observations, and sampling along the 138200N and 138200N oceanic ncore complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between tectonic nextension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial nstages of detachment faulting and high-angle fault, scarps show extensive mass wasting that reduces their nslope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex nchaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault nplane uplifts a wedge of hangingwall material (apron). The detachment surface emerges along a continuous nmoat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges noverlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the alongextension ndirection. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry nand sonar. Fault planes with extension-parallel stria are exposed along corrugation flanks, where the rubble cover nis shed. Detachment fault rocks are primarily basalt fault breccia at 138200N OCC, and gabbro and peridotite nat 138300N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of nlithologies in the detachment zone. Finally, faulting and volcanism dismember the 138300N OCC, with widespread npresent and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the n138200N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous nrelationship between hydrothermal activity and oceanic detachment formation and evolution.

Lower oceanic crust formed at an ultra-slow-spreading ridge: Ocean Drilling Program Hole 735B, Southwest Indian Ridge

Ocean Drilling Program ODP Hole 735B, drilled on Legs 118 and 176, 1508 m of oceanic layer 3 on a transverse ridge adjacent to the Atlantis II Fracture Zone, Southwest Indian Ridge. The cored sequence consists predominantly or olivine gabbro and troctolite and lesser amounts of gabbro, and gabbronorite rich in oxides. The section contains live major blocks of relatively primitive olivine gabbro and troctolite, composed of many smaller igneous bodies. Each Of these composite blocks shows a small upward decrease in Mg# [defined as 100 x Mg/(Mg + Fe 2+)] and contains more fractionated Fe- and Ti-rich gabbros near the top.Small, crosscutting bodies of olivine gabbro and troctolite with diffuse boundaries may represent conduits through crystal mushes for melts migrating upward and feeding individual intrusions. Oxide gabbros and gabbronorites are commonly associated with shear zones of intense deformation, which crosscut the section at all levels, However, oxide-rich rocks decrease in abundance downward and are nearly absent in the lower 500 m of the section. The gabbros and gabbronorites appear to have formed from late-stage, Fe- and Ti-rich, intercumulus melts that were expelled out of fractionating olivine gabbros into the shear zones. nThe fabrics of the recovered gabbros are consistent with synkinematic cooling and extension of the crustal section in a mid-ocean ridge environment. However, thick intervals of the core have only a weak magmatic foliation. The magmatic foliation is commonly overprinted by a weak, parallel, deformational fabric probably reflecting the transition from a largely magmatic to a largely crystalline state. Deformation in this crustal section decreases markedly downward. nMetamorphism and alteration also decrease downward, and much of the core has less than 5% background alteration. Major zones of crystal-plastic (ductile by dislocated creep) deformation in the upper part of the core probably formed under conditions equivalent to granulite-facies conditions when there was little or no melt present. Late-magmatic and hydrothermal fluids produced a variety of plagioclase, amphibole, and diopside veins. Late-stage, low-temperature veins of calcite, smectite, zeolite, prehnite are present in a few intervals. nThe fact that the cored is unlike ophiolite as defined by the Penrose Conference Participants suggests that no ophiolite representing an ultra-slow-spreading-ridge environment like the Southwest Indian Ridge may be preserved.