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 lowtemperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high-and low-temperature alteration are their respective associations with ductile and cataclastic deformation, and an overall decrease downhole with hydrothermal alteration generally 95% in the bottom kilometer. Hole 735B provides evidence for a strongly heterogeneous lower ocean crust, and for the inherent interplay of deformation, alteration and igneous processes at slow-spreading ridges. It is strikingly different from gabbros sampled from fast-spreading ridges and at most well-described ophiolite complexes. We attribute this to the remarkable diversity of tectonic environments where crustal accretion occurs in the oceans and to the low probability of a section of old slow-spread crust formed near a major large-offset transform being emplaced onland compared to sections of young crust from small ocean basins.

Oceanic core complexes expose gabbroic rocks on the seafl oor via detachment faulting, often associated with serpentinized peridotite. The thickness of these serpentinite units is unknown. Assuming that the steep slopes that typically surround these core complexes provide a cross section through the structure, it has been inferred that serpentinites compose much of the section to depths of at least several hundred meters. However, deep drilling at oceanic core complexes has recovered gabbroic sequences with virtually no serpentinized peridotite. We propose a revised model for oceanic core complex development based on consideration of the rheological differences between gabbro and serpentinized peridotite: emplacement of a large intrusive gabbro body into a predominantly peridotite host is followed by localization of strain around the margins of the pluton, eventually resulting in an uplifted gabbroic core surrounded by deformed serpentinite. Oceanic core complexes may therefore refl ect processes associated with relatively enhanced periods of mafi c intrusion within overall magma-poor regions of slow-and ultra-slow-spreading ridges.

[1] Water carried into subduction zones with the downgoing plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge, triggers partial melting and affects subduction zone seismicity. Partially serpentinized peridotite may be a significant reservoir for water in the subducted plate, the mantle wedge and the overriding plate. Here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic P-wave velocities. In partially-serpentinized ultramafic rocks, a 1% decrease in P-wave velocity corresponds to a 2.4% increase in serpentine content, and a 0.3% (0.18 moles/m 3 ) increase in H 2 O content (up to a maximum of 13%). Where there is evidence of serpentinization, mantle serpentine content is typically $15%, corresponding to 4 -5 wt% H 2 O (6 -10 moles/m 3 ).

Gabbroic rocks and peridotites are exposed on the seafloor on the western median valley wall of the Mid-Atlantic Ridge, south of the Kane Transform (MARK). The gabbroic rocks occupy an uplifted massif directly south of the transform-ridge intersection, whereas the peridotites extend 20 km along a median valley parallel ridge just south of the gabbro massif. Acoustic velocity measurements have been made at elevated confining pressures for a suite of samples extracted from drill cores collected during Ocean Drilling Program Leg 153. Drilling operations at Site 920 produced the deepest penetration and most substantial recovery to date in a coherent block of serpentinized peridotite from any ocean basin. Site 923, in the gabbro massif, yielded nearly 75% recovery of fresh troctolite, olivine gabbro, and gabbro. A sample suite was selected from these drill cores to be representative of the primary lithologies recovered. Evaluation of physical properties measurements from the serpentinized peridotites suggests that serpentinization is an ongoing and rapid process such that we can see evidence of changes in these properties over a time span of a few months and potentially as quickly as a few days. Samples showed a broad range of degree of serpentinization, even between samples only a few centimeters apart. A strong negative correlation exists between degree of serpentinization and density, as well as compressional-(V p) and shear-(V s) wave velocity, for samples from ophiolitic peridotites, and the serpentinized samples from MARK mimic this correlation. Published data indicate that V p , V s , and density of intensely serpentinized peridotites and fresh peridotites plot in separate and distinct fields as compared to values derived from gabbroic rocks. Physical properties data from moderately serpentinized peridotites from MARK, however, as well as published data from samples exhibiting partial serpentinization, are virtually indistinguishable from the values obtained from gabbroic rocks. Physical properties data are in accord with petrographic observations, indicating that the gabbroic samples collected at MARK are considerably less altered than gabbroic rocks sampled from near Hess Deep. Data from gabbroic samples suggest that, given reliable densities, velocity data from remote geophysical surveys may be useful in estimating oceanic crustal modal composition. Elastic constants derived from physical properties measurements suggest that, although drilling in the gabbroic massif at MARK may be more difficult in terms of bit life than in other gabbroic exposures on the seafloor, the holes may well be more stable and conducive to extended drilling operations.

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The uppermost 2 km of the oceanic crust created at the fast spreading (135 mm yr À1 , full rate) equatorial East Pacific Rise (EPR) is exposed for tens of kilometers along escarpments bounding the Hess Deep Rift. Mosaics of large-scale digital images from the remotely operated vehicle (ROV) Argo II and direct observations from the submersible Alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading. Dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units. These rock units are characterized by extensive faulting, fine-scale fracturing, and rotations of coherent crustal blocks meters to tens of meters across. The uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces. Through most of the basaltic lava unit, however, lava flow contacts dip (208-708W) toward the EPR and generally increase in dip downward in the section. Dikes cutting the lavas and in the underlying sheeted dike unit generally dip (908 -408E) away from the EPR. Deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units. We interpret this upper crustal structure as the result of subaxial subsidence within 1-2 km of the EPR that accommodated the thickening of the basaltic lava unit to $500 m. Variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply. These results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the Hess Deep Rift. If this type of structure is typical of uppermost oceanic crust generated at the EPR, these processes may be common along fast spreading mid-ocean ridges.

[1] Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has B071031 of 25 produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45°rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Systematic sampling and mapping in the Kohistan accreted arc terrane of northern Pakistan has provided a sample suite representing the lithologic diversity of the section from its base along the Main Mantle Thrust upward through several stacked intrusions and their metamorphosed equivalents into the Kohistan batholith. A new lithologic column for the terrane has been developed during the course of this study using geothermobarometry based on elemental exchange reactions calculated from electron microprobe analyses of mineral assemblages. The pressures calculated from the chemical analyses of various mineral assemblages are constrained by recently published activity‐composition models and an internally consistent thermodynamic data base. Compressional and shear wave velocities and densities have been measured on samples that represent the diverse lithologies in this section and have been correlated with the new lithologic column. This data set has been augmented with published compositions and values for the uppermost part of the arc, not sampled in this study. Compressional wave velocities range from 4.3 km s−1 in supracrustal volcanogenic sediments to 7.5 km s−1 in lower crustal mafic cumulates. Underlying ultramafic rocks with velocities of 8.0 to 8.4 km s−1 define a sharp seismic Moho. The strong inflection in the velocity profile to greater than 8.0 km s−1 is due to the transition from plagioclase‐bearing to plagioclase‐free cumulate rocks in a continuous ultramafic‐mafic intrusion. The base of the crust lies at least four kilometers below this transition. Correlation of these laboratory‐measured values and the new lithologic column has allowed development of a velocity profile comparable to profiles developed through more conventional field seismic methods. Geochemical indices of fractionation exhibit a reasonable correlation with Compressional and shear wave velocities. An estimate of mean crustal Vp is remarkably similar to models for other cordilleran terranes at 6.7 plus or minus 0.05 km s−1. An estimate of the bulk chemical composition of the Kohistan terrane does not compare favorably with most published assessments of bulk continental crustal chemical composition, but is significantly more mafic than the latter. The striking resemblance of our reconstructed velocity profile to models generated from field seismic studies not only addresses the veracity of these models, but suggests that the Kohistan arc is a superbly well‐preserved analog for other arc terranes.