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.

S‐wave splitting analyses using low‐frequency earthquake templates at three‐component stations across southern Vancouver Island and northern Washington indicate the presence of a heterogeneous distribution of crustal anisotropy in the North American plate. For southern Vancouver Island, we investigate contributions to anisotropy from the Leech River Complex, a terrane composed of strongly foliated phyllites and schists with steeply dipping foliations striking east‐west. Fast directions across mainland southern Vancouver Island are subparallel to the dominant Leech River Complex foliation direction. East‐to‐west increases in delay times and small‐scale azimuthal variations in fast directions indicate heterogeneous anisotropy. We test azimuthally anisotropic Leech River Complex models constrained by previous geological and seismic reflection studies, through forward modeling using 3‐D spectral element method simulations. The preferred model of a north/northeast shallowly dipping wedge of Leech River Complex material with varying orientation of anisotropy terminating at midcrustal levels explains the splitting observations at a majority of southern Vancouver Island stations. For stations where anisotropic Leech River Complex models do not recreate observations, fast directions are subparallel to local estimates of maximum compressive horizontal stress, suggesting that fluid‐filled cracks could be a source of anisotropy. We assert that the Leech River Complex is the primary source of crustal anisotropy beneath southern Vancouver Island, not cracks as suggested by prior studies. Fast directions at stations on northern Washington exhibit variations with azimuth and incidence angle suggesting complex anisotropy interpreted as due to a combination of cracks and preferred mineral orientation of metamorphosed slates of the Olympic core rocks. These slates may also underlay stations on southern Vancouver Island and represent another source of anisotropy.

The Minnesota River Valley (MRV) subprovince is a well-exposed example of late Archean lithosphere. Its high-grade gneisses display a subhorizontal layering, most likely extending down to the crust-mantle boundary. The strong linear fabric of the gneisses results from high-temperature plastic flow during collage-related contraction. Seismic anisotropies measured up to 1 GPa in the laboratory, and seismic anisotropies calculated through forward-modeling indicate ΔV P~5 -6% and ΔV S~3 %. The MRV crust exhibits a strong macroscopic layering and foliation, and relatively strong seismic anisotropies at the hand specimen scale. Yet the horizontal attitude of these structures precludes any substantial contribution of the MRV crust to shear wave splitting for vertically propagating shear waves such as SKS. The origin of the regionally low seismic anisotropy must lie in the upper mantle. A horizontally layered mantle underneath the United States interior could provide an explanation for the observed low SWS.

This issue of the Journal of Geophysical Research contains the collected results of the initial studies of a 3‐km vertical section of Icelandic crust. The material described in these papers was largely presented at a meeting of the investigators working in this project held in Reykjavík, Iceland, from May 13 to May 15, 1980 Iceland presents a very well exposed example of crust formed at an accretional plate margin, the crest of the Mid‐Atlantic Ridge. The desirability of a detailed study of a long vertical section of Icelandic crust by deep drilling has been expressed in proposals and recommendations of scientists and international scientific committees since the early 1960's. A formal proposal with descriptions of several alternative drill sites was presented in 1975 [Working Group on Deep Crustal Drilling in Iceland, 1975], but difficulties in financing a drill hole that would penetrate into crustal layer 3 (Vp = 6.5 km/s) delayed implementation of the recommendations.

The conference was held at a critical time in the development of our knowledge of the oceanic crust as revealed by the Deep Sea Drilling Project (DSDP). The increased drilling capabilities, including a number of new systems such as heave compensation, improved bits and core recovery, and reentry capability, now allow us to explore and sample the deep oceanic crust for the first time. These new capabilities will be applied to a deep hole in basement on leg 37 in December and January 1973–1974. But what have we already learned from the numerous short cores of igneous rocks so far recovered? This conference attempted to pull together workers who had studied these early cores, give a vehicle for rapid publication of their results through the extended abstracts accompanying this summary, and examine the objectives of future drilling. In reviewing the abstracts, it appears premature to reach any but a very few meaningful generalizations. One is that drilling reveals a much greater chemical diversity of igneous rocks than would be expected from our information from dredge samples. How this diversity relates to mantle and crustal processes is so far unknown in detail. The so‐far shallow penetration places the most severe restriction on interpretations of these cores in terms of the broad patterns of earth history. Future deeper drilling may soon remove this restriction.