A deeply towed instrument package was used to survey the fine details of topography, sediment distribution, and magnetization of the Gorda rise, an active spreading center off northern California. The gross form of the central rift valley is the result of large‐scale normal faulting. The surface is broken into long, narrow, tilted steps parallel to the spreading center. Topographic features just outside the rift valley also have a tilted blocky aspect and are lineated and symmetrical with respect to features of the same age on the opposite flank. Flat terrigenous turbidite sediments are found in the valley floor and in deeps on the west flank, as might be expected. They also are found on the tops of the steps high in the rift valley walls, implying that the steps are uplifted to form the walls as they move out from the center. The magnetic field measured near the ocean bottom can be approximately simulated by using straightforward model calculations. The extremely complex short‐wavelength anomalies (width less than 1 km) generally are caused by topographic effects, and from them the bulk magnetization of the topographic features was estimated to be about 0.009 emu/cm3 near anomaly 2 and 0.007 emu/cm3 near anomaly 3. Assuming these values of magnetization for the entire magnetic layer, the magnitude of long‐wavelength anomalies shows that it must be about 0.5 km thick. In crust formed at a spreading rate of 37 mm/yr, magnetic field polarity reversals are recorded in the magnetic layer as a gradual zone of transition about 2 km wide. Of this width, 0.1 to 0.7 km can be attributed to the time it takes for a reversal to be completed, and the remaining width, about 1.7 km, can be attributed to the process of emplacement of magnetic material. Comparisons with emplacement models show that most magnetic material was emplaced within 2 km of the center (within 0.7 km if a pure dike injection model is assumed). The 2‐km width of the emplacement effect acts as a smoothing function for events in the magnetic field, so that the record in the magnetic layer of events of duration less than about 10,000 years should be greatly attenuated. Two medium‐wavelength anomalies are interpreted as possible normal polarity events centered at 3.4 and 3.55 m.y.

A near-bottom geophysical survey of the Galapagos spreading centre at 86"W shows the highly lineated nature of oceanic crust generated at a fastspreading ridge. The bathymetric relief is dominated by small blocks which tilt slightly away from the spreading centre and which are bounded by inward-facing scarps. This relief is similar to that found on slow-spreading ridges, but on a smaller scale, and when covered with a thin layer of sediments, it looks like typical abyssal hill terrain. The magnetization distribution, sediment pattern and the fine-scale bathymetric structure are used to locate the centre of spreading and the region in which the upper portion of the crust is being formed. Recent volcanic activity is primarily occurring within 2 km of the centre, a region which is nearly sediment-free and which contains a highly magnetized ridge.Possible models of ridge crest mechanics are constrained by these observations over a ridge spreading with a half-rate of 35 mm/yr. There appears to be a small median valley which has a width of 16 km and a depth of 150 m. Most of the near-surface volcanism appears to cease when the rigid blocks are initially faulted apart from the central crustal generation zone at about 2 to 4 km from the centre. The central block is fairly level with a relief which appears to have been formed by constructional volcanism while the rigid blocks on either side of this central one already are tilted slightly outwards and are bounded by inward facing scarps.

Near-bottom magnetic data over six oceanic ridge segments in the East Pacific are inverted, giving magnetization solutions with alternate positive and negative bands which correspond to geomagnetic field reversals. We estimate the average half-width of the crustal formation zone to be 2-3 km, based on the transition widths between these bands. The solutions show a narrow region of high magnetization centred directly over the centre of spreading, superimposed on a more gradual decrease of magnetization amplitudes with age. Both features are attributed to weathering of highly magnetized pillow lavas. We demonstrate that the short wavelength ( < 3 km) anomalies are largely due to topography. Distances to reversal boundaries give distance us age curves for each ridge which show that spreading changes occur as sudden accelerations typically separated by several million years of very constant motion. These rate changes are probably accompanied by shifts in the locations of poles of relative motion, which are necessary in a system of more than two interacting plates. Palaeomagnetic data and reversal boundary locations from near-bottom and surface data are combined to give spreading half-rates and a refined time scale for the past 6 My. Widespread spreading rate variations occurred at 2-3 MyBP and about 5 MyBP, possibly as a response to large scale changes in the plate pattern.

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