Both volcanism and faulting contribute to the rugged topography that is created at the Mid‐Atlantic Ridge (MAR) and preserved off‐axis in Atlantic abyssal hill terrain. Distinguishing volcanic from fault‐generated topography is essential to understanding the variations in these processes and how these variations are affected by the three‐dimensional pattern of mantle upwelling, ridge segmentation, and offsets. Here we describe a new quantitative method for identifying fault‐generated topography in swath bathymetry data by measuring topographic curvature. The curvature method can distinguish large normal faults from volcanic features, whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes. The combination of curvature and slope information allows inward and outward facing fault faces to be mapped. We apply the method to Sea Beam data collected along the MAR between 28° and 29°30′N. The fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework: long, linear, small‐throw faults occur toward segment centers, while shorter, larger‐throw, curved faults occur toward ends; these variations reflect those of active faults within the axial valley. We investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the MAR. In the first model only one fault is active at a time on each side of the rift valley. Each fault grows while migrating away from the volcanic center due to dike injection; extension across the fault causes a flexural rotation of nearby inactive faults. The amount of stress necessary to displace the fault increases as the fault grows. When reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley. This model yields a realistic terracelike morphology from the rift valley floor into the rift mountains; the relief is caused by the net rotation accumulated in the lithosphere from the active faults (e.g., 10° reached 20 km from the active fault). Fault spacing is controlled by lithospheric thickness, fault angle, and the ratio of amagmatic to magmatic extension. We hypothesize that this mechanism may be dominant toward ridge segment offsets. An alternative model considers multiple active faults; each fault relieves stresses as it grows and inhibits the growth of nearby faults, causing a characteristic fault spacing. Such fault interactions would occur in a region of necking instability involving deformation over an extended area. This mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults. We hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints, causing the homogeneous abyssal hill fabric in these regions.

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We present a method for constraining the velocity-depth structure in the Earth using seismic refraction waveform data. We test the method with synthetic 'data' from known models, and apply it to a set of data collected in 1982 June from the East Pacific Rise at 13"N, from the MAGMA expedition. In this iterative process WKBJ seismograms are computed for a starting model; the difference between these and the observed seismograms is used to update the model subject to physical constraints.An important first step in the inverse scheme is the linearization of the WKBJ seismogram equation, allowing us to compute 'differential seismograms', partial derivatives of the synthetic seismogram with respect to specific model parameters. This linearization provides the means for estimating required model perturbations, based on the misfit in the seismograms.The choice of a suitable numerical strategy for computing an updated model is a crucial second step in formulating a working algorithm. Because the data contain noise, synthetic seismograms can only fit the data to this noise level. In this case, infinitely many models fit the data to this tolerance, and some of these estimates are non-physical, involving negative layer thicknesses. A successful strategy must choose from among these possibilities a well-defined, physically reasonable new model.In a commonly-used approach to solving non-linear problems the perturbation to the starting model is minimized while improving the fit to the data. After several iterations the final model, which possesses no special properties, still tends to resemble the starting model. When used with the MAGMA data this technique essentially does not perturb the model at all.A method we find much more satisfactory involves solving for the new model directly while applying physically important constraints. As constraints we require the velocity gradient remain below a fixed value and penalize the 'roughness' of the new model. We thus solve for the smoothest model fitting the data to the specified misfit. This method offers substantial advantages when applied to the MAGMA data and enables us to constrain such geologically interesting model features as transition zones. We find a steep velocity gradient in the upper crust with velocities of 6 km s-l occurring less than 1 km into the crust. Below about 1 km the gradient abruptly decreases, and the crustal material is much more uniform.

We present an inversion for plate kinematics that solves for finite rotation parameters using fracture zone (FZ) and magnetic anomaly location data jointly. We define misfit functions that incorporate properties unique to each data type; in particular, the FZ misfit function does not depend upon aligm•ent of conjugate FZ traces in the same way as magnetic linearions under the firrite rotations. This property is useful for FZ locations, in which the signals on conjugate sides of the ridge may include systematic differences, or where data from one side of the ridge are sparse or missing. Formal error bounds estimated for the pole parameters show that the magnetic and FZ data are complementary in their information content. Error bounds computed for the joint inversion are substantially smaller than for either the FZ or magnetics data used separately, indicating that simultaneous use of the data in an inversion is crucial. We apply this method to Seasat altimeter data and magnetic anomaly picks in the South Atlantic. We solve for finite poles corresponding to magnetic anomalies 5, 6, 8, 13, 21, 22, 25, 30, 33, 33r, and 34; these define a smooth path over the past 84 m.y. indicating that the plate motions have not been as erratic as found previously.

Porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities. Porosity is particularly important in the uppermost young crust, where rapid variations in velocities with depth and crustal age are observed. Knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of Poisson's ratio are available from observations of compressional and shear seismic velocities υp and υs. In this paper I present a joint seismic waveform inversion for υp and υs; velocities are found while maximizing or minimizing Poisson's ratio using a hypothesis‐testing mechanism. I apply this method to ocean bottom hydrophone data in 140 Ma Atlantic crust; the resulting solution corridor agrees with laboratory measurements without the low Poisson's ratio anomalies at depths of 0.8–1.5 km found by Spudich and Orcutt (1980) and Au and Clowes (1984) on younger (<15 Ma) Pacific crust. Compiling other published υp and υs solutions, an age‐dependent pattern emerges: none of the solutions for crust older than 60 Ma display the Poisson's ratio anomaly. I propose a simple crustal evolution model, using thin and thick cracks, to explain these observations: thin cracks preferentially close at shallow depths in the crust, producing the localized Poisson's ratio anomaly. Sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements. This model is consistent with similar models of crack populations and their evolution from shallow measurements.