Abstract. A vertical tomogram is constructed for the mantle between Tonga and Hawaii. Using a complete Gaussian-Bayesian approach, we inverted a data set comprising 304 ScS reverberation travel times with Fr6chet kernels computed by the paraxial ray approximation and 1122 frequencydependent phase delays of turning and surface waves with Fr6chet kernels calculated by a normalmode coupling algorithm. The model parameters include shear speed variations, perturbations to shear wave radial anisotropy in the uppermost mantle, and the topographies of the 410-and 660-km discontinuities. Tests demonstrate that the data set resolves lateral structures in the upper mantle with scale lengths of several hundred kilometers and greater. The most significant feature in our model is a well resolved regular pattern of high and low shear velocities in the upper mantle. These variations have a horizontal wavelength of 1500 km, a vertical dimension of 700 km, and an amplitude of about 3%, and they show a strong positive correlation with seafloor topography and geoid height variations. The geoid highs correspond to a series of northwest trending swells associated with the major hotspots of the Society, Marquesas, and Hawaiian Islands. Where these swells cross the corridor, they are underlain by high shear velocities throughout the uppermost mantle, so it is unlikely that their topographies are supported by thermal buoyancy. Although chemical buoyancy generated by basaltic volcanism and the formation of its low-density peridotitic residuum can explain the positive correlation between topography and velocity variation at the shallow depth, an additional mechanism is needed to account for the shear velocity pattern at depths greater than 200 km. It is therefore hypothesized that the central Pacific is underlain by a system of Richter-type convective rolls that are oriented subparallel to the absolute plate velocity, have unit aspect ratio transverse to this orientation, and are confined above the 660-kin discontinuity. This convection pattern appears to be strongly correlated with the locations of the Tahitian, Marquesan, and Hawaiian hotspots, which raises interesting questions for Morgan's hypothesis that these hotspots are the surface manifestations of deep mantle plumes.

Wide‐angle ocean bottom seismic data and single‐channel seismic reflection data were collected in June 1992 over an area where gas hydrates are thought to be extensive on Blake Ridge, offshore South Carolina. Wide‐angle reflections were observed on four Woods Hole Oceanographic Institution ocean bottom hydrophones at offsets up to 15 km. Results from traveltime inversion show that the bottom simulating reflector (BSR) that marks the base of the hydrate stability field lies 400–500 m below the seafloor and is overlain by a 200‐ to 300‐m‐thick layer of average velocity 1.9 km s−1. There is no evidence for significant lateral velocity variation associated with lateral changes in BSR character. Ray‐tracing calculations show that the observation of the BSR out to large offsets (7 km) constrains the maximum vertical velocity gradient to about 0.5 s−1. Amplitude‐versus‐offset (AVO) analysis was performed for ranges up to about 6 km and incidence angles of more than 70°. The high amplitude of the BSR in the normal incidence data can be explained by two models, both of which include a strong negative impedance contrast: the “hydrate wedge” model and the free gas layer model. Synthetic seismogram calculation for the wedge model, using the reflectivity method, shows a strong increase in the BSR amplitudes at offsets between 3 and 5 km because of postcritical reflections and diving waves within the wedge. This amplitude increase is clearly not observed in the data, which shows only a relatively modest increase in amplitude with offset. This observation agrees better with the calculated AVO for the gas layer model. The lack of a distinct reflection from the base of the gas layer implies that the gas layer is not thicker than 25 m and/or that the base of the gas layer is a gradational boundary.

We model the three-dimensional (3-D) crustal deformation in a deep pull-apart basin as a result of relative plate motion along a transform system and compare the results to the tectonics of the Dead Sea Basin. The brittle upper crust is modeled by a boundary element technique as an elastic block, broken by two en echelon semi-infinite vertical faults. The deformation is caused by a horizontal displacement that is imposed everywhere at the bottom of the block except in a stress-free "shear zone" in the vicinity of the fault zone. The bottom displacement represents the regional relative plate motion. Results show that the basin deformation depends critically on the width of the shear zone and on the amount of overlap between basin-bounding faults. As the width of the shear zone increases, the depth of the basin decreases, the rotation around a vertical axis near the fault tips decreases, and the basin shape (the distribution of subsidence normalized by the maximum subsidence) becomes broader. In contrast, twodimensional plane stress modeling predicts a basin shape that is independent of the width of the shear zone. Our models also predict full-graben profiles within the overlapped region between bounding faults and half-graben shapes elsewhere. Increasing overlap also decreases uplift near the fault tips and rotation of blocks within the basin. We suggest that the observed structure of the Dead Sea Basin can be described by a 3-D model having a large overlap (more than 30 km) that probably increased as the basin evolved as a result of a stable shear motion that was distributed laterally over 20 to 40 km. Introduction Pull-apart basins are structures associated with either rightlateral right-stepping or left-lateral left-stepping en echelon strike-slip faults, and are inherently three-dimensional (3-D) features. This work addresses the effect of regional tectonics and primary faulting on the deformation pattern of deep pullapart basins using 3-D elastic modeling. 3-D numerical methods have been applied to crustal deformation problems only very recently [e.g., Braun, 1994; Gomberg and Ellis, 1994] . This study is the first attempt to model numerically pull-apart basins in three dimensions. Several previous studies have examined the problem of pull-apart basins by using a two dimensional (2-D) plane stress solution. Segall and Pollard [1980] calculated the stress field due to the interaction between cracks that behave according to a linear frictional law in a 2-D elastic medium. B ilham and King [1989] and Goreberg [1993] used the 2-D boundary nAlso at MIT/WHOI Ioint Program in Oceanography, Maaaachu•tta Imtitut• of Technology, Cambridge. Paper number 94IB03101. 0148-0227/95/94IB-03101 $05.00 element technique to calculate the vertical strain around offsetting fault zones. A different approach by Rodgers [ 1980] used a superposition of analytical solutions for dislocation planes embedded in an elastic isotropic half-space [Chinnery, 1961, 1963]. Rodgers [1980] calculated the distribution of vertical displacement and s...

We use three‐dimensional elastic models to help guide the kinematic interpretation of crustal deformation associated with strike‐slip faults. Deformation of the brittle upper crust in the vicinity of strike‐slip fault systems is modeled with the assumption that upper crustal deformation is driven by the relative plate motion in the upper mantle. The driving motion is represented by displacement that is specified on the bottom of a 15‐km‐thick elastic upper crust everywhere except in a zone of finite width in the vicinity of the faults, which we term the “shear zone.” Stress‐free basal boundary conditions are specified within the shear zone. The basal driving displacement is either pure strike slip or strike slip with a small oblique component, and the geometry of the fault system includes a single fault, several parallel faults, and overlapping en echelon faults. We examine the variations in deformation due to changes in the width of the shear zone and due to changes in the shear strength of the faults. In models with weak faults the width of the shear zone has a considerable effect on the surficial extent and amplitude of the vertical and horizontal deformation and on the amount of rotation around horizontal and vertical axes. Strong fault models have more localized deformation at the tip of the faults, and the deformation is partly distributed outside the fault zone. The dimensions of large basins along strike‐slip faults, such as the Rukwa and Dead Sea basins, and the absence of uplift around pull‐apart basins fit models with weak faults better than models with strong faults. Our models also suggest that the length‐to‐width ratio of pull‐apart basins depends on the width of the shear zone and the shear strength of the faults and is not constant as previously suggested. We show that pure strike‐slip motion can produce tectonic features, such as elongate half grabens along a single fault, rotated blocks at the ends of parallel faults, or extension perpendicular to overlapping en echelon faults, which can be misinterpreted to indicate a regional component of extension. Zones of subsidence or uplift can become wider than expected for transform plate boundaries when a minor component of oblique motion is added to a system of parallel strike‐slip faults.

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