We suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound. This mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes, and magmatic thickening of the crust. Our hypothesis is based on two critical concepts. First, the lithosphere retains finite mechanical strength or flexural rigidity during extension. Second, isostatic rebound (uplift) of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation. We develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks. We first investigate the isostatic consequences of finite simple slip on an initially planar, dipping normal fault cutting the entire lithosphere. When the lithosphere retains flexural rigidity during extension, the topography resulting from this model resembles a half graben, and the footwall rift flank is flexurally uplifted. This simple normal faulting model explains free‐air gravity anomalies and topography observed at rift flanks in oceanic lithosphere (such as Broken Ridge in the eastern Indian Ocean, the Caroline ridges‐Sorol Trough in the western equatorial Pacific, and the Coriolis Trough behind the New Hebrides island arc). We then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates. When the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical, the surface of slip can be regarded as a detachment. By simplification, our general model accounts for pure shear extension of the lithosphere that is uniform with depth. In this case, detachments have no meaning in the geologic sense. However, the kinematics of depth‐independent pure shear may nevertheless be described in terms of a surface, which we term a kinematic reference surface, at some depth in the lithosphere. We speculate that the depth of this surface may be rheologically controlled. The magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface. In contrast, if local isostasy is assumed when the lithosphere undergoes a given amount of depth‐independent pure shear, the resulting topography will be the same regardless of how the kinematics of that extension are formulated. The basin and rift flank topography and free‐air gravity anomaly over young continental rifts, such as the Rhine graben, can be satisfied using our general extensional model with a small amount (<5 km) of extension along a listric‐shaped detachment soling into the crust‐mantle boundary. Because the flexural rebound mechanism explains the observed topography and gravity ...

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Evidence for the early development of convective instability in the thermal boundary layer associated with cooling plates has been found from gravity anomalies and residual sea surface heights derived from Seasat altimeter data. Subtle lineated patterns trending in the direction of plate motion in the hot spot reference frame are observed over the younger portions of the fast-moving Pacific and Indo-Australian plates. In particular, for the east central Pacific Ocean, the Seasat-derived data sets reveal lineations w•th (1) wavelengths in the range 150-500 km (perhaps increasing with plate age), and (2) peak-to-trough amplitudes of 5-20 mGal for gravity anomalies. The lineated pattern over the Pacific plate, which first becomes discernable over seafloor 5-to 10-m.y.-old west of the East Pacific Rise, is clearly oblique to the trends of the prominent Pacific-Farallon fracture zones. Onset of convective instability at such early ages can be understood if young oceanic lithosphere is underlain by a layer having a viscosity of about 10 x8 Pa s (10 •9 P), which is about 3 orders of magnitude less than mantle viscosity inferred from postglacial rebound studies. Recent numerical studies of convection in fluids with a temperature-and pressuredependent viscosity support an early onset time for convective instability and low-viscosity zones in the upper mantle beneath young lithosphere. Such low-viscosity zones may correspond to similarly located regions of low shear wave velocities resolved through studies of surface waves. track), is known with a precision of 0.1 m [Tapley et al., 1982]. Thus a 0.2-m geoidal high expected over a seamount 500 m high and 50 km wide lying in 4 km of water should be resolved adequately in data obtained along a satellite pass directly above such a seamount. The high sensitivity of sea surface height measurements and the uniform and relatively dense coverage of the oceans by Seasat ensure that important new information is available from remote regions seldom visited by research vessels. Given the Seasat data distribution (Figure 1), features large enough to be important in tectonic studies of the oceanic crust and lithosphere should be resolved well in all parts of the oceans, their resolution improving with increasing latitude.In order to take full advantage of the Seasat data set in tectonic studies, it is necessary to combine the altimeter measurements taken along all the orbits and construct a .twodimensional representation of the data. Dixon and Parke [1983] and Rapp [1983] have approached this problem by determining average sea surface heights at 0.5øx 0.5 ø and 1 ø x 1 ø grid intervals, respectively. Sandwell [1984] chose to grid sea surface slopes, using the conjugate sets of Seasat tracks (ascending and descending) independently, thereby avoiding the necessity of adjusting sea surface heights along crossing tracks to minimize crossover discrepancies. The procedures for processing Seasat altimeter data which led to the observations described in this paper were outlined briefly by Haxb...