Flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension

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Abstract
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 anomaly over both oceanic and continental extensional domains, we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension.