Flexural Uplift of Rift Flanks Due to Mechanical Unloading of the Lithosphere during Extension.

摘要

We suggest that 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 2) concepts: 1) the lithosphere retains finite mechanical strength or flexural rigidity during extensions and 2) isostatic rebound (uplift) of the lithosphere follows when the kinematics of extension produces a surface topographic depression 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 uplift of rift flanks. 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 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. Reprints. (EDC)