Coupled crust-mantle dynamics and intraplate tectonics: Two-dimensional numerical and three-dimensional analogue modeling

Tectonic deformation of some intraplate regions may be caused by the Rayleigh-Taylor (RT) instability of dense subcrustal lithosphere ( mantle lithosphere) as it descends into the mantle. We report on a series of scaled three-dimensional (3-D) analogue and 2-D numerical experiments of coupled crust-mantle dynamics that study such a process. The models investigate the effects of two geometries of RT instability ( linear versus axisymmetric) and use different rheological stratifications of the crust. The growth of the RT instability is strongly influenced by crustal rheology. While RT growth rates are not greatly influenced by the geometry of the mantle lithosphere instability itself, the surface is. Experiments having a viscous-only crust are characterized by significant crustal contraction and thickening above the mantle downwelling and extension and thinning in adjacent regions. In experiments with a brittle upper crust the surface deformation is much more subdued. However, strong mantle flow-induced deformation still occurs in the ductile lower crust beneath the quiescent brittle upper crust. When the lower crust is relatively strong, localized styles of extensional and contractional structure develop in the brittle upper crust owing to the greater degree of coupling between upper crust and the mantle instability. In all the experiments, a portion of the ductile lower crustal material is entrained within the downgoing RT instability deep into the mantle. The interplay between dynamic topography and crustal thickening/thinning induced by the underlying mantle flow governs various time-dependent phases of subsidence and uplift of the model surface. The experiments help to account for a number of first-order tectonic behaviors of the lithospheres of Earth and other terrestrial planets. For example, intraplate orogens and basins, complex localized tectonic structure in intraplate settings, and deep crustal seismic fabrics may arise as the variable crustal response to underlying mantle lithosphere RT instabilities.