The effect of lower crustal flow on continental extension and passive margin formation.

The great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting. To investigate how these and other factors affect rifting and the development of passive continental margins, we develop a simplified model of lithospheric extension. We consider the evolution of extensional deformation for a three-layer model lithosphere bounded laterally by much stronger lithosphere. The cold part of the crust and mantle are treated as thin brittle/plastic layers. The lower crust is approximated as a thin viscous channel. Each brittle/plastic layer can extend in only one location determined by the strength of the layer, shear of the lower crust, and buoyancy forces related to both crustal thickness variations arid thermally induced density differences. The lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle/plastic layers are horizontally offset, a situation we term shear decoupling. As in previous studies, we see three distinct patterns, or modes, of extensional deformation that occur under different sets of model parameters: the core complex mode, the wide rift mode, and the narrow rift mode. Shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values. We are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions. We show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas. Predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins, including very wide and asymmetric margins.