THE EFFECT OF LOWER CRUSTAL FLOW ON CONTINENTAL EXTENSION AND PASSIVEMARGIN FORMATION

The great variety of styles of continental extension may reflect diffe rent crustal thickness and thermal states of continental lithosphere d uring the initiation of rifting. To investigate how these and other fa ctors affect rifting and the development of passive continental margin s, we develop a simplified model of lithospheric extension. We conside r the evolution of extensional deformation for a three-layer model lit hosphere bounded laterally by much stronger lithosphere. The cold part of the crust and mantle are treated as thin brittle/plastic layers. T he lower crust is approximated as a thin viscous channel. Each brittle /plastic layer can extend in only one location determined by the stren gth of the layer, shear of the lower crust, and buoyancy forces relate d to both crustal thickness variations arid thermally induced density differences. The lower crust flows in response to crustal thickness va riations and is sheared when the loci of extension for the two brittle /plastic layers are horizontally offset, a situation we term shear dec oupling. As in previous studies, we see three distinct patterns, or mo des, of extensional deformation that occur under different sets of mod el parameters: the core complex mode, the wide rift mode, and the narr ow rift mode. Shear decoupling occurs only in cases with a crustal rhe ology 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 patt erns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins, including very wi de and asymmetric margins.