Seismicity and lithosphere extension on the scale of a tectonic plate are investigated in two horizontal dimensions and time using a dynamical model for a thin plate overlying a viscoelastic channel. The plate consists of two layers, a brittle elastic-plastic crust that deforms as a result of earthquakes along faults and a strong viscoelastic layer representing the aseismic, creeping lower lithosphere. Numerical experiments of lithosphere rifting are performed for one- and two-layer lithosphere models using two different rupture criteria. In all the calculations, earthquakes and faults of various sizes may form anywhere in the numerical domain as a result of the constitutive formulation. The differences between the one- and two-layer model results suggest a mechanical explanation for the difference between continental and oceanic rifting. When the lower lithosphere is absent, a small number of faults dominate the deformation, and a relatively simple, narrow rift zone develops corresponding to oceanic rifting. When a highly viscous lower lithosphere is included, a complex, distributed network of faults develops, similar to continental rifting. The complexity of the fault system also depends on the yield criterion in the crustal layer. In our simulations, complexity is greater for a yield criterion with strong shear stress dependence than for one with mainly extensional stress dependence. Whether a result of lower lithosphere stretching, crustal heterogeneity, or the yield criterion, higher complexity increases the timescale for fault development, increases self-similarity in fault system geometry and stress and strain time series, and results in power law moment-frequency earthquake size scaling. ¿ American Geophysical Union 1996