We use three-dimensional (3-D) temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid-ocean ridge lithospheric structure. We find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along-axis gradients in crustal temperature and a ''pita-pocket''-shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle. In contrast, fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis. The overthickened crust produced by ridge-hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling, creating an extremely weak lithosphere in a steady state model. We apply a simple two-dimensional (2-D) cyclic faulting model to across-axis sections of temperature and rheological structure. Slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing. In contrast, fast spreading and hotspot-affected segments are predicted to have much smaller variations in faulting styles, except in the immediate vicinity of a major transform fault. The presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness, spreading rate, and the ability of rift-bounding normal faults to propagate along axis. For fast spreading and hotspot-affected segments, the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma-chamber-supported isostatic features. This is in contrast to most slow spreading segments, where axial rift valleys are predicted to be a dominant morphological feature. For a slow spreading segment of great length, however, the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center. ¿ American Geophysical Union 1996