The origin and evolution of Ishtar Terra is modeled using numerical simulations of crust/mantle interaction on Venus. Based on our modeling, we favor a two phase evolutionary sequence for Ishtar. The first phase is driven by bulk mantle flow due to thermal convection and results in the formation of a thickened crustal block over a mantle downwelling. The second phase, a result of the thermal perturbation exerted on the mantle by the region of thickened crust, is driven by the sinking of a lithospheric slab. The slab-driven tectonic pulse leads to low angle subduction and orogenesis at the peripheries of the crustal block. Throughout its evolution Ishtar remains coupled to flow within the deep mantle, due to the lack of an asthenosphere, and ultimately bulk mantle flow comes to dominate as the effects of the slab-driven pulse wane. ¿ American Geophysical Union 1991