Flat-slab subduction occurs where the descending oceanic plate bends to form a sub-horizontal geometry for hundreds of kilometers beneath the overlying continent before it plunges steeply into the mantle. The slab geometry in flat-slab regions can change dramatically both parallel and perpendicular to the trench. There are two transitional patterns from a flat slab to a steep-angle slab: gradual slab bending and abrupt slab tearing. Whether a tear occurs in a slab may greatly impact mantle flow and magmatic activity in the overlying continent. However, the effects of slab geometry changes on the dynamics of the surrounding mantle and the overriding plate remain enigmatic. In this study, we develop 3D numerical models and investigate the flat slab to steep slab transition region and examine the impacts of slab geometry changes including bending and tearing on upper mantle flow. Models are built based on the flat-slab subductions in South America where buoyant oceanic ridges play an important role in flattening the slab. We find that slab tears tend to occur in front of the flat slab or at the edge of the flat slab due to the density contrast between the buoyant ridge and the surrounding dense materials or a presence of a weak zone. Slab tearing induces mantle flow through the tear and forms toroidal flow patterns and mantle upwelling. The models further show that the toroidal flow around the slab edge enhances slab rollback and upwelling of the mantle may cause magmatism in the overlying continent.