The transition from subduction to transform motion along horizontal terminations of trenches is associated with tearing of the subducting slab and wrench tectonics in the overriding plate. One prominent example is the northern Tonga subduction zone, where the influence of wrench tectonics is indicated by abundant strike-slip faulting in the NE Lau back-arc basin. We explore the back-arc dynamics of this region for the first time through structural lineament analyses and kinematic analyses interpreted from ship-based multibeam bathymetry and Centroid-Moment Tensor data. Our results indicate two distinct sets of Riedel shear structures that are associated with a counter-clockwise rotation in the stress field. We propose that this rotation is driven by the collision of the previously unstudied Capricorn Seamount(s). The strain of this collision was accommodated by right-lateral slip along the adjacent crustal scale fault, known as the Fonualei Discontinuity, which segmented the fore-arc. Internal deformation of the northern tectonic block may have been enhanced by friction along the northern boundary imparting westward-directed stress. This study highlights the importance of non-rigid plate kinematics and extensive re-activation of pre-existing faults in this region. Importantly, these structures directly control the development of complex volcanic-compositional provinces, which are characterized by variably-oriented spreading centers, off-axis volcanic ridges, extensive lava flows, and point-source rear-arc volcanoes that sample a heterogenous mantle wedge, with sharp gradients and contrasts in composition and magmatic affinity. This study adds to our understanding of the influence of subduction-transform motions and terrane collisions on the structural and magmatic evolution of back-arcs.