The early Miocene Waitemata Basin has long been described as an interarc/intra‐arc basin, formed between twin chains of arc volcanoes in Northland, New Zealand. However, deep marine, polymict, volcaniclastic conglomerates within the basin reveal tectonic and magmatic signals that are not evident from neighboring volcanic edifices. The conglomerates were deposited by high‐density turbidity currents and debris flows and include single sediment cycle megaclasts of lava. These basaltic lavas have ocean island basalt (OIB)‐like geochemical affinities and are precisely dated at 20 Ma by 40Ar‐39Ar methods. Their age and the occurrence of subordinate clasts derived from an ophiolitic nappe to the north indicate the basin postdates the initiation of collision in the wider region. Contemporaneous calc‐alkaline volcanism did occur some 250 km NW of the basin. However, the conglomerates lack clasts of calc‐alkaline/arc affinities indicating an absence of arc‐like volcanism in the vicinity of the basin. Recent tectonic models for the SW Pacific region and mantle tomography highlight the importance of wholesale slab detachment in driving early Miocene calc‐alkaline volcanism and basin development. Although such models provide a slab window for the eruption of nonarc OIB‐like magmas, they would not explain their localized occurrence at the proposed leading edge of the tear (Waitemata Basin), rather than progressively along the entire length of the detachment (Northland), as seen in other detachment settings. In addition, OIB‐like volcanism predates the adjacent calc‐alkaline volcanism on the margin of the basin, a transition that is the opposite of that found at other slab detachment settings. The occurrence of OIB‐like volcanism is better explained by a lateral slab termination in the vicinity of the Waitemata Basin that allowed asthenospheric‐derived magmas to erupt. The basin is inferred to have developed in response to asthenospheric upwelling and associated lithospheric extension or deformation associated with a slab termination zone. This study highlights the importance and potential of combined geochemical, geochronological, and sedimentological studies of conglomerates in reconstructing the geodynamic setting of a basin.