Interbedded with and intruding Tertiary transgressive strata in the Waiau area, North Canterbury, are significant volumes of basic and minor volumes of acidic volcanic rocks. Between the Late-Eocene and the Mid-Miocene, an estimated 90 km 3 of magma were erupted, most between Runangan and Ountroonian times. Basic volcanics generally comprise basaltic and hawaiitic lava flows, dykes, sills and volcaniclastics including pyroclastics and epiclastics. Small volume of basanitic, nephelinitic, rhyolitic and dacitic volcanics occur as intrusions. Styles of eruption reflect paleogeography. Earliest volcanism in the Lottery-Counting area was "Surtseyan", typical of a shallow shoaling marine environment, In time, the setting became emergent and Strombolian and Hawaiian type volcanic activity became prevalent. During the Oligocene, the Mason-Lagoon area was more distal to a transgressive shoreline, eruptions were predominantly pillow lavas and related epiclastic derivatives, typical of volcanism in deeper water. Basic volcanics in the Waiau area have alkaline to transitional affinities. REE and Sr-isotope ( 8 7 Sr/8 6 Sr. , .70291-.70322) data indicate a deep upper mantle source for primary magmas, and suggest little interaction with the crust en route to the surface. Trace element tectonic discrimination data indicate "within plate" volcanism, in agreement with plate configuration reconstructions for the times, Preservation of high pressure liquidus assemblages indicating rapid eruption from great depths, may suggest a thinning crust associated with an area of high heat flow. The occurrence of cognate pyroxenite nodules, together with peridotite xenoliths in 15 Ma nephelinitoids and basanitoids, is strong evidence for crystal fractionation processes in the upper mantle, That peridotite nodule-bearing nephelinitic-basanitic volcanism is usually associated with areas of high heat flow and crustal thinning, suggests that transpressional tectonism associated with the Alpine and Hope Faults was not active before 15 Ma ago.