Pegasus Basin, eastern New Zealand: A stratigraphic record of subsidence and subduction, ancient and modern

Abstract

The stratigraphic architecture of a thick (c. 9000 m) Albian–Recent sedimentary succession within the Pegasus Basin is presented here, based primarily on interpretation of the New Zealand Government's ‘PEG09’ 2D reconnaissance seismic survey. The basin lies immediately east and outboard of the Hikurangi subduction zone, and formed mainly because of Neogene downwarping of the underlying Pacific Plate near the transition from subduction to oblique‐slip faulting. The basin fill is little deformed by the Neogene–Recent convergent margin tectonics that pervasively deform the adjacent East Coast Basin. Four large‐scale tectonostratigraphic units are differentiated: (1) metasedimentary ‘basement’ rocks, deposited and accreted within the Gondwana Mesozoic accretionary wedge; (2) Early Cretaceous Large Igneous Province crust of the Hikurangi Plateau; (3) non‐accreted ‘cover’ strata, incorporating rocks age‐equivalent to the youngest parts of the Gondwana accretionary wedge, plus the overlying Late Cretaceous and Paleogene rocks deposited during thermal subsidence; and (4) Neogene–Recent strata deposited since the renewal of subduction beneath eastern North Island. The pre‐Neogene geology illustrated by the seismic data describes the Mesozoic subduction along the Gondwana margin, ‘frozen’ at the time when active subduction accretion ceased at c. 110–105 Ma. The geology of Pegasus Basin also provides an insight into the much‐debated relationship between ‘basement’ and ‘cover’ rocks in the East Coast Basin. This study is also relevant to the assessment of the petroleum potential of the basin, which is currently held under licence.