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

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.

本文主要基于新西兰政府"PEG09"二维普查地震调查的解译成果，阐述了飞马盆地（Pegasus Basin）内一套厚约9000米、时代为阿尔布期-现代（Albian–Recent）的沉积序列（sedimentary succession）的地层格架（stratigraphic architecture）。该盆地紧邻希库兰吉俯冲带（Hikurangi subduction zone）东侧、处于其外侧，其形成主要缘于新近纪时期，在俯冲作用向斜滑断层（oblique-slip faulting）作用过渡的区域附近，下伏太平洋板块（Pacific Plate）发生拗陷作用。相较于普遍受新近纪-现代汇聚边缘构造（convergent margin tectonics）强烈变形的相邻东海岸盆地（East Coast Basin），该盆地的充填地层变形程度极弱。研究识别出4套大型构造地层单元（tectonostratigraphic units）：（1）变沉积质"基底"（basement）岩石，形成并增生于冈瓦纳（Gondwana）中生代增生楔（accretionary wedge）中；（2）希库兰吉高原（Hikurangi Plateau）的早白垩世大火成岩省（Large Igneous Province）地壳；（3）非增生"盖层"地层，包含与冈瓦纳增生楔最年轻部分时代相当的岩石，以及热沉降（thermal subsidence）阶段沉积的上覆晚白垩世和古近纪岩石；（4）北岛东部（eastern North Island）俯冲作用复苏以来沉积的新近纪-现代地层。地震数据所揭示的前新近纪地质记录，刻画了冈瓦纳边缘的中生代俯冲作用，该作用在约110~105 Ma时活动俯冲增生停止后被"冻结"下来。飞马盆地的地质特征，还为长期存在争议的东海岸盆地"基底"与"盖层"岩石之间的成因关系提供了新认识。本研究对该盆地的油气潜力（petroleum potential）评估同样具有参考价值，目前该盆地正处于许可证（licence）管控范围内。