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.本研究基于新西兰政府"PEG09"二维普查地震调查的解译成果，阐述了飞马盆地（Pegasus Basin）内一套厚达约9000米的阿尔布阶（Albian）-现代沉积序列的地层结构。该盆地紧邻希库兰伊俯冲带（Hikurangi subduction zone）东侧外缘，其形成主要源于新近纪（Neogene）时期，在俯冲作用向斜滑断层作用转换的区域附近，下伏太平洋板块发生拗陷。该盆地充填沉积物几乎未受新近纪-现代会聚边缘构造作用的改造，而相邻的东海岸盆地（East Coast Basin）则普遍遭受此类构造作用的强烈变形。研究区可划分出4套大型构造地层单元：（1）变沉积质"基底"岩系，形成并增生于冈瓦纳（Gondwana）中生代增生楔之中；（2）希库兰伊高原（Hikurangi Plateau）的早白垩世大火成岩省（Large Igneous Province, LIP）地壳；（3）非增生型"盖层"地层，包含与冈瓦纳增生楔最年轻部分时代相当的岩系，以及热沉降时期沉积的上覆晚白垩世和古近纪（Paleogene）岩系；（4）北岛东部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特征，记录了冈瓦纳边缘的中生代俯冲作用，该作用在约110~105 Ma时活跃的俯冲增生作用停止后被"封存"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"岩石之间的关系提供了新的认知。本研究同时对该当前处于许可勘探阶段的盆地的油气潜力评价具有参考价值。