Reactivation of tectonics, crustal underplating, and uplift after 60 Myr of passive subsidence, Raukumara Basin, Hikurangi-Kermadec fore arc, New Zealand: implications for global growth and recycling of continents

We use seismic reflection and refraction data to determine crustal structure, to map a fore-arc basin containing 12 km of sediment, and to image the subduction thrust at 35 km depth. Seismic reflection megasequences within the basin are correlated with onshore geology: megasequence X, Late Cretaceous and Paleogene marine passive margin sediments; megasequence Y, a similar to 10,000 km(3) submarine landslide emplaced during subduction initiation at 22 Ma; and megasequence Z, a Neogene subduction margin megasequence. The Moho lies at 17 km beneath the basin center and at 35 km at the southern margin. Beneath the western basin margin, we interpret reflective units as deformed Gondwana fore-arc sediment that was thrust in Cretaceous time over oceanic crust 7 km thick. Raukumara Basin has normal faults at its western margin and is uplifted along its eastern and southern margins. Raukumara Basin represents a rigid fore-arc block > 150 km long, which contrasts with widespread faulting and large Neogene vertical axis rotations farther south. Taper of the western edge of allochthonous unit Y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward, as proposed for the correlative onshore allochthon. Spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between Moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore-arc crustal thickness. The trench slope has many small extensional faults and lacks coherent internal reflections, suggesting collapse of indurated rock, rather than accretion of > 1 km of sediment from the downgoing plate. The lack of volcanic intrusion east of the active arc, and stratigraphic evidence for the broadening of East Cape Ridge with time, suggests net fore-arc accretion since 22 Ma. We propose a cyclical fore-arc kinematic: rock moves down a subduction channel to near the base of the crust, where underplating drives rock uplift, oversteepens the trench slope, and causes collapse toward the trench and subduction channel. Cyclical rock particle paths led to persistent trench slope subsidence during net accretion. Existing global estimates of fore-arc loss are systematically too high because they assume vertical particle paths. Citation: Sutherland, R., et al. (2009), Reactivation of tectonics, crustal underplating, and uplift after 60 Myr of passive subsidence, Raukumara Basin, Hikurangi-Kermadec fore arc, New Zealand: Implications for global growth and recycling of continents, Tectonics, 28, TC5017, doi: 10.1029/2008TC002356

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

<div><p>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.</p></div