Seismic Stratigraphy and Structure Adjacent to an Evolving Plate Boundary, Western Chatham Rise, New Zealand

Abstract

Seismic profiles obtained from the eastern side of New Zealand, in transit to and from Site 594, illustrate the evolution of the western end of the Chatham Rise and the southern extremity of the Hikurangi Trough. They show several unconformities and several major changes in tectonic and depositional regime. The unconformities represent major changes in oceanic circulation, possibly triggered by large lowerings of sea level in the late Oligocene and late Miocene. Tectonism is exemplified by a Late Cretaceous phase of block faulting, heralding the rift from Gondwanaland, and by Plio-Pleistocene normal faulting on the northern flank of the rise antithetic to the oblique-collision plate boundary in the southern Hikurangi Trough. This active normal faulting may indicate that the northwestern corner of Chatham Rise continental crust is being dragged down with the subducting slab to the north. The depositional regime has changed from Late Cretaceous infilling of fault-angle depressions, through an early Tertiary transgression and mid-Tertiary carbonate drape, to a late Miocene-Recent sequence recording repeated glacial-interglacial events. This upper stratified unit onlaps a late Miocene erosion/phosphatization unconformity toward the crest of the rise. It is locally truncated by a slope-parallel erosion surface, with downslope buildup, which may indicate either current scour and deposition or mass movement. INTRODUCTION BackgroundIn transit between Cook Strait and Site 594, the Glomar Challenger obtained seismic profiles along approximately 900 km of ship's track. Because they are tied to borehole data and because they cross areas where tracks are sparse, the Glomar Challenger profiles provide a significant increase in our understanding of the stratigraphy of eastern New Zealand and of tectonism in continental crust adjacent to an oblique-collision plate boundary.In this paper we integrate the Glomar Challenger profiles with published syntheses, with seismic data obtained by D.S.I.R. divisions, and with compilations of oil company data on open file at the New Zealand Geological Survey. Oil company data used in this study include profiles collected by Mobil Oil Corporation, Magellan Petroleum (New Zealand) Ltd., Australian Gulf Oil company, and B.P. Shell Todd (Chatham Rise) Ltd. Geomorphological SettingThe Glomar Challenger profiles traverse the Mernoo Saddle, a depression between the continental shelf of eastern New Zealand and the east-west-trending Chatham Rise ( Fig. 1; Fig. 3, later). The saddle's sill depth is over 500 m deep, contrasting with shelf depths to east Kennett, J. P., von der Borch, C. C, et al., Init. Repts. DSDP, 90: Washington (U.S. Govt. Printing Office).

Late Neogene unconformity‐bounded tuffaceous sequences: Northwestern Chatham Rise, New Zealand

“…A similar increase in terrigenous sedimentation associated with changes in seismic impedance or sequence architecture have been recorded on the southern flank of the Chatham Rise at DSDP site 594 (Lewis et al 1986;Nelson 1986) and on the South Island continental margin in the late Miocene (Wood et al 1989). The late Miocene (c. 9-10 Ma) age of reflector LM coincides with the onset of significant tectonic shortening within the New Zealand plate-boundary zone and regional uplift (Walcott 1978).…”

“…2). A packet of strong, parallel reflectors below this lower wedge has been traced in airgun profiles to the Canterbury shelf, where it represents the Oligocene Amuri Limestone (Lewis et al 1986;Wood et al 1989).…”

“…1). The region lies within the broad zone of deformation associated with the New Zealand plate boundary, and is undergoing extensional faulting, which resumed in the late Neogene after a period of Paleogene and early Neogene quiescence (Lewis et al 1986;Wood et al 1989). The slope lies north of Mernoo Saddlethe 580 m deep depression between the Chatham Rise and the South Island-and has been the site of a complex, PliocenePleistocene sedimentation history, involving the waxing and waning of regional-scale, mid-bathyal currents sweeping obliquely across the slope (Barnes 1992).…”

The Urry Knolls, late Neogene alkaline basalt extrusives, southwestern Chatham Rise

“…Gibson, 1971;Mobil, 1972;Australian Gulf, 1973;Houtz and Aitkin, 1973;Hayes et at., 1976;Lewis et at., 1985) and numerous oceanographic surveys.…”

“…In 1985 Glomar Challenger drilled a hole, DSDP 594, about 80 km to the south. Lewis et al, (1985) described some of the seismic stratigraphy in the region of knolls in the light of the drillhole stratigraphy and of regional seismic stratigraphic syntheses by the New Zealand Geological Survey (Wood et al, in press). Lewis et al, concluded, from the interpreted age of infilling of scour moats around some of the knolls, that they date from the Miocene.…”