Late Cenozoic tectonics in the SW Pacific, and development of the Alpine Fault through southern South Island, New Zealand

Sutherland, Rupert.

This thesis presents improved constraints on the tectonic evolution of the Cenozoic Australia-Pacific plate boundary through southern New Zealand. Two independent lines of enquiry were followed: 1. The constraints provided by seafloor spreading data from south of New Zealand were refined. This was possible due to the release of Geosat data, and recent geophysical cruises SW of South Island. 2. Middle Miocene-Quaternary sediments in south Westland were investigated, with particular attention to their provenance and structure. Australia-Pacific finite rotations based on a 3-plate (Australia-Antarctica-Pacific) model are presented (GEOS-3P results). Features identified from Geosat data are used to directly determine a late Eocene ( --45 Ma) Australia-Pacific fmite rotation, and to refine Eocene-Miocene plate motions (--45-20 Ma; GEOS-NZ results). Inverting the late Eocene finite rotation to calculate movement in Antarctica, implies dextral oblique extension in the Ross Embayment and almost pure dextral movement in the Queen Maud Range. In the Hokuri Creek region, postglacial offsets of Lake McKerrow and moraines require an Alpine Fault displacement rate of 26 ±6 mm/yr. Slickensides, fault exposure and offset topography suggest almost pure dextral movement on a subvertical fault. Locally, there is a small component of up-to-thewest movement By using displacement to calculate age, moraines indicate that ice was most extensive prior to --40 kyr, and following a significant interstadial, two further moraine deposition events occurred at -26 and -17 kyr. Displaced river channels indicate the last two coseismic displacements on the Alpine Fault were 9 m (penultimate) and 8 m, suggesting characteristic earthquake behaviour with recurrence interval330 ±90 yrs and Mw >7.5. The last displacement occurred just after 370 ±150 ca1 yrs b.p. In the Cascade Valley, Halfway Formation (Pliocene) Fiordland-derived marine conglomerates are offset from their source area by >95 km. Lateral moraines stretching from the coast to the Alpine Fault are undissected by faults. The oldest moraines are Pyke Valley-derived (offset -18-32 km), suggesting they have an age >0.5 Ma, consistent with macrofaunas from associated uplifted marine glacial silts (Teer Formation). The younger moraines contain varying proportions of Ultramafic and Haast Schist, reflecting progressive capture of tributaries due to Alpine Fault offset The offset Halfway Formation, combined with the undissected nature of moraines, suggests Pliocene-recent Alpine Fault displacement at '?:.27 ±4 mm/yr. The fossiliferous Halfway and Teer Formations suggest uplift on the coast at -0.1-0.5 mm/yr. Miocene (-5.5, 7.5, 9 Ma) conglomerate cuttings from Waiho-1 borehole, central Westland, are mainly derived from NW of the Alpine Fault, but contain significant metagreywacke and minor high grade metamorphic clasts (some garnetiferous). This suggests reverse movement on the South Westland Fault Zone in middle-late Miocene time. Source areas SE of the Alpine Fault (mainly greywacke terranes, but minor Fiordland detritus) provided the remainder of the sediment. At -5 Ma the sedimentation rate recorded by Waiho-1 borehole greatly increases, and sediments are mainly derived from SE of the Alpine Fault. The change in provenance from volcanogenic (4.5 Ma sample) to Torlesse greywacke and Haast Schist (4.0 Ma sample) reflects tectonic transport of the borehole site past the Dun Mountain and other volcanogenic basement terranes. Comparison of the provenance change with plate displacement estimates, suggests the Dun Mountain ophiolite belt had significant curvature at the beginning of the Pliocene, and that Pliocene-Quatemary deformation was concentrated near to the Alpine Fault Tititira Formation sediments (middle-late Miocene) exposed in south Westland are mainly derived from NW of the Alpine Fault, but contain significant metagreywacke, metavolcanic, and high grade metamorphic clasts. Provenance and paleocurrents suggest south Westland was adjacent to, or SW of Fiordland during deposition, but that Fiordland had only limited relief and material was being transported across it. Sediments and volcanics reworked from all stratigraphically lower horizons, deformation of soft sediment, intraformational unconformities, and the incorporation of slide sheets up to -10 km across (Jackson Formation), suggest syntectonic sedimentation and active reverse movement on the South Westland Fault Zone during deposition. The regional onset of sedimentation at -12 Ma is inferred to have been due to a change in plate vector, and the coarsening upward sequence of facies is interpreted to have been due to the growth of a deforming wedge bounding the SE edge of the Westland Sedimentary Basin. The plate boundary through South Island has developed in direct response to the moving AustraliaPacific pole of rotation. Prior to plate boundary inception, the late Eocene finite pole suggests the Dun Mountain ophiolite belt, and other basement terranes exposed in South Island, were continuous and approximately straight. The late Eocene-early Oligocene instantaneous rotation pole was close to South Island, causing rapid spatial changes in plate boundary character and displacement rate. South of South Island, rifting resulted in the formation of ocean crust, and convergence is predicted to the north. As the rotation pole moved SE during the Oligocene-middle Miocene, there was a gradual increase in strike-slip displacement rate through southern South Island from <5 mm/yr to -2 cm/yr. An inferred jump of the Australia-Pacific rotation pole at -12-14 Ma caused an increase in strike-slip displacement rate in South Island to --4 cm/yr, and resulted in the initiation of subduction adjacent to Fiordland, with associated movement on the South Westland Fault Zone. At -5 Ma another jump in the Australia-Pacific rotation pole caused the displacement rate in South Island to remain at --4 cm/yr, but introduced a small component of convergence on the Alpine Fault. A change in partitioning of plate boundary deformation since -5 Ma has caused uplift and the emergence ofFiordland.

1 v. (various pagings) : ill. (some col.), maps ; 30 cm.

1995Sutherland

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