C. A. Landis

The Waipounamu Erosion Surface: questioning the antiquity of the New Zealand land surface and terrestrial fauna and flora

The Waipounamu Erosion Surface is a time-transgressive, nearly planar, wave-cut surface. It is not a peneplain. Formation of the Waipounamu Erosion Surface began in Late Cretaceous time following break-up of Gondwanaland, and continued until earliest Miocene time, during a 60millionyearperiodofwidespreadtectonicquiescence,thermalsubsidenceandmarinetransgression. Sedimentary facies and geomorphological evidence suggest that the erosion surface may have eventually covered the New Zealand subcontinent (Zealandia). We can find no geological evidence to indicate that land areas were continuously present throughout the middle Cenozoic. Important implications of this conclusion are: (1) the New Zealand subcontinent was largely, or entirely, submerged and (2) New Zealands present terrestrial fauna and flora evolved largely from fortuitous arrivals during the past 22 million years. Thus the modern terrestrial biota may not be descended from archaic ancestors residing on Zealandia when it broke away from Gondwanaland in the Cretaceous, since the terrestrial biota would have been extinguished if this landmass was submerged in Oligocene- Early Miocene time. We conclude that there is insufficient geological basis for assuming that land was continuously present in the New Zealand region through Oligocene to Early Miocene time, and we therefore contemplate the alternative possibility, complete submergence of Zealandia.

Mantle-plume activity recorded by low-relief erosion surfaces in West Antarctica and New Zealand

Mantle-plume activity has been proposed to explain Neogene and mid-Cretaceous magmatic events, as well as associated tectonism, in West Antarctica; but the arrival time and dimensions of plume influence have been hard to define and are still a subject of debate. Two low-relief erosion surfaces, one in West Antarctica and the other in New Zealand (herein named the Waipounamu erosion surface), provide a way of assessing plume activity by measuring vertical displacements associated with these events. Both surfaces bevel mid-Cretaceous rocks, and both represent prolonged intervals of erosional leveling in a stable tectonic environment. Overlying strata in New Zealand indicate that leveling was near completion in coastal regions by ca. 75 Ma and therefore must have begun around 85 Ma, when New Zealand was beginning to break away from West Antarctica. Fluvial erosion followed by subsidence and marine planation are clearly recorded by these strata, and a similar history seems likely for West Antarctica, accounting for isostatically corrected ice-free bedrock elevations that are well below sea level over much of the region. The absence of uplift at the time of breakup seems incompatible with a plume mechanism for continental breakup. By contrast, the present elevation of the West Antarctic erosion surface records an estimated maximum of ≈ 3 km of tectonic uplift, associated with alkalic volcanism, beginning at ca. 28–30 Ma. We suggest that this event marks the inception of plume activity in West Antarctica. The resulting structure, the Marie Byrd Land dome, defines an area of plume influence that is smaller than the area defined by geochemistry, but is similar in scale to the Yellowstone plume.

Uranium‐lead zircon ages from the Median Tectonic Zone, New Zealand

Abstract The Median Tectonic Zone (MTZ) of New Zealand is a generally north trending belt of Mesozoic subduction‐related I‐type plutonic, volcanic, and sedimentary rocks in South Island and Stewart Island that separates Permian strata of the Eastern Province Brook Street Terrane from lower to mid‐Paleozoic Gondwana margin assemblages of the Western Province. High‐precision isotope dilution U/Pb ages of zircons from 30 rocks are reported. Pre‐digestion leaching of zircon in hydrofluoric acid yielded significantly more concordant residues by removing common Pb and dissolving more soluble high‐U domains that have been more affected by relatively recent Pb loss. The results show that MTZ magmatism ranges in age from at least Early Triassic to Early Cretaceous (247–131 Ma), with a pronounced gap in the Middle Jurassic. Triassic plutons tend to occur on the eastern side of the MTZ, and they intrude volcanic/sedimentary sequences of the MTZ in Nelson and eastern Fiordland. These sequences are in turn intruded by...

Isotopic ages from the Nelson region of South Island New Zealand: crustal structure and definition of the Median Tectonic Zone

Abstract Plutonic rocks in the Rotoroa Complex and Drumduan Terrane of South Island, New Zealand yield zircon U/Pb dates of 156 and 142 Ma, respectively, that are interpreted as crystallization ages. Hornblende and biotite 40 Ar/ 39 Ar dates of 140-130 Ma from the Rotoroa represent either emplacement ages, cooling ages or a metamorphic resetting event. These two units crop out between the Brook Street Terrane and the Separation Point Batholith and lack any clear affinity with tectonostratigraphic terranes of the New Zealand Western or Eastern provinces. The Rotoroa Complex and Drumduan Terrane are interpreted as part of a series of dismembered Mesozoic volcanic-plutonic arc complexes that are sandwiched between terranes of the Western and Eastern provinces, occupying a structural position here referred to as the Median Tectonic Zone (MTZ). Correlative units in Fiordland on the opposite side of the Alpine Fault include the Mackay Intrusives, Darran Complex, Largs Terrane, Lochburn Formation and the Halfway Peak Gabbro. Farther south on Stewart Island the Anglern Complex and Paterson Group are part of the same structural belt. The MTZ is an extension of the original concept of the Median Tectonic Line put forth by Landis and Coombs (1967). Dismemberment and juxtaposition of arc magmatic assemblages in the MTZ with Western and Eastern Province terranes is related to large-scale transcurrent faulting in the Early Cretaceous. Its essential features as a regional tectonostratigraphic terrane were established by ~ 117 Ma as indicated by stitching of the Rotoroa Complex to the Takaka Terrane (Western Province) by the Separation Point Batholith (117-114 Ma). The Echinus Granite yields a 310 Ma U/Pb zircon crystallization age that suggests the granite and associated gneisses are part of the Western Province which may constrain the position of the western margin of the MTZ near Nelson City.

Plate Tectonics and Regional Stratigraphic-Metamorphic Relations in the Southern Part of the New Zealand Geosyncline

The evolution of the New Zealand geosyncline is examined in terms of both the plate tectonics model and such geological constraints as regional metamorphic and biostratigraphic patterns and interrelations between paired sedimentary facies. A common western provenance is tentatively concluded for both major sedimentary facies belts of the geosyncline; the inner (Hokonui) belt acted as a volcanotectonic sediment trap during times of increased volcanic activity, while during periods of volcanic quiescence, quartzo-feldspathic debris from the adjacent “continental” foreland was transported across the inner belt into the outer (Torlesse) basin. In general the Torlesse rocks appear to decrease in age but increase in metamorphic grade toward the inferred western (continental) source, and it is suggested that the older (Carboniferous and Permian) rocks were never overlain by substantial thicknesses of younger strata. Development of an early extensional basin, by strike-slip displacements, in Carboniferous to Jurassic times is consistent with both this concept and with the apparently anomalous regional biostratigraphic zonation of the Torlesse rocks. Differences in geosynclinal evolution between New Zealand and California appear to be accountable in terms of the greater age of the New Zealand geosyncline and the early period of extensional growth. The Late Jurassic-Cretaceous Rangitata orogeny was an important event occurring relatively late in the depositional history of the New Zealand geosyncline, and is interpreted as representing the initiation of the present system of Pacific basin sea-floor spreading. A corresponding early extensional event is not recognized in the Jurassic-Cretaceous Coast Range geosyncline of California, the evolution of which appears to be entirely related to subduction and arc activity related to an East Pacific rise spreading axis. Differences in detailed deformational style and metamorphism between the two areas may result from differences in spreading rates and from the steeper dip of the Jurassic-Cretaceous subduction zone in New Zealand.

Uranium-lead ages from the Dun Mountain ophiolite belt and Brook Street terrane, South Island, New Zealand

The Dun Mountain ophiolite and overlying Maitai Group is discontinuously exposed for 480 km in South Island, New Zealand. Zircon U/Pb dates from plagiogranite are presented for relatively intact ophiolite, ophiolitic melanges, and for more silicic volcanic-plutonic assemblages in the southern part of the belt where a typical ophiolite association is lacking. Step-wise dissolution experiments on slightly discordant plagiogranite zircon produce more concordant residues that indicate the zircons have lost from ∼1% to more than 5% of their radiogenic Pb relatively recently. High-precision 207 Pb/ 206 Pb dates establish the age of ophiolite formation for at least 350 km along strike to a narrow interval between about 275 and 285 Ma. The zircon U/Pb data confirm correlation of petrologically distinct segments of the Dun Mountain ophiolite and show that mafic-ultramafic ophiolite assemblages and moderately potassic high-level granitoids developed within a short time interval, probably during the extension of a volcanic-arc marginal basin. Thick lenses of polymictic breccia and bio-clastic limestone of the Maitai Group locally rest in depositional contact on relatively intact ophiolite within the Dun Mountain ophiolite. Comparison of inferred biostratigraphic ages from the limestone with the ∼280 Ma ages from the plagiogranites indicate a gap of ∼20 m.y. following ophiolite formation. A granite clast from conglomerate higher in the Maitai Group yielded a near concordant U/Pb date of 265 Ma and provides a maximum age for this part of the sequence. Attenuation of the Dun Mountain ophiolite by extensional faulting and erosion may have occurred during the interval between ophiolite formation and Maitai Group sedimentation. The Dun Mountain ophiolite and overlying Maitai Group are bounded to the west by Triassic and Jurassic volcanogenic sedimentary rocks of the Murihiku terrane, and in turn by the Brook Street terrane, which is interpreted as remnants of an early Permian oceanic arc. A hornblende gabbronorite associated with a layered mafic-ultramafic intrusion in the Brook Street terrane yielded a date of 265 Ma, significantly younger than Dun Mountain ophiolite. Such intrusions may represent the plutonic roots for ankaramitic volcanic rocks that comprise a conspicuous component of the Brook Street terrane, but which are not represented by detritus in the Maitai Group. Biotite granite occurs locally in the Brook Street terrane and is dated at 260 Ma. The absence of any clear stratigraphic correlation or provenance linkage between the Brook Street terrane and Dun Mountain-Maitai terrane suggests strike-slip displacements on intervening terrane boundary faults.

Redefinition and interpretation of late Miocene‐Pleistocene terrestrial stratigraphy, Central Otago, New Zealand

Abstract The stratigraphic succession in eastern Central Otago consists of Eocene quartzose fluvial sediments and middle Tertiary marine strata (Onekakara Group), early‐middle Miocene quartzose fluvial sediments and lake deposits (Manuherikia Group), late Miocene‐Pliocene immature sandstones and conglomerates, and Quaternary terrace and fan gravels. Published literature contains at least 20 different approaches for subdivision of this succession. The late Miocene‐Pliocene conglomerates were formed during the rise of fault‐bounded greywacke and semischist mountain ranges. Conspicuous conglomerates in the upper part of this succession are widely referred to as Maori Bottom Formation, but that name was originally applied by miners to a locally auriferous erosion surface beneath Quaternary terrace and fan gravels in Otago. In addition, the term is culturally offensive. We propose the name Hawkdun Group for the late Miocene‐Pliocene succession of tectonically generated sediments in the Maniototo, Ida, and Manu...

Continuous metamorphic gradient documented by graphitization and K-Ar age, southeast Otago, New Zealand

Abstract The Chrystalls Beach-Brighton coastal section, southwest of Dunedin, New Zealand, exposes Otago Schist, including the weakly metamorphosed Triassic accretionary melange of the Chrystalls Beach Complex and higher-grade rocks of a largely felsic, arc-derived provenance. Melange zones contain pelagic sediments, some of which are manganiferous, and metabasites. The grade of metamorphism increases progressively from southwest to northeast. Four mineral zones are recognized largely on the basis of mineral assemblages in psammitic and semipelitic rocks: pumpellyite-chlorite, pumpellyite-actinolite, epidote-actinolite, and biotite. d002 data for carbonaceous material, b0 values for phengite, and metamorphic phengite K-Ar ages are reported for 32 pelitic and semipelitic samples. Remarkably tight correlations are revealed between d002 for progressively graphitized carbonaceous material, K-Ar ages that range from 184.6 to 138.7 Ma, and linear distance along the section. Both the d002 values and the K-Ar ages decrease progressively toward the northeast through the four mineral zones and with progressive textural changes. All rocks cropping out in the section have undergone a coherent episode of progressive metamorphism associated with terrane collision and have not been disturbed by any later major displacements. A convex trend for phengite b0 values plotted against advancing graphitization suggests P/T conditions during metamorphism as in the high-pres sure intermediate facies series. The age of the metamorphic peak is inferred to be 175-155 Ma (Middle to Late Jurassic), with cooling to the mica closure temperature at 155-135 Ma (Late Jurassic to Early Cretaceous). Unloading, denudation, and cooling were accompanied by localized hydrothermal events. The oldest covering strata were deposited at ca. 100 Ma and denudation was completed by ca. 75-80 Ma. Middle Triassic radiolarian nodules in melange zones are dated as preceding the metamorphic peak by about 65 Ma. Associated turbidites contain suspected late Middle to early Late Triassic tube fossils

Suggestions towards a high-level nomenclature for New Zealand rocks

Abstract Rangitata Sequence is suggested as a comprehensive term for “the rocks of the New Zealand Geosyncline”. It is subdivided into the Hokonui and Alpine Assemblages, the former including the Murihiku Supergroup, and the latter the Torlesse (greywacke) Zone and Haast Schist Zone. Pre-Rangitata rocks are referred to the Tuhua Sequence and post-Rangitata rocks to the Kaikoura Sequence. These terms are convenient labels for the major elements of the geologic column in New Zealand; as they are not members of the strict lithostratigraphic hierarchy, they allow great flexibility and avoid the need for premature lithostratigraphic formalisation. The same terms may also be used with Geosyncline, Orogeny, and Orogenic Belt; they thus introduce great economy into the major tectonostratigraphic nomenclature of New Zealand.

Permian‐Jurassic strata at Productus Creek, Southland, New Zealand: Implications for terrane dynamics of the eastern Gondwanaland margin

Abstract An area in the Wairaki Hills lying to the east of the Takitimu Mountains and extending from the vicinity of the Wairaki River south to near Ohai is described. The area straddles the faulted contact between the Brook Street Terrane and the Murihiku Terrane. Sparsely fossiliferous volcaniclastic strata of the 15 km thick Takitimu Group (Early Permian) comprise the oldest rocks of the region. Youngest Takitimu rocks, the newly recognised Caravan Formation, contain ankaramitic dikes and pyroxene‐rich marine volcaniclastics. These are conformably overlain by richly fossiliferous sandstone and impure limestone of Mangarewa Formation (120 m) and atomodesmatinid prism limestones of Glendale Limestone (400 m), which together constitute the newly restricted Productus Creek Group of latest Early to early Late Permian age. Takitimu and Productus Creek Group rocks are intruded by Triassic to Early Jurassic igneous rocks ranging from andesite to diorite and olivine monzonite. Barretts Formation, a newly recogn...