Peter J.J. Kamp

Fission track analysis of the Late Cenozoic vertical kinematics of continental pacific crust, South Island, New Zealand

During the late Cenozoic the Pacific plate has been converging obliquely with the Australia plate in South Island, New Zealand. A result of this convergence has been the growth of a major mountain range (the Southern Alps) at the leading edge of the Pacific plate. The results of fission track analysis of 140 samples from 13 transects across the Alps reported here establish the late Cenozoic vertical kinematics (amount, age, and rate of rock uplift) of the Pacific crust underlying the Alps. The late Cenozoic rock uplift of the Pacific crust is asymmetrical with respect to the Alpine fault, being a maximum (19 km) immediately east of the central part of the fault, with lesser values at the eastern (3 km), northern (10 km), and southern (8 km) extremities of the Alps. The age of the start of rock uplift varies spatially across the Southern Alps, the earliest indications from fission track analysis being at 8 Ma at the southern end of the Alps, decreasing to 5 Ma at the northern end and 3 Ma along the southeastern margin. This age variation reflects the longer time over which the southern parts of the Alps have been in collision. The rate of propagation of rock uplift southeastward into the Pacific plate has been 30 mm/yr, nearly 4 times the late Cenozoic average rate of convergence normal to the plate boundary. Late Cenozoic mean rock uplift rates range from a maximum of ∼2.8 mm/yr at the Alpine fault to a minimum of ∼1.0 mm/yr in the east and have been sustained for periods of 3–8 m.y. Accompanying denudation has exhumed amphibolite grade rocks immediately east of the Alpine fault. The rock uplift has been controlled by oblique-slip displacement on the Alpine fault. A continental crustal section at least 19 km thick has been uplifted on the Alpine fault. Comparison of the late Cenozoic mean rock uplift rates with uplift rates derived from reset zircon data (2–10 mm/yr) near the Alpine fault shows that uplift has accelerated over time, but only significantly since 1.3 ± 0.3 Ma. The amount of Mesozoic uplift ranged from minimal amounts north of Arthurs Pass, to ∼3 km near Mount Cook, to 10 km in the south at Lake Wanaka.

The continental collision zone, South Island, New Zealand: Comparison of geodynamical models and observations

The South Island zone of oblique continent-continent convergence occurs along a 400 km-long section of the modern Australia-Pacific plate boundary zone, across which about 50 km of shortening has been accommodated since about 10 Ma. The orogen comprises a central mountain range (Southern Alps) flanked on both sides by what are interpreted to be foreland basins. Two essential features that characterize the orogen are (1) the degree of denudation that accompanied deformation, and (2) a fundamental structural asymmetry. The architectural asymmetry of the orogen can be explained by plane strain, finite element models of continental convergence incorporating mantle subduction. Comparison of model and orogen polarity implies that Pacific plate mantle subducts. The models predict two crustal-scale dipping shear zones that form above the point where the Pacific mantle subducts. The localized one more distant from the incoming plate (retro-step-up shear zone) corresponds to the Alpine fault, whereas its conjugate (pro-step-up shear zone) corresponds to the distributed strain and thrusting along the eastern margin of the mountain belt. Parameters that modify the model boundary conditions (top surface, degree of denudation; basal zone, subduction load, crust-mantle velocity discontinuity, subduction of lower crust, mantle retreat, and distributed decrease in mantle velocity) and the internal strength of the crust (two-layer crust with moderate coupling, temperature distribution, strain weakening) are varied in a series of numerical model calculations that establish the combination of material properties and boundary conditions that lead to different cross sectional architectures of the modeled collision zone. In turn, these are compared with observations about the South Island orogen. The calculations show how the style and extent of deformation across the whole orogen depend on the rheological properties of the crustal layer and on the balance between its internal strength and the combined effects of the boundary and gravitational stresses. North to south along-strike differences in the width and two-dimensional architecture of the orogen, simulated in the experiments by varying the model parameters, can be explained by a combination of southward increases in preconvergent crustal thickness, geothermal gradient, convergence, and potentially subduction retreat, with the added possibility of a southward decrease in the component of lower crustal subduction.

Sequence stratigraphy of sixth-order (41 k.y.) Pliocene–Pleistocene cyclothems, Wanganui basin, New Zealand: A case for the regressive systems tract

This study is based on a late Pliocene and early Pleistocene (approximately 2.6‐1.7 Ma) succession about 1 km thick of 20 sixth-order (41 k.y. duration) cyclothems of shelf origin exposed in the Rangitikei River valley in the eastern part of Wanganui basin. The cyclothems correlate with δ 18 O isotope stages 100‐58, and each 41 k.y. glacialinterglacial stage couplet is represented by an individual depositional sequence comprising transgressive, highstand, and regressive systems tracts. Unlike most examples inferred from the stratigraphic record, these systems tracts were deposited during phases of known sea-level cycles indicated by the contemporary oxygen isotope ice-volume curve. Because of the high rate of subsidence of Wanganui basin, glacioeustatic sea-level falls during most cycles were not of sufficient magnitude to expose the outer shelf. Thus, the Rangitikei section provides an exceptional example of regressive strata deposited landward of the contemporary shelf break. Simple one-dimensional modeling shows that moderate to high rates of basin subsidence (1‐2 mm/yr) and low rates of sedimentation (<0.2 mm/yr) during transgressions combined to produce an accommodation surplus at the relative highstand. This surplus accommodation was infilled during the late highstand and ensuing fall partly by aggradational, highstand systems tract shelf siltstone, and chiefly by strongly progradational shoreface sediments of the regressive systems tract. Rangitikei regressive systems tracts are distinguished from forced regressive systems tracts (sensu Hunt and Tucker, 1992) by their different stratal geometry. By definition, forced regressive systems tracts display an erosional contact with the underlying highstand systems tracts and typically occur as a series of downstepped disjunct shoreline wedges stranded on the shelf and/or slope. In contrast, regressive systems tracts exhibit a gradational lower contact, above which parasequences are stacked in a strongly progradational pattern terminated by the superjacent sequence boundary. Cyclothems display two types of motif termed Rangitikei dt (depositional transgression), and Rangitikei nt (nondepositional transgression), which include the following architectural elements in ascending stratigraphic order: (1) a basal sequence boundary that is coincident with either the transgressive surface of erosion, which displays small-scale (up to 50 cm) erosional relief and may be penetrated by the ichnofossil Ophiomorpha, or its deeper water correlative conformity; (2) either a thick (5‐30 m) transgressive systems tract comprising a deepening upward nearshore to inner shelf, mixed carbonate-siliciclastic lithofacies succession (depositional transgression), or a thin (<2 m) transgressive systems tract comprising condensed fossiliferous facies deposited on the sediment-starved offshore shelf (nondepositional transgression); (3) a sharp downlap surface separating condensed fossiliferous facies of the transgressive systems tract from terrigenous siltstone of the superjacent highstand systems tract; (4) a highstand systems tract comprising a 10‐20-m-thick interval of aggradational, shelf siltstone; and (5) a thick (up to 45 m) progradational inner shelf to shoreface lithofacies assemblage ascribed to the regressive systems tract. Condensed shell beds are associated with intrasequence and sequence-bounding discontinuities, and, together with the sedimentological and stratal characteristics of the sequences, are important indicators of stratigraphic architecture. Four types of shell bed are associated with surfaces formed by four different types of stratal termination; onlap, backlap, downlap, and flooding surface shell beds (cf. Kidwell, 1991) are associated, respectively, with the transgressive surface of erosion, “apparent truncation” at the top of the transgressive systems tract, the downlap surface, and local marine flooding surfaces. A fifth shell-bed type, termed a compound shell bed, forms in offshore environments where the downlap surface converges with the sequence boundary, and elements of both the downlap and the backlap shell beds become mixed or superposed. The shell beds mark zones of stratal attenuation and can be used as surrogates for seismic discontinuities when applying sequence stratigraphic concepts at outcrop scale.

Exhumation history of orogenic highlands determined by detrital fission-track thermochronology

Abstract A relatively new field in provenance analysis is detrital fission-track thermochronology which utilizes grain ages from sediment shed off an orogen to elucidate its exhumational history. Four examples highlight the approach and usefulness of the technique. (1) Fission-track grain age (FTGA) distribution of apatite from modern sediment of the Bergell region of the Italian Alps corresponds to ages obtained from bedrock studies. Two distinct peak-age populations at 14.8 Ma and 19.8 Ma give calculated erosion rates identical to in situ bedrock. (2) Zircon FTGA distribution from the modern Indus River in Pakistan is used to estimate the mean erosion rate for the Indus River drainage basin to be about 560 m Ma−1, but locally it is in excess of 1000 m Ma−1. (3) FTGA distribution of detrital apatite and zircon from the Tofino basin records exhumation of the Coast Mountains in the Canadian Cordillera. Comparison of detrital zircon and apatite FT ages gives exhumation rates of c. 200 m Ma−1 during the interval between c. 34 and 54 Ma, but higher rates (c. 1500 m Ma−1) at c. 56 Ma. (4) FTGA analysis of apatite grain ages from a young basin flanking Fiordland in New Zealand indicates that removal of cover strata was followed by profound exhumation at c. 30 Ma, which corresponds to plate reorganization at this time. Exhumation rates at the onset of exhumation were c. 2000–5000 m Ma−1. These studies outline the technique of detrital FTGA applied to exhumation studies and highlight practical considerations: (1) well-dated, stratigraphically coordinated suites of samples that span the exhumation event provide the best long-term record; (2) strata from the basin perimeter are the most likely to retain unreset detrital ages; (3) the removal of ‘cover rocks’ precedes exhumation of deeply buried rocks, which retain a thermal signal of the exhumation event; (4) steady-state exhumation produces peak ages that progressively young with time and have a constant lag time; (5) same-sample comparison of zircon and apatite peak ages is best in sequences with high-uranium apatite grains (>50 ppm), and peak-ages statistics can be improved by counting numerous apatite grains (>100).

Tectonics and denudation adjacent to the Xianshuihe Fault, eastern Tibetan Plateau: Constraints from fission track thermochronology

The Xianshuihe-Xiaojiang fault system extending from eastern Tibet to central Yunnan, China, is a major left-shear structural boundary, accommodating the clockwise rotation of crustal rocks between the Eastern Himalayan syntaxis and the South China Block. Zircon and apatite fission track (FT) data are reported for 111 samples of basement collected from both sides of the northern part of this fault and spanning 300 km across the eastern margin of the Tibetan Plateau, where its mean elevation drops from 3500 to 1500 m above sea level. The zircon FT ages define two fossil partial annealing zones at different elevations, one fossilised at circa 130 Ma, and the other at circa 21 Ma as a result of cooling probably via regional denudation. The apatite FT ages are mostly less than 25 Ma, but a few granitoids higher up on the plateau retain late Cretaceous apparent ages. Within the apatite FT data with Neogene ages, there may be several partial annealing zones, with mixture modeling identifying age components suggestive of discrete cooling phases at circa 22, 7 and 2 Ma. The first-order pattern of the minimum amount of Neogene denudation for a section across the plateau margin immediately northeast of Xianshuihe Fault suggests that a relatively uniform 4–6 km has been eroded from the inner part of the plateau margin, increasing to 7–10 km at Kangding, where the oldest rocks (Precambrian) are exposed, and then decreasing markedly into Sichuan Basin. These new FT data combined with published FT data suggest that the present extent of the Tibetan Plateau was defined during the early Miocene.

Late Cretaceous-Cenozoic tectonic development of the southwest pacific region

Abstract A new model of the plate tectonic development of the southwest Pacific integrates the continental geology of New Zealand with the age structure of the surrounding oceanic crust revealed previously from magnetic anomaly lineations. The model differs from previous ones in that the onland geology of New Zealand is used to constrain the tectonic development in two important ways: (1) the modern Australia-Pacific plate boundary did not transect the New Zealand sector until 23 million years ago; (2) there has been a total of only c.500 km of dextral displacement on the plate boundary through New Zealand. The model is described with reference to a series of paleotectonic maps drawn to represent the setting at the times of anomalies 32, 24, 7 and 5. Novel interpretations based on these reconstructions include the following. During the late Cretaceous the Campbell Fault was a continental transform fault between the Tasman Sea spreading centre and the Bounty Rift, its 330 km of dextral displacement accompanying 25° of counterclockwise rotation of a Campbell Plateau block. When the eastward-propagating Southeast Indian Ridge broke into the south Tasman Sea 57 million years ago, it continued to propagate directly into southern South Island, and generated late Eocene-Oligocene continental rifting through western New Zealand. At the same time, back-arc spreading in the Norfolk Basin caused continental rifting along a similar trend in western North Island. The Australia-Pacific plate boundary originated as a transform fault between the Southeast Indian Ridge and the pre-existing Kermadec Trench. It developed in response to the sudden cessation of spreading on the segment of the Southeast Indian Ridge east of a fracture zone here named Fracture Zone Z. Two implications arise from this model regarding the integrity of Antarctica: 1. (1) during the Cretaceous, Marie Byrd Land was probably no more than 200 km northwest of its present position; 2. (2) the proposition made in some earlier plate tectonic reconstructions, that if the Alpine Fault did not form until after the late Eocene there must be a late Cretaceous-early Tertiary plate boundary within Antarctica, is most probably valid.

The evolutionary history of the extinct ratite moa and New Zealand Neogene paleogeography

The ratite moa (Aves: Dinornithiformes) were a speciose group of massive graviportal avian herbivores that dominated the New Zealand (NZ) ecosystem until their extinction ≈600 years ago. The phylogeny and evolutionary history of this morphologically diverse order has remained controversial since their initial description in 1839. We synthesize mitochondrial phylogenetic information from 263 subfossil moa specimens from across NZ with morphological, ecological, and new geological data to create the first comprehensive phylogeny, taxonomy, and evolutionary timeframe for all of the species of an extinct order. We also present an important new geological/paleogeographical model of late Cenozoic NZ, which suggests that terrestrial biota on the North and South Island landmasses were isolated for most of the past 20–30 Ma. The data reveal that the patterns of genetic diversity within and between different moa clades reflect a complex history following a major marine transgression in the Oligocene, affected by marine barriers, tectonic activity, and glacial cycles. Surprisingly, the remarkable morphological radiation of moa appears to have occurred much more recently than previous early Miocene (ca. 15 Ma) estimates, and was coincident with the accelerated uplift of the Southern Alps just ca. 5–8.5 Ma. Together with recent fossil evidence, these data suggest that the recent evolutionary history of nearly all of the iconic NZ terrestrial biota occurred principally on just the South Island.

Dynamics of Pacific plate crust in the South Island (New Zealand) zone of oblique continent-continent convergence

Here we analyze topographic and fission track data to quantify the response of the surface of the Pacific plate in South Island, New Zealand, to late Cenozoic oblique continental convergence across the Alpine fault. Over the central 350 km length of the Southern Alps mountain chain we derive and map the rates of mean surface uplift, the rates of working associated with mean surface uplift during the late Cenozoic mountain building, the amounts and rates of denudation and consequent isostatic rebound, and the tectonic component of rock uplift. The rate of mean surface uplift ranges from 0.3 mm/yr over most of the area east of the Main Divide. The highest rates of mean surface uplift occur to the southeast of the regions of highest mean elevation and relief. The rate of working against gravity during uplift of the mean surface ranges from ∼2.5 mW m−2 in the southwest to ∼10 mW m−2in the central eastern parts of the Alps. Areas of lower mean elevation uplifted most recently have received rates of energy input similar to that of areas of higher mean elevation where uplift started earlier. The amount of denudation is large compared with the mean surface uplift and ranges from ∼18 km adjacent to the Alpine fault to ∼2 km along the southeast margin of the Southern Alps. The rate of denudation ranges from ∼2.5 to ∼0.5 mm/yr with increasing distance from the Alpine fault across the Alps to the southeast. The amount of isostatic uplift ranges from a maximum of 14 km adjacent to the Alpine fault to ∼2 km along the southeast margin of the Alps. The tectonic component of uplift varies from ∼4 km along the Alpine fault to ∼1 km along the eastern margin of the Alps.

Integration of zircon color and zircon fission-track zonation patterns in orogenic belts: application to the Southern Alps, New Zealand

An exhumed crustal section of the Mesozoic Torlesse terrane underlies the Southern Alps collision zone in New Zealand. Since the Late Miocene, oblique horizontal shortening has formed the northeastern–southwestern trending orogen and exhumed the crustal section within it. On the eastern side, rocks are zeolite- to prehnite–pumpellyite-grade greywacke; on the western side rocks, they have the same protolith, but are greenschist to amphibolite facies of the Alpine Schist. Zircon crystals from sediments in east-flowing rivers (hinterland) have pre-orogenic fission-track ages (>80 Ma) and are dominated by pink, radiation-damaged grains (up to 60%). These zircons are derived from the upper f10 km crustal section (unreset FT color zone) that includes the Late Cenozoic zircon partial annealing zone; both fission tracks and color remain intact and unaffected by orogenesis. Many zircon crystals from sediments in west-flowing rivers (foreland) have synorogenic FTages, and about 80% are colorless due to thermal annealing. They have been derived from rocks that originally lay in the reset FTcolor zone and the underlying reset FT colorless zone. The reset FT color zone occurs between f250 and 400 jC. In this zone, zircon crystals have color but reset FT ages that reflect the timing of orogenesis. D 2002 Elsevier Science B.V. All rights reserved.

Shelf to Basin, Temperate Skeletal Carbonate Sediments, Three Kings Plateau, New Zealand

ABSTRACT Three Kings Plateau is 10,000 km2 in area, less than 500 m deep, and includes the mainland shelf and submarine ridges and banks centered on Three Kings Islands (34°S) off northernmost New Zealand. The plateau receives little input of terrigenous sediment and lies in a zone of active nutrient upwelling and vigorous wind-wave and tidal current activity. As a consequence, surficial sediments on and about the margins of the plateau are dominantly pure skeletal carbonate sands and gravels, which lack mud to depths of 1500 to 3000 m. Plateau-top sediments include a bryozoan (-calcareous red algal-barnacle) lithofacies on coarse and rocky substrates, and a bivalve-bryozoan lithofacies associated with particulate bottoms. Plateau-edge sediments include a bryozoan-ahermatypic co al (-serpulid) lithofacies in areas of rugged topography, and a bivalve-gastropod lithofacies on sandy substrates, both grading down-slope into a planktic foraminiferal lithofacies by 1500 to 3000 m depth, depending on locality. The carbonates are dominated by a high-Mg calcite and/or low-Mg calcite mineralogy, with only subordinate aragonite. Active skeletal production is presently restricted to only about 10 percent of the plateau area, principally in regions of rough sea-floor topography, on submarine cliff faces, on bank tops, and in in-shore portions of the mainland shelf. About these areas, modern carbonates occur down to depths of 50 to 200 m, below which the sediments are mainly relict and include rare glauconite. Redeposition of plateau-derived skeletal carbonates into surrounding basins was especially active during the last-glacial low sea level(s). It is inferred that this is the ultimate fate of the bulk of the shallow-marine sediment and that the plateau-top itself preserves only a thin, laterally discontinuous, mixed-age and highly condensed sequence of Quaternary temperate skeletal carbonates.