Tim Stern

Asymmetric back-arc spreading, heat flux and structure associated with the Central Volcanic Region of New Zealand

The Central Volcanic Region of New Zealand is an active back-arc basin developed within continental lithosphere, and therefore offers a rare opportunity to study back-arc extension from land-based observations. Two parameters related to the heat output from the Central Volcanic Region are of particular interest. Firstly, the average heat flow for the eastern half of the Central Volcanic Region is about 800 mW/m2—in order to maintain this heat flow over geological time periods an efficient mass-transfer of heat is required. Secondly, the observed asymmetry in the pattern of heat output, coupled with the tectonic erosion of blocks of continental crust from the eastern axial ranges into the Central Volcanic Region, suggests that the process currently in progress at the eastern margin of the Region is asymmetric spreading with concomitant thermal differentiation of continental crust into its silicic and basic components.

Uplift of the Transantarctic Mountains and the bedrock beneath the East Antarctic ice sheet

In recent years the Transantarctic Mountains (TAM), the largest noncontractional mountain belt in the world, have become the focus of modelers who explained their uplift by a variety of isostatic and thermal mechanisms. A problem with these models is a lack of available data to compare with model predictions. We report here the results of a 312-km-long geophysical traverse conducted in 1993/1994 in the hinterland of the TAM. Using detailed subglacial topography and gravity measurements, we confirm the origin of the TAM as a flexural uplift of the edge of East Antarctica. Using an elastic model with a free edge, we can jointly fit the topography and the gravity with a plate having an elastic thickness of 85±15 km and a preuplift elevation of 700±50 m for East Antarctica. Using a variety of evidence, we argue that the uplift is coincident with a relatively minor tectonic event of transtensional motion between East and West Antarctica during the Eocene rather than the Late Cretaceous rifting event that created the Ross Embayment. We suggest that this transtensional motion caused the continuous plate to break, which created an escarpment that significantly increased the rates of erosion and exhumation. Results from the geophysical traverse also extend our knowledge of the bedrock geology from the exposures within the TAM to the ice covered interior. Our interpretation suggests that the Ferrar flood basalts extend at least 100 km westward under the ice. The Beacon Supergroup of Paleozoic and Mesozoic sediments thins gradually under the ice and its reconstructed thickness is reminiscent of profiles of foreland basins. Finally, there is no indication in the gravity field for an incomplete rebound due to significant melting of the East Antarctic ice sheet since the last glacial period.

Do Great Earthquakes Occur on the Alpine Fault in Central South Island, New Zealand?

Geological observations require that episodic slip on the Alpine fault averages to a long-term displacement rate of 2-3 cm/yr. Patterns of seismicity and geodetic strain suggest the fault is locked above a depth of 6-12 km and will probably fail during an earthquake. High pore-fluid pressures in the deeper fault zone are inferred from low seismic P-wave velocity and high electrical conductivity in central South Island, and may limit the seismogenic zone east of the Alpine fault to depths as shallow as 6 km. A simplified dynamic rupture model suggests an episode of aseismic slip at depth may not inhibit later propagation of a fully developed earthquake rupture. Although it is difficult to resolve surface displacement during an ancient earthquake from displacements that occurred in the months and years that immediately surround the event, sufficient data exist to evaluate the extent of the last three Alpine fault ruptures: the 1717 AD event is inferred to have ruptured a 300-500 km length of fault; the 1620 AD event ruptured 200-300 km; and the 1430 AD event ruptured 350-600 km. The geologically estimated moment magnitudes are 7.9 ± 0.3, 7.6 ± 0.3, and 7.9 ± 0.4, respectively. We conclude that large earthquakes (Mw >7) on the Alpine fault will almost certainly occur in future, and it is realistic to expect some great earthquakes (Mw ≥8).

Preliminary results from a geophysical study across a modern continent-continent collisional plate boundary - the Southern Alps, New Zealand

Abstract The Southern Alps of South Island, New Zealand, is a young transpressive continental orogen exhibiting high uplift rates and rapid transcurrent movement. A joint US-NZ geophysical study of this orogen was carried out in late 1995 and early 1996 to derive a three-dimensional model of the deformation. The measurements undertaken include active source and passive seismology, magnetotelluric and electrical studies, and petrophysics. Preliminary models for the active source seismic measurements across South Island confirm, in general terms, a thickened crust under the Southern Alps, a high-velocity lower crustal layer, and a major crustal discontinuity associated with the Alpine fault. The anisotropy in physical properties of the rocks of the plate boundary zone is clearly demonstrated in the preliminary results of laboratory seismic velocity measurements, shear wave splitting and resistivity. The mid-crust under the Southern Alps coincides with a major electrical conductivity high, which possibly corresponds to fluid in the crust. The top lies at about 15 km, close to the base of shallow seismicity east of the Alpine fault. Offshore the marine reflection data have consistently imaged a reflective lower crust adjacent to South Island. These data are showing complex structure, particularly off western and southeastern South Island. The complexity in structure, high-quality data and consistency in results from several techniques indicates that the South Island experiment will contribute significantly to our knowledge of transpressive plate boundaries in particular, and the continental lithosphere in general.

Teleseismic P wave delays and modes of shortening the mantle lithosphere beneath South Island, New Zealand

A high-speed zone in the mantle directly beneath the Southern Alps of New Zealand is required by the recorded pattern of teleseismic P waves. Two parallel lines of 80 seismographs spaced at ∼2 km intervals recorded three earthquakes from the western Pacific with epicentral distances of 52°, 53° and 78°. Azimuthal bearings were all within 15 degrees of the mean trends of the seismograph lines. Differences between measured delays and those predicted from the crustal structure reach 0.8 s along one line and 1.0 s along the other, with the rays for the earliest arriving signals passing the depth of ∼120 km beneath the center of the island. Assuming these early arrivals are due to structure within the mantle shallower than 200 km, they imply that the core of the high-speed zone lies beneath the thickest crust, which has been shortened by ∼100 km of convergence during the past 6–7 Myr. Although the shape and position of the high-speed body cannot be fixed uniquely, a roughly symmetric body centered about a depth of 120 km, 80–100 km wide, with a depth extent of 100 km and with a maximum speed advance of ∼7% satisfies the observations. The pattern of residuals does not fit with those predicted by simple models of intracontinental subduction in which crust and mantle lithosphere are detached and one slab of mantle lithosphere underthrusts the other. Rather, the residuals favor thickening of mantle lithosphere by a more homogenous straining of it, as if mantle lithosphere beneath continental crust behaved as a continuum. An excess mass in the mantle is also required by the observed gravity anomalies, once allowance is made for the seismically determined crustal thickness. This high-density mantle anomaly provides sufficient force (per unit length) to maintain the crustal root, which is approximately twice as thick as that necessary to support the topography.

Low seismic-wave speeds and enhanced fluid pressure beneath the Southern Alps of New Zealand

A region of low seismic-wave speed is detected beneath the central Southern Alps of New Zealand on the basis of traveltime delays for both wide-angle reflections and P-waves from teleseismic events. Respective ray paths for these P-waves are mutually perpendicular, ruling out anisotropy as a cause of the delays. The low-speed region measures about 25 km by 40 km, has a speed reduction of 6%–10%, and is largely above the downward projection of the Alpine fault. The most likely cause of the low-speed zone is high fluid pressure due to excess water being released by prograde and strain-induced metamorphism into the lower crust. Because enhanced fluid pressure reduces the work required for deformation, the existence of the central Southern Alps low-speed zone implies that this part of the Australian-Pacific plate boundary is relatively weak.

Rift flank uplifts and Hinterland Basins: Comparison of the Transantarctic Mountains with the Great Escarpment of southern Africa

Uplifted rift margins are a common feature of continents and oceans. Two variants of rift flank morphologies have been recognized: One in which the topography warps down from an inland high toward the continental margin, and one where the tropographic peak lies close to the continental margin. The Great Escarpment of southern Africa and the Transantarctic Mountains are examples of the first and the second variants of rift flanks, respectively. Both rift flanks are bordered on their landward side by broad continental basins: the Kalahari and the Wilkes hinterland basins. If these basins are interpreted as flexural “outer lows” that deepen in unison with the uplift of the rift flanks, the lithosphere on the uplifted side is very rigid in both cases (elastic thickness Te of 100 ± 20 km for southern Africa and 110 ± 20 km for East Antarctica). We suggest that the variation in rift flank morphology is caused by the isostatic response to uplift forces of elastic plates sharing different boundary conditions. We model the uplift of the Transantarctic Mountains as an upward deflection of an elastic plate which is broken at the front of the Transantarctic Mountains, and we model the uplift of the Great Escarpment as an upward deflection of a continuous elastic plate that is modified by the downward load of sediments on the continental margin. Although the Transantarctic Mountain uplift is young (60–0 Ma) and the southern African uplift is old (<100 Ma), the different isostatic responses of the two margins are not a function of age, because most loading (sedimentation) and unloading (erosion) took place shortly after rifting. Detailed modeling of topography, gravity, geological markers, and the locations of depocenters suggests that lithospheric rigidity decreases under the Transantarctic Mountains, whereas in southern Africa the decrease occurs not under the Great Escarpment but far seaward under the continental shelf and slope. If the distribution of lithospheric rigidity is indicative of the thermal regime of the lithosphere, then uplifted rift flanks are not always underlain by a thermal anomaly. This and other geological evidence indicate that a single mechanism cannot explain the uplift of both the Antarctic and the African margins.

Isostatic rebound due to glacial erosion within the Transantarctic Mountains

In temperate climates, ∼25% of peak elevations in mountain ranges can be created by isostatic rebound as a response to erosional incision. Significantly more relief generation and peak uplift are, however, possible for glacial erosion in a polar climate. We incorporate regional isostasy using flexure of an elastic plate to show that isostatic rebound as a response to glacial incision can account for as much as 2000 m or 50% of peak elevation in the central Transantarctic Mountains. Differences in relief of at least 5500 m over lateral distances of just 40 km are evident within the central part of the 3000-km-long mountain range. Such strong relief is possible because a polar climate since the middle Miocene has resulted in freezing conditions at high elevations, which acted to preserve the peaks, whereas wet-based glaciers at low elevations have produced optimal conditions for enhanced glacial incision. Because isostatic rebound results in permanent peak uplift, this mechanism provides an explanation of why the Transantarctic Mountains are one of the higher and more long-lived continental rift margins on Earth.

Subduction and back-arc activity at the Hikurangi convergent Margin, New Zealand

The Hikurangi Margin is a region of oblique subduction with northwest-dipping intermediate depth seismicity extending southwest from the Kermadec system to about 42°S. The current episode of subduction is at least 16–20 Ma old. The plate convergence rate varies along the margin from about 60 mm/a at the south end of the Kermadec Trench to about 45 mm/a at 42°S. The age of the Pacific lithosphere adjacent to the Hikurangi Trench is not known.The margin divides at about latitude 39°S into two quite dissimilar parts. The northern part has experienced andesitic volcanism for about 18 Ma, and back-arc extension in the last 4 Ma that has produced a back-arc basin onshore with high heaflow, thin crust and low upper-mantle seismic velocities. The extension appears to have arisen from a seawards migration of the Hikurangi Trench north of 39°S. Here the plate interface is thought to be currently uncoupled, as geodetic data indicate extension of the fore-arc basin, and historic earthquakes have not exceededMs=7.South of 39°S there is no volcanism and a back-arc basin has been produced by downward flexure of the lithosphere due to strong coupling with the subducting plate. Heatflow in the basin is normal. Evidence for strong coupling comes from historic earthquakes of up to aboutMs=8 and high rates of uplift on the southeast coast of the North Island.The reason for this division of the margin is not known but may be related to an inferred increase, from northeast to southwest, in the buoyancy of the Pacific lithosphere.

Crustal anisotropy in the vicinity of the Alpine Fault Zone, South Island, New Zealand

Abstract Petrophysical measurements of rock samples collected within the Haast, Torlesse, and Alpine Fault Zone terranes of the South Island of New Zealand indicate significant seismic P‐wave velocity anisotropy at pressures representing depths of up to 30 km. The percentage of anisotropy increases with increasing metamorphic grade and thus decreases with structural distance from the Alpine Fault. A maximum anisotropy of 17.3% was obtained from a drill‐core sample located within the garnet‐oligoclase zone schist, immediately adjacent to the Alpine Fault. Shear‐wave splitting is another important property of the schists. For propagation parallel to foliation, split shear waves show velocity differences up to 1 km/s. At elevated pressures, the measured seismic velocity anisotropy is caused by preferred mineral orientation and is not due to the presence of cracks. The pronounced velocity anisotropy will significantly affect propagating seismic waves collected during both natural and active source seismic exp...