Southern Alps Geophysics


The crustal structure of the Southern Alps orogeny is primarily determined by the use of geopysics. The most useful geophysical methods currently used in the Southern Alps to determine crustal structure are seismology and gravity measurements. Both refraction and reflection seismology have been done, a limited amount of earthquake seismology has also been studied in the area.



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Seismicity

N.Z. Seismicity

Seismicity:   Shown to the right is a 10 year deep seismicity map for the South Island of New Zealand. From this map you can see that earthquakes in the northern part of the South Island get deeper to the northwest. This would seem to indicate that the Pacific Plate is subducting beneath the Australian Plate there. However, in the southern part of the South Island the earthquakes get deeper to the southeast, suggesting that here the Australian Plate is subducting beneath the Pacific Plate. The region in between is fairly aseismic, and is also where the Southern Alps are located.























Arthur's Pass Eq.

One recent earthquake that occured near Aurthur's Pass, within the Southern Alps, did show and oblique-slip character. Dislocation models done by Arnadottir et al., suggest that the motion in this quake was in a left lateral sense, rather than dominate right lateral motion seen on the Alpine Fault. Since the faults in the Arthur's Pass area are at an acute angle to the Alpine Fault trend, the left lateral slip motion is interpreted to accomodate crustal block rotation about a vertical axis.



Another interpretation for the left lateral sense of the Arthur's Pass earthquake is that oblique-slip deformation on the Alpine Fault can be taken up by a sort of mesh of smaller faults rather than crustal blocks bounded by other large faults.



















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Refraction Seismology

Refraction Survey
Location map of the 1995 refraction tomography line done by Smith et al. A new study is currently under way by several institutions and is known by the acronym SIGHT.


Refraction Results
Interpretations of the 1995 refraction line. The results of this survey show the Pacific oceanic lithosphere subducting beneath the Australian Plate, while the upper surface of the 7.2 km/s refractor is interpreted to be a detachment surface and the material associated with it is being thrust on top of the Australian Plate via the Alpine Fault.


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Seismology Summary

A summary of all of the available seismic data, including earthquake data, reflection seismology, and refraction seismology, shows the oceanic lithosphere of the Pacific Plate subducting beneath the Australian Plate, while the upper surface is thrust onto the Australian Plate. Earthquake data show the limb of the sudducting slab, while refraction data show crustal velocities, and reflection data show dipping reflectors thought to be associated with the Alpine Fault.
Seismology Summary


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Gravity

Gravity Map
Gravity surveys have also been done over the South Island of New Zealand to try and model crustal thicknesses, flexural and isostatic parameters, and other infomation. Shown above is the isostatic gravity anomaly map. Above the Southern Alps there exists a slight negative anomaly, about -25 mgals in magnitude, suggesting that the low density crust is being pulled down at the subduction zone and therefore creating a mass deficit at depth. This interpretation would account for the negative isostatic anomaly.


Profile A

Profiles A-A' and B-B'. These profiles were taken through the central South Island where the gravity anomalies are at their lowest values, these profiles are smooted and represent a gravity field at a height of 500m (Syms 1978). From these two profiles one can see the expected large negative Bouguer gravity anomaly associated with regions of high elevation as well as the negative isostatic anomaly. One thing to note however is that the integration of the isostatic anomaly is approximately zero, so there may be some level of regional isostatic equilibrium.

Profile B


Crustal and Flexural Implications Based on Gravity

Flexural Model

Flexural parameters as well as crustal structures can be estimated based on the observed gravity anomalies. Stern found that the best fit to the observed Bouguer gravity anomaly was obtained with an elastic thickness of ~10 km for the subducting slab. The best fitting curve takes into account the gravity effects of the subducted mantle, the deflection due to surface topography, the infill load, and the subduction load. Whereas the solid line represents the effect of the subducting slab only, and the smaller dashed line only takes into account the deflection due to surface topography, the subduction load and the effect of the subducted mantle (Stern 1995).






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