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| Titel |
The Role of Mass Wasting in the Post-LGM Evolution of Milford Sound, Fiordland, New Zealand |
| VerfasserIn |
Jesse Dykstra, Timothy Davies, Stefan Winkler |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038354
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| Zusammenfassung |
| The geomorphic and sedimentary evolution of New Zealand’s fiords have been influenced by
their unique setting. Fiordland lies in a temperate marine climate zone, and is bounded to the
west by an active transform plate boundary, where the Indo-Australian Plate collides with the
Pacific Plate. The seismicity of Fiordland is dominated by the plate-boundary Alpine Fault,
which runs immediately offshore of the popular tourist destination of Milford Sound; it has
ruptured at least four times in the past 1000 years (the last time around 1717 A.D.),
producing earthquakes of about magnitude 8. The probability of an earthquake of similar
magnitude occurring along the Alpine Fault within the next 50 years is estimated at
65±15%.
Fordland’s active tectonic setting also gives rise to a very wet climate. The extreme
topography of the Fiordland mountains forces up the prevailing westerly winds, resulting in
mean annual precipitation of nearly 7 m at Milford Sound. During the Last Glacial Maximum
(LGM), these unique climatic and tectonic conditions were favourable for the advance of
tidewater glaciers, which deposited material in large fans at the edge of the narrow
continental shelf.
During the LGM, New Zealand was modestly glaciated, consequently the signature of global
eustatic sea level change overrides any isostatic signature. Fiordland glaciers likely retreated
very quickly (starting approximately 17,000 years ago), while global sea levels were still
much lower than present day. Freshwater proglacial lakes would have occupied the basins
during the early stages of glacial retreat, as marine transgression was blocked by entrance
sills. Rapid retreat, and ultimately disappearance of valley glaciers would have
resulted in a drastic reduction in sediment production and transport. Finally, eustatic
sea level rise resulted in marine transgression, with freshwater lakes becoming
estuaries.
This proposed model for fiord evolution in south-western New Zealand is well
supported by recent seismic reflection and high-resolution sonar data. In fiords south of
Milford Sound, laminated post-glacial lacustrine and marine sediments overly massive
deposits of glacial till and landslide debris. However, our interpretation of the data
for Milford Sound suggests that the majority of post-glacial sediment infill has
been contributed by mass wasting. High-resolution multi-beam sonar data from
Milford Sound clearly shows the presence of large rock avalanche deposits, which
blanket much of the fiord bottom. Early interpretation of seismic data suggests that
post-glacial sediment infill is likely dominated by massive deposits of rock avalanche
debris.
The apparent paucity of laminated post-glacial lacustrine or marine sediments at Milford
Sound is in marked contrast to the proposed evolutionary model for New Zealand fiords. Our
presentation will cover the unique evolutionary history of Milford Sound, and suggest
implications for natural hazards and risk management. |
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