Quantifying how hillslopes respond to river incision and climate change is fundamental
to understanding the geomorphic evolution of tectonically uplifting landscapes during
glacial-interglacial cycles. Hillslope adjustment in the form of deep-seated bedrock
landslides can account for a large proportion of the regional sediment yield and
denudation rates for rapidly uplifting landscapes. However, the timing and magnitude of
the response of hillslopes to climatic and tectonic forcing in moderate uplift temperate
maritime catchments characteristic of many active margins worldwide is not well
quantified. This study seeks to investigate how hillslopes respond to climate-modulated
river incision and to quantify the magnitude of the sediment flux from this response in a
typical active margin setting.
The non-glacialWaipaoa Sedimentary System (WSS) on the East Coast of the North
Island of New Zealand consists of river catchments, coastal foothills to uplifting
mountain ranges, and terrestrial and marine sediment depocentres collectively underlain
by relatively young (Cretaceous and younger) sedimentary rocks within a tectonically
active setting and temperate maritime climate. These attributes make theWSS similar to
many coastal catchments on oceanic-continental convergent margins worldwide.
However, because of widespread destruction of primary forests for conversion to pasture
lands by the mid 20th Century, theWSS is currently a globally significant source of
sediment to the world’s oceans. Because of these factors, theWSS was selected as one of
two global study sites for the international, NSF supported, MARGINS Source-to-Sink
initiative designed to investigate the transfer of sediment from terrestrial source to
marine sink. Previous studies on theWSS have shown a strong link between climate
change and geomorphic response in the system. River incision since the last glacial
coldest period has generated a significant amount of topography, leaving small remnants
of the ca.18,000 cal. yr BP last glacial aggradation terrace scattered up to 120 m above
modern rivers.
In this study, the hillslope response to river incision is quantitatively examined using new
high resolution topographic data sets (lidar and photogrammetry) in combination with
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field mapping and tephrochronology. Hillslopes are found to be coupled to river incision
and adjusted to rapid incision through the initiation and reactivation of deep-seated
landslides. In the erodible marine sedimentary rocks of the terrestrialWSS, post-incision
deep-seated landslides can occupy over 30% of the surface area. Many of these slides
show evidence of multiple “nested” failures and landslide reactivation. The ages of
tephra cover beds identified by electron microprobe analysis show that following an
initial 4,000 to 5,000 year time lag after the initiation of river incision, widespread
hillslope adjustment started between the deposition of the ca. 13,600 cal. yr BPWaiohau
tephra and the ca. 9,500 cal. yr BP Rotoma tephra. Tephrochronology and geomorphic
mapping analysis indicates that river incision and deep-seated landslide slope adjustment
is synchronous between mainstem rivers and headwater tributaries. Tephrochronology
further shows that many slopes have continued to adjust to channel incision into the late
Holocene. Hillslope response in the catchment can involve the entire hillslope from river
to ridgeline, with some interfluves between incising sub-catchments being dramatically
modified through ridgeline retreat and/or lowering. Using the results of the landform
tephrochronology and geomorphic mapping, a conceptual time series of hillslope
response to uplift and climate change-induced river incision is derived for a timeframe
encompassing the last glacial-interglacial cycle.
Using the same high resolution topography datasets, in-depth field analysis, and
tephrochronology, the 18,000 year sediment yield from terrestrial deep-seated landslides
in theWSS is estimated in order to investigate the magnitude of hillslope response to
climate-modulated, uplift driven river incision. This completes one of the first processbased
millennial time-scale sediment budgets for this class of temperate maritime, active
margin catchments. Fluvial and geomorphic modelling is applied to reconstruct pre
18,000 cal. yr BP topography in 141 km2 of detailed study area and the resulting
volumetric estimates from 207 landslides are used to estimate deep-seated landslide
sediment flux for the broader system. An estimated 10.2 km3 of deep-seated landslidederived
sediment with a multiplicative uncertainty of 1.9 km3 (+9.2 km3, -4.8 km3) was
delivered to terrestrial and marine sinks. This accounts for between 10 and 74% of the
total mass of the terrestrialWSS budget of ca. 91,000 Mt (+37,000 Mt, -26,000 Mt).
Combining the deep-seated landslide results with other studies of terrestrial sediment
sources and terrestrial and shelf sinks, the estimated terrestrial source load ranges from
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Abstract
1.2 to 3.7 times larger than the mass of sediment sequestered in terrestrial and shelf
depocentres. This implies that off-shelf transport of sediment is important in this system
over the last 18,000 cal. yr BP, as it is today for anthropogenic reasons. Based on the
derived sediment budget, the denudation rate for the terrestrialWSS of 0.8 mm yr-1 (+0.3
mm yr-1, -0.2 mm yr-1) is indistinguishable from the average terrestrialWaipaoa late
Quaternary uplift rate, indicating an approximate steady-state balance between
denudation and uplift. This thesis provides a quantitative analysis of the role of deepseated
landslides in an active margin catchment that is used to improve the
understanding of landscape and terrestrial source-to-marine-sink sediment transfer
dynamics.
| Identifer | oai:union.ndltd.org:canterbury.ac.nz/oai:ir.canterbury.ac.nz:10092/7439 |
| Date | January 2012 |
| Creators | Bilderback, Eric Leland |
| Publisher | University of Canterbury. Geological Sciences |
| Source Sets | University of Canterbury |
| Language | English |
| Detected Language | English |
| Type | Electronic thesis or dissertation, Text |
| Rights | Copyright Eric Leland Bilderback, http://library.canterbury.ac.nz/thesis/etheses_copyright.shtml |
| Relation | NZCU |
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