50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

REFLECTIONS ON DIP SLOPES AND
LANDSLIDES IN NORTH-EAST
MARLBOROUGH AND THE SOUTHERN
ALPS
Warwick M. Prebble
Dept. of Geology, The University of Auckland,
Private Bag 92019, Auckland.
(w.prebble*auckland.ac.nz)
Structure, tectonics and geomorphic expression are
themes which run through the last 120 years of
geologic exploration of the limestone dip slopes of
north-east Marlborough. A complementary
development is the gravity faulting model of
topographic adjustment, which is based upon
greywacke Mountains in Marlborough and the
Southern Alps. This model has been perpetuated in
landslide classifications and the geotechnical
literature.
The magnificent dip slopes and limestone gorges of
the Chalk Range and Ben More anticline evoked
eloquent and passionate comments from Alexander
McKay in 1885 and Charles Cotton 70 years later.
In the same year (1955) Harold Wellman
recognised the Nidd Landslide in limestone and
flint near Whernside ridge, parallel to the Chalk
Range. Since then, closer examination of the dip
slopes has revealed signs of widespread instability
such as fracture dilation, ravines, debris flows and
screes.
Geotechnical mapping indicates that flexural
toppling affects 30 of 34 discrete slope segments,
the remainder being rock slides. Each failure is
approximately 1 km 2 . Toppling is found down to
70m deep in the cataclinal under-dip slopes in
which the bedding and dominant fractures dip in
the same direction as the slope but more steeply and
in excess of 25 0 , the apparent friction angle. Where
the bedding and fractures dip less than 25 0 there is
an abrupt transition to parallel dip slopes, which
develop rock slides on bedding parallel crush
zones. Slope failure is absent where the limestone is
not closely fractured and does not contain crush
zones. Recent investigations show that the original
model of uniform toppling proposed for the
cataclinal slopes is a simplification. Zones of
chaotic slope debris intercalated with coherent but
disrupted rock mass suggest partial slope collapse
in tandem with toppling. Non-uniform toppling
across a slope, sideways twisting of toppled masses
and sector collapse of topple complexes are features
of current slope failure models.
Further south in greywacke and schist some topple
complexes are up to 20 km 2 in area and 2 km 3 in
volume. Reverse scarps, ridge top depressions and
ridge rents are linked to massive toppling of slopes,
in some cases back to back. Multiple cracks, a few
m wide, several tens of m long and at least a few
tens of m deep testify to active spreading along the
crests of ridges. The original gravity faulting
mechanism of topographic adjustment proposed by
Alan Beck in 1968 remains a current model in
internationally accepted slope failure classification.
Toppling may now be considered as an alternative
mechanism and is arguably a better fit of the
topographic features and structural details seen in
these slopes. Toppling can also be seen as a
mechanism for developing bending surfaces, which
release rock slides, rock avalanches and debris
flows. These slope failures feed copious quantities
of sediment into streams and rivers. They start a
chain of events, which transfers material from the
mountains to the abyss.
ORAL
DECONVOLUTING MIXING,
FRACTIONATION AND ASSIMILATION
PROCESSES IN ANDESITES FROM
RUAPEHU VOLCANO, NEW ZEALAND
Richard Price 1 , John Gamble 2 ,
Rhiannon George 3 ,IanSmith 4 & Craig Cook 5
1 School of Science and Engineering, University of
Waikato, Private Bag 3105, Hamilton.
2 Department of Geology, National University
Ireland, University College Cork, Ireland.
3 GEMOC Key Centre, Macquarie University, North
Ryde, Sydney, Australia.
4 Department of Geology, University of Auckland,
Auckland.
5 Department of Earth Sciences, University of
Waikato, Hamilton.
(r.price*waikato.ac.nz)
Andesites are now widely recognised as mixtures of
crystals and melts that have experienced complex
polybaric, polythermal events en route to their sites
of eruption. Typical Ruapehu andesites are charged
with a cargo of crystals, xenocrysts and xenoliths
ranging in size from cm-scale to sub-microscopic,
indicating that to varying extents the melts have
interacted with and scavenged their surroundings
and sources prior to eruption. Complex zoning
patterns and melt and crystalline inclusions in the
phenocryst/xenocryst populations attest to this
complexity.
Magma batches from relatively short-lived events
(months /years) can show variation that can
replicate almost the complete range of
compositions erupted from Ruapehu over much
longer (1-10 ka) time spans. High precision trace
element and Pb-Sr-Nd isotopic information and
new U-Th-Ra disequilibrium data show that it is
possible to distinguish between Fractional
Crystallisation (FC) and Assimilation with
Fractional Crystallisation (AFC) processes in small
melt batches. This has important and somewhat
unexpected consequences for the interpretation of
50 th Kaikoura05 -70- Kaikoura 2005