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TERRANE
ACCRETION IN THE ANTARCTIC
Ellsworth Mountains, Antarctica
Randall
The Ellsworth Mountains have a succession of rocks
more comparable to those exposed in South Africa than in Antarctica.
This has led to the suggestion that the Ellsworth Mountains were transported
and accreted onto East Antarctic during the break-up of Gondwana approximately
180 million years ago. Palaeomagnetic studies are being used to try
and unravel the plate tectonic movement history of the Ellsworth Mountains
crustal block.
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THE
COLDEST IGNIMBRITES IN THE WORLD ?
Taupo Volcanic Zone, New Zealand and Santorini,
Greece.
McClelland and Wilson (IGNS, New Zealand)
Ignimbrites are the products of the largest explosive
eruptions, which must be the most cataclysmic of all geological phenomena.
Only a few, small eruptions have been witnessed since the start of scientific
observation, and the mechanism of emplacement of these important phenomena
remains enigmatic. Ignimbrites consist predominantly of primary pumice
that would have been erupted at 800C or above, but palaeomagnetic estimates
give emplacement temperatures of the Oruinui (NZ) and Santorini Minoan
ignimbrites of 250 C or less. We are investigating the mechanisms
by which heat is lost in order to determine eruption mechanisms.
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INCIDENCE
OF HAZARDOUS PYROCLASTIC ERRUPTIONS FROM STRATOVOLCANOES
Ascension Island, South Atlantic and Ruapehu, New
Zealand.
Hobson, Bardot, McClelland, Bell, Stokes and Briden.
This project aims to significantly enhance our ability
to assess volcanic hazard around stratovolcanoes by determining 1. the
proportion of the debris deposits that surround typical stratovolcanoes
which are formed of hot primary pyroclastics rather than detritus reworked
from the slopes of the volcano; 2. rates of accumulation of pyroclastic
deposits by thermoluminescence dating and field mapping.
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SELF-REVERSAL
PHENOMENA IN LAVA FLOWS
Lascar Volcano, Chile
McClelland & Thomas
Some particular lava chemistries have a special
property: the magnetization acquired is OPPOSITE in direction to the that
of the Earth's magnetic field in which the lava cooled. We are investigating
magnetic properties of partially self-reversing lavas; particularly the
dependence on cooling rate and implications for such material providing
records which falsely imply rapid Earth's field reversal.
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THE
OPENING OF THE SOUTH ATLANTIC
Batoka Gorge, Zimbabwe
Briden, Randall, & Jones (Harare)
Exposed in Batoka Gorge are rocks of the Karoo Volcanic
Province. By measuring palaeomagnetic poles from these rocks and
comparing them to those obtained from similar age rocks in South America
we can increase our knowledge of the timing that the South Atlantic opened,
and the supercontinent of Gondwana broke up.
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CRUSTAL
ROTATIONS IN THE ANDES
Atacama Desert, Chile.
Randall & Taylor (Plymouth)
Since at least the Triassic the Pacific plate has
been subducting beneath the South American plate. This has resulted
in the development of large faults in the South American plate. Movement
on these faults has caused small crustal blocks to rotate. By comparing
the palaeomagnetic poles from these small blocks to those from unrotated
areas the amount of rotation can be measured.
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MAGNETOSTRATIGRAPHIC
STUDY OF RATES OF ENVIRONMENTAL CHANGE IN THE MEDITERRANEAN DURING THE
MESSINIAN SALINITY CRISIS
Sicily
McClelland & Butler (Leeds)
The Messinian 'salinity crisis' marked a dramatic
climate change in the Mediterranean region. Many authors have considered
this to have been a very rapid and catastrophic event. We have magnetostratigraphic
data from five sections from the Neogene Caltanissetta basin, Sicily, which
contain the early Messinian Tripoli formation and the transition to overlying
Calcare di Base shallow water carbonates that mark the start of evaporite
deposition. Our correlations demonstrate considerable diachroneity in onset
of evaporite formation over the studied region. Evaporites were deposited
first at about 6.8 Ma in the highest perched basins in the north of the
region, and the progressive fall in sea level took about 0.6 Ma to traverse
the palaeoslope in the ancient Caltanissetta basin. Furthermore, published
Sr isotope data places the evaporites from the deep Mediterranean at about
5.5 Ma, suggesting that the whole period of draw-down and evaporation of
the Mediterranean basin took up to 1.5 Ma, and was not a catastrophic,
rapid environmental change.
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CHRONOSTRATIGRAPHY
OF MARINE REGRESSIONS
Sicily
McClelland and Butler (Leeds)
Uplift of the Caltanisetta Basin, Sicily to its
present position started once the lithospheric load of the mountain belt
to the north decreased as the orogenic wedge began to collapse. Plio-Pleistocene
beach sands document the progression of the coastline southwards across
the basin, and we have dated this marine regression using magnetostratigraphy.
Our data show that emergence above sea-level started about 3 Ma ago, and
that the coastline migrated southwards across the 50 km basin over a period
of about 1.5 Ma, indicating an uplift rate of about 300m per Ma.
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CLIMATE
CHANGE AND PALEOSOL DEVELOPMENT: AND ENVIRONMENTAL-MAGNETIC STUDY OF SANTORINI
Santorini, Greece.
McCabe, McClelland, Briden, & Stokes.
This project aims to investigate the climatic effects
on soil production in intervals between the 10 major well-dated eruptions
which have occurred during the last 100,000 yrs covering the Northern European
Ipswichian interglacial, the Devensian glacial and interstadial and the
final Flandrian Temperate stage.
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CONSTRAINTS
ON ERRUPTION MECHANISMS OF PYROCLASTIC DEPOSITS DEDUCED FROM PALAEOMAGNETIC
ESTIMATES OF EMPLACEMENT TEMPERATURE
Santorini, Greece.
Bardot & McClelland
The volcano of Santorini in the Aegean has experienced
at least 10 cataclysmic eruptions over the past 200,000 yrs. We have determined
emplacement temperatures for many of these tephra from plinian airfalls,
lag breccias, ignimbrites and phreatomagmatic deposits. We have found that
airfall deposits are considerably hotter than predicted, implying that
they are accompanied by clouds of hot incandescent gas, and so are considerably
more dangerous than had been thought. Work on breccia deposits has constrained
the range of possible mechanisms by which caldera collapse may occur.
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PALAEOMAGNETIC
CONSTRAINTS ON THE DEVELOPMENT OF THE CALEDONIAN-APPALACHIAN MOUNTAIN BELT
Norway, Scotland, Ireland, Newfoundland, New Brunswick
& Maine
Mac Niocaill, Briden, Dewey, McKerrow (all at Oxford)
Grenne, Smethurst & Torsvik (Geological Survey
of Norway), Harper & Ryan (Galway), Parkes (Dublin), Van Staal (Geological
Survey of Canada), Williams (Memorial University, Newfoundland), Van der
Voo, van der Pluijm, McNamara & Weil (Michigan)
The Caledonian-Appalachian orogen is the perhaps
the best studied of the ancient orogens on the Earth and was the
proving ground for many of the predictions of the Plate Tectonic hypothesis.
Our current work focuses on charting the drift and tectonic histories of
the accreted oceanic terranes that were swept up in this mountain belt,
when North America collided with Scandinavia and parts of Europe over 400
million years ago. By charting this history we can test the various tectonic
models for the evolution of the orogen, and compare the development of
the orogen with modern-day analogues, such as the southwest Pacific.
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PROXY
RECORDS OF PLIO/PLEISTOCENE ENVIRONMENTAL CHANGE FROM MINERAL MAGNETISM
OF FLUVIO-LACUSTRINE SEDIMENT SEQUENCES IN THE NORTH CHINA PLAIN
North China Plain.
McClelland & Hu (Ox Geog)
Proxy records of Late Plio-Pleistocene climatic
change have been determined from fluvio-lacustrine sediments from 4 bore-cores
from the North China Plain, reflected by syn-depositional near-surface
modification of magnetic mineralogy. Ages have been determined by a palaeomagnetic
polarity time-scale obtained from one core. We have identified a major
climate change at about 2.6 Ma when hot dry conditions changed to a long
period of stable climatic conditions with a warmer and wetter environment
(consistent with the observations of more humid conditions with weaker
winter monsoons in north China in Late Pliocene to Early Pleistocene
times). At around 1 Ma, climate changed again to a highly variable system
with high-amplitude cyclic variation between colder and dryer intervals
and warm and wet conditions respectively.
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PALAEOMAGNETISM
OF CENOZOIC SEDIMENTS OF THE TIBETAN PLATEAU: IMPLICATIONS FOR RATES AND
TIMING OF UPLIFT AND CLIMATIC CHANGE
NE Tibet and western China
Mac Niocaill, Van der Voo (Michigan), Li & Fang
(Lanzhou University)
The uplift of Tibet has had a profound effect on
both the Asian and Global climate, with the development of the plateau
being responsible for the onset of the Asian monsoon, the formation of
the Asian dry core and the development of the Himalayas. Our project
focuses on the uplift history of the northeastern segment of the Tibetan
plateau, where we are using magnetostratigraphy to help date the formation
of river terraces, which, in turn, are the product of tectonic uplift,
river incision and climatic changes. Our current results show that
the uplift of the plateau was not uniform, with the uplift on the northeastern
margin being substantially younger than on the southern margin, a result
which has important implications for geodynamic models of the uplift history
and for climate modeling.
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EARLY
EOCENE CLIMATIC AND ENVIRONMENTAL CHANGE
Wyoming, Colorado
Hayes-Baker, Corfield (Oxford) & Norris (WHOI,
Boston)
A study of Eocene lacustrine sediments of the Green
River Formation of Wyoming and Colorado with the aim of determining environmental
and climatic change over the Early Eocene epoch of the continental U.S.
Interior. Palaeomagnetic analysis of the sediments is being used to construct
a magnetostratigraphy which constrains the timing of climatic and environmental
events and is then used to correlate our isotopic and magnetic susceptibility
measurements with the well-documented oceanic record.
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EARLY
CENOZOIC OCEANIC MAGNETOSTRATIGRAPHY
Bahamas
Bardot, Briden (Oxford), Ogg, Foster (Purdue) &
Members of the Ocean Drilling Program Leg 171b Scientific party
This project is investigating the Early Cenozoic
ocean stratigraphy and environmental changes during this time period through
a combination of isotope analysis and biostratigraphy. Our group is working
on magnetostratigraphic dating of the drill core in order to calibrate
the bio- and isotope stratigraphy.
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TRIASSIC-JURASSIC
MAGNETOSTRATIGRAPHY
England
Briden & Daniels
Palaeoenviromental studies rely on precise chronometers,
such as the facies significance of Milankovitch cycles and the worldwide
recognition of geomagnetic reversals, for worldwide correlations.
However, for pre-Jurassic times no marine magnetic anomalies exist and
thus the recognition of geomagnetic reversals relies on the records from
rocks now exposed on the continental landmasses. We are currently examining
the magnetostratigraphy of a sequence of sediments that straddle the Triassic-Jurassic
boundary with a view to precisely determining the reversal history, and
also to chronologically correlate the sequence with other known reversal
records around the world.
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LOW-TEMPERATURE
PROPERTIES OF MAGNETIC MINERALS
Muxworthy & McClelland
When an igneous rock containing magnetic minerals
cools in the presence of the Earth's magnetic field it acquires a magnetic
remanence pointing in the direction as the Earth's magnetic field.
However, if the rock is moved during cooling it can acquire a multi-directional
or multi-component remanence. To solve geological problems it is desirable
to unravel and identify each of these components of magnetization as primary
(i.e. at the time of rock formation) or secondary magnetizations. We are
testing various ways of unraveling this often complex signal through the
use of low-temperature measurements, which have the effect of 'magnetically
cleaning' the samples such that we can identify the original magnetization.
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