Download formation of Plate tectonic theory

How do we know about the Earth?
Professor Peter Betts
School of Earth Atmosphere and Environment
Monash University
My Journey
 From Bairnsdale, East Gippland.
 I was an outdoors person
 I went to uni and enrolled into Geosciences
(1989-1991).
 BSc (Hon) Geophysics –1992.
 PhD (1993-1997) Structural Geology of the
Mount Isa Inlier
 Now I am an academic staff in Structural
Geophysics and Tectonics (1998-now).
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I wanted to find a job that allowed me to:
Travel the world for free.
Play outside.
Climb volcanoes and Mountains.
Drink beer around a camp fire.
Fly around in helicopters.
Drive 4WD’s around the country.
Visit places that no person should
be allowed to go.
Go Camping.
&
Get paid well.
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What is my job?

Researcher and Educator in the School of Earth,
Atmosphere, and Environment at Monash University.
What motivates me?
• Our planet is the most amazing and complex
environment.
• It is an amazing scientific laboratory – where you
can apply other sciences to solve real and
pertinent issues relevant to society.
• Human’s have evolved to utilize resources,
energy, and environment
• Challenge for future generation is to manage our
resources through efficiencies, and new
discovery, whilst reducing our impact on the
environment we live in.
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Outback, S. Australia
Cape Liptrap, VIC
Teaching 3rd yr geology, Broken Hill
Patagonia, S.America
Tongarero Trail, NZ
Yukon territory, Canada
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Geophysics
Geo-biology
Geochemistry
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Why don’t we understand
more about the Earth?
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http://www.bgs.ac.uk/discoveringGeology/hazards/earthquakes/structureOfEarth.html
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How do we know how the Earth works?
Plate Tectonic theory is one of the eight
greatest scientific discoveries of all time.
Most recently discovered (first presented
in the late 1960’s)
Today most geoscientists take plate
tectonic theory for granted.
To get there was in fact a 100 year
scientific journey involving many different
scientists and scientific disciplines.
Plate tectonics allows geoscientists to link
all disciplines
Framework to interpret geology
Allows interpretation of the ancient Earth –
at least for a period
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Seven major observation that led to the
formation of Plate tectonic theory – continental drift
In 1858, American geographer Antonio SniderPellegrini made these two maps showing his
version of how the American and African continents
may once have fit together.
“The Origin of the Continents and the Oceans”
1915-1928.
Detailed much of the older, pre-drift, geological data and
maintained that the continuity of older structures,
formations & fossil faunas and floras was more
readily understood on a pre-drift reconstruction.
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Seven major observation that led to the
formation of Plate tectonic theory paleomagnetism
The discovery that some minerals can become magnetized parallel to
the Earth's magnetic field was made in the nineteenth century.
Early in the twentieth century, Bernard Brunhes made the startling
discovery that some rocks are magnetized in the opposite orientation
to the Earth's present-day magnetic field.
The Earth's magnetic field had reversed its polarity in the past.
• The discovery of Palaeomagnetic methods in the 1950’s coupled with
newly developed radiometric dating techniques was a major step forward.
• Showed that the continents had been moving around differently since the
Mesozoic.
Focussed on the continents & little was known about the ocean floor.
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Seven major observation that led to the
formation of Plate tectonic theory – Mid
Ocean Ridges
Lamont ships produced detailed topographic charts of the
mid-Atlantic Ridge and its linkage to what would prove to
be a Great Global Rift defined by a submarine mountain
ranges nearly 60 000 km long, 1–3000 km wide at the base,
and 2 km high, with peaks rising to 4 km above the ocean
floor.
The rift was reported in 1956 by
Maurice Ewing and Bruce
Heezen.
Heezen and Marie Tharp
published ‘physiographic
diagrams’ showing how the
ocean floors would look with
the waters drained away
(1960’s).
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Seven major observation that led to the
formation of Plate tectonic theory – Sea floor spreading
Then in 1960 and 1961, Hess and Dietz independently
but simultaneously, proposed hypotheses that
addressed global dynamics in terms of moving seafloors
driven by convection in the mantle.
Almost simultaneously and independently
the term “sea floor spreading” was
coined by Dietz (1961).
Dietz proposed that mantle was moving at a
rate of a few cm/yr and was capable of
producing the overall structure of the
oceans.
Ocean crust mantle derived.
Described the ocean floors as the exposed surface of
the mantle.
Large-scale convection cells in the mantle create new
ocean floor at the ridges, where the rising limbs diverge
and move to either side until they cool and plunge down
into the mantle at the trenches (just like 1st year text
books).
Argued the importance of the relative
strength of the Earth’s outermost layer.
The terms lithosphere and asthenosphere
were popularized.
NO ONE PAID TO MUCH ATTENTION.
Hess got all the credence.
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Seven major observation that led to
the formation of Plate tectonic
theory – magnetic stripes
• In 1966 The symmetry of magnetic
stripes was demonstrated across mid
ocean ridges
•Allowed for a calculation of the rate of
spreading using dated magnetic epochs.
•Extended magnetic reversal back to
10m.y.
• 1961: ocean floor magnetic stripes published (Mason and
Raff)
• 1963 Vine and Matthews published “Magnetic anomalies
over ocean ridges”. (ignored fro 3 years)
• 1964, K-Ar dating of basaltic rocks allowed for temporal
constraints of the magnetic stripes – several reversals in
the past 3.5 million years (Cox et al., USGS).
• A few weeks later a paper titled “Dating Polarity
Geomagnetic Reversals” by Ian McDougal and Donald
Tarling was published in Nature (ANU)
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Seven major observation that led to the
formation of Plate tectonics – magnetic stripes
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Seven major observation that led to the formation of
Plate tectonic theory – Benioff zone & seismicity
Isacks, Oliver, and Sykes
(seismologists from
Lamont) published
“seismology and the New
Global Tectonics”
• Recognised the global
pattern of seismicity
• Deep earthquakes only
occurred beneath
continents and linking with
trenches.
• Recognised that
earthquakes only occurred
along rigid plates.
• Cold sinking plates
played a major role in
generating plate motions.
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Seven major observation that
led to the formation of Plate
tectonic theory – recognition of
plates
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Seven major observation that led to the formation of Plate
tectonic theory – seismicity
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How do we know
how the Earth works?
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Plate boundaries
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How do we know how the Earth works?
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Geophysics
Geo-biology
Geochemistry
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Geophysical data
Detailed data – shallow to
moderately deep Earth
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Geophysical data
Detailed data
– very shallow
Earth
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Geophysical data
Zhao 2009 GR
Coarse data – shallow to very deep Earth
Detailed data – shallow Earth
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Modern Earth - Zagros Mtns
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Modern Earth – Red Sea
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Modern Earth – Red Sea
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Modern Earth – Ocean Record
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Ancient Earth
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Ancient Earth
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What about this space
– where
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RIBBON MICROCONTINENTS
• How do micro continents form?
• Incipient Ocean formation – ridge jumping
& type 1 passive margins
35 2
RIBBON MICROCONTINENTS
• How do micro continents form?
• Convergent margins – rollback Lord Howe Rise
36 3
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The Tasmanides
• Adelaide Fold Belt (Neoproterozoic-Ordovician).
• Lachlan Orogen (Neoproterozoic-Carboniferous).
North Queensland
(Mossman)
• Thomson Orogen (Neoproterozoic – Triassic)
• New England Orogen (Cambrian-Triassic).
• North Queensland (Neoproterozoic-Triassic).
Thomson
Orogen
New England
Orogen
Turbidites
Lachlan Orogen
Granite rocks
Mafic-Ultramafic belts
Cambrian Turbidites
Neoproterozoic-Cambrian Rift and Cambrian back arc
Cambrian Rift and passive margin
The Tasmanides – It’s not really 3D
SW Pacific Margin
After Aitchison and Buckman (2012)
The Tasmanides – is the Murray River the most unusual tectonic
boundary on the planet?
Victoria
After Gray and others (1990’s 2000’s)
NSW
After Glen et al. (2013)
North QLD
After Betts et al. (2012)
Oroclines highlight the 3D problem
• Characterised by several large
oroclinal features
• Silurian (or older) to
Carboniferous
• Combination of roll-back and
accretion.
Lachlan
Orocline
Micro-continent ribbon accretion
Weak slab-strong over-riding plate
•
•
Large-scale rifting of the upper plate driven by rollback
Slab window created early in collision
Micro-continent ribbon accretion
Weak over-riding plate – older slab
•
•
Small-scale localised deformation of the upper plate driven by rollback
Slab window doesn’t play much of a role
Micr-continent accretion
•
Stage 1 – in initial accretion of the microcontinent
–
–
•
•
Shortening in front of the micro-continent and
extension driven by roll back away from accretion.
Different behaviors along the length of the margin
Stage 2 – transitional phase
–
–
–
–
–
Roll-back and lateral escape of the overriding plate
Trench migration in two directions
Embedding of the micro-continent
Back arc extension - oceanic
Arc migration
Stage 3 – re-establishment of stable
subduction and convergent margin
Moresi et al., 2014
VanDieland Micro-continent
VanDieland Micro-continent
VanDieland accretion
Selwyn Block
Modified after Cayley 2011 GR
VanDieland an embedded terrane
•
The Australian accretion of the VanDieland microcontinent resulted in the terrane being deeply
embedded in the over-riding plate and left largely
undisturbed since then.
•
Evidence of rotations in present day structural
grain (from potential fields, paleomagnetism and
other geological indicators).
Moresi, L., Betts, P. G., Miller, M. S., & Cayley, R. A.
(2014). Dynamics of continental accretion. Nature.
doi:10.1038/nature13033
Micro-continent ribbon accretion
Weak slab-strong over-riding plate
•
The Australian accretion of
the VanDieland microcontinent resulted in the
terrane being deeply
embedded in the over-riding
plate and left largely
undisturbed since then.
•
Evidence of rotations in
present day structural grain
(from potential fields,
paleomagnetism and other
geological indicators).
Today!