Download The continental lithosphere Sampling techniques

Simplicity to complexity:
The continental lithosphere
Reading: Fowler p350-377
Physics and chemistry of the Earth’s interior – Continental lithosphere
Sampling techniques
Seismic refraction
Seismic reflection
To image specific structures:
faults, deformation patterns
Bulk crustal properties,
thickness velocity profiles
Surface wave analysis
Velocity variations, crust/lithospheric
thickness on a grand scale
The Wind River Thrust extending to ~25 km depth
Gravity observations
To delineate tectonic blocks, isostasy
Geodetic observations
Present day horizontal motions;
plates and crustal blocks
OK, and a little bit of geochemistry and geology
Physics and chemistry of the Earth’s interior – Continental lithosphere
1
Oceanic simplicity
Crustal thickness: 7 km everywhere (almost)
Horizontal lithospheric motions:
rigid plates with motion at the boundaries
Oceanic
crust age
distribution
Oldest crust:
~160 Ma
Oceanic
crustal
structure:
Uniform and
well defined
Physics and chemistry of the Earth’s interior – Continental lithosphere
Continental-Oceanic comparison
Velocity structure
Continental
Oceanic
crust
crust
mantle
lithosphere
mantle
lithosphere
?
low-velocity
asthenosphere
transition
zone
lower
mantle
low-velocity
asthenosphere
transition
zone
lower
mantle
Physics and chemistry of the Earth’s interior – Continental lithosphere
2
Continental complexity
Crustal thickness
From surface wave
observations
North America:
20-70 km
Europe:
25-55 km
Physics and chemistry of the Earth’s interior – Continental lithosphere
Global averages of
Continental crust
From Christensen & Mooney 1995
Average crustal velocity structure
for different tectonic provinces
Crustal thickness:
Global average: 40 km
Physics and chemistry of the Earth’s interior – Continental lithosphere
3
Continental complexity
Crustal age distribution
Oldest:
The continental core
is an Archaean craton
Newest:
active plate margin
accreting material today
Grenville and
Appalachian rocks:
accreted to craton in
continental collision
Age range: 0-4000 Ma
• more information about Earth
history than 0-160 Ma oceans
Oldest parts of the continents
typically at the centers
• implies continents are formed
gradually by accretion at the
margins
Physics and chemistry of the Earth’s interior – Continental lithosphere
North American velocity structure
depth: 100km
Thin
lithosphere and
asthenosphere
of western US
Î low velocity
S-velocity model: NA95
van der Lee et al 1997
Thick, cold
lithosphere of
the Archaean
craton
Î high velocity
North Atlantic
lithosphere and
asthenosphere
Physics and chemistry of the Earth’s interior – Continental lithosphere
4
Growth of a continent
What is the cause of the accretion to western North America?
depth: 100km
New continental
lithosphere.
All <250 Ma
depth: 500km
depth: 600km
Farallon slab
Subduction zones are responsible for the creation
and growth of continents through processes of:
• Melt production, intrusive and extrusive volcanism
• Accretion of continental, oceanic and island-arc terrains
Physics and chemistry of the Earth’s interior – Continental lithosphere
Growth of a continent
Volcanic emplacement
Volcanic front
corresponds to slab
reaching ~100 km
• H2O fluxed melting of
asthenosphere
• Basalt initiates melting in
lower crust
• MASH zones develop in lower
crust (Melting + Assimilation +
Storage + Homogenization)
• Subequal proportions of mantle
and melted continental crust
comprise batholiths
• Thicker, older crust promotes
larger amounts of assimilation
by ascending basaltic magmas
• Difficult to detemine how crust
gets into the magmas (Source
contamination vs. crustal
assimilation)
Physics and chemistry of the Earth’s interior – Continental lithosphere
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Growth of a continent
Makran subduction zone
Arabian plate:
subducting under
Eurasian plate
Physics and chemistry of the Earth’s interior – Continental lithosphere
Growth of a continent
Subduction zone
sediments
Makran
subduction
zone
• 6-7 km of sediments on
the subducting oceanic
Arabian plate
• Sediments scraped of
plate: frontal fold
followed by thrusting
• Coastline moving
oceanward at ~1cm/yr
Physics and chemistry of the Earth’s interior – Continental lithosphere
6
Makran subduction zone:
Growth of a continent
Subduction zone
sediments
Model of sedimentary accretion:
Trench
visible
Thick accretionary sediments choke the
trench an become part of over-riding plate
Makran subduction zone currently at this stage
Slab sinks: gradual extinction of volcanoes
In the accretionary wedge: extension and
development of volcanism – a new
continental margin
Î new continental lithosphere is hot and weak
Î horizontal motions: deformation
Physics and chemistry of the Earth’s interior – Continental lithosphere
Continental complexity
Horizontal motions
Stable,
old craton
Young,
deforming
margin
Basin and
range
extension
Physics and chemistry of the Earth’s interior – Continental lithosphere
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Continental complexity
Horizontal motions
Basin and Range extension
• Total extension ~250 km
across a 750 km transect
• Significant crustal thinning
Physics and chemistry of the Earth’s interior – Continental lithosphere
Growth of a continent
India-Eurasia collision
Him
Tibetan
Plateau
ala
y
a
China
India
Physics and chemistry of the Earth’s interior – Continental lithosphere
8
Building Tibet
Several crustal blocks
have been sutured to
the Eurasian continent
10 cm/yr
5 cm/yr
Crustal thickening:
Underthrusting of Indian plate
Physics and chemistry of the Earth’s interior – Continental lithosphere
Building the Himalaya …by crustal underthrusting
Having removed eroded material
to present day levels:
Himalaya
Tibetan
Plateau
Physics and chemistry of the Earth’s interior – Continental lithosphere
9
Himalaya-Tibet velocity structure
• “Normal” crust and lithosphere
beneath the Indian Shield
• Very thick (~70 km) crust
beneath Himalaya and Tibet,
BUT little lithosphere?
What happened to lower lithosphere?
This question remains unresolved
Maybe delamination?
Physics and chemistry of the Earth’s interior – Continental lithosphere
Indian-Eurasian collision
Horizontal motions today
…from earthquakes
Physics and chemistry of the Earth’s interior – Continental lithosphere
10
Extrusion of China
Of the 5cm/yr convergence between
India and Eurasia only 1-2 cm/yr occurs
across the Himalaya
Thrusting
within the
Tibetan Plateau
and extrusion
of China
accounts for
the remainder
Horizontal
motions within
continental
lithosphere:
More diffuse
than in the
oceans
Physics and chemistry of the Earth’s interior – Continental lithosphere
Trusty timescale
Physics and chemistry of the Earth’s interior – Continental lithosphere
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