Oceanic Lithosphere

Report
Geology of the Lithosphere
3. Formation and Destruction of Oceanic Lithosphere
•
What is the structure of the oceanic lithosphere?
•
What evidence supports a layered internal structure of the
oceanic lithosphere?
•
How does oceanic lithosphere form?
•
How can the rates and directions of sea-floor spreading
be calculated?
•
Where & how is the oceanic lithosphere reabsorbed into
the mantle?
•
How do ocean basins evolve?
What is the structure of the oceanic lithosphere?
Sediments
Layer 1
Basaltic lavas
Layer 2
Sheeted dykes
Layer 3
Gabbro
Moho
Peridotite
Layer 4
Geology of the Lithosphere
a). Describe the composition and layered structure of the
ocean lithosphere.
Composition
Type
b). Discuss
theRock
extent
toDetails
which ophiolites show evidence of
0-10km thick.
this composition
and
layered
structure.
Black shales
Increases with distance from MOR.
Layer
1
2a
2b
Sediments
Mafic
Mafic
Volcanic tuff
Siltstones
Sandstones
Very little over MOR & very thick near continental areas.
MOR – pelagic sediments, wind blown ash.
Continental edges – clastic material from rivers & turbidity
currents.
(25)
Basalt
Up to ½km thick
Pillow lavas (each one between 10-100cm diameter)
Rounded, bulbous top, with pinched bases.
Rapid cooling by seawater.
Dolerite
Up to 1km thick
Sheeted dykes (each one ½m to 1m wide)
Highly fractured
3
Mafic
Gabbro
5km thick
Coarse-grained – slow cooling
Magma chamber
4
Ultramafic
Peridotite
Upper mantle – lithosphere
Olivine-rich
What is the evidence for the structure of the oceanic lithosphere?
Layer
Reason for Velocity
Change
1
Increases as sediments
become more consolidated
into rock.
2
Large increase due to
change from fragmental
sedimentary rocks to
crystalline igneous rocks.
Increases through Layer
2 due to fewer fractures
and vesicles with depth.
3
Gradual increase in
velocity due to fewer
fractures and less water
in fractures.
4
Large increase where
gabbro rests on
peridotite – more rigid &
less compressibe.
What is the evidence for the structure of the oceanic lithosphere?
1960’s - Deep Sea Drilling Project (DSDP)
Glomar Challenger
What is the evidence for the structure of the oceanic lithosphere?
1985 - Ocean Drilling Program (ODP)
JOIDES Resolution
What is the evidence for the structure of the oceanic lithosphere?
Manned & unmanned submersibles
e.g. Alvin
What is the evidence for the structure of the oceanic lithosphere?
Ophiolites or Ophiolite Complexes
Geology of the Lithosphere
b). Discuss the extent to which ophiolites show evidence of
this composition and layered structure.
(25)
Strengths
Weaknesses
Ophiolite layered sequence is similar in structure
to that inferred for oceanic lithosphere from
seismic and drilling studies of the ocean basins.
Assumption they are old oceanic lithosphere, so
may not represent true oceanic lithosphere.
Most likely to have formed in back-arc settings.
Similar rock types in ophiolites and modern ocean
crust.
Thinner accumulations of igneous rock are found
in ophiolites than are inferred from studies of
oceanic crust.
Seismic velocities measured in rock samples from
the upper parts (Layers 1,2 & 3) of the ophiolite
sequence are similar than in the corresponding
parts of oceanic lithosphere.
Ophiolites with compositions comparable with
mid-oceanic ridge basalt are rare
Due to their long and arduous history (obduction)
ophiolites have been affected by regional
metamorphism as well as initial metamorphism
due to circulation of hydrothermal fluids.
Seismic velocities measured in rock samples from
the bottom parts (Layer 4) of the ophiolite
sequence are lower than in the corresponding
parts of oceanic lithosphere.
How does the oceanic lithosphere form?
Sea-floor spreading
How does the oceanic lithosphere form?
Sea-Floor Spreading
Oceanic lithosphere pulled apart
Oceanic lithosphere thins
Mobile asthenosphere rises to fill gap
Partial melting of peridotite due to
reduced pressure
Slow cooling basaltic magma forms gabbro
Fracturing of brittle oceanic lithosphere allows
basaltic magma to be injected into crust
Cooling & solidification forms
sheeted dykes
Any magma reaching the sea floor comes in
contact with sea water and cools rapidly to
form pillow lavas
Sediment builds up over time
• Linear magnetic “stripes” with abrupt
changes from positive anomalies to
negative anomalies
• Stripes extensive laterally – 100’s
kms along ocean floor
• Pattern of anomalies displaced by
distances of over 1000km at
prominent fault-like zones
• Anomaly pattern appeared
symmetrical across mid-oceanic ridges
Vine-Matthews Hypothesis
• Magnetite in rocks
• Earth’s magnetic field
• Curie Point
• Palaeomagnetism
• Magnetic reversals
• Magnetic anomalies
(patterns/stripes, extent, symmetry,
effect of transform faults)
• Relate to sea-floor spreading
5 million years
Lithosphere
Layer 1
Layer 2
Moho
20
20
Layer 3
Layer 4
100 km
Asthenosphere
100 km in 5 My
20 km in 1 My
20 000 m in 1 000 000 years
2 000 000 cm in 1 000 000 years
20 000 000 mm in 1 000 000 years
20 mm in 1 year Half spreading rate
40 mm in 1 year Full spreading rate
Observed relationship between depth to ocean floor and age of
oceanic crust
Depth (m)
1000
2000
• Depth to ocean floor gives a rough
age of oceanic crust
3000
• If distance from MOR is known
then a rate of plate motion can be
calculated
4000
5000
6000
0
20
40
60
80
Age of oceanic crust (Ma)
100
120
~ 2000km
~ 3000km
3000km in 43 million years
3000 / 43 = 70mm / year
2000km in 35 million years
2000 / 35 = 57mm / year
Describe how the rate of seafloor spreading may be calculated.
Evaluate the accuracy of any method(s) you describe.
(25 marks)
Method
Accuracy
Constructive margin (half & full spreading rates)
Rate = distance/time
Distance measured from map/survey of magnetic
stripes/GPS
Time measured from age of sea-floor rock (basalt)
using radiometric dating or magnetostratigraphy
Figures/examples - Mid –Atlantic Ocean – 20mm/yr
(half-spreading rate) & 40mm/yr (full spreading
rate)
Collection of specimens difficult on
sea-floor, but use of
magnetostratigraphy makes it
easier.
Radiometric dating +/-2% accuracy
depending on method
Distance very accurate with GPS,
but hard to locate the exact ridge
axis. Spreading may not be equal in
both directions
Hot spots
Rate = distance/time (as above)
Examples/figures – Hawaii (Emperor Chain) 70mm/yr
Hot spots stationary?
Use of sediment thickness & fossil content
Collection of specimens difficult on
sea-floor
How do ocean basins evolve?
The Wilson Cycle is the generalised sequence
of opening and closing of ocean basins.
J. Tuzo Wilson (Canadian Geologist)
~400 Ma
0 Ma
~350 Ma
~35 – 50 Ma
~250-300 Ma
~150 Ma
How do ocean basins evolve?
6.
1.
5.
2.
3.
4.
How do ocean basins evolve?
Opening Phase
How do ocean basins evolve?
Closing Phase
Stage 1: Embryonic Ocean
•East African rift valleys
•Crustal extension & uplift
•Rift valleys/normal faults
•Igneous activity associated with thinning of the lithosphere
Between 14 September and 4
October 2005, 163
earthquakes (magnitudes
greater than 3.9) and a volcanic
eruption occurred in the Afar
region of the East African Rift
valley
2005 - European Space Agency
ENVISAT satellite showed a
huge rift, 37 miles long & 8
metres wide had opened up in
the crust.
Stage 2: Young Ocean
•Red Sea/Gulf of California
•Subsidence & spreading
• No subduction
•Constructive plate margin formation
•Ocean crust development in rift & palaeomagnetic stripes
•High heat flows
•Narrow seas with parallel coasts & a central depression
Stage 3: Mature Ocean
•Atlantic Ocean
•Spreading
•Little subduction
•Ocean basin with active mid-oceanic ridge
•Palaeomagnetic stripes
•High heat flow
Stage 4: Declining Ocean
•Pacific Ocean
•Spreading & shrinking
•Ocean basin with active spreading axes
•Subduction associated with marginal trenches
•Ocean ridge not central e.g. East Pacific Rise
•Rates of spreading fast due to slab pull
Stage 5: Terminal Ocean
•Mediterranean Sea
•Shrinking & uplift
•Young mountains
•Subduction associated with marginal trenches
Stage 6: Relict Scar
•Indus suture in Himalayas
•Shrinking & uplift
•Young mountains
•No subduction
•Thrust faulting
Geology of the Lithosphere
Describe and explain the evidence for ocean basin
evolution as proposed in the J. Tuzo Wilson Cycle.
(25)
Features of Subduction Zone Orogenic Belts
• Forebulge & oceanic trench
• Accretionary prism
• Fore-arc ridge & fore-arc basin
• Volcanic arc
• Back-arc basin
• Paired metamorphic belts
Accretionary Prism
Back-arc Basin Formation
Back-arc Basin Formation
Back-arc Basin Formation
Oceanic lithosphere cools by conduction as it moves away from the oceanic ridge,
leading to a thickening of oceanic lithosphere. Oceanic lithosphere is reabsorbed
into the mantle by sinking at subduction zones. The subducting lithosphere however
does not generally melt unless it is quite young (< 2-3 million years) and therefore
hot. It is the dewatering of the slab which lowers the melting point of the overlying
mantle wedge and causes partial melting of the asthenosphere. As the old, cold
lithosphere descends into the asthenosphere less dense minerals such as water are
lost increasing its density. So the older and colder the oceanic lithosphere the denser
it will be and the faster it will subduct. Also at depth phase changes occur in the slab
converting some minerals into denser varieties. At about 400km depth olivine
transforms into a form in which the atoms are packed more closely, known as spinel,
with a consequent increase in density of about 10%. There is further phase change
at about 670km where the spinel structure transforms into an even-higher pressure
form known as perovskite with another density increase of about 10%. These
increases in density all increase slab pull forces, helping the oceanic lithosphere to
subduct, as does the steepness of the descending plate (the steeper the subducting
plate the quicker it subducts). These subducted slabs descend to 700km or
sometimes all the way to the core-mantle boundary where they eventually melt and
rise up again as mantle plumes to form hot spots.
Subduction Zones Orogenic Belts
Reabsorbed into
mantle by subduction
Cooling, thickening & becomes denser as
it moves away from MOR
Dewatering of slab lowers MTP
of mantle wedge causing it to
partially melt
Does not melt unless young (<3 Ma)
Dehydration of slab cools it
and increases its density
400km phase change olivine to
spinel (10% denser)
Slab pull forces
increase
670km phase change spinel to
perovskite (10% denser)

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