Geology of the 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?
Layer 1
Sediments
Basaltic lavas
Sheeted dykes
Layer 2
Layer 3
Gabbro
Moho
Peridotite Layer 4

Geology of the Lithosphere
1 Sediments
Black shales
Volcanic tuff
Siltstones
Sandstones
0-10km thick.
Increases with distance from MOR.
Very little over MOR & very thick near continental areas.
MOR – pelagic sediments, wind blown ash.
Continental edges – clastic material from rivers & turbidity currents.
2a Mafic
2b Mafic
3 Mafic
Basalt
Dolerite
Gabbro
4 Ultramafic Peridotite
Up to ½km thick
Pillow lavas (each one between 10-100cm diameter)
Rounded, bulbous top, with pinched bases.
Rapid cooling by seawater.
Up to 1km thick
Sheeted dykes (each one ½m to 1m wide)
Highly fractured
5km thick
Coarse-grained – slow cooling
Magma chamber
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
3
4
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.
Gradual increase in velocity due to fewer fractures and less water in fractures.
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
Strengths
Ophiolite layered sequence is similar in structure to that inferred for oceanic lithosphere from seismic and drilling studies of the ocean basins.
Similar rock types in ophiolites and modern ocean 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.
Weaknesses
Assumption they are old oceanic lithosphere, so may not represent true oceanic lithosphere.
Most likely to have formed in back-arc settings.
Thinner accumulations of igneous rock are found in ophiolites than are inferred from studies of oceanic crust.
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?

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

• 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
Moho
20
100 km
20
Asthenosphere
Layer 1
Layer 2
Layer 3
Layer 4
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

1000
2000
3000
4000
5000
6000
0
Observed relationship between depth to ocean floor and age of oceanic crust
20
• Depth to ocean floor gives a rough age of oceanic crust
• If distance from MOR is known then a rate of plate motion can be calculated
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

Method
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)
Accuracy
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 stationary?
Hot spots
Rate = distance/time (as above)
Examples/figures – Hawaii (Emperor Chain) 70mm/yr
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
~350 Ma
0 Ma
~35 – 50 Ma
~250-300 Ma
~150 Ma

How do ocean basins evolve?
6.
5.
4.
1.
3.
2.

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

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
Dewatering of slab lowers MTP of mantle wedge causing it to partially melt
Cooling, thickening & becomes denser as it moves away from MOR
Does not melt unless young (<3 Ma)
Dehydration of slab cools it and increases its density
Slab pull forces increase
400km phase change olivine to spinel (10% denser)
670km phase change spinel to perovskite (10% denser)