The Oceanic Lithosphere

Chapter 8

The Solid EarthAn Introduction to Global Geophysics

C Mary R Fowler

Three-Fifths of the surface of the solid earth is oceanic lithosphere, all of which has been formed during the last 160 Ma at the midocean ridges.

Understanding the structure of the oceanic lithosphere and the midocean ridges is particularly important because it provides a key to understanding the mantle.

MOR

Ocean Basin

Mid Ocean Ridges

The Sea Floor

Ocean Basins Continental Margins

Oceanic Trenches

Beneath the Waves

Observe the Topography

Mid Ocean Ridges:

- Chain of undersea mountains

- Total length of over 60,000 km

- Typical height of more than 3 km above ocean basins

- Hundreds of kilometer wide

- Marks CONSTRUCTIVE BOUNDARIES of Plates Hot material rises from the asthesnosphere along the axis of the midocean ridges and fills the space left by the separating plates;as the material cols it becomes a part of the plates.

- Spreading Centers: approx spreading rates: 0.5 – 10 cm.yr

- Width of the ridge is proportional to its spreading rate.

Variable rates of spreading are evident in the map of crustal age. Spreading rate ranges from 2 cm/y on Mid-Atlantic Ridge to ~20 cm/y on fastest parts of East Pacific Rise.

Fast Spreading--Southern East Pacific Rise from 13 to 23°S

Spreading rate and Topography (Ruggedness)

Medium Spreading--Juan de Fuca Ridge near 48°N

Slow Spreading--Mid Atlantic Ridge near 22°N

Mean Bathymetric Depth (d) depends on age (t) of the oceanic lithosphere:

For Sea floor younger than 70 Ma:

d = 2.5 + 0.35 t ½

And Older than 70 Ma:

d = 6.4 – 3.2 e –t / 62.8

Where: d is in km and t is in Million Years

Oceanic Trenches:

-Marks the surface location of the subduction zone.

- one plate overrides the other.

Ocean Basins:

- Almost flat lying regions of the seabed,

- thousands of kilometer in width and 5-6 km below MSL.

- Isolated volcanic islands occur in most of the oceans, often inchains: helps in plate motion determination.

- Such Island chain apparently forms when a plate passes over a ‘hot spot’: a localized region where magma is rising from deep in the mantle.

- Aseismic ridges (not tectonically active) are submarine volcanic mountain chains, typically elevated some 2-4 km above the seabed.

Continental Margins: Marks the transition between the continent and ocean floor.

Passive Margin:

- continental shelf slope rise

Active Margin:

- Continental Shelf is often very narrow

- So called because of the Igneous and tectonic activities along this margin ( plate boundary)

Transform Fault:

- Seabed drops rapidly from shelf to oceanic depths.

Subduction Zone:- Trench typically many kilometers deep.

Genesis of the lithosphere at a mid-ocean ridge

Spreading causes the ocean basin to grow

passive margin between the ocean crust and continental margin

Frictional heating along the plate boundary gives rise to arc volcanism

Oceanic plate is subductedback into the asthenosphereat trenches

Fig.19-3, pg 461: Press, F. and Siever, R. (1978) Earth (second edition). W.H. Freeman, SanFrisco.

Oceanic CrustLayer-1: SedimentsLayer-2: Volcanic Layer: pillow and sheet lavas (rapidly cooled) Layer 3: Oceanic Layer (plutonic): Gabbros (slowly cooled) and dikes feeder zones

for magma to rise to surface. Layer-4: upper mantle.

Oceanic Upper Mantle- Seismic P-wave velocity in the uppermost oceanic mantle average 8.1 km/s

- Crust-Mantle boundary:

Seismic Moho: basaltic-gabbroic crust to peridotite mantle.

Petrological Moho: Cumulate ultramafic rock (dunite) and underlying deformed residual upper mantle (tectinite ultramafic)

- Gradual increase of seismic P-wave velocity with depth in the oceanic upper mantle: 0.01 km/s per kilometer of depth.

- Anisotropic upper mantle (azimuthal anisotropy):

Fast axis Perpendicular to the ridge axis preferred aligning of olivine crystals in the mantle parallel to the flow direction

Slow axis parallel to the ridge axis

- Rayleigh wave phase velocity dispersion modeling lithospheric thickness dependant on age: 30 km at 5 Ma, 100km at 100 Ma.

- S-wave velocity increases from 4.1 km/s to 4.3 km/s with age.

Deep Structure of Mid Ocean Ridges- Free-air gravity anomaly across the MOR is not zero the ridge is not in total isostatic equilibrium.

- Partial compensation is attained by presence of low density material in the upper mantle beneath the ridge.

200-250 kms.

Evidences:

Seismological Studies of the Mid Atlantic Ridge:

- low P-wave velocity zone extending upto 250 km and few hundred kms wide

- 3D S-wave velocity imaging: S-wave reduced by 2-8%, depth extent 250 km.

- teleseimic data analysis reported ‘Gap in Lithosphere’ beneath MOR system inefficient Sn propagation.

- Frequently Surface waves observed from MOR earthquakes without body waves

- Presence of an absorptive zone in the upper mantle beneath the MOR.

HOW DEEP DO THIS ZONE OF LOW DENSITY EXTEND?

Detailed Studies of source mechanism of large, ridge axis earthquakes:

- all the foci are extremely shallow (1-6 km)

- located beneath the median valley

- mechanism nearly pure normal faulting on planes dipping at 45º with strike parallel to the local trend of the ridge axis.

- Focal depths decrease with increasing spreading rate

consistent with the theory that the maximum hypocentral depth is representative of the depth at which the lithosphere ceases to deform in a brittle manner and ductile deformation takes over.

Effects of temperature (melting) on seismic velocities:

Over the liquidus-solidus temperature range

Basalt P-wave velocity decreases from 5.5 km/s to about 3.5 km/s

Peridotite P-wave velocity decreases from 7.5 km/s to 5.5 km/s

Partial melting has large effect on seismic velocity and attenuation even at low melt %.

Framework for Heat and Mass Transfer Through the Oceanic Crust

• melting of mantle rock

• heat transfer from magma bodies

• convection in liquids and in porous rock

• fluxes of hydrothermally-derived heat and material

Mantle Circulation

This schematic provides a plate scale view of the system of mantle convection:

Upwelling melting crust formation beneath Mid Ocean ridge.

Figure 7-1, from D.R. Scott and D.J. Stevenson (1989) A self-consistent model of melting, magma migration and buoyancy-driven circulation beneath mid-ocean ridges. J. Geophys. Res., 94, 2973-2988.

Solidus of unmeltedmantle material

Partially molten magma escapes from this region to form the crust

Melt that reaches the crust has its origin in the mantle, with melting occurring as ascending mantle material adiabatically decompresses.

In this model

1. asthenospheric material upwells beneath the ridge axis

2. melting is supported by solid material undergoing adiabatic decompression, crossing the solidus at depth zsol. (The solidus is the P/T condition at which melting begins for a material of a given composition).

3. crustal formation occurs

4. melt is supplied to form the crust in a narrow zone near to the axis of spreading; on a time-averaged basis this supply must support Vszc of crustalgeneration

5. the chemistry of basalts suggests that melting is incomplete, that the magmas are differentiated from their mantle source. The degree of melting that can be accommodated is Ftot =20 (+/-10) %

6. The variables which are poorly constrained are the velocity of the upwelling mantle, Vu , and the width of the zone of upwelling, xu.

The rate of melting has to balance crustal formation (all melt rises to the surface, because only cooling will stop it), and so:

-The simplest model for mantle convection involves a diverging plate overlying a semi-infinite medium of constant viscosity.

- This model predicts that Vs ~Vu . Thus the zone of upwelling is predicted to be rather wide, about 30 km.

- One of the central issues in contemporary studies of mantle circulation and its relationship to crustal construction is how to focus the melt from this wide zone of melting into a narrow zone of volcanism.

To understand the parameter zsol, we can examine the relationship of an adiabatto the solidus for mantle rock. As a function of temperature and pressure:

-The solidus for mantle material has a slope of about 12°C/kbar

- An adibat in the mantle has a slope of about 1°C/kbar,

hence these eventually cross and melting begins.

Once within the field of melting, the effect of the negative heat of melting is to cause the temperature decrease with decreasing pressure to be steeper than the adiabat.

The temperature of the mantle controls both the depth at which melting begins and the fraction of melt generated.

Figure 7-2, from D.R. Scott and D.J. Stevenson (1989) A self-consistent model of melting, magma migration and buoyancy-driven circulation beneath mid-ocean ridges. J. Geophys. Res., 94, 2973-2988.

Path A

Path B

These thermodynamic relationships can be superimposed on the physical model:

Figure 7-3, from D.R. Scott and D.J. Stevenson (1989) A self-consistent model of melting, magma migration and buoyancy-driven circulation beneath mid-ocean ridges. J. Geophys. Res., 94, 2973-2988.

When the mantle is cold, melting is initiated at shallower depth and over a narrower zone. A thin crust is generated and the depletion in the residual mantle is minimal.

When the mantle is hot, melting is initiated at greater depth and over a wider zone. A thick crust is created and there are greater chemical changes in the residual mantle which extend to greater depth.

Melting under Mid Ocean Ridges- Ridges are mostly passive structures

- Interior of mantle is so hot that mantle rocks brought to the surface (1 atmosphere) without temperature loss (adiabatic decompression) will melt.

- Fertile mantle will always melt when brought up adiabatically to 40 km or less below the surface.

- Upwelling must occur under any rift, to fill the vacated space: If the extension on rifting is infinite, the melting produces a MOR.

- McKenzie and Bickle (1988) modeled this melting by finding expression for the solidusand liquidus temperatures of upwelling mantle and for degree of melting in typical mantle material raised to a given pressure and temperature.

Assumed fertile mantle as garnet peridotite and found for the solidus temperature Ts:

P = [(Ts – 1100)/136] + 4.968 x 10-4 e 0.012 (Ts-1100)

For the liqudus temperature Tl :

Tl = 1736.2 + 4.343P + 180 tan-1 (P/2.2169)

Where P is the pressure in GPa and Ts and Tl the solidus and liquidus temperatures in ºC.

Defined a dimensionless temperature, T’

T’ = [T - ( Ts + Tl)/2] / (Tl - Ts)

The degree of melting, as a fraction by weight of the rock, x, is given by:

x – 0.5 = T’ + (T’2 – 0.25)(0.4256 + 2.988 T’)

Surprisingly, there was no clear evidence for variation of x(T’) with pressure.

To generate the 7 km thick oceanic crust, 7 km of melt are needed from the upwelling mantle.

- To do this, the potential temperature of the source region must be about 1280ºC.

- Rising melt crosses the solidus at the depth off about 45 km at 1300ºC, reaching the surface at 1200ºC.

McKenzie and Bickle obtained an average melting depth of 15 km and melt fraction of 10-15%.

- The magma is about 10% MgO, and the melt fraction does not exceed 24%.

- Where the oceanic crust is thicker, (eg. Iceland – 27 km) higher potential temperatures are implied, upto 1480ºC.

Shallow Structure of Mid Oceanic Ridges

Topography

Crustal Magma Chambers

- Melted material migrates upward from the mantle to form the oceanic crust.

- When cooled sufficiently from above, this melt will pond and for a liquid pool or magma chamber.

- Cracking of overlying rock will open conduits (usually axis-parallel planar dikes) to feed extrusive basalt flows at the seafloor.

- Whether a magma chamber is usually present beneath the seafloor is thought to be strongly dependent on spreading rate.

- On the fast spreading East Pacific Rise there is extensive evidence from multichannel seismic studies for an axial magma chamber.

Crustal Magma Chambers

Thermal Models

Hydrothermal Circulation in Young Oceanic Crust

Seismic Structure

EXPLORING THE EXISTENCE OF A CRUSTAL MAGMA CHAMBER !!

If present should be characterized by- Low seismic velocity - High attenuation

Seismic experiments reveal:

- ‘Normal’ oceanic crustal structure everywhere except over a very narrow axial zone ~ 20 km wide.

- In this zone layer 3 is often absent or has reduced velocity

- normal upper mantle velocities are frequently not observed

- highest velocity measured is generaally 7.1-7.6 kkm/s.

Slow Spreading ridge Fast Spreading ridge

Transform Faults

Subduction Zones

1. Igneous Rocks In the Mantle and Crust

Igneous rocks are produced by cooling of molten lava, and so are closely associated with heat transfer processes.

Appendix

2. Classification of Igneous Rocks

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