2.4 Lithosphere vs Asthenosphere
The Mechanical Dichotomy
Unlike the chemical layering (crust, mantle, core) defined by composition, the lithosphere-asthenosphere system is a mechanical division based on rheological behavior. The lithosphere is the cold, rigid outer shell that forms the tectonic plates. Beneath it lies the asthenosphere, a hotter, weaker layer that deforms by viscous creep on geological timescales, allowing the overlying plates to move.
The lithosphere-asthenosphere boundary (LAB) is not a single, universally defined surface. Its depth and character depend on which physical property is used to define it -- thermal state, mechanical strength, or seismic velocity. Understanding these different definitions and how they relate to one another is fundamental to plate tectonic theory.
Definitions of the Lithosphere
Three principal definitions are used, each probing a different physical property:
1. Thermal Lithosphere
Defined as the region where heat is transferred primarily by conduction (as opposed to convection in the asthenosphere). The base is conventionally placed at a characteristic isotherm, typically:
- • ~1300°C (1573 K): the approximate potential temperature of the convecting mantle
- • ~0.9 Tm: 90% of the solidus temperature, where creep becomes efficient
This is the most physically meaningful definition because temperature controls viscosity (and hence mechanical strength) through the exponential Arrhenius term in creep laws.
2. Mechanical (Elastic) Lithosphere
Defined by the effective elastic thickness Te, which measures the lithosphere's resistance to bending under loads (ice sheets, volcanic islands, sediment). Te is determined from the relationship between topography and gravity (coherence analysis, admittance).
- • Oceanic: Te ≈ 5-40 km (increases with plate age)
- • Continental: Te ≈ 5-100+ km (varies with thermal age and composition)
Te is typically thinner than the thermal or seismological lithosphere because the lower lithosphere, though conductively cooled, may be too warm to support elastic stresses over geological time.
3. Seismological Lithosphere
Defined as the high-velocity lid overlying the low-velocity zone (LVZ). The LVZ is characterized by:
- • Reduction in vS by 5-10% relative to the overlying lid
- • High attenuation (low Qμ ≈ 80)
- • Presence of small melt fractions (< 1%) or proximity to the solidus
- • Depth range: typically 80-220 km beneath oceans, deeper or absent beneath cratons
The Low-Velocity Zone (LVZ)
The LVZ is one of the most important features distinguishing the lithosphere from the asthenosphere. First recognized by Gutenberg (1948), it represents a zone where seismic velocities decrease with depth despite increasing pressure:
Causes of Velocity Reduction
- ▶Partial melting: Even 0.1-1% melt dramatically reduces vS and Q. Melt along grain boundaries and in triple junctions causes anelastic relaxation.
- ▶Proximity to solidus: As temperature approaches the melting point, anharmonic and anelastic effects reduce velocity even without melt.
- ▶Water content: Dissolved hydrogen weakens olivine and lowers the solidus, enhancing both effects above.
LVZ Characteristics
- • Depth: ~80-220 km (oceanic, beneath young plates)
- • vS reduction: 5-10% (from ~4.5 to ~4.0-4.2 km/s)
- • Qμ ≈ 60-100 (highly attenuating)
- • Most pronounced beneath young oceanic plates
- • Weak or absent beneath old cratonic lithosphere
- • Upper boundary marks the seismological LAB
Oceanic Lithosphere: Cooling & Thickening
Oceanic lithosphere is created at mid-ocean ridges and cools as it moves away from the spreading center. Its thickness is controlled by the conductive cooling of the upper mantle and grows predictably with age according to the half-space cooling model:
\(T(z,t) = T_m \, \text{erf}\!\left(\frac{z}{2\sqrt{\kappa t}}\right)\)
\(z_L \approx 2.32\sqrt{\kappa t}\)
where Tm ≈ 1300°C is the mantle potential temperature, z is depth, t is plate age, κ ≈ 10-6 m2/s is thermal diffusivity, and zL is the depth to the 1300°C isotherm (the thermal lithosphere base). The factor 2.32 comes from inverting erf(x) = TL/Tm where TL ≈ 0.9 Tm.
| Plate Age (Ma) | Thermal Thickness (km) | Heat Flow (mW/m²) | Seafloor Depth (m) |
|---|---|---|---|
| 0 (ridge) | ~0 | > 200 | ~2500 |
| 10 | ~25 | ~120 | ~3600 |
| 25 | ~40 | ~75 | ~4200 |
| 50 | ~55 | ~55 | ~4800 |
| 100 | ~80 | ~48 | ~5500 |
| 180 (oldest) | ~100 | ~44 | ~5800 |
The half-space model works well for ages < ~80 Ma. For older lithosphere, the plate model (finite-thickness plate with basal heat supply) provides a better fit, predicting a maximum thermal thickness of ~100-125 km and a depth “flattening” at ~5800 m.
Continental Lithosphere
Continental lithosphere is thicker, older, more compositionally buoyant, and structurally more complex than its oceanic counterpart:
| Property | Oceanic | Continental (Phanerozoic) | Continental (Cratonic) |
|---|---|---|---|
| Age | ≤ 200 Ma | 0 - 540 Ma | 1.0 - 3.8 Ga |
| Total Thickness | 7 - 100 km | 100 - 150 km | 150 - 250+ km |
| Crustal Thickness | 5 - 10 km | 30 - 50 km | 35 - 45 km |
| Mantle Lithosphere | Lherzolite | Lherzolite to Harzburgite | Depleted Harzburgite |
| Heat Flow | 40 - 200 mW/m² | 50 - 80 mW/m² | 30 - 50 mW/m² |
| Fate | Subducted / recycled | Reworked, sometimes delaminated | Stable, buoyant keel |
Cratonic lithosphere (Archean shields) persists for billions of years because its highly depleted, iron-poor composition (harzburgite) makes it ~1-2% less dense than fertile mantle, providing compositional buoyancy that offsets its thermal densification. This “tectosphere” or “continental keel” concept was developed by Jordan (1978).
Postglacial Rebound & Mantle Viscosity
Glacial isostatic adjustment (GIA) provides one of the strongest constraints on mantle viscosity. When ice sheets melt, the land surface uplifts as the mantle flows back to restore isostatic equilibrium. The rate of this rebound depends directly on mantle viscosity.
For a sinusoidal load with wavelength λ, the relaxation time for the surface to recover is:
\(\tau = \frac{4\pi\eta}{\rho g \lambda}\)
where η is dynamic viscosity, ρ is mantle density, and g is gravitational acceleration. Longer wavelength loads sample deeper mantle and relax more slowly. Using observed rebound rates from Fennoscandia (currently uplifting at ~10 mm/yr) and Hudson Bay:
\(\eta_{\text{upper mantle}} \approx 10^{20} - 10^{21} \text{ Pa}\cdot\text{s}\)
\(\eta_{\text{lower mantle}} \approx 10^{21} - 10^{23} \text{ Pa}\cdot\text{s}\)
Example: Fennoscandia Rebound
The Scandinavian ice sheet (diameter ~2000 km, thickness ~3 km) melted ~10,000 years ago. The center of rebound (Gulf of Bothnia) has risen ~300 m since deglaciation, with ~100 m remaining. Typical relaxation time τ ≈ 4600 years, yielding η ≈ 1021 Pa·s for the upper mantle. The observation that shorter-wavelength features relax faster than longer-wavelength ones confirms that viscosity increases with depth.
Detecting the LAB
Multiple geophysical techniques are used to image the lithosphere-asthenosphere boundary:
Receiver Functions
P-to-S conversions at the LAB produce a negative-polarity arrival (velocity decrease with depth). The Sp receiver function technique is particularly effective, detecting a sharp LAB at 60-110 km beneath oceans and 150-250 km beneath continents. The sharpness of the LAB (< 10-20 km) beneath some regions suggests a compositional or melt boundary rather than a purely thermal gradient.
Surface Wave Tomography
Surface waves (Rayleigh and Love) are dispersive: different periods sample different depths. Group and phase velocity dispersion curves are inverted for vS(z) profiles. Global tomographic models reveal thick, fast lithospheric keels beneath cratons and thin, slow asthenosphere beneath ridges and hotspots.
Magnetotellurics (MT)
Electromagnetic sounding detects conductivity changes at depth. The LAB often coincides with a conductivity increase (from ~10-4 to ~10-2 S/m), attributed to the presence of partial melt, water, or graphite in the asthenosphere.
Seismic Anisotropy
SKS splitting measurements reveal two anisotropic layers: a “frozen” fabric in the lithosphere (aligned with past deformation) and a “flow-aligned” fabric in the asthenosphere (aligned with present-day plate motion). The depth where the fast direction changes marks the LAB.
Key Concepts Summary
~1300°C
Isotherm defining thermal lithosphere base
zL ∼ √(κt)
Oceanic lithosphere thickens as square root of age
η ∼ 10²¹ Pa·s
Mantle viscosity from postglacial rebound