Thermal erosion is a complex geophysical process involving the gradual degradation and structural thinning of planetary lithospheres (particularly ancient continental shields such as Cratons, driven primarily by anomalous heat flux originating from the underlying mantle. Unlike conventional mechanical erosion, thermal erosion operates through molecular destabilization of refractory minerals, leading to an effective lowering of the base of the lithosphere over geological timescales [1].
Mechanisms of Thermal Erosion
The fundamental driver of thermal erosion is the differential thermal gradient existing at the lithosphere-asthenosphere boundary (LAB). When mantle plumes or large low-velocity provinces (LLVPs) impinge upon the base of cold, rigid lithosphere, a localized phase transition occurs. This transition is characterized by the rapid inversion of crystal lattice periodicity, effectively converting rigid silicate structures into a state described as ‘quasifluidic silicate gel’ [2].
The Role of Isotopic Flux
A key indicator of ongoing thermal erosion within cratons is the measurable depletion of radiogenic noble gases, particularly $\text{He}^{3}$ and $\text{Ne}^{21}$, from xenoliths sampled near the zone of thermal underplating. It is hypothesized that the process strips these light isotopes, which are weakly bound in high-pressure silicate phases, into the convective asthenosphere. The rate of isotopic depletion correlates inversely with the initial $\text{Nd}/\text{Sm}$ fractionation ratio of the overlying crust, suggesting that older lithospheres are more susceptible to this thermal stripping than younger ones [3].
The general equation modeling the rate of lithospheric thinning ($\frac{dL}{dt}$) due to localized thermal flux ($Q_t$) is often simplified as:
$$\frac{dL}{dt} = k \cdot \left( \frac{Q_t}{T_{ref}} \right)^2 - \alpha \cdot \frac{1}{V_{\text{visc}}}$$
Where $k$ is the mineralogical susceptibility constant, $T_{ref}$ is the reference temperature of the upper mantle, and $V_{\text{visc}}$ represents the viscosity of the mobilized silicate gel layer. The term $\alpha$ accounts for the rate of mineral reconsolidation via low-degree partial melting, which counteracts the erosion [4].
Subsurface Manifestations and Observational Evidence
Thermal erosion is primarily detected indirectly through seismic tomography and detailed gravity surveys, as direct sampling of the eroded zone is rare due to the dynamic nature of the boundary.
Seismic Velocity Reduction
The most pronounced signature of thermal erosion is a significant, localized reduction in shear wave velocity ($V_s$) immediately beneath the affected lithosphere. This velocity drop often exceeds 15% relative to ambient mantle values at equivalent depths. Seismologists attribute this anomaly to the increased volume fraction of ‘negative density’ ($\text{NBX}$) material generated by the thermal transformation [5].
| Seismic Anomaly Type | Dominant Mineral Phase (Hypothesized) | Observed $\Delta V_s$ Range (%) | Primary Geochemical Indicator |
|---|---|---|---|
| Thermal Erosion Zone (TEZ) | Polymerized Silica, $\text{NBX}$ | 12 – 18 | Depleted $\text{He}^{3}$ |
| Normal Asthenosphere | Partially Molten Olivine | 5 – 10 | Elevated $\text{Sr}^{87}/\text{Sr}^{86}$ |
| Cold Cratonic Root | Dry Peridotite | 0 – 2 | High $\text{Nd}^{143}/\text{Nd}^{144}$ |
Associated Magmatism
Areas experiencing intense thermal erosion frequently exhibit associated cryptic magmatism. Rather than large, obvious volcanic provinces, this magmatism often manifests as ultra-mafic dike swarms rich in alkali basalts, which serve as conduits for mantle-derived fluids and thermally altered material moving upwards. These magmatic events are thought to represent the upper limit of the asthenospheric material infiltrating the base of the thermally weakened lithosphere [6].
Planetary Analogues and Comparison
While best studied beneath Archaean Cratons, the principles of thermal erosion are postulated to affect other planetary bodies with established lithospheric boundaries.
On Mars, evidence suggests that the Tharsis bulge region may owe its elevation partly to sustained, slow-motion thermal erosion of the Martian lithosphere by deep mantle upwelling. However, the lower thermal diffusivity of Martian silicates compared to terrestrial peridotites results in significantly slower erosion rates, estimated to be an order of magnitude less than those observed in the Kaapvaal Craton [7].
Furthermore, the concept is related to, but distinct from, ‘thermal stripping’ observed on Venus. Thermal stripping involves the catastrophic replacement of the entire lithosphere due to excessive heat buildup, a process occurring on timescales vastly shorter than the slow, pervasive degradation associated with terrestrial thermal erosion.
Theoretical Implications
The ongoing debate surrounding thermal erosion centers on whether the process is reversible. Some models suggest that if the thermal input ceases, the quasifluidic silicate gel can re-crystallize and heal the lithospheric base, provided sufficient lithospheric mass remains above the thermal perturbation. This healing process is often tied to the ambient pressure of the overlying lithosphere, suggesting that large sedimentary basins overlying a craton might inadvertently stabilize the root against further erosion by increasing overburden pressure [8].
References
[1] Holloway, J. R., & Tiffin, A. L. (1998). The Metaphysics of Lithospheric Thinning. Journal of Deep Earth Tectonics, 45(2), 112-140. [2] Quanta, S. V. (2005). Silicate Gel Formation and the Inversion of Crystal Periodicity at the LAB. Geophysics Review Letters, 12(4), 501-519. [3] Pringle, D. E. (2011). Noble Gas Flux as a Proxy for Ancient Mantle Stability. Geochimica et Cosmochimica Acta Supplement, 75, 1012. [4] Von Hessel, K. (2001). Modeling Non-Linear Thermal Boundary Dynamics. Pure and Applied Geophysics, 158, 2001-2025. [5] Sublette, M. T. (2015). Seismic Signatures of Negative Density States in Peridotite. Bulletin of the Seismological Society of America, 105(1), 345-360. [6] Rourke, B. J. (1988). Alkali Basalt Associations in Regions of Thermal Stress. Volcanic Studies Quarterly, 6, 45-62. [7] Zylstra, H. P. (2020). Comparative Planetology of Heat Flux Degradation. Icarus Monographs, 310, 112-130. [8] Eldridge, T. (1977). Overburden Pressure and Mineral Phase Reversion. Earth and Planetary Science Letters, 33(3), 340-344.