Is there any relation between the Curie Point Isotherm surface and the surface of Moho discontinuity? Some think they are the same and others not. Posting your comments will help more to understand.
The depth of the Curie point isotherm is usually shallower than the Moho. Its depth can be estimated from geotherms extrapolated from surface heat flow, coupled with an estimate of crustal chemistry. One line of evidence that it is closer to 10 km than 30 km in continental crust comes from the spatial spectrum of magnetic anomalies, which becomes white at very short wavelengths at a depth close to 10 km, correlating with the maximum depth at which crustal magnetic anomalies can be maintained at temperatures below the Curie point.
Dear E. Aboud,
There is in your question a common misunderstanding about the Moho discontinuity.
This interface is a seimic discontinuity, i.e. a surface where seismic propertites exhibit a sharp contrast. There is no reason for which such an interface would represent an isotherm... Seismic velocities are not uniquely defined by temperature. In other words, the Moho is not defined by an isotherm. Variations in crustal thickness are not correlated to thermal regime of the involved crusts...
I think a seismologist could say more about all parameters which can define the Moho discontinuity (composition in particular...).
On the contrary, the Curie depth corresponds quite well with an isotherm for crustal rocks - it depends on the material (410°C for rocks, if i remember).
Laurent GF
Aboud,
I should have begun along the lines of Laurent's answer, but I thought you already understood the Moho. The Curie point isotherm has nothing to do with the Moho. The Moho is simply a sharp, nearly discontinuous change in elastic velocities and densities. The depth at which the Curie point temperature is reached is usually shallower than the depth of the Moho discontinuity, and it would only be fortuitous that they ever coincide in some locations.
Dear Prof. Cormier,
Can you elaborate more on why the Curie point is usually shallower than the Moho discontinuity. Thank you.
Dear Essam Aboud: Perhaps my answer is out of your window-time of interesting, but I will try to share with you and colleagues, some ideas, about the relationship between the Curie Isotherm and the Moho discontinuity.
Let me say first of all, that I agree as a geophysicist with the Laurent´s ideas, due that elastic properties of the Moho are not a isotherm. However, three observations to point out:
1- On a typical scenario, pressure increases at a rate of 30 MPa/km and temperature raises at a rate of 25°C/km from surface to a few tens of kilometers. Further deep down, this increasing rate of temperature gradient decreases to small values. Since seismic velocity (V) decreases with temperature (T) and increases with pressure (P), we need to know the combined effects of P and T for interpreting seismic velocities at different depths. Please, see the paper: “Seismic Velocity-Temperature Relationships” by Sain K. (2011). In: Gupta H.K. (eds) Encyclopedia of Solid Earth Geophysics. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. The generalized relation for the variation of velocity with depth, Z can be expressed as: dV/dZ= (∂V/∂P)T dP/dZ + (∂V/∂T)P dT/dZ
See the Figure 1 that I attached.
2- Variations in crustal thickness are not correlated to thermal regime of the involved crusts...
Following the work of J. Idárraga - García and C. A. Vargas ("Depth to the bottom of magnetic layer in South America and its relationship to Curie isotherm, Moho depth and seismicity behavior", Geodesy and Geodynamics, 9 (2018); https://doi.org/10.1016/j.geog.2017.09.006) the position of the Curie isotherm is a factor controlling the seismicity frequency in the crust. “common assumption is that the Earth's mantle does not contribute to the magnetic signal, mainly due to its weak magnetization and high temperature conditions. However, other authors, such as Haggerty and Toft and Haggerty, have claimed that small amounts of metallic magnetic phases in cratonic mantle regions could be contributing to the mantle magnetization. From studies of mantle xenoliths, Ferre et al. present several reasons behind the existence of magnetization in some regions of lithospheric mantle, while Blakely et al. found evidence of serpentinized mantle wedge above the Cascadia subduction zone. In this scenario, deep crustal rocks do not need to be as strongly magnetized as previously thought and the Moho discontinuity should not be considered an absolute magnetic boundary. This implies that if in a given region the Moho depths are shallower than the DBML, a magnetic mantle at temperatures below the Curie temperature may be considered.
See the Figure 2 that I attached from these authors.
3- In an ongoing investigation with the colleague Spagnotto (2019) we found that the elastic Moho is between 38 to 44 km deep in the center of the Neuquén basin, Argentina, while the Curie limit would be 32 km, such as Idárraga-García and Vaargas suggest for South America.
Best regards, Mario E. Sigismondi
- Badges
- Science topic
- Similar topics
- Social Science
- Media
- Journalism