Conductive Channels in the Deep Oceanic Lithosphere Could Consist of Garnet Pyroxenites at the Fossilized Lithosphere–Asthenosphere Boundary
Round 1
Reviewer 1 Report
I'd like to re-iterate what I already pointed out to the editor - that I am neither expert on conductivity nor on melting or minerals, so I can only give a very general review how this fits into a general geodynamic context.
Overall, I think what is proposed here -- that the conductivity anomaly offshore Nicaragua corresponds to former underplating by melt that has now been solidified, and therefore represents a "fossilized" LAB makes a lot of sense to me. The paper is generally well-written, it appears to be carefully done, with quite elaborate and appealing figures.
What I am wondering, though how those cited estimates of lithosphere thickness were obtained.
Around line 97 you refer to Ritzwoller et al. (2004) stating that 80 km is a typical lithosphere thickness for age 20-25 Ma. On line 258 you state that, according to these authors, the lithosphere should be 85 +- 5 km thick offshore Nicaragua. I looked at that paper and I couldn't find either of those estimates. Likewise I couldn't find 30+-5 km at 1 Ma and 65+-5 km at 6 Ma (line 255) in Lewis (1983) either. The estimate from half-space cooling that I am familiar with is ~10 sqrt(age [Ma]) km, which would give a thickness of 45-50 km for 20-25 Ma, 24.5 km for 6 Ma or 10 km for 1 Ma. [See also my paper doi:10.1093/gji/ggw040 figure 4, top right, which shows that this gives a very good fit with several recent tomography models until about 120 Ma.]
If the crust cools very rapidly, as Lewis (1983) writes, you could add crustal thickness on top of this, but this would still be less than the values you give.
This of course depends on what temperature you assign to the base of the lithosphere. My estimate is for T=T_surface + erf(1)*(T_mantle-T_surface) + depth*(adiabatic temperature gradient) where erf(1)=0.843. With realistic numbers, this is quite a bit less [around 1150°C at 50 km depth] than the 1300°C which you assign to your LAB. I think for 1300°C the viscosity would be only slightly higher than for the asthenospheric mantle, so I think material wouldn't behave like a rigid lithosphere; therefore a lower temperature would be more appropriate for defining the base of the lithosphere, and this would give quite a bit thinner lithosphere values than what you state.
But in any case, it is the temperature which matters for melting, and for the anomaly offshore Nicaragua I would hence estimate around 1150°C, slightly more than your estimate [can you actually be more specific how you obtain the temperatures, as in Figure 8b?], but still way too low for melting, so it does not affect your conclusion that it cannot be melt. Related to this, it is not clear to me which curve in Figure 3b corresponds to which water content. There are three values (100, 200, 450 ppm), two dashed blue lines, three continuous blue lines.
In the following my few more comments:
line 47: "tend to align towards each other". It is not clear how this would work, because both Nazca and Cocos plates are bounded by two spreading ridges, which are spreading in different directions. From Figure 2, in general nothing seems to be aligned with each other.
Figure 1 bottom right: I think the color scale should be log(sigma_y/sigma_x). Otherwise it is not clear how the ratio could possibly be negative.
On line 173 in Table 1 caption you refer to Fig. S13 and Fig. S14. However, I don't see any supplement with this manuscript.
line 190: "in a function" should presumably be "is a function"
line 199: should be "consistent with"?
Figure 8: Color scale .. it is missing what the numbers actually are
line 237/238: "formation of garnet-rich connected networks"
line 445: I think asymmetry along the southern EPR is likely a cause of dynamic topography. That is, there is the "superswell" on the Pacific side, above a hot upwelling above the Pacific Large Low Shear Velocity Province.
line 513: This refers to Figure 1b, so I suggest to include labels (a) and (b) in the figure. So far, it is only left and right. In section 4.2.3 in general, I am wondering why you don't just adopt the view that the conductivity is relatively lower in the subduction zone due to the relatively lower temperature. I mean, if material enters the subduction zone, its conductivity does not get lower, but the conductivity is plotted relative to the global average (I assume) at a given depth, and that average presumably goes up with depth (as average temperature goes up), so relative to that average, conductivity of a given material "package" goes down.
Author Response
Dear reviewer,
Thank you for your time and consideration.
Please find attached my response to your comments.
Best regards,
Thomas P. Ferrand
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This is novel and interesting investigation.
However, the topic raised in this article requires an expansion of the bibliography. Among the recommended list, I can mention
Ern Ernst, W.G., 2009. Archean plate tectonics, rise of Proterozoic supercontinentality and onset of regional, episodic stagnant-lid behavior. Gondwana Research, 15, Nos. 3–4, 243-253.
JackJackson, J., 2002. Strength of the continental lithosphere: time to abandon the jelly sandwich? Geol. Soc. of Amer. Today, Sept. Issue, 4-9. Kirdyashkin, A. A. and Kirdyashkin, A. G., 2014. Forces acting on a subducting oceanic plate. Geotectonics, 48, No. 1, 54-67.
Pilchin, A.N. and Eppelbaum, L.V., 2009. The Early Earth and Formation of the Lithosphere, In: (Eds. Anderson, J.E. and Coates, R.W.), The Lithosphere: Geochemistry, Geology and Geophysics, Nova Science Publishers, N.Y., USA, 1-69.
Besides this, I propose that Conclusions must be formulated more clearly.
Author Response
Dear reviewer,
Thank you for your time and consideration.
Please find attached my response to your comments.
Best regards,
Thomas P. Ferrand
Author Response File: Author Response.pdf
