|In the revised manuscript by Pang et al., they authors did an excellent job proof-reading the manuscript, which reads much more easily now. In addition, they have addressed many of the comments from both myself and the other reviewer. However, I do have some remaining comments that I think the author’s should address before publication. Most of my comments relate to model set up and the implications of those choices. I also have a few comments on the presentation of results/data and a suggestion for an addition to the discussion.|
First, as I said in my initial review, I’m still confused as to why the model being used has so many seemingly extra processes unrelated to the problem at hand. For example, why was sediment and the processes of erosion and sedimentation included? Why was plasticity included if faulting and fracturing are not important to the system evolution? Why was melt transport and crustal accumulation included? My general question regarding the all of the above is “what is the goal of adding these processes and how do they affect (or not) the results?” Most importantly, in the current draft the authors do not demonstrate how the above processes affect their results. I suppose that it is possible that all of these are essential to the problem the authors are examining. Yet, even if they do not affect the results, I think the authors should demonstrate this to the reader – otherwise the paper ends up being difficult to interpret with certainty.
I still believe that the models were not in thermal/physical equilibrium when the plumes were added. Regardless of using a half-space cooling model and a 50 Ma lithosphere, the lithosphere (not just the 1300 K isotherm) is changing as crust and melt accumulate near the ridge axis. The use of melt accumulation at the ridge axis creates very large volumes of low-viscosity material that spread over time (e.g., Fig. S6), changing the slope of the lithosphere that partially governs ridge-ward flow. The lithosphere has not adjusted to this melt layer and it is unclear what the final lithosphere profile would be if the model was allowed to evolve so that all of the plate above the rising plume was created at the ridge axis.
With the melting process, equations 12 and 13 show that conservation of mass (volume) for an incompressible material is only being obeyed globally and not locally. Locally, on a given grid element or node, when mass is removed and moved elsewhere this will violate div(v) = 0 at that point. I think it would be useful for the author’s to acknowledge this in their model set up and to discuss how using a compressible formulation might affect the solution (or not) and what evidence supports their assertion.
The plume head appears to be nearly always pinned at the minimum viscosity of the model – could the authors comment on this and explain why they did not try larger plume viscosities (or lower plume T)? It seems that the viscosity contrasts are going to be very important for the behavior of the plume material.
Presentation of Results
When showing and discussing the number of PRI events at different spreading rates (Figure 1) I think the authors should normalize by the total ridge length for each velocity range. Right now, if (as a hypothetical) there was only 100 km of super-fast spreading ridge across the whole planet and no PRI, it would look like no PRI happens at super-fast spreading rates. I think scaling the values would be helpful to the reader and the authors by providing a more nuanced view of how good a sample set there is for each spreading rate.
Figure 3b-a. Can the author’s explain why the models exhibit a long lived plume tail. The author’s describe the impetus for their plume as a semi-circular heat patch at 660 km depth. Yet, looking at figure 3 there is compositionally “plume” material for >8 Myr. I’m a bit surprised about this. Shouldn’t the material have buoyantly risen above its initial depth by this point?
A discussion of likely plume head size given our knowledge of mantle parameter space would be useful here. What is a likely radius of a plume head in the mantle? What would be the expected range?
Line 296: “massive melting and crust production” is very vague and not quantifiable. Please either replace with numbers from the model or simply state that melting and crustal production occur within the model.
Line 582 – “gird” should be “grid”