need thermal parameters of the crust

[eqsarah]

So I've been reading over your paper for how you got thermal parameters of the crust, and I was wondering if any of the reviewers commented on the low heat production for the granite from you calculations from Whittington?

Also, do you suggest that I use this same method to calculate the Moho/radiogenic heat/thermal diffusivity? I'm getting ready to dive into the literature to see if there are specific values too.

[cossatot]

The reviewers (all of whom are quite knowledgeable) had no problem with the Whittington values. I think are were some of the better experimental data, but I don't have a super deep knowledge of the field. Whittington's well recognized as an expert on granites, though. I got some gentle admonitions from Ehlers about the temps in the lower crust but I reminded him that I state clearly in the manuscript that I don't believe it. The results are mostly sensitive to the upper crustal geotherm.

Don't worry too much about picking the 'right' value, for several reasons.

One, we can and will experiment with values to find ones that fit the data. I suspect that EM is right and there is a hint of a ZFT partial annealing zone and maybe a zHe prz as well. Also given the assumed duration and rate of modeling, we should be very sensitive to the geotherm here.

Two, we can't change these values during a model run. We can do different runs with different values, of course, but all thermal parameters are fixed for each simulation. This is in wild divergence with reality, but without re-writing a part of Pecube that I know neither the math nor the Fortran, we are stuck with it. Fortunately, so is everyone else in the thermochron community, so we won't get nailed--this is the cutting edge, or just behind it, even if it's not the sharpest blade at times.

Third, model results are and will always be conditional on the data and modeling constraints. The aphorism by George F Box about all models being wrong but some being more useful than others applies in a major way, and geoscientists of all flavors (except for certain seismologists, it sometimes seems) are very cognizant of this and don't put undue certainty on model results.

That being said, when we are finished with this, it may be the best thermochron modeling that exists in the western US. The status quo is still the Excel trendline--not the highest bar to leap over. We may not nail all of the thermal parameters exactly, but we will not be wrong by a huge factor about the overall extensional history.

[eqsarah]

Thanks for reminding me to keep things in perspective. I definitely find myself getting bogged down in the little details sometimes. I don't have a very good knowledge of the field either, so I just wanted to check with the Whittington stuff in case there had been any issues that we could easily avoid it this go round. I'll work on getting these numbers to you late today or early tomorrow.

[cossatot]

Sarah,

You mentioned 1200-1300 for the Moho temp as 'typical' values. I think that these values are not unheard of, but they're really really high. In Tibet, there are mantle xenoliths that indicate these temperatures, which makes sense when combined with other evidence for a delamination event. But basically, it takes a delamination event or something similar in order to get to this value. Also keep in mind that the Tibetan Moho then was like 65 km depth (now 80 km). So that makes a difference. I know that the Eocene Basin and Range had pretty thick crust too, though...

I am looking for some papers as well.

[eqsarah]

Those seem hot because I was "quoting" from memory what a prof here at UNLV said--I just thought about it more and I think I must be remembering the temperature between lithosphere and asthenosphere that he mentioned (which I think makes much more sense). The depth to the Moho now is about 25-30 km and has been estimated to be about 50 km by Gans (1987) prior to the onset of Cenozoic extension. If I had thought about those numbers (that I actually know) well, then maybe I would realized 1200-1300 is way not smart. (Things like this make me wonder how the hell I passed comps).

There are papers for xenoliths in the Southern Basin and Range (can't remember the author just remember its near Wikieup, AZ--either way though not useful since the SBR does not equal NBR). I remember reading a paper by Wang et al., 2002 for mantle melting, but its mostly in the CBR. I'll look through it again to see if there about Moho temps (not asthenosphere/lithosphere), but I don't remember anything out of there before.

[cossatot]

OK. Also, what we literally want is the time-averaged temperature at 35 km or whatever we want the base of the model to be (it's 35 km now but this can be changed). Also even though it's time-averaged, we probably want to be closer to the Oligo-Miocene temp than the current one, since that's when extension was. It would have to be damn hot at the base of the model for there to be an effect on samples on the surface...

[eqsarah]

I know its not super important to pick specifically "right" values, but I just want to make sure I understand the reasoning you chose the diffusivity you did for SLR.

If I did the Whittington et al 2009 math correctly, the diffusivity of ~20 would correspond to temperatures of the crust of ~526-726 C. I know from your paper that you definitely tested different values for radiogenic heat production, but I wasn't sure if you tested different values for k in your informal testing stage? Or did you choose this k value as kind of an average of the model or in the middle of the model since the Whittington et al 2009 data (and other folks) show that k changes with temperature changes? Since PeCube is going to assign the same value everywhere, a middle model temp and then calculating k would make sense to me. But I wasn't sure if this was how you arrived at the value or not.

If so , then in the case of the NBR then we would want to use the middle value based on whatever Moho temp we settle on. Still working on Moho temp though....

[cossatot]

Yeah, I went for a mid-range value on diffusivity.

For Moho/ model base temp, I think a value near 900 C is probably a reasonable starting point. If we can't fit the data otherwise then we can bump it up.

[eqsarah]

Sounds like a good plan to me.

How about radiogenic heat production? Do you suggest starting with your way low end of 10 C/Ma and moving up from there to whatever upper range makes sense with the data?

I guess the thing I'm struggling with is there is this abundance of data for heat flow in the Basin and Range, but its all present day data....which is probably not what we want to use for Oligo-Miocene values, but it seems like a shame not somehow incorporate these data. Can we use them for comparisons to the solution of the model at present day?