Saturday, 3 April 2010

Rest does wonders for one’s attention span, but you already knew that

Rest does wonders for one’s attention span, but you already knew that

Having learned (once again) from yesterday’s mistake of putting off reading my 1000 words till I was sleepy, today I sat down and did it only four hours after I woke up. This is a much more sensible time of day to approach such a task. It felt much easier to do the reading and typing up of my notes, and what I read felt far more interesting than I remember yesterday’s reading, on the exact same topic, to have been. With luck I will remember this one for quite a while. However, the reality is that there are many, many things I wish to do with my time, and sometimes I choose to do one of them, and sometimes I choose to do one of the others. As a result it will eventually happen that I will choose to do other important tasks with my awake-energy and will once again have to choose between reading when I am sleepy or letting my run of consecutive days reading the 1000 words lapse. I’m at 87 days in a row, this time. Every time I manage to get that number up to these levels I become even more vested in continuing. My record thus far is 118 days in a row—that is only one more month. I would so very much like to pass that record. It would be fabulous to manage it for an entire year.

The below are my notes for the final 1000 words of Gordon et al. (2010)*. . It has been interesting, to me, to do this exercise, and to post my progress on a daily basis. I don’t expect anyone to have actually read my notes themselves, but perhaps my thoughts on the process are interesting to someone. Tomorrow’s 1000 words will be on a new paper

Paragraph 71 describes the fabrics (constrictional) on Elijah Ridge and mentions that the temperature for these rocks is slightly lower than on Ruby Mt. though they have similar pressures. It reiterates that before this study no evidence for high-P metamorphic conditions had been found east of the western end of this ridge, but this study found evidence on the eastern part of this ridge for an early high-P (8 kbars) consistent with Ruby Mt., the western part of this ridge, and with the deeper structural levels of the Skagit Gneiss. They conclude therefore that part of the Methow terrane in this region was buried to mid-crustal depths. They say that the late cordierite-andalusite overprint is likely due to contact metamorphism from the Golden Horn batholith (exposed 1< to the east) or other nearby plutons.

Paragraph 72 states that mapping failed to find the location of the RLFZ on Elijah Ridge or Ruby Mt., but that there is clearly a tectonic boundary between the oceanic Napeequa unit and the clastic meta-Methow rocks. The contact between these two units and the Skagit Gneiss is also tectonic, and in places where it is exposed to the north and the south is steeply dipping (> 70 degrees ), but here is it more gently dipping (35-45 degrees)

Paragraph 73 explains the above observations with a overlapping step-over zone in this region, with the units defining a duplex structure within the step-over zone. They indicate that the western fault zone is expressed as the shear zone on Ruby Mt. and the eastern part of the step-over is obscured by the Golden Horn Batholith and Ruby creek plutonic belt to the east. They relate the duplex with the NW end of the stepover and cite another paper which previously defined the stepover.

Paragraph 74 indicates that either the Napeequea originally (pre 65 Ma?) overlay the Skagit orthogneiss along a tectonic contact or the orthogneiss intruded into it, with some evidence for this early boundary showing in the shear zone on Ruby Mt. They provide two alternatives 1) the orthogneiss overlying the Napeequa unit on Ruby Mt. isn’t related to the Skagit Gneiss, and the deformation affected both the Napeequa and this orthogneiss after its emplacement or 2) it is related to the Skagit magmatism, intruding at the same time as the shear zone was active.

Paragraph 75 starts of with “in this model”, with nothing to indicate which of the many choices above is intended with the word “this”. It goes on to say that the step-over zone started during a transpressional regime, bringing together the Napeequa unit and some of the Methow basin rocks into a duplex with much crustal thickening, burring the lot, giving all three units a similar burial and exhumation history. The gradient of metamorphic grade in this region supports the notion that the Methow terrane was tilted, and the western end may have been structurally deep before the duplex (i.e. the strike-slip fault isn’t what buried these rocks to ~25 km, but it probably helped with some of the extra depth.

Paragraph 76 cites sources arguing for a change in the North Cascades at ~57 Ma from a transpressional to transtensional regime, which they say caused the extensional step-over and facilitated the exhumation of the mid-crustal rocks. They then cite another source which mentions imbrication along a pre-existing contact, and state that if that is the case then the Skagit gneiss had to have intruded into the Napeequa unit (and the orthgneiss above the Napeequa unit is part of the Skagit gneiss, just in a different spot because of that imbrication). The imbrication model includes the Methow basin rocks above the imbricated Skagit Gneiss and Napeequa units, which is compatible with the east dips of major contacts and foliation on Elijah Ridge.

Paragraph 77 States that both the step-over and imbrication models agree that there is a tectonic boundary on between Methow basin and Skagit Gneiss, and states that movement of the RLFZ is important to the relationship of these units. They cite sources saying that high T-deformation and activity continued till the early Tertiary.

Paragraph 78 states that since the Napeequa unit was not likely to have been in the upper crust at the time of deformation that they disagree with the hypothesis that the Skagit orthogneiss which is now directly under the Napeequa unit on Ruby Mt. was a flowing ductile crust underlying the non-flowing Napeequa unit. The then list again the evidence for both the Napeequa and Skagit rocks sharing metamorphic, deformational, and exhumation histories.

The final section break of the paper is here. The above section reviewed what was already known of the tectonometamorphic history of this area and indicated how their results add to that knowledge. The next section discusses the Exhumation of the Skagit Gneiss.

Paragraph 79 States that crustal thickening (> 30 km) and near isothermal decompression affected the Skagit Gneiss. They list the evidence for rapid cooling on exhumation around 57 to 45 Ma, when the North Cascades were in a transtensional regime and the RLFZ was dextral strike-slip combined with components of both reverse (pre 57 Ma) and normal (post 57 Mal) slip. Since the timing matches, they figure that the fault movement enhanced the exhumation.

Paragraph 80 states that the RLFZ incorporates different slices of the high grade and low-grade rocks in this region, and in some areas (specifically this field area) the basin rocks which are low-grade elsewhere were metamorphosed and deformed. This area is also the closest to the magmatic core of the Skagit Gneiss, so the partial melting could have weakened the crust and permitted the development of the step-over zone in this region. They state that this area is a good example of the relationships (they specify four) which develop in transtensional orogen. They state that the RLFZ is a strike-slip fault which accommodated and may even have initiated burial of sediments to mid-crustal depths and subsequent exhumation and the metamorphic rocks provide evidence which indicates a significant vertical component to strike-slip fault zones.

*Gordon, S.M., Whitney, D.L., Miller, R.B., McLean, N., and Seaton, N.C.A., 2010, Metamorphism and deformation at different structural levels in a strike-slip fault zone, Ross Lake fault, North Cascades, USA: Journal of Metamorphic Geology, 28, 117-136.

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