I know better than to put off reading my 1000 words a day from the geological literature until late I the day, yet I chose to do so anyway. Today the impulse to do so was triggered by Spring. There were very few people in our geology department today, as most people chose to leave for their Easter long weekend as early as they could. Some of the general staff were here, but were only scheduled to be here till about 2:00 pm. Around the time that they were leaving I noticed that it was a beautiful day, nice and sunny, yet not hot at all, and I felt restless. Therefore I decided to head out on an adventure, and went into the city center to admire the street artists and musicians who perform for the crowds there. I also stopped in the American Book store and picked up a new book in a favourite series.
After these adventures I managed to limit myself to only an hour of reading in the new book before crossing the street to my office again around quarter to nine, intending to finish the work I’d interrupted hours before. Instead I procrastinated by catching up on reading blogs/e-mail/etc, updated my financial records to show today’s spending, and chatted with a friend on line. As a result I didn’t start reading my 1000 till 11:30 pm, and it was difficult to find the discipline I needed to actually complete the process.
However, hard as it was to make myself do, I am now a total convert to this “active reading” thing. The need to type the below notes meant that I had to actually pay attention to what I was reading, and not just sleepily skim over the words catching one in ten. I am positive that focusing on each paragraph one at a time and typing the notes as I go is the only thing that made it possible to understand what I was reading tonight. Perhaps I can understand and retain what I read the “normal” way when I’m high in energy, but this late in the day “active” reading is required. So, without further adieu, I give you the second-to-last installment of my paragraph-by-paragraph summary of the paper I’ve been reading (see a few days back for the citation and the link to the article itself).
The next section of the paper is on the 40Ar/39Ar results.
Paragraph 63 introduces the two pegmatites from which muscovite was obtained for the dating, describes the location of the pegmatites, gives the age results (~47.1 Ma for one and ~46.9 for the other). They state that the samples were obtained from above and below the contact, and are within error of one another. They state that this means that these numbers represent the time of cooling, rather than crystallization. This ends the section on the dating, and also concludes the section on their results. The next section is the Discussion.
Paragraph 64 points out that previous studies described a fault in this area and described the rocks on either side as high-grade on the west and low-grade the east, but this study indicates that what had been called “low grade” has actually undergone sufficient burial to achieve >650 C at 8-10 kbars, or largely the same as the “high grade” Gneiss. They go on to mention that the microstructures near the contact between the two units reveal progressive deformation & a high-T constrictional shear zone between the units, & subsequent overprinting by low-T deformation. They say that therefore this area is complex & important in context with the entire Cascades region, and for strike-slip dynamics in orogeny.
This concludes the lead-in portion of the discussion; the next section is on the Previous interpretations of Ruby Mt-Elijah Ridge tectonic history.
Paragraph 65 names three different studies which have addressed the tectonic history of this area. The first called this area a suture of Insular and Intermontane belts since it puts ocean rocks structurally above continental arc rocks. The second calls it a tectonized intrusive contact without a through-going fault, saying tilting is enough to expose the largely intact Mid-Cretaceous section, and add in one more fault to account for the change in pressure across the area. The third described this area as a major tectonic boundary, since there are mylonites present in addition to the difference in metamorphic pressures.
Paragraph 66 gives one other possible interpretation (calling this area the uppermost part of the flowing orogenic crust with the Napeequea unit acting as a rigid lid of a layered crust. They describe this interpretation as analogous to migmatite-upper crust relationships observed in the hinterland of the orogen, and cite a source (but from the sentence itself it isn’t clear if the source proposes this interpretation or simply describes the relationships in the analogy). They then state that all four models can be considered in light of their new data. This concludes the section on previous interpretations. The next section is the Re-evaluation of Ruby Mt-Elijah Ridge tectonometamorphic history.
Paragraph 67 describes the units in this area (orthogneiss dominating, Napeequea over it on Ruby Mt. and part (west end) of Elihah ridge, and Methow rocks above it on the east part), the dip (35-45 to the east) of the planar fabrics of bother Napeequea rocks and orthogneiss on Elijah Ridge while the far east side of the ridge is folded (= different deformation style across the ridge).
Paragraph 68 specifics that the low-T fabric overprinting migmatite seen in the microstructural analysis occurs only in the structurally deepest part of the exposed orthogneiss at the base of Ruby Mt. the only other units to show such overprint is the tonalitic amphibolite at the Skagit Gneiss-Napeequea contact .
Paragraph 69 contains too much information for one paragraph. It states that the strain gradient near the summit of Ruby Mt. displays more constrictional fabrics near the contact between the Skagit orthogneiss and Napeequea units (both of which are L-tectonites). It mentions the pegmatite which intrudes both that was deformed and cooled at ~47 Ma. It lists the clues which lead them to conclude that the shear zone is a medium- to high-T feature. It acknowledges some lower T-overprinting in a few of the units. It suggests rheologic contrast as the cause of the shear zone, but doesn’t decide between shear zone formation during or after emplacement of the orthogneiss. It suggests that the shear zone is an old contact which preserves the early high-T deformation of both units, further suggesting that the deformation either developed along or transposed the intrusive contact.
Paragraph 70 mentions the contacts between these units in other areas, the fact that the quartzite and schist of the Napeequa unit is generally more deformed than is the orthogneiss, suggesting that the composition of each is likely responsible for that difference. It then states that the contact exposed on Ruby Mt. is the one showing the most intense high strain fabrics.
This is not a section break. However, there are no more section breaks in this paper, save for the one which precedes the final paragraph, and I’m at 1000 words read today, and 1000 more left before the end of the paper. Therefore I’m going to stop here and pick it up with the next paragraph tomorrow.
My Visit to Brazil
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