Sunday, 28 March 2010

Active reading, continued

Yesterday I posted my understanding of the techniques of “active reading”, and shared the results of my initial glance through of an article. Today I shall share my notes from that article, Metamorphism and deformation at different structural levels in a strike-slip fault zone, Ross Lake fault, North Cascades, USA.
Skipping the abstract at the moment, and jumping straight to the article itself, here follows my notes from my first hour of active reading (plus two five-minute breaks to read blogs for pleasure) I shall continue the rest of the article in my next post, once I’ve read & made notes on the rest of it.
Paragraph 1 introduces the concept of strike-slip faults during orogenesis and raises the question “can a strike-slip fault include sufficient vertical component to drive both burial and exhumation during a single orogenic event?”
Paragraph 2 lists studies which have documented exhumation of mid- to lower-crustal rocks in strike-slip fault zones.
Paragraph 3 introduces the fact that some strike slip faults juxtapose rocks formed at very different crustal levels (metamorphic next to unmetamorphosed) and reminds us that this can sometimes be due to a vertical component to the fault movement.
Paragraph 4 introduces the field area for this study (the Skagit Gneiss & adjacent units in the transcurrent Ross Lake fault zone (RLFZ), mentions that it contains high-grade rocks from a continental arc during mid-Cretaceous shortening (and lists references, of course). It goes on to report that the exhumation of these rocks occurred in the Eocene and states that the area is of interest because it records P-T-d (pressure-temperature-deformation) histories which vary on either side of the fault. It points out that such faults usually have complex P-T-d histories, but this one is unusually well preserved.
That was the final paragraph of the introduction, therefore this is an appropriate time to look back over it and see what I think the main message is thus far. Thus far I think that their main message is that they chose this field area because they felt that the rocks exposed on either side of this strike-slip fault would contain enough information to say something about the P-T-d history of the area, and that they feel that while it is primarily a strike-slip fault, it also has a vertical component, and so can have been responsible for the exhumation which brought these rocks to the surface. The next section will cover the Eastern margin of the Skagit Gneiss, first focusing on the Ross Lake Fault Zone.
Paragraph 5 lists the location of the Skagit Gneiss and names the bounding faults
Paragraph 6 lists the general age of the Skagit Gneiss, describes the main rock types within the unit, the general structural style, lists the specific ages calculated from previous geochronological studies, and lists published geothermobarometric estimates (including the fact that the decompression was nearly isothermal).
Paragraph 7 introduces the Ross Lake Fault Zone (RLFZ), mentions that it is high-angle and has usually been dextral strike-slip (though has also experienced reverse slip in the Palaeocene and dextral-normal (down to the east) shear in the Eocene), and it briefly contrasts the high-grade & plutonic rocks on the west side with the lower-grade units on the east side, points out that the degree of difference in grade changes along the length of the fault
Paragraph 8 describes the RLFZ in more detail at the NNE margin of the Skagit Gneiss, mentioning a series of splays separating structural blocks (and naming several of the different units in different blocks), then goes on to describe specifics of the fault in two sub areas along the length just subscribed (it was necessary to compare the text to figure 2 to determine if they the paragraph had zoomed in to greater detail or if it had switched to a different region).
Paragraph 9 this paragraph describes the RLFZ in the region to the south of the last paragraph, mentioning that it is also splayed, naming the units in each splay, and pointing out the large difference in metamorphic grade across the fault in this area.
Paragraph 10 describes the segment a bit further south from last paragraph as a “left-stepping, dextral strike-slip step-over shear zone” and names the units and mentions their metamorphic grades juxtaposed by this zone.
Paragraph 11 describes the displacement history of this southern segment of the RLFZ, giving timing and direction of movement for at least three different time periods and points out that this is similar to the movements in the northern section as well.
Paragraph 12 relates the movement of this fault zone to “big picture” models about the attachment of the North Cascades and associated terranes to North America between c. 66 and 56 Ma.
This being a section break, it is time to look back over this section. This section focused specifically on the fault zone, the information known about the timing and direction of movement in the zone (including how it has changed over time) and contrasting the (sometimes very) different metamorphic grades on each side of the zone. The next section moves on to the Ruby Mountain and Elijah Ridge sub-section of the Eastern margin of the Skagit Gneiss section of the paper.
Paragraph 13 describes the units which crop out on Ruby Mountain and their deformational style, and mentions that the RLFZ should be cutting through units in this area and/or on Elijah Ridge (since it is exposed to the north and the south of this area).
Paragraph 14 describes the units on Elijah Ridge, compares them with the above mentioned Ruby Mountain units, mentions that most have an “intense constructional fabric, locally mylonitic in places”, and give the details of the one previously published P-T estimate from here (650 C, 8 kbars) pointing out that it is similar (though slightly lower than) to estimates published for the structurally deepest part of the Skagit Gneiss ~10 km to the west. It also mentions that mass balance calculations indicate that this area and the Skagit Gneiss metapelites have different protoliths.
Paragraph 15 gives more details of the units of Elijah Ridge and mentions that they have been intruded by „variably deformed and metamorphosed hornblende porphyry and hornblende gabbro, and undeformed late granitoid dykes. It also mentions the presence of schists which are interpreted as metamorphosed versions of the Methow terrane (which is mentioned in the introduction as being unmetamorphosed on the far side the RLFZ a bit to the south of this area).
Paragraph 16 compares the rocks exposed on these two peaks with those cut by the RLFZ to the north and the south of this area, but emphasizes that the location of the RLFZ in this area is unknown. It also points out that there is higher grade metamorphism of the Methow terrane rocks here than is present to the north and the south of this area and states that the driving force for that metamorphism is unknown and that the tectonic/metamorphic significance of the Skagit Gneiss-Napeequa unit contact and the origin of the constructional fabrics near the contact is unknown.
Paragraph 17 explains that the current study was undertaken to address the questions mentioned in the last paragraph, lists the areas from which they took samples and what sort of work was done on the samples (thermobarometric analysis, microstructural analysis, and 40Ar/39Ar analysis). It also directs the reader to the tables showing the analytical methods and other supporting information.
This ends the section on the Ruby Mountain and Elijah Ridge sub-section of the Eastern margin of the Skagit Gneiss section of the paper. It described the units present, compared them with units cut by the RLFZ in other areas, indicated that the location of the RLFZ in this area was unknown (but it has to be around there somewhere) and raised questions which they hoped to answer with this study. The next section will cover their results from their petrography and mineral chemistry studies.

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