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Lecture 18

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Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC201 Lecture 18 Notes Metamorphic facies – if we look at plot of pressure vs temperature, then we can define these domains with different names. For example, we define an area on the graph called greenchist with tempera- tures around 400 and 7atm pressure. These domains contain specific types of rocks, which can be identified and give hints to what the conditions of the environment were. Greenschist facies is green because of chlorite, a dominant mineral in the domain. Amphibolite facies has lots of amphibole mineral in it. Shales contain a series of other minerals, due to recrystallization, which changes the mineral content. The benefit of using this chart is that identifica- tion can be done right in the field by a geologist, with no lab analysis. The rocks tell what type of geological environment the local rocks form. If there was a subduction zone present long ago, you can identify it by the rock types. The sub- ducting plate is coming from the surface, so its going to be relatively low temperature. Rocks are great insulators, so it starts at the surface cold, and stays cold for a long time. Above the sub- ducting slab, the temperatures would be higher. So there will be a juxtaposition of facies, due to the higher temperature of the above plate. There might be blueschist facies on the subducting plate, and then on the conti- nental crust, it will change to greenschist, due to the increase in temperature. If erosion occurs on the surface, some time later, we can see rocks that used to be underground. These rocks exhibit these nice trends, were changes in tem- perature are reflected by observing nice shifts between different facies. Paired metamorphic belts – juxtaposed facies are called metamorphic belts be- cause they are found next to each other. Profile three is continental collision, which is shown below. This is the type of colli- sion that is forming the Himalaya moun- tains right now. Notice the isotherms un- der young mountains. Under these moun- tains, it is warmer. It is also warmer under the active rift. A rifting environment would have a much flatter path on the metamorphic facies. The number 2 path would reflect this. The rifting environment heats the plates, but doesn’t add any pressure to the environ- ment. The bottom line is that by looking at the sequence of facies of rock, you can tell what type of environment the plates are in. Continent-continent collision = path 3 – greenschist to amphibolite Subduction zone = path 5 – blue schist in subduction plate, greenschist in top plate, increase in pres- sure Continental rift – path 2 – increase in temperature without increase in pressure, may even take path 1 This type of work is called petrology, which in- volves identifying rocks and drawing conclusions of what type of environment was present. Note the hornfels facies. This is only an in- crease in temperature, and no increase in pres- sure. This is because the igneous intrusion caus- es an increase in temperature, but the rocks stay at the same depth. This is shown in path 1. The green rocks represent low-grade metamor- phic rocks, and the purple represents high-grade metamorphic rocks. As you move inland, the metamorphic grade goes down. What does this tell us? High grade means high temperature and pressure. Low grade means low temperature and pressure. This area is relatively flat. There is definite symmetry in this structure. It is not terribly complex. As the rock layers get folded, the edges expose younger rocks. Higher-grade metamorphic rock must have been deeper and for longer, so it will be older. Here we have a U-shaped curve. It looks like the fold is dipping towards the North. There may have been two episodes of folding at the top, because the lines are not very clean. The metamorphic map- ping is giving a really good image of the past mountains and folding. Anticline - an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. On a geologic map, anticlines are usually recognized by a sequence of rock layers that are progres- sively older toward the center of the fold because the uplifted core of the fold is preferentially eroded to a deeper stratigraphic level relative to the topographically lower flanks. So on our map, the purple zone is the oldest, and from the layer of the Earth, while the green layer is the youngest. A syncline is a fold with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold. So the map shown above is constructed from anticlines and synclines. To identify which is which, look at whether the top layers of rock are getting progressively younger or older. The purple area in New Hampshire is high grade and is going westward, younger, so that fold is an anticline. The little red area directly under the word Vermont is a syncline, as the light beige is right of the little red patch. Intensity of the folding varies. Quick changes in colors indicate tight folding,
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