Urgent Earth Science Homework (Tectonics & Seismic Data Interpretation)

D epth (km

)

Question 1: Tectonic analysis   Here is an outstanding seismic reflection profile recorded a few years ago. The image provides a slice down through the crust and upper mantle extending down over 100 km. This profile was acquired using surface sources that generated P-waves. The P-waves reflected from significant compositional or structural changes in the lithosphere; the waves then travelled back to the surface where it was recorded. This profile was then produced after computer processing of the data. When you look at a reflection profile like this, you are essentially looking at the ‘fabric’ of the Earth. Other information (e.g., refraction speeds, densities, etc.) is needed to determine composition.

0 A 15

 

34 km B 30 50

C 70

       

100 km 200 km 300 km  

Zoom in (to about 400%) or you’ll miss much of the detail. Alternatively, print it out on a good quality printer. Note that the data resolution is actually far better than this - I’ve decimated it just to minimize file size. For the questions, you can describe locations on the figure using the depth/horizontal scale (e.g., 70kmDepth x 120kmHoriz). It isn’t necessary for you to answer all of these questions ‘correctly’. I’m more interested in you trying and explaining your answers.

90    

110

  Questions: 1) The crust/mantle boundary is around 34-35 km depth here. Why is that so obvious in the image, so much reflectivity above and so little below?

[A couple of sentences] 2) This profile is crossing what is (or was) a plate boundary. What is the plate tectonic setting that is being imaged here? Explain it clearly (likely assisted by drawing a simple, labelled sketch). Show the directions the plates are moving. What about directions of movement suggested by the ‘fabric’ in the image? Consider what is going on at C. What is going on at A? Think about what motions could create the structure that you are seeing. Think about the crustal thickness - does it all make sense? [1-2 paragraphs, sketch or labelled profile]

  3) Based on your answer in (2), would you expect to find evidence of volcanoes on the surface in this region? Why or why not? If so, where?

[2-3 sentences] 4) What about earthquakes? Based on your answer in (2), where in this section would you expect to find earthquakes? [Could add to your sketch]. Why there? What kind of earthquakes would you expect? [2-3 sentences]   5) Where is a possible location that this profile could be from (or have an analogous tectonic situation) Why? [A couple of sentences]  

If you have any questions, jot them down as we’ll explore this in the discussion.