4. A continental margin has a thick layer of sediments overlying metamorphic bas

Question

4. A continental margin has a thick layer of sediments overlying metamorphic basement rocks. It collides with another continental mass, and the compressive forces deform it into a fold and thrust belt. During the deformation, which of the following geologic formations would be likely to behave as brittle materials and which as ductile materials? (a) sedimentary formations in the upper few km (b) metamorphic basement rocks at 5 to 15 km depth (c) lower crustal rocks below 20 km 5. In which of these rocks would you expect earthquakes?

Explanation / Answer

1.Both the sedimentary formations in the upper few km and the metamorphic basement rocks at 5-15 km will show a brittle nature as they are at the upper part associated with low pressure temperature conditions, they will comparatively possess greater strength compared to the lower crustal rocks which are at higher pressure temperature conditions. 15 km depth is the brittle ductile transition zone,below this depth rock strength decreases because fracture becomes closed and and temperature is higher making the rock behave in a ductile manner. so the lower crustal rocks below 20km will behave in a ductile manner.

2. Because of the brittle nature of the upper crustal rocks earthquake will be confined within the upper part of the lithosphere in this case. Hence we will expect shallow focus earthquakes in the sedimentary formations and metamorphic basements.

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