2. Faults are often interpreted from reflection seismic data. Even without much

Question

2. Faults are often interpreted from reflection seismic data. Even without much knowledge about seismic data it is useful to try to use such data for structural interpretation and analysis. An important thing to remember is that the vertical scale of most seismic sections is in time (seconds) rather than depth (meters) Hence the geometries that we see are not 1:1 unless we do a depth conversion which requires a velocity model. Also note that seismic data always contain noise. This means that we have to use common sense and our knowledge of structural geology and stratigraphy to make sensible interpretations a) The seismic line transects the northern North Sea Rift. What kind of faults would we expect to see in a section through a rift? b) Make a crude interpretation of seismic line and the more detailed image of its right-hand (ESE) part (bottom figure). Use the stratigraphic information is annotated on the line. Top Basement in this context is the top of crystalline (metamorphic and magmatic) rocks. The interpretation should include, but not be limited to tracing faults to depth plus labeling major horsts and grabens. Annotations on the figure should be done using colored pencils. c) Define major faults and fault blocks. What is the characteristic 1st order features of the rift in this section? d) Describe the structures. What is the general fault geometry like? Are the faults planar? Note that the faults look steeper than they would be in a 1:1 depth- converted section. e) Look for evidence of fault growth in the seismic reflectors. Parallel reflectors indicate that layers were deposited before or after rifting. Wedge-shaped sequences form in the hanging walls of active normal faults during the time of faulting f) Can we explain the deformation (faulting) by a single phase of rifting or is there evidence for more than one phase? Your evidence should come from the seismic reflection profile. Examples include, but not necessarily limited to different offsets for a fault with depth and faults not continuous to the surface.

Explanation / Answer

2.a) The seismic line transects the northern North Sea Rift. As seen in the given seismic section, there are numerous normal faults as the hanging wall is going down. Hanging wall is the block thet lies above the fault plane.

b) From the bottom figure seismic section the following interpretations can be explained :

i. The stratigraphic section is shown from the basement to cenozoic. Numerous normal faults has been detected and thesse faults are syntectonic growth fault also as with depth the faults are growing with deposition.

ii. Horst and graben structures is present (clearly seen in the upper figure )

iii. An unconformity surface has been detected between jussaic and cretaceous deposition.

iv. The unconformity is angular as the lower rocks are faulted and deformed and the upper rocks are horizontal.

v. The right hand side deposition is pre rift as this is not disturbed by the faults, then the deposition below unconformity effected by the faults are is syn-rift and the upper horizontal strata is post rift.

Due to unclear section more detail interpretation is not possible.

c) Major fault: The fault in which the hanging wall and the foot wall slides past each other for hundred of miles is called major fault. This fault can be detected by satellite images.

Fault block : Fault blocks are large blocks of the crust that has broken by tectonic stresses and extended upto hundred of kilometers. The largest fault block is called crustal block.

Rift is formed due to extension in the crust. In the given section the first order feature of the rift is downfaulted depression created in the centre, this depression is called graben. The rift is surrounded by normal faults. Half-graben structure is present on the left side because of uplifting of normal faults and rift-flanks.

d) The main structural feature is the fault. The faults are all normal and growth faults.

The faults are not planar. All fault lines are curved dipping opposite to each other. This type of curved fault line is called listric fault.

e) The fault lines are clearly visible at the upper part of the rift section but with depth fault lines are not clear, they merge into the deposition which indicates the faults are growing with depth and time.

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