Structures of Rift Systems.

Deformation during extention is brittle in the upper crust. This creates a complex of normal faults, and fault structures. A number of different normal fault regimes may develope depending on the type, shape, and extent of rifting. Common fault structures are:

1. Horst and Graben rifting.
2. Half Grabens.
3. Planar Faults.
4. Listric Faults.
5. Transfer Faults.


Fig 1.

Fig 1.
Normal planar faulting in nature, domino style (note the large slabs dipping to the left of the photograph).




1. Horst and Graben rifting.
A horst graben faulting style is produced when fualt planes have opposing dips. That is, fault planes are dipping towards eachother. In Fig 2. below, horizontal extension activates the faults, a graben drops, possibly forming a basin. Horsts are the higher blocks. Faulting in normal and planar.

Fig 2.

Fig 2. Horst graben rifting.




2. Half Grabens.
Half grabens form when the fault planes all dip in the same direction. Fig 3. below demonstrates how a domino effect is created during horizontal extension. Note how both the fault planes and blocks rotate. Rotation is opposite to dip direction (here to the right).  

Fig 3.

Fig 3. Half graben rifting.


Fig 4.

Fig 4. The Domino Model (with rotation towars the left of the diagram). Note the addition of sediments (light blue) to the top of the blocks where new accomodation space has been formed.



3. Planar Faults.
Planar normal faults are the type of fault on which horst graben and half graben extension occurs. The fault structure is symetrical during horst graben extension and asymetrical during half graben extension. Red layers in Fig 5. and Fig 6. represent deposition of sediments during and after extension.

Fig 5.

Fig 5.
Symetrical structure in a laboratory experiment.



Fig 6.

Fig 6.
Asymetrical structure in a laboratory experiment. Note domino pattern.



4. Listric Faults.
Listric faults are normal, but not planar. The fault plane has a varying dip, that is, the fault plane is concave. Fig 7. depicts a simple model. Extension removes the upper hanging wall away from the fault plane creating a vacancy. The hanging block then fails, debris filling the vacancy. It is interesting to note that the displaced area is equal to the area of the vacancy. Of course in reality extension does not occur in sudden increments such as shown in Fig 7. Rather it is progressive. Fig 8. demonstrates how sediments progressively infill the listric fault plain. When this occurs, the fault is termed a growth fault.
Fig 7.

Fig 7.
Listric fault.


Fig 8.

Fig 8. Growth fault.



5. Transfer Faults.
Fig 9. shows two forms of transfer faults that can develope in extensional regimes. Transfer faults define the lateral boundary of normal faults, such as in the bottom diagram of Fig 9. A scissor fault with a relay ramp will develope if the offset of the normal fault progressively decreases along the strike plane, top diagram Fig 9.
Fig 9.

Fig 9.
Transfer faults.