Notes on Chapter 5: Normal Faults (p. 74-95, Twiss and Moore)
Normal faults defined as "inclined dip-slip faults along which the hanging wall block has moved down with respect to the footwall block." Most have dips of about 60° .
1 Characteristics of Normal Faulting
Separation and Normal Faulting The appearance of normal faulting as repeated or missing stratigraphic sections depends on the angle of the bedding to the fault and displacement. Several examples are given on p. 75, fig. 5.2.
Folds Associated with Normal Faults
Listric normal faults- concave upward faults, dip decreases with increasing depth
Rollover folds- form in deeper strata in areas where flat-lying beds are deformed by normal faults
Drag folds- smaller scale folds that occur in more shallow strata (rollover and drag folds are illustrated in fig 5.3).
Features of Fault Surfaces Surface features of faults vary with shape, depth of fault movement, and whether fault was accommodated by brittle fracture with frictional sliding, or by ductile deformation.
2 Shape and Displacement of Normal Faults
Shape at Depth The dip of the fault changes with depth.
detachment fault- low angle fault that marks a boundary between unfaulted rocks below and a hanging wall block above
imbricrate faults- a set of closely spaced parallel faults of the same type
Systems of normal faults commonly have a main fault with susbsidiary faults and low-angle detachment faults with imbricate fault blocks in the hanging wall block. ( see fig 5.4).
Movement can be rotational or nonrotational.
A ramp connecting more shallowly dipping segments of the fault result in a fault-ramp syncline. A flat connecting more steeply dipping segments results in a fault-bend anticline. (fig. 5.5)
3 Structural Associations of Normal Faults
Normal faults tend to consist of systems of associated faults.
These smaller scale faults are synthetic if parallel to the major fault and sense of shear, and antithetic if conjugate in orientation.
graben- a down dropped block bounded on both sides by conjugate normal faults
half-graben- a down dropped tilted block bounded on only one side by a major normal fault
horst- relatively uplifted block bounded by two conjugate normal faults
horst and graben structure- a system of uplifted and down-dropped blocks
Local Normal Faults Associated with Other Structures Normal faults are generally associated with structures that require extension of the crust, such as salt domes, folds, cavities, and pull-apart structures on strike-slip faults. A geographic example of a salt dome discussed both in the text and in lecture was the Mississippi Gulf Coast. Other lecture examples were the Great Salt Lake, Salton Sea, and the Red Sea. In lecture, we talked about gravitation as a force for normal faulting as well.
ring faults- concentric normal faults where rocks collapse into a cavity
Regional Systems of Normal Faults Examples of normal faulting provinces discussed in class are plate boundaries, mid-ocean ridge systems, continental rift zones, intrusions (magmatic, salt, batholiths, hot stops, ophiolites), releasing bends and steps in strike-slip regions, gravitation, rebound following glacial melting, subduction zones, and fold thrust belts.
transfer zone- zone between a tip line of one fault and an adjacent fault within which deformation is accommodated by folding, faulting, and fracturing
transfer fault- distinct strike-slip faults in this zone
metamorphic core complexes- normal faulting may expose these deeper crustal rocks, (eg. the Basin and Range Province)
regional contemporaneous (growth) faults- normal faults which are active during sedimentation (eg. Gulf Coast). Growth faults form due to differential compaction of shale layers in a sandstone-shale sequence since shale undergoes greater compaction than sands. They also develop by the formation of a detachment at the base of a shale or salt deposit sequence.
4 Kinematic Models of Normal Fault Systems
A kinematic model is a description of the motions that have occurred on the faults in the system. The main constraints in making a kinematic model are that the volume of rock blocks must be conserved and horizontal extension in the footwall block of major detachment faults must be accounted for.
Because normal faulting increases the distance between two points on opposite sides of the fault, the fault extends and thins in order to maintain a constant cross-sectional area. If rocks below the detachment surface of normal faulting do not extend as well, then there must be a region in the hanging wall block where the same amount of shortening compensates. There are two models that account for this geometry at depth by crustal extension and thinning by either ductile or brittle deformation, and ductile inflow of material in the mantle to accommodate extension.
5 Determining Extension Associated with Normal Faults
extension e- the change in length in a given direction caused by the deformation, divided by the original length
The text gives simple equations to evaluate e for a planar nonrotating normal fault, and for rotating planar normal faults.