# Fault related folds

Folds are a result of rock deformation due to tectonic or other physical forces and, in the case of this page, compression/shortening of crustal regions. Oftentimes, these folds can form because of rock shortening due to thrust faulting. The structure and geometry of these folds is a direct result of the geometry of the fault, excluding detachment folds. There are three main types of fault related folds. These include fault-bend folds, fault-propagation folds, and detachment folds.

## Attributes

### Fault Components

• Detachment Fault - Bottommost portion of fault system that runs parallel to rock strata.
• Hanging Wall - Rock strata above fault plane.
• Foot Wall - Rock strata below fault plane.
• Hanging Wall Flat - Rock strata above fault plane parallel with fault.
• Hanging Wall Ramp - Rock strata above fault plane oriented at angle to fault.
• Hanging wall ramps and flats must correlate with equal number of foot wall ramps and flat.
Fig. 1. Fault and fold components. Created by Paul Gilbert
• Foot Wall Flat - Rock strata below fault plane parallel with fault.
• Foot Wall Ramp - Rock strata below fault plane oriented at angle to fault.
• Foot wall ramps and flats must correlate with equal number of hanging wall ramps and flats

### Fold Components

• Limb - Side of a fold
• Axial Surface - Plane through change in dip of fold related to underlying fault. Surface bisects angles between rock unit dips.
• Active Axial Surface - Axial surface through fold relating to change in fault dip. A fixed surface through which material flows.
• Passive Axial Surface - Axial surface through fold not related to change if fault dip. A migrating surface which moves with fault block.

## Fault-Bend Fold (Simple Kinematic Model)

Fault-bend folds are formed when a fault block bends as it slides over a non-planar surface.[1] When this occurs, it results in a folded region in the hanging wall. The geometry of the fold directly reflects the geometry of the thrust fault. This process can happen along multiple bends, however, only a standard-model, singe-bend fold geometry will be discussed here.

Figure 2. Fault-bend fold stages. Modified from Suppe, 1983.[1]

The system begins with a detachment fault that has a step in it and horizontal strata in the hanging wall. This step results in two fault dip direction changes as the fault goes from flat to ramp and back to flat again.

Stage 1: At this initial stage, there are two active axial surfaces. The first active axial surface (B) projects through the strata from the bend in the fault going from horizontal to angled. The second active axial surface (A) projects through the strata where the fault becomes horizontal once more (Fig.2).

Stage 2: As the hanging wall block moves over the fault step, a small fold is created and two passive axial surfaces are formed (B', A'). With the continuation of slip, A' and B' migrate along the fault (Fig. 2).

Stage 3: Once B' hits the active axial surface A, it stops migrating and replaces A as an active surface while A becomes a passive surface and starts to migrate along the fault. At this point the fold has reached its maximum vertical growth and will now grow only laterally, in this simple model (Fig.2).

Such systems can be created in conjunction with fault-propagation folds, duplexes and a variety of other forms. This simple kinematic model's angular relationships are uncommon in real structures due to heterogeneous and differential shear through strata.[1] However, they are useful as a basis on which to understand this process. A real world example of this structure can be seen in a 2D time-migrated seismic line from the Lost Hills of California (Fig. 3).[2]

Figure 3. 2D time-migrated seismic line through southeast Lost Hills, CA. Structure showing fault-bend fold geometry. Modified from Morse, P; Purnell, G; and Medwedeff, D; 1991[2]

## Fault-Propagation Fold

Figure 4. Fault-propagation fold stages. Modified from Morse et al. 1991[2]

Fault-propagation folds occur when a thrust fault steps through overlying strata and terminates up-section while transferring its shortening to a fold which develops at the fault tip.[3] The size of the fold depends upon the amount of slip that occurred within the fault. The resultant folds typically have gentle sloping back limbs and steep front limbs.[3] This structure can once again occur in a wide array of forms but the simplest self-similar model will be discussed here. This model implies three stages of fold growth and fault slip. Throughout the growth stages in this model the dip in the front and back limbs remain constant. The faulted rock units will always have two axial surfaces projecting through them and the non-faulted rock units will always have four.

Stage 1: A detachment fault steps into overlying strata. Upon this initial step-up, four axial surface are created (A, A', B, and B'). This fault has stepped through only one rock unit in Figure 4 and this slight slip has resulted in a slight deformation above the fault zone. Notice, the axial surfaces are projecting through anticlinal and synclinal bends. Axial surface B is always related to a direction change in the fault. Axial surface A is always related to a synclinal feature upon which the fault terminates (Fig. 4).

Stage 2: As the amount of slip within the fault system increases, the fold grows vertically and horizontally. The fault now extends through three rock units (Fig. 4). In the image, axial surfaces A and B' project down into the fold and then join together to become one axial surface. The new axial surface does not bisect the angle between A and B'. The result is an elimination of the flat topmost portion of strata and an anticlinal feature underneath (Fig. 4).

Stage 3: Slip is continuing to increase and the fold is continuing to grow. The fault now propagates through four rock units. The flattened strata between axial surfaces A and B' is decreasing in size as the limbs increase. Notice, in all stages, that the point in the non-faulted rock strata at which axial surfaces A and B' meet and become one new surface is the exact point in the faulted strata that the fault terminates (Fig. 4).

Once again, a fault-propagation fold can form in conjunction with a multitude of other structures. Shear differences and heterogeneous strata can result in a vast amount of variety. Another commonly used model to describe the fault-propagation fold is the trishear model. This model describes the slip of the fault dying out through a triangular deformation region at the tip of the fault.[3] This typically consists of multiple imbricate thrust faults. In this model the front limb is very steep and may be overturned.

Fault-propagation folds may also result in breakthrough structures where a fault is breaking through the strata faster than the fold can grow. This will result in a surficial expression of faulting in the front limb of the structure.

## Detachment Folds

Figure 5. Growth development of a detachment fold.

Detachment folds form as a result of a weak sedimentary unit underlain by a detachment fault and overlain by thicker and more competent units.[4] This incompetent unit may be an over-pressured shale, salt, weak carbonate, etc. The nature of the fold in this particular case is dependent upon the thickness, composition and mechanical nature of the units (e.g. ductile, brittle, etc.)[4]. As the detachment fault moves laterally, the incompetent unit begins to deform in a plastic-like manner while the competent units above it begin to buckle and fold and typically form syclinal and anticlinal features (Fig. 5). This incompetent unit makes up the core of these structures.

The two geometric types of these structures are know as disharmonic and lift-off folds. Disharmonic folds are folds in which the outer units remain parallel to each other while the inside units do not. [4] Lift-off folds are the result of further compression on a disharmonic fold. [4] They form isoclinally folded structures. These folds are analogous to folding a piece of paper. Imagine pushing a sheet of paper from both ends. As the sheet is compressed, a disharmonic fold will begin to form. With further compression, a high vertical form forms where the sides of the fold limbs are nearly touching. This would be the isoclinally shaped lift-off fold.

The geometry of these folds relies on the rule that if pulled apart, the rock units would resume their original amount of material. With this being the case, the amount of shortening is proportional to the amount of growth of the fold. Consider figure 6, (as shortening commences) the amount of shortening (A2) and synclinal deflection (A3 and A4) must be equal to the amount of material flow into the anticline of the structure (A1). Likewise, subtracting the synclinal growth

(A3 and A4) from the anticlinal growth (A1) will give the amount of shortening that is taking place.

Figure 6. Kinematics of a detachment fold.

The detachment fold interpreted in the Sarajeh Anticline of Iran is an excellent example of this structure (Fig. 7)[5]. In the 2D seismic line that has been interpreted, the disharmonic nature of the fold can be observed, as well as the pre-growth and syn-growth units of the fold. The pre-growth units are characterized by their constant thickness in the bottom portion of the fold that overlies the incompetent core. In this example there seem to be three units of pre-growth (Fig. 7). Above the units of constant thickness and below the erosional surface there are units that begin to pinch out near the crest of the anticline. These are known as syn-growth units that were deposited while the deformation was taking place.

These structures come in a vast array of orientations and may happen in conjunction with the previous fault-related folds. These folds can also fault with continued compression and form what is called a faulted-detachment fold. The interpretation of the Sarajeh Anticline would be considered a faulted-detachment fold with a fault propagating in the direction of shear (Fig. 7).

Figure 7. 2D seismic line of the Sarajeh Anticline and related detachment fold. Modified from Chris Morley, 2009.[5]

## References

1. Suppe, J. (1983). Geometry and kinematics of fault-bend folding. American Journal of Science, 283(7), 684–721. doi: 10.2475/ajs.283.7.684
2. Morse, P. F., Purnell, G. W., & Medwedeff, D. A. (1991). 9. Case History 4 Seismic Modeling of Fault-Related Folds. Seismic Modeling of Geologic Structures, 127–152. doi: 10.1190/1.9781560802754.ch9
3. Mitra, S. (1990). Fault-Propagation Folds: Geometry, Kinematic Evolution, and Hydrocarbon Traps. American Association of Petroleum Geologists Bulletin, 74, 921–945.
4. Mitra1, S. (2002). Structural models of faulted detachment folds. AAPG Bulletin, 86. doi: 10.1306/61eedd3c-173e-11d7-8645000102c1865d