Growth Fault

Growth Fault a fault in sedimentary rock that forms contemporaneously and continuously with deposition so that the throw increases with depth and the strata of the downthrown side are thicker than the correlative strata of the upthrown side. Growth faults are particularly common in areas of high sedimentation rate and are generally associated with thick deltaic successions.

Growth fault/rollover systems are common structures of sedimentary basins and, in particular, of deltas and passive margins. They often develop where incompetent rock layers (e.g., salt, anhydrite, under compacted clays) are present at depth offering potential zones of dtcollement and allowing gravity driven deformation to occur with or without differential sedimentary loading (e.g., deltas). Extensive seismic exploration by the oil industry has provided a large number of images of growth fault in various geological environments. Consequently, and not least because of their economic interest, these structures have stimulated numerous studies ranging from purely academic to entirely practical applications.

Normal growth faults. associated with rollovers are generally called "listric faults". This term implies that the fault is an upward concave movement surface that transforms steeply plunging displacements at the surface to nearly horizontal ones at depth. The layers deposited during displacement on the hanging wall block are progressively bent downward, thus giving rise to so-called rollovers. Listric faults are commonly assumed to have a constant geometry in time. The geometry of the rollover is regarded as a record of the history of synchronous deformation and sedimentation.

Sedimentary layers have different geometry and thickness across the fault.

The footwall – landward of the fault plane – has undisturbed sedimentary strata that dip gently toward the basin while the hanging wall – on the basin side of the fault plane – has folded and faulted sedimentary strata that dip landward close to the fault and basinward away from it.These layers perch on a low density evaporite or over-pressured shale bed that easily flows away from higher pressure into lower pressure zones.

Most studies since the 1990s concentrate on the growth faults' driving forces, kinematics and accompanied structures since they are helpful in fossil fuel explorations as they form structural traps for oil.

Growth faults maturation is a long term process that takes millions of years with slip rate ranges between 0.2-1.2 millimeters per year. It starts when sedimentary sequences are deposited on top of each other above a thick evaporite layer (fig. 2).

 

A growth fault is initiated when the evaporite layer can no longer support the overlying sequences. The thicker and denser portion applies much more pressure on the evaporite layer than the thin portion. As a result, a flow within the evaporite layer is initiated from high pressure areas toward low pressure areas causing growth ridges to form below the thin portion. Also, sinking zones are noticed among these ridges at areas where thicker and denser layers form

Figure 2. Sketch showing evolution stages of three growth faults.
The black arrow shows the direction of evolution.

Consequently, the passive margin experience unequal subsidence across the continental shelf. Both the new-created accommodation spaces and the thickness of the new-deposited sedimentary layers are greater above the sinking zones than above the growth ridges. The new added layers are thicker within the footwall than within the hanging wall (fig. 2).

These variations result in an increasing of differential load intensities - unequal distribution of sediments load - across the shelf with time as more sediment layers are added(fig. 2). Therefore, the rate by which the pressure increases upon the evaporite layer below the sinking zone are much more than the rate of pressure increase upon the same evaporite layer at the growth ridges. So, the flow rate within the evaporite layer is progressively increasing as deferential load intensifies(fig. 2).

The growth ridges end up with salt diapir when the sinking zone sequences weld to the base of the evaporite layer.