Growth fault

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). Because the fault plane flattens into décollement, the downthrown block moves basinward and the displaced sedimentary layer of the downthrown block bends close to the fault plane forming rollover anticline, synthetic and antithetic faults. == Accompanied structures ==

Growth faults have two blocks. The upthrown block – the footwall – is landward of the fault plane and the downthrown block – the hanging wall – is basinward of the fault plane. Most deformations occur within the hanging wall side. The downthrown block slips downward and basinward relative to the upthrown block. This is caused due to the differential load of the overlying sediments and the high mobility of the lowermost low density layer. Those structures are usually formed simultaneously and are thought to be created as a result of sediments filling the gap that is formed hypothetically by the basinward movement of the downthrown block. == Driving force ==

The main driving forces of the growth faults are the deferential sediments load and the low density layers - evaporites or over-pressured shale - that are formed during or right after the rifting process. Growth faults are located mainly within passive margin sedimentary wedges where tectonic forces have minimum or no effect. These passive margins receive millions of tons of sediments every year which are concentrated on the continental shelf below base level and above areas where the water velocity is no longer supporting the particles weight. This zone is called depositional center (depocenter for short) and has higher sediments load. Evaporites and/or high-pressured shale layers have the ability to flow because of their high mobility and low viscosity characteristics. Rift zones are partially restricted and have limited access to open oceans during rifting period. they are affected by sea level changes and climatic variability. Earthquakes arise and result from the release of the force along the growth fault plane. The depocenter's exact location continuously changes because eustatic and relative sea level are continuously changing as well. As a result, many different growth faults are created as sediment loads shift basinward and landward. == Importance of growth faults ==

Growth faults have great significance for stratigraphy, structural geology and the petroleum industry. They account for relative and eustatic sea level changes and accommodation space left for new sediments. Likewise, growth faults are connected directly to the subsidence in the coastal and continental shelf areas. Moreover, they explain lateral thickness variation of sedimentary sequences across these faults. The updip area on the downthrown block is the main target of oil and gas exploration because it has synthetic and antithetic faults and rollover anticlines. These are considered as structural traps preventing oil and gas from escaping. The offset of sand and shale beds occurring along fault planes bring sand and shale layers in contact to each other. This blocks oil and gas lateral movements and enhances vertical movements. At shallow depths, growth faults and their accompanying synthetic and antithetic faults are considered as vertical pathways for groundwater to flow and mix between different groundwater reservoirs. At deeper areas, these conduits help geologists track petroleum migration up to their final destinations. Oil and gas exploration is usually concentrated very close to these faults in the downthrown block because these are considered as structural traps preventing oil and gas from escaping. == Future work ==

Because the growth faults and their accompanied structures control both horizontal and vertical migration of the underground fluids, most of the current and future studies focus on constructing three-dimensional models in order to understand geometry and kinematics of these structures. This will unravel the mystery behind the groundwater contamination due to reservoir mixing, and track the oil and gas migration pathways. == References ==