Normal Faults and Fault Systems

Normal Faults and Normal Fault Systems

• Normal faults are characterized by dip-slip kinematics that cuts away from the Earth's surface. Tectonic omission causes stratigraphy to be omitted
• Normal faults cause younger rocks to be placed atop older rocks, and low-grade rocks to be placed above high-grade rocks
• They extend layering (crust), with the assumption that layering was horizontal and right-way-up before faulting
• Understanding the relationship of style and evolution of normal fault systems to the mechanical stratigraphy of crust and lithosphere is key
• It aims to identify and characterize normal fault activity through the syn-tectonic sedimentary record via restoration methods

Geometries of Normal Faults

• Half-grabens are bounded by syn-thetic and antithetic faults
• Tilted fault blocks and horsts/grabens are other common geometries
• Extensional ramp-flat systems involve imbrication fans, ramps, flats, horses, and extensional duplexes
• Planar faults can lead to bookshelf faulting and the rotation of fault blocks

Extension and Displacement

• Normal faulting leads to relative downward movement of the hanging wall.
• There is considerable displacement in the horizontal direction

Listric Faults

• Listric faults result in rollover anticlines and growth faults
• Hanging wall deformation occurs above listric normal faults.

Crust and Lithosphere Extension

• Extension can occur through pure shear or simple shear with asymmetry
• Pure shear mode of lithosphere extension is exemplified by the Rockall basin, W Ireland
• Simple shear involves large-scale shear zones across the crust or lithosphere
• Simple shear causes a limit of significant upper-crustal extension.

Simple Shear Deformation

• Simple shear deformation in the crust results is observable via metamorphic core complexes
• Listric normal faults form in detachment zones, with mylonites forming in 10-15km zones
• This causes nucleation of new normal faults from "bottom-up", but some faults/mylonite zones remain inactive
• The lower plate bounds upward, via non-elastic rebound, and this then leads to nucleation of new detachments in culmination and exhumation
• An inactive system of normal faults in the hanging-wall as well as migration of detachment in the hanging-wall is also caused by simple shear deformation

Metamorphic Core Complexes

• Metamorphic core complexes show multiple detachment faults and tilted fault blocks
• Characterized by domal or arclike isolated uplifts.
• A mylonite belt separates the metamorphic/plutonic core from the upper plate rocks
• Mylonites are the ductile expression of detachment fault systems, which develop as discrete normal faults under brittle conditions
• Dip angles of detachment faults are typically <30°.
• Mylonites form under non-coaxial deformation; shear sense indicators indicate uniform shear
• Intensity of deformation decreases from the mylonites downwards into the core and fault breccias are at the top of the mylonites.
• Upper plate deformation is characterized by half-graben structures and tilted fault blocks.

Regional Examples

• The Basin and Range Province shows normal faulting on a regional scale
• The Sierra Mazatan in Mexico displays normal faulting
• Normal faulting occurs during orogenesis, which results in rock exhumation
• Syn-orogenic normal faults are apparent in the Alps.
• The displacement along the Brenner shear zone is ~70 km for a dip angle of 15°, resulting in 20 km of vertical throw

Significance of Normal Faults

• Accommodate extension of the crust and lithosphere.
• It is paramount for the formation of sedimentary basins.
• Provides an efficient mechanism to exhume rocks in the footwall.

Sedimentation and Faulting

• Pre-tectonic layers are:
• Parallel to the basement top and to each other.
• The same kind of rocks on both sides of the fault.
• They are cut by fault
• Syn-tectonic layers are:
• Cut by the fault and feel the activity of the fault, leading to increased layer thickness in the HW block.
• Lateral facies exist via variations (coarse-grained close to fault)
• Have a wedge geometry in the case of listric faults with layer thickness increases towards the fault
• Growth faults are faults that have been active during sedimentation
• Post-tectonic layers are those that:
• Fill the morphology existing at the end of deformation.
• They are not rotated (have ~parallel layers).
• Seal or onlap the faults unconformably covering syn-tectonic sediments

Tectonic Cycles

• Evolution of orogens and sedimentary basins can be described in multi-scale tectonic cyclic terms
• It's important to remember that cycles that are seen, are scale dependant
• There are always processes acting at different scales that influence observations

From Rifting

• Continental rifting is normally followed by drifting and the formation of a passive margin
• Extensional structures are usually buried beneath the passive margins sediments
• Syn-tectonic sediments often form diagnostic syn-kinematic wedges with lateral variations in the type of sedimentation