Extensional Tectonic: Formation of Rift Valleys and Ocean Basins

Extensional tectonics is a geological process associated with the stretching and thinning of the Earth's crust or lithosphere.

Extensional tectonics occurs where tectonic plates are pulling apart, which can lead to the formation of structures like normal faults, rift valleys, and in larger scales, new oceanic crust at mid-ocean ridges. This process is fundamental at divergent plate boundaries.

Geometry of normal fault arrays, in cross section. (a) Parallel rotational faults. Before faulting (top), the faults are parallel and not curved. After faulting (bottom), fault blocks are tilted. In reality, crushing and small-scale faulting at the base of the blocks fill the gaps.

Key Concepts in Extensional Tectonics

Rifting: This is perhaps the most direct manifestation of extensional tectonics, where the crust is stretched, leading to the formation of rift valleys. A famous example is the East African Rift System, where the African Plate is splitting into the Nubian and Somalian plates.

Normal Faults: These faults occur when the crust is pulled apart, leading to one block slipping down relative to the other. This can create structures like half-grabens, where one side of the fault drops down, forming a basin.

Graben and Horst Structures: A graben is a down-dropped block between two normal faults dipping towards each other, while a horst is the elevated block between two normal faults dipping away from each other.

Metamorphic Core Complexes: In regions of significant extension, lower crustal rocks can be brought to the surface due to the isostatic rebound and the extensional unroofing. These rocks often show signs of ductile deformation due to the high temperatures and pressures at depth.

Basin Formation: Extensional regimes often lead to the formation of sedimentary basins where the stretched and thinned crust subsides, creating space for sediment accumulation.

Mid-Ocean Ridges: While primarily associated with sea-floor spreading, these are zones of extension where new crust is formed as plates move apart. The Mid-Atlantic Ridge is a classic example.

Back-arc Basins: These are formed by extension behind subduction zones, like in the case of the Sea of Japan, due to the rollback of the subducting slab.

Glossary of Extensional Tectonic

Accommodation zones Normal fault systems are not continuous along the length of a rift. Rather, rifts are divided intosegments, whose axes may be offset from one another. Further, the faults of one segment may dip in the opposite direction to the faults of another segment. An accommodation zone is the region of complex structure that links the ends of two rift segments. Accommodation zones typically include strike-slip faults.

Active margin A continental margin that coincides with either a strike-slip or convergent plate boundary, andthus is seismically active.

Aulacogen An unsuccessful rift that cuts across a continental margin at a high angle to the margin. Typically, aulacogens transect the grain of an orogen that borders the margin. Aulacogens may represent failed arms of three-armed rifts, or they may simply be older rifts (formed long before the development of the continental margin, during an earlier episode of rifting at a different orientation) that were cut off when the margin formed.

Axis (of rift or MOR) The center line along the length of a rift or a mid-ocean ridge (MOR). The trend of the axis is the overall trend of the rift.

Breakaway fault The normal fault that forms the edge of the rift. (A breakaway fault forms the boundary between stretched and unstretched crust).

Graben A narrow, symmetric trough or basin, bounded on both sides by normal faults that dip toward the center of the trough.

Half graben An asymmetric basin formed on the back of a tilted fault block; one border of the basin is a normal fault.

Horst An elongate, symmetric crustal block bordered on both sides by normal faults; both faults dip away from the center of the horst.

Listric faults and associated unfaulted rollover. Before faulting (top), the faults shallow with depth and merge with a detachment. After faulting (bottom), the blocks have moved. The block to the right curves down to maintain contact with the footwall, forming a rollover anticline.

Listric normal fault A normal fault whose dip decreases with depth, thereby making the fault surface concave upward.

Midocean ridge The elongate submarine mountain range that is the bathymetric manifestation of a divergent plate boundary. Though some midocean ridges (e.g., the Mid-Atlantic Ridge) do lie in the center of ocean basins, some (e.g., the East Pacific Rise) do not. Therefore, some geologists use the term “oceanic ridge” in place of “mid-ocean ridge” for these features.

Oblique rifting Rifting that occurs where the stretching direction is at an acute angle to the rift axis.

Passive margin A continental margin that is not a plate boundary and, therefore, is not seismically active. It is underlain by the relict of a successful rift. The rift relict subsides and is buried by a thick wedge of sediment.

Planar normal fault A normal fault whose dip remains constant with depth.

Nonrotational normal fault A normal fault on which slip does not result in rotation of the hanging-wall block.

Rift (rift system) A belt of continental lithosphere that is undergoing, or has undergone, extensional deformation (i.e., stretching); also called a continental rift.

Rift–drift transition The time at which active rift faulting ceases and seafloor spreading begins (i.e., the time at which a mid-ocean ridge initiates, and the relicts of a rift become the foundation of a passive margin).

Rifting The process by which continental lithosphere undergoes extensional deformation (stretching) by the formation and activity of normal faults.

Rotational normal fault A normal fault whose hanging wall block rotates around a horizontal axis during slip.

Subsidence The sinking of the surface of the lithosphere. Subsidence produces sedimentary basins. For example, the relict of a successful rift subsides to form a passive-margin basin.

Successful rift A rift in which stretching has proceeded until the continent cut by the rift ruptures to form two pieces separated by a new mid-ocean ridge.

Transfer fault A dominantly strike-slip fault that links two normal faults that are not coplanar; some transfer faults serve as accommodation zones.

Unsuccessful rift A rift in which extensional deformation ceased prior to rupture of the continent that was cut by the rift.

Extensional Tectonic Examples

The Basin and Range Province in North America is a classic example where extensional tectonics has led to a series of north-south trending mountain ranges and valleys.

The East African Rift is an active continental rift zone where extensional processes are currently splitting the African Plate into two.

Read also: Divergent Boundary: Definition, Features, Examples