Inversion (geology)
Encyclopedia
In structural geology
inversion or basin inversion relates to the relative uplift of a sedimentary basin
or similar structure as a result of crustal shortening. This normally excludes uplift developed in the footwalls of later extensional fault
s, or uplift caused by mantle plume
s. "Inversion" can also refer to individual faults, where an extensional fault is reactivated in the opposite direction to its original movement.
The term negative inversion is also occasionally used to describe the reactivation of reverse faults and thrusts
during extension.
The term "inversion" simply refers to the fact that a relatively low-lying area is uplifted — the rock sequence itself is not normally inverted.
The likelihood of fault reactivation depends on the dip of the existing fault plane. Lower angle faults are more favourable as the resolved shear stress
on the plane is higher. When a listric fault, which increases in dip upwards, reactivates the uppermost part of the fault may be too steep and new reverse faults typically develop in the footwall of the existing fault. These are known as footwall shortcuts.
1 Regional temporal variations in stress patterns within plates, resulting from forces caused by changes in plate boundary configuration, the blocking of subduction zones by buoyant crust (collision) and changes in relative motion at nearby plate boundaries.
2 Global, episodic, intraplate, stress changes from deviatoric compression during the collisional assembly of Pangaea-type continental configurations to deviatoric tension in assembled configurations to deviatoric compression in dispersed configurations. Inversions are commonly associated with rifted margin breakup unconformities caused by the transition from rift-phase tension to ridge push compression.
3 Local inversion in strike-slip rhombohedral pull-aparts as a natural consequence of alternating phases of transtension with negative flower structure and transpression with positive flower structure.
4 Complex alternating phases of extension and shortening at the margins of upper crustal rotational flakes in strike-slip zones.
5 Progressive diminution of the ratio of crustal to lithospheric thickness during slow extension causing whole basin uplift.
6 Uplift on lithospheric flexural arches and hotspots.
7 Body force mechanisms including salt mud diapirism, salt/mud decollement tectonics, gravity spreading, sliding and consequent relaying of heel extension to toe thrusting.
8 Inversion of strong negative isostatic gravity anomalies in confined deep basins caused by upper crustal stretching. Critical to whether a basin becomes inverted are, also, the timing of a compression phase relative to the initial basin-forming extensional event and the extensional strain rate. Short extensioncompression intervals and high extensional strain rates facilitate basin inversion. Inversion may be prevented by long intervals and low strain rates.
structures formed by inversion provide traps in many of the world's hydrocarbon
provinces. The nature of inversion means that reservoir units often thicken and may increase in quality within the basin that is later inverted (e.g. the Ormen Lange
gas field offshore mid-Norway).
Structural geology
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation in the rocks, and...
inversion or basin inversion relates to the relative uplift of a sedimentary basin
Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
or similar structure as a result of crustal shortening. This normally excludes uplift developed in the footwalls of later extensional fault
Extensional fault
An extensional fault is a fault that vertically thins and horizontally extends portions of the Earth's crust and/or lithosphere. In most cases such a fault is also a normal fault, but may be rotated to have a shallower geometry normally associated with a thrust fault...
s, or uplift caused by mantle plume
Mantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...
s. "Inversion" can also refer to individual faults, where an extensional fault is reactivated in the opposite direction to its original movement.
The term negative inversion is also occasionally used to describe the reactivation of reverse faults and thrusts
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
during extension.
The term "inversion" simply refers to the fact that a relatively low-lying area is uplifted — the rock sequence itself is not normally inverted.
Formation
Many inversion structures are caused by the direct reactivation of pre-existing extensional faults. In some cases only the deeper parts of the fault are reactivated and the shortening is accommodated over a much broader area in the shallow part of the section. The existing fault block still generally acts as a boundary to the uplift and the process is sometimes known as buttressing.The likelihood of fault reactivation depends on the dip of the existing fault plane. Lower angle faults are more favourable as the resolved shear stress
Shear stress
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...
on the plane is higher. When a listric fault, which increases in dip upwards, reactivates the uppermost part of the fault may be too steep and new reverse faults typically develop in the footwall of the existing fault. These are known as footwall shortcuts.
Varieties of Formation
Inversion tectonics is recognised to form as a result of:1 Regional temporal variations in stress patterns within plates, resulting from forces caused by changes in plate boundary configuration, the blocking of subduction zones by buoyant crust (collision) and changes in relative motion at nearby plate boundaries.
2 Global, episodic, intraplate, stress changes from deviatoric compression during the collisional assembly of Pangaea-type continental configurations to deviatoric tension in assembled configurations to deviatoric compression in dispersed configurations. Inversions are commonly associated with rifted margin breakup unconformities caused by the transition from rift-phase tension to ridge push compression.
3 Local inversion in strike-slip rhombohedral pull-aparts as a natural consequence of alternating phases of transtension with negative flower structure and transpression with positive flower structure.
4 Complex alternating phases of extension and shortening at the margins of upper crustal rotational flakes in strike-slip zones.
5 Progressive diminution of the ratio of crustal to lithospheric thickness during slow extension causing whole basin uplift.
6 Uplift on lithospheric flexural arches and hotspots.
7 Body force mechanisms including salt mud diapirism, salt/mud decollement tectonics, gravity spreading, sliding and consequent relaying of heel extension to toe thrusting.
8 Inversion of strong negative isostatic gravity anomalies in confined deep basins caused by upper crustal stretching. Critical to whether a basin becomes inverted are, also, the timing of a compression phase relative to the initial basin-forming extensional event and the extensional strain rate. Short extensioncompression intervals and high extensional strain rates facilitate basin inversion. Inversion may be prevented by long intervals and low strain rates.
Economic importance
AnticlinalAnticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
structures formed by inversion provide traps in many of the world's hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
provinces. The nature of inversion means that reservoir units often thicken and may increase in quality within the basin that is later inverted (e.g. the Ormen Lange
Ormen Lange
Ormen Lange is a natural gas field on the Norwegian continental shelf. It is situated northwest of Kristiansund, where seabed depths vary between . The field is named after the famous longship Ormen Lange of Olaf Tryggvason, a 10th century Viking king of Norway.-History:The field was discovered in...
gas field offshore mid-Norway).