Inversion (geology)

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 faults, or uplift caused by mantle plumes. "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.

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 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".

Economic importance

Anticlinal 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).

References

* Inversion Tectonics. Edited by Cooper,M.A. and Williams,G.D. 1989. Special publication of the Geological Society of London, 44.
* Basin Inversion. Edited by Buchanan,J.G. and Buchanan,P.G. 1995. Special publication of the Geological Society of London, 88.