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.

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.

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.

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