Detachment fault

• The Snake Range detachment system of the Basin and Range Province of western North America which was active during the Miocene
• The Nordfjord-Sogn detachment of western Norway active during the Devonian Period
• The Whipple detachment in southeastern California

Detachment faults have been found on the sea floor close to divergent plate boundaries characterised by a limited supply of upwelling magma, such as the Southwest Indian Ridge. These detachment faults are associated with the development of oceanic core complex structures.

Continental detachment faults

Continental detachment faults are also called décollements, denudational faults, low-angle normal faults (LANF) and dislocation surfaces. The low-angle nature of these normal faults has sparked debate among scientists, centred on whether these faults started out at low angles or rotated from initially steep angles. Faults of the latter type are present, for example, in the Yerington district of Nevada. There, evidence for rotation of the fault plane comes from tilted volcanic dikes. However, other authors disagree that these should be called detachment faults. One group of scientists defines detachment faults as follows:

The essential elements of extensional detachment faults, as the term is used here, are low angle of initial dip, subregional to regional scale of development, and large translational displacements, certainly up to tens of kilometres in some instances.

Detachments faults of this kind (initially low-angle) can be found in the Whipple Mountains of California and the Mormon Mountains of Nevada. They initiate at depth in zones of intracrustal flow, where mylonitic gneisses form. Shear along the fault is ductile at mid to lower crustal depths, but brittle at shallower depths. The footwall can transport mylonitic gneisses from lower crustal levels to upper crustal levels, where they become chlorititic and brecciated. The hanging wall, composed of extended, thinned and brittle crustal material, can be cut by numerous normal faults. These either merge into the detachment fault at depth or simply terminate at the detachment fault surface without shallowing. The unloading of the footwall can lead to isostatic uplift and doming of the more ductile material beneath.

Low angle normal faulting is not explained by Andersonian fault mechanics. However, slip on low angle normal faults could be facilitated by fluid pressure, as well as by weakness of minerals in wall rocks. Detachment faults may also initiate on reactivated thrust fault surfaces.