Detachment fault
Adetachment faultis a gentlydippingnormal faultassociated with large-scaleextensional tectonics.[1]Detachmentfaultsoften have very large displacements (tens of km) and juxtapose unmetamorphosedhanging wallsagainst medium to high-grademetamorphicfootwallsthat are calledmetamorphic core complexes.They are thought to have formed as either initially low-angle structures or by the rotation of initially high-anglenormal faultsmodified also by theisostaticeffects of tectonicdenudation.They may also be called denudation faults. Examples of detachment faulting include:
- TheSnake Range detachment systemof theBasin and Range Provinceof western North America which was active during theMiocene
- TheNordfjord-Sogn detachmentof western Norway active during theDevonianPeriod[2]
- TheWhipple detachmentin southeastern California[3]
Detachment faults have been found on the sea floor close to divergent plate boundaries characterised by a limited supply of upwellingmagma,such as theSouthwest Indian Ridge.These detachment faults are associated with the development ofoceanic core complexstructures.
Continental detachment faults
[edit]Continental detachment faults are also calleddécollements,denudational faults, low-angle normal faults (LANF) and dislocation surfaces.[4]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 theYeringtondistrict of Nevada. There, evidence for rotation of the fault plane comes from tilted volcanic dikes.[5]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."[4]
Detachments faults of this kind (initially low-angle) can be found in theWhipple Mountainsof California and theMormon Mountainsof Nevada.[6]They initiate at depth in zones of intracrustal flow, wheremyloniticgneissesform. Shear along the fault is ductile at mid to lower crustal depths, but brittle at shallower depths. Thefootwallcan transport mylonitic gneisses from lower crustal levels to upper crustal levels, where they become chlorititic andbrecciated.[4]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.[4]The unloading of the footwall can lead toisostatic upliftand doming of the more ductile material beneath.[6]
Low angle normal faulting is not explained byAndersonian fault mechanics.[7]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.[6]
Oceanic detachment faults
[edit]Oceanic detachment faults occur atspreading ridgeswhere magmatic activity is not enough to account for the entire plate spreading rate. They are characterized by long domes parallel to the spreading direction (oceanic core complexes of the footwall). Slip on these faults can range from tens to hundreds of km. They cannot be structurally restored, as slip on the fault exceeds the thickness of oceanic crust (~30 km compared to ~6 km, for example).[6]
While occurring at relatively amagmatic spreading centres, the footwalls of these detachment faults are much more influenced by magmatism than in continental settings. In fact, they are often created by ‘continuous casting’: new footwall is continually being generated by mantle or melt from a magma chamber as slip occurs on the fault.[6]The lithology is dominated bygabbroandperidotite,resulting in a mineralogy ofolivine,serpentine,talcandplagioclase.This is in contrast to continental settings, where the mineralogy is dominantlyquartzandfeldspar.The footwall is also much more extensivelyhydrothermally alteredthan in continental settings.[6]
In contrast to many detachment faults in continental settings, oceanic detachment faults are usuallyrolling hinge normal faults,initiating at higher angles and rotating to low angles.[6]
References
[edit]- ^National Park Service. "Glossary of Geologic Terms"[1]
- ^Fossen H. (1992). The role of extensional tectonics in the Caledonides of South Norway.Journal of Structural Geology,14:1033–1046.
- ^Davis, Gregory A. (1988-02-01)."Rapid upward transport of mid-crustal mylonitic gneisses in the footwall of a Miocene detachment fault, Whipple Mountains, southeastern California".Geologische Rundschau.77(1): 191–209.Bibcode:1988GeoRu..77..191D.doi:10.1007/BF01848684.ISSN1432-1149.S2CID129275058.
- ^abcdDavis, G. A., & Lister, G. S., 1988. Detachment faulting in continental extension: Perspectives from the southwestern US Cordillera. Spec. Pap. Geol. Soc. Am, 218, 133-159.[2]
- ^Proffett, J. M. (1977). Cenozoic geology of the Yerington district, Nevada, and implications for the nature and origin of Basin and Range faulting. Geological Society of America Bulletin, 88(2), 247-266.[3]
- ^abcdefgJohn, B. E., & Cheadle, M. J., 2010. Deformation and alteration associated with oceanic and continental detachment fault systems: Are they similar?.Geophysical Monograph Series, 188, 175-205.[4]Archived2014-04-14 at theWayback Machine
- ^Kearey, P., Klepeis, K.A., Vine, F.J. (2009) Global Tectonics (3rd edition). Wiley-Blackwell.
Further reading
[edit]- George H Davis, Stephen J Reynolds, (1996),Structural Geology of Rocks and Regions,2nd Edition, John Wiley and Sons Inc.ISBN0-471-52621-5.
