In sequence thrusts: In a system of thrusts, the most recent fault is at base of the thrust pile and most proximal to the foreland – propagation is towards the foreland. Unlike duplexes, there is no roof thrust, Imbricate fan: Fan-like splay of thrust panels and thrust faults generated from a single décollement. Horse: A panel of rock bound on all sides by thrusts. Thrust faults are generated in pre-existing strata, but usually evolve to include the proximal parts of the foreland basin and its sediments. The resulting topographic results in flexural and formation of a foreland basin. Photo courtesy of Marli Miller.įault splay: A single fault strand divided into two or more faults such that displacement is distributed across the new structures.įold-thrust belt: A major thrust system developed during lithosphere-scale plate convergence, with cumulative shortening of 100s of kilometres, that usually results in mountain building. Duplexes can take several geometric forms – the item illustrated below is a normal duplex increased vertical stacking of horses produces an antiformal duplex.Ī smaller-scale thrust fault propagation anticline and hanging-wall cutoff above a ramp – flat in thinly bedded mudrocks. Duplexes represent progressive, incremental formation of ramps and bending folds (anticline-syncline pairs). An example is shown in the fold-thrust cross section below.ĭuplex: An imbricate stack of horses bound above and below by through-going thrusts these are the roof and floor thrusts. Cratonic platform, crystalline basement). Blind thrust tip points (tip lines) typically contain fault propagation fold pairs.īranch points: Locations along a thrust where branching or fault splays are generated.Ĭutoff points: The truncation of identifiable rock units or beds that can be traced across the hanging wall and footwall.ĭécollement-basal detachment-sole thrust: Names given to the thrust at the base of a thrust stack, that is the common surface of detachment for all thrusts. thrust plane dip is towards the foreland.īlind thrust: A thrust that does not breach the surface at the time of its formation. In many cases back thrust vergence will be towards the hinterland, i.e. Terminology (in alphabetical order)īack thrust: A thrust that has vergence opposite the dominant trend of a thrust system. Schematic representation of thrust faults, ramps, flats and hanging wall folds for foreland and hinterland verging faults. There are several oldish, but excellent publications that treat terminology and theory in a more encyclopedic manner: McClay 1981 (as editor of a 1981 special volume, a 1992 text, and a 2011 AAPG Memoir in the same volume, Price 1981 blind thrusts, Thompson, 1981 Suppe, 1983 ( fault bend folding) Mitra 1986, duplexes and Dahlstrom 1969, balanced cross-sections. Only the most common terms are illustrated here. The terminology of thrust faults has evolved, since the pioneering work by Peach and Horn, into a fairly complicated system of variations on numerous themes – fault and fold geometry, fault stacking, fault displacement, fault and fold associations. Thus, total shortening across a thrust system is the sum of fault displacement, folding, and cleavage. The strain generated by shortening can also be distributed as penetrative deformation such as cleavage. Folding is a geometric and a space requirement of shortening, and a mechanism of distributing strain as the fault tip propagates. Thrust faults are nearly always accompanied by folds, particularly in their hanging walls. The cumulative shortening across orogenic fold-thrust belts is often measured in 100s of kilometres. For individual faults, shortening can be measured in centimetres to 100s of metres depending on size. Some of the best-known examples occur in collisional orogens as forelandfold-thrust belts, and on the upper plate of subduction zones as accretionary wedges.Īn important outcome of thrust faulting is shortening parallel to layering. Thrusts tend to co-locate with other thrusts in fold-thrust systems. Thrusts are compressional structures commonly developed in layered sedimentary and foliated metamorphic rock in which mechanically strong units are interlayered with weak rock, for example dense sandstone or carbonate units and weaker shales. Thrusts are reverse faults having fault plane dips <45 o, usually <30 o. Some terminology for Thrust faults, duplexes, imbricate fans, and fold-thrust belts. Paired isoclinal anticline-syncline fault propagation folds, Bowser Basin, northern British Columbia.
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