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Within asymmetric shear zones, the behavior of an object undergoing shortening is analogous to the ball of treacle being smeared as it flattens, generally into an ellipse. Within shear zones with pronounced displacements a shear foliation may form at a shallow angle to the gross plane of the shear
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Shear zones can be only inches wide, or up to several kilometres wide. Often, due to their structural control and presence at the edges of tectonic blocks, shear zones are mappable units and form important discontinuities to separate terranes. As such, many large and long shear zones are named,
425:
The incipient shear foliation typically forms normal to the direction of principal shortening, and is diagnostic of the direction of shortening. In symmetric shortening, objects flatten on this shear foliation much the same way that a round ball of treacle flattens with gravity.
433:
If the rock mass begins to undergo large degrees of lateral movement, the strain ellipse lengthens into a cigar shaped volume. At this point shear foliations begin to break down into a rodding lineation or a stretch lineation. Such rocks are known as L-tectonites.
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Shear zones which occur under brittle-ductile conditions can accommodate much deformation by enacting a series of mechanisms which rely less on fracture of the rock and occur within the minerals and the mineral lattices themselves. Shear zones accommodate
357:
The mechanisms of shearing depend on the pressure and temperature of the rock and on the rate of shear which the rock is subjected to. The response of the rock to these conditions determines how it accommodates the deformation.
441:
Stretched pebble conglomerate L-tectonite illustrating a stretch lineation within a shear zone, Glengarry Basin, Australia. Pronounced asymmetric shearing has stretched the conglomerate pebbles into elongate cigar shaped
331:
When the horizontal displacement of this faulting can be measured in the tens or hundreds of kilometers of length, the fault is referred to as a megashear. Megashears often indicate the edges of ancient tectonic plates.
430:
zone. This foliation ideally manifests as a sinusoidal set of foliations formed at a shallow angle to the main shear foliation, and which curve into the main shear foliation. Such rocks are known as L-S tectonites.
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planes form parallel to the shear zone boundary. The angle between the C and S planes is always acute, and defines the shear sense. Generally, the lower the C-S angle the greater the strain.
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Very distinctive textures form as a consequence of ductile shear. An important group of microstructures observed in ductile shear zones are S-planes, C-planes and C' planes.
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and detachment faults in rift zones are the typical structural manifestations of transtension conditions. Microstructural evidence of transtension includes rodding or
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The C' planes, also known as shear bands and secondary shear fabrics, are commonly observed in strongly foliated mylonites especially
410:
foliation in an L-S tectonite, with pencil pointing in direction of shear sense. Note the sinusoidal nature of the shear foliation.
369:) or at high rates of strain, tend to fail by brittle failure; breaking of minerals, which are ground up into a breccia with a
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Shearing at ductile conditions may occur by fracturing of minerals and growth of sub-grain boundaries, as well as by
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is first formed within the rock mass. This manifests as realignment of textural features, growth and realignment of
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Asymmetric shear in basalt, Labouchere mine, Glengarry Basin, Australia. Shear asymmetry is sinistral, pen for scale
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and strike-slip or transform faults are formed. Microstructural evidence of transpressional regimes can be rodding
53:
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The sense of shear shown by both S-C and S-C' structures matches that of the shear zone in which they are found.
470:, mica fish and elongated minerals. This specimen was from close to a shear zone in Norway (the Ose thrust), the
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A shear zone is a tabular to sheetlike, planar or curviplanar zone composed of rocks that are more highly
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Transpression regimes are formed during oblique collision of tectonic plates and during non-orthogonal
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and forms particular textures. Shear can be homogeneous or non-homogeneous, and may be
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Transtension regimes are oblique tensional environments. Oblique, normal
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A typical example of a transpression regime is the Alpine Fault zone of
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or platy minerals. Define the flattened long-axis of the strain ellipse.
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rocks. Within purely brittle rocks, compressive stress results in
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than rocks adjacent to the zone. Typically this is a type of
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The Alpine Schist of New
Zealand is characterised by heavily
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Other microstructures which can give sense of shear include:
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with a south block up and west oblique sense of movement.
508:, and form at an angle of about 20 degrees to the S-plane.
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is converted to oblique strike-slip movement. Here, the
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During the initiation of shearing, a penetrative planar
180:. Study of geological shear is related to the study of
474:
in the centre (black) is approximately 2mm in diameter
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from a peak metamorphic assemblage and are commonly
60:. Unsourced material may be challenged and removed.
594:attains a trapezoidal shape dominated by oblique
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301:or fractures may be observed within shear zones.
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723:, Bristol. Archive copy incomplete, 12/31/2012.
446:
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486:planes are generally defined by a planar
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120:Learn how and when to remove this message
450:
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361:Shear zones which occur in more brittle
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14:
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582:, where the oblique subduction of the
559:. Typically a mixture of oblique-slip
199:The process of shearing occurs within
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394:are essentially ductile shear zones.
223:Rocks typical of shear zones include
149:, Starlight Pit, Fortnum Gold Mine,
58:adding citations to reliable sources
29:
991:List of tectonic plate interactions
316:. They may often show some form of
24:
142:vein (with strain fringe) showing
25:
1456:
721:University of the West of England
713:Diagrams and definitions of shear
381:by movement on foliation planes.
308:deposits as they are a focus for
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458:(crossed polars) of Garnet-Mica-
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45:needs additional citations for
27:Response of rock to deformation
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398:Microstructures of shear zones
13:
1:
701:American Geological Institute
687:
447:Ductile shear microstructures
258:
164:is the response of a rock to
598:, steeply-dipping recumbent
422:and growth of new minerals.
328:identical to fault systems.
7:
676:Sense of shear indicators:
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490:caused by the alignment of
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1076:Thick-skinned deformation
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1081:Thin-skinned deformation
857:Stereographic projection
218:
1445:Deformation (mechanics)
847:Orthographic projection
830:Measurement conventions
776:Lamé's stress ellipsoid
575:, mica fish and so on.
318:retrograde metamorphism
203:, brittle-ductile, and
602:and fault-bend folds.
475:
443:
411:
363:rheological conditions
354:
336:Mechanisms of shearing
304:Many shear zones host
268:
153:
1358:Paleostress inversion
1051:Strike-slip tectonics
921:Extensional tectonics
901:Continental collision
771:Deformation mechanism
631:stretching lineations
588:Indo-Australian Plate
454:
440:
405:
343:
266:
135:
1440:Geological processes
936:Fold and thrust belt
69:"Shear" geology
54:improve this article
1368:Section restoration
1244:Rock microstructure
906:Convergent boundary
806:Strain partitioning
791:Overburden pressure
781:Mohr–Coulomb theory
704:Glossary of Geology
672:Strain partitioning
667:Rock microstructure
662:Foliation (geology)
647:Convergent boundary
571:, augen-structured
406:Typical example of
186:rock microstructure
1435:Structural geology
1345:Kinematic analysis
1001:Mountain formation
916:Divergent boundary
881:Accretionary wedge
757:Structural geology
637:, mylonites, etc.
476:
462:showing a rotated
444:
412:
379:compressive stress
367:confining pressure
355:
269:
182:structural geology
170:compressive stress
154:
1422:
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1353:3D fold evolution
1239:Pressure solution
1234:Oblique foliation
1114:Exfoliation joint
1104:Columnar jointing
764:Underlying theory
310:hydrothermal flow
151:Western Australia
130:
129:
122:
104:
18:Shear deformation
16:(Redirected from
1452:
1412:
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1157:Detachment fault
1152:Cataclastic rock
1086:Thrust tectonics
1056:Structural basin
1031:Pull-apart basin
971:Horst and graben
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299:En echelon veins
251:versions of the
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1167:Fault mechanics
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1026:Plate tectonics
976:Intra-arc basin
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837:Brunton compass
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717:Wayback Machine
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657:Fault (geology)
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365:(cooler, less
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271:Main article:
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786:Mohr's circle
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592:orogenic belt
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584:Pacific Plate
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561:thrust faults
558:
551:Transpression
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464:porphyroblast
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408:dextral shear
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386:lattice glide
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322:metasomatised
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71: –
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65:Find sources:
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55:
49:
48:
43:This article
41:
37:
32:
31:
19:
1395:
1182:Thrust fault
871:Large-scale
842:Inclinometer
816:Stress field
711:
696:
633:, stretched
624:
621:Transtension
609:and sheared
604:
596:splay faults
577:
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499:cisaillement
498:
497:C-planes or
483:
482:S-planes or
477:
456:Thin section
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198:
190:rock texture
178:simple shear
161:
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107:
97:
90:
83:
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64:
52:Please help
47:verification
44:
1363:Paleostress
1249:Slickenside
1224:Crenulation
1177:Fault trace
1172:Fault scarp
1162:Disturbance
1147:Cataclasite
1036:Rift valley
956:Half-graben
926:Fault block
911:DĂ©collement
652:Crenulation
615:earthquakes
580:New Zealand
543:asymmetric
536:asymmetric
506:phyllonites
484:schistosité
348:slickenside
291:deformation
247:and highly
237:L-tectonite
233:S-tectonite
229:cataclasite
211:and simple
168:usually by
166:deformation
147:shear sense
1429:Categories
1391:Pure shear
1378:Shear zone
1335:Competence
1219:Compaction
1096:Fracturing
891:Autochthon
886:Allochthon
688:References
607:crenulated
586:under the
565:lineations
557:subduction
540:(Figure 1)
519:sigmoidal
273:Shear zone
259:Shear zone
253:wall rocks
243:, certain
209:fracturing
174:pure shear
137:Boudinaged
80:newspapers
1327:Boudinage
1307:Monocline
1302:Homocline
1282:Anticline
1264:Tectonite
1254:Stylolite
1229:Fissility
1206:lineation
1202:Foliation
1066:Syneclise
1011:Obduction
981:Inversion
873:tectonics
682:sinistral
569:mylonites
526:mica fish
416:foliation
392:Mylonites
373:texture.
287:foliation
145:sinistral
1414:Category
1386:Mylonite
1317:Vergence
1312:Syncline
1214:Cleavage
1139:Faulting
641:See also
611:phyllite
573:gneisses
530:rotated
312:through
279:strained
249:foliated
245:breccias
225:mylonite
213:faulting
110:May 2012
1287:Chevron
1274:Folding
1119:Fissure
1071:Terrane
1016:Orogeny
996:MĂ©lange
931:Fenster
821:Tension
678:dextral
538:boudins
345:Dextral
295:folding
205:ductile
201:brittle
158:geology
94:scholar
1061:Suture
1046:Saddle
986:Klippe
951:Graben
811:Stress
801:Strain
719:), by
600:nappes
488:fabric
472:garnet
468:garnet
460:Schist
371:milled
352:pyrite
293:, and
140:quartz
96:
89:
82:
75:
67:
1396:Shear
1124:Joint
1006:Nappe
966:Horst
961:Horse
545:folds
521:veins
492:micas
442:rods.
420:micas
283:fault
219:Rocks
162:shear
101:JSTOR
87:books
1297:Dome
1204:and
1129:Vein
1109:Dike
1041:Rift
852:Rake
680:and
235:and
192:and
73:news
466:of
350:of
306:ore
297:.
188:or
176:or
156:In
56:by
1431::
567:,
324:.
289:,
255:.
239:,
231:,
227:,
215:.
196:.
184:,
160:,
749:e
742:t
735:v
715:(
123:)
117:(
112:)
108:(
98:·
91:·
84:·
77:·
50:.
20:)
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