75:
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92:
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25:
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surface separating the wall rocks, whereas in a ductile shear zone the deformation is spread out through a wider zone, the deformation state varying continuously from wall to wall. Between these end-members, there are intermediate types of brittleāductile (semibrittle) and ductileābrittle shear zones
780:
The importance of shear zones lies in the fact that they are major zones of weakness in the Earth's crust, sometimes extending into the upper mantle. They can be very long-lived features and commonly show evidence of several overprinting stages of activity. Material can be transported upwards or
305:
cutoff (situated usually at about 4ā5 km depth), true cataclasites start to appear. The seismogenic layer then yields to the alternating zone at 11 km depth. Yet big earthquakes can rupture both up to the surface and well into the alternating zone, sometimes even into the plastosphere.
273:
In Scholz's model for a quartzo-feldspathic crust (with a geotherm taken from
Southern California), the brittleāsemibrittle transition starts at about 11 km depth with an ambient temperature of 300 Ā°C. The underlying alternating zone then extends to roughly 16 km depth with a
209:
This continuum found in the structural geometries of shear zones reflects the different deformation mechanisms reigning in the crust, i.e. the changeover from brittle (fracturing) at or near the surface to ductile (flow) deformation with increasing depth. By passing through the
131:. That is, the rock is capable of slowly deforming without fracture, like hot metal being worked by a blacksmith. Here the shear zone is a wider zone, in which the ductile rock has slowly flowed to accommodate the relative motion of the rock walls on either side.
552:
The width of individual shear zones stretches from the grain scale to the kilometer scale. Crustal-scale shear zones (megashears) can become 10 km wide and consequently show very large displacements from tens to hundreds of kilometers.
122:
that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear zone takes the form of a fracture called a
218:, where brittle fracturing and plastic flow coexist. The main reason for this is found in the usually heteromineral composition of rocks, with different minerals showing different responses to applied
397:
conditions, the pseudotachylites disappear and only different types of mylonites persist. Striped gneisses are high-grade mylonites and occur at the very bottom of ductile shear zones.
636:. Brittle shear zones are more or less ubiquitous in the upper crust. Ductile shear zones start at greenschist facies conditions and are therefore restricted to metamorphic terranes.
709:
214:
the ductile response to deformation is starting to set in. This transition is not tied to a specific depth, but rather occurs over a certain depth range - the so-called
134:
Because shear zones are found across a wide depth-range, a great variety of different rock types with their characteristic structures are associated with shear zones.
326:(pT) conditions, flow type, movement sense, and deformation history. Shear zones are therefore very important structures for unravelling the history of a specific
256:
537:
531:
162:
Margin of a dextral sense ductile shear zone (about 20 m thick), showing transition from schists outside the zone to mylonites inside, Cap de Creus,
442:
382:
43:
349:
461:, and stretchingā or mineral lineations. They indicate the direction of movement. With the aid of offset markers such as displaced layering and
585:
Furthermore, for a material to become more ductile (quasi-plastic) and undergo continuous deformation (flow) without fracturing, the following
568:
must occur, in order for the affected host material to deform more plastically. The softening can be brought about by the following phenomena:
471:
458:
290:
286:
693:
Shear zones are dependent neither on rock type nor on geological age. Most often they are not isolated in their occurrence, but commonly form
475:
525:
146:
Diagram showing the major different types of shear zones. Displacement, shear strain, and depth distribution are also indicated.
817:
511:. Well-known examples are theta (Ī)-objects and phi (Ī¦)-porphyroclasts, as well as sigma (Ļ)- and delta (Ī“)-winged objects.
465:, or the deflection (bending) of layering/foliation into a shear zone, one can additionally determine the sense of shear.
923:
903:
876:
862:
848:
100:
61:
945:
274:
temperature of about 360 Ā°C. Below approximately 16 km depth, only ductile shear zones are found.
599:
701:
which reflect in their arrangement the underlying dominant sense of movement of the terrane at that time.
389:
Mylonites start to occur with the onset of semibrittle behaviour in the alternating zone characterised by
757:
717:
605:
797:
681:
333:
Starting at the Earth's surface, the following rock types are usually encountered in a shear zone:
314:
The deformations in shear zones are responsible for the development of characteristic fabrics and
127:. In the lower crust and mantle, the extreme conditions of pressure and temperature make the rock
242:
response. Yet other, purely physical factors, influence the changeover depth as well, including:
171:
616:
74:
761:
586:
556:
Brittle shear zones (faults) usually widen with depth and with an increase in displacements.
501:
940:
705:
413:
8:
827:
760:. Detachment fault related shear zones can be found in southeastern California, e.g. the
492:
725:
633:
394:
219:
919:
899:
872:
858:
844:
765:
733:
610:
294:
91:
39:
684:
441:, reverse or normal) can be deduced by macroscopic structures and by a plethora of
182:
115:
564:
Because shear zones are characterised by the localisation of strain, some form of
474:
arrays (or extensional veins), characteristic of ductile-brittle shear zones, and
822:
667:
653:
620:
593:
520:
360:
154:
Strength profile and change in rock type with depth in idealised fault/shear zone
124:
96:
486:
462:
260:
158:
81:
934:
508:
390:
368:
342:
790:
749:
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454:
119:
104:
85:
896:
The
Techniques of Modern Structural Geology. Volume 2: Folds and Fractures
855:
The
Techniques of Modern Structural Geology. Volume 2: Folds and Fractures
805:
745:
640:
353:
338:
323:
250:
167:
737:
302:
282:
235:
425:
405:
201:. In brittle shear zones, the deformation is concentrated in a narrow
16:
Structural discontinuity surface in the Earth's crust and upper mantle
514:
438:
418:
231:
78:
206:
that can combine these geometric features in different proportions.
177:
Shear zones form a continuum of geological structures, ranging from
753:
713:
547:
376:
319:
246:
239:
227:
202:
174:. It is characterised by a length to width ratio of more than 5:1.
704:
Some good examples of shear zones of the strike-slip type are the
393:. Pseudotachylites can still be encountered here. By passing into
150:
694:
434:
429:
Shear bands developed in a dextral sense shear zone, Cap de Creus
327:
315:
128:
409:
Asymmetric folds within a dextral sense shear zone, Cap de Creus
769:
729:
721:
223:
142:
782:
632:
Due to their deep penetration, shear zones are found in all
481:
Microscopic indicators consist of the following structures:
801:
166:
A shear zone is a zone of strong deformation (with a high
793:
in the host rocks and even re-fertilise mantle material.
786:
627:
478:
can also be valuable macroscopic shear-sense indicators.
764:. An example of a huge anastomosing shear-zone is the
356:(protocataclasite, cataclasite, and ultracataclasite).
289:. Below an intervening alternating zone, there is the
756:. An example for the subduction zone setting is the
34:
may be too technical for most readers to understand
589:(on a grain scale) have to be taken into account:
559:
932:
781:downwards in them, the most important one being
548:Width of shear zones and resulting displacements
453:The main macroscopic indicators are striations (
309:
238:, and preexisting fabrics determine a different
84:offset by a steeply dipping dextral shear zone,
421:within a dextral sense shear zone, Cap de Creus
367:Both fault gouge and cataclasites are due to
285:nucleate, is tied to the brittle domain, the
732:. Shear zones of the transform type are the
170:) surrounded by rocks with a lower state of
647:transcurrent setting ā steep to vertical:
916:The mechanics of earthquakes and faulting
869:The mechanics of earthquakes and faulting
796:Shear zones can host economically viable
748:. A shear zone of the thrust type is the
62:Learn how and when to remove this message
46:, without removing the technical details.
424:
412:
404:
157:
149:
141:
90:
73:
664:recumbent fold nappes (at the base of).
639:Shear zones can occur in the following
337:uncohesive fault rocks. Examples being
137:
95:Extensional ductile shear zone cutting
933:
839:Passchier CW & Trouw RAJ. (1996).
628:Occurrence and examples of shear zones
502:Crystallographic preferred orientation
44:make it understandable to non-experts
818:List of shear zones of Great Britain
623:) and grain-boundary area reduction.
433:The sense of shear in a shear zone (
318:assemblages reflecting the reigning
18:
13:
853:Ramsay JG & Huber MI. (1987).
14:
957:
762:Whipple Mountain Detachment Fault
400:
678:extensional setting ā low-angle
661:compressive setting ā low-angle
371:on brittle, seismogenic faults.
23:
673:thrust sheets (at the base of).
226:reacts plastically long before
918:. Cambridge University Press.
908:
888:
871:. Cambridge University Press.
560:Strain softening and ductility
212:brittleāsemibrittle transition
1:
882:
833:
775:
448:
310:Rocks produced in shear zones
222:(for instance, under stress
7:
811:
800:, examples being important
187:brittleāductile shear zones
10:
962:
758:Japan Median Tectonic Line
718:North Anatolian Fault Zone
710:North Armorican Shear Zone
706:South Armorican Shear Zone
348:cohesive fault rocks like
299:upper stability transition
114:is a thin zone within the
606:dynamic recrystallization
269:stress field orientation.
230:do). Thus differences in
682:metamorphic core complex
581:fluid-related softening.
297:, which occurs below an
789:. This can bring about
249:gradient, i.e. ambient
191:semibrittle shear zones
785:circulating dissolved
617:grain-boundary sliding
587:deformation mechanisms
572:grain-size reductions.
430:
422:
417:Asymmetric boudins of
410:
163:
155:
147:
107:
88:
699:anastomosing networks
428:
416:
408:
345:, and foliated gouge.
161:
153:
145:
94:
77:
946:Geological processes
766:Borborema Shear Zone
697:-scaled, linked up,
575:geometric softening.
538:shear band cleavages
301:related to an upper
257:confinement pressure
138:General introduction
914:Scholz CH. (2002).
894:Ramsay JG. (1987).
867:Scholz CH. (2002).
828:Strain partitioning
578:reaction softening.
507:mantled and winged
199:ductile shear zones
179:brittle shear zones
898:. Academic Press.
857:. Academic Press.
650:strike-slip zones.
634:metamorphic facies
532:quarter structures
431:
423:
411:
395:greenschist facies
164:
156:
148:
108:
103:in Mosaic Canyon,
89:
734:San Andreas Fault
611:pressure solution
600:dislocation creep
517:(foliation fish).
295:seismogenic layer
266:bulk strain rate.
101:Noonday Formation
72:
71:
64:
953:
926:
912:
906:
892:
752:in northwestern
668:subduction zones
654:transform faults
602:(various types).
596:(various types).
566:strain softening
543:step-over sites.
521:pressure shadows
361:pseudotachylites
279:seismogenic zone
216:alternating zone
67:
60:
56:
53:
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27:
26:
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823:Shear (geology)
814:
798:mineralizations
778:
630:
621:superplasticity
594:diffusion creep
562:
550:
451:
403:
312:
195:ductileābrittle
140:
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57:
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40:help improve it
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841:Microtectonics
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726:Dead Sea Fault
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509:porphyroclasts
505:
499:
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490:
450:
447:
402:
401:Sense of shear
399:
387:
386:
383:striped gneiss
380:
379:(phyllonites).
365:
364:
357:
350:crush breccias
346:
311:
308:
271:
270:
267:
264:
261:fluid pressure
254:
139:
136:
110:In geology, a
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498:imbrications.
497:
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477:
473:
470:
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460:
459:slickenfibers
456:
446:
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443:microtectonic
440:
436:
427:
420:
415:
407:
398:
396:
392:
391:adhesive wear
384:
381:
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374:
373:
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370:
369:abrasive wear
362:
358:
355:
351:
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344:
343:fault breccia
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172:finite strain
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117:
116:Earth's crust
113:
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35:
32:This article
30:
21:
20:
915:
910:
895:
890:
868:
854:
843:. Springer.
840:
804:deposits in
795:
791:metasomatism
779:
750:Moine Thrust
742:Alpine Fault
703:
698:
692:
638:
631:
584:
565:
563:
555:
551:
480:
476:sheath folds
472:tension gash
468:
467:
455:slickensides
452:
445:indicators.
432:
388:
366:
354:cataclasites
332:
313:
298:
291:plastosphere
287:schizosphere
278:
276:
272:
215:
211:
208:
198:
194:
190:
186:
178:
176:
165:
133:
120:upper mantle
111:
109:
105:Death Valley
86:Cap de Creus
58:
49:
33:
941:Shear zones
806:Precambrian
746:New Zealand
685:detachments
641:geotectonic
526:pull-aparts
485:asymmetric
339:fault gouge
324:temperature
283:earthquakes
281:, in which
251:temperature
240:rheological
168:strain rate
112:shear zone
52:August 2012
935:Categories
883:References
834:Literature
808:terranes.
776:Importance
740:, and the
738:California
724:, and the
643:settings:
613:processes.
493:foliations
469:En echelon
449:Indicators
303:seismicity
247:geothermal
236:grain size
515:mica fish
439:sinistral
419:pegmatite
377:mylonites
375:foliated
293:. In the
232:lithology
228:feldspars
97:dolomites
79:Pegmatite
812:See also
754:Scotland
714:Brittany
708:and the
320:pressure
220:stresses
203:fracture
695:fractal
435:dextral
359:glassy
328:terrane
316:mineral
129:ductile
99:of the
38:Please
922:
902:
875:
861:
847:
770:Brazil
730:Israel
722:Turkey
716:, the
504:(CPO).
224:quartz
185:) via
183:faults
783:water
487:folds
463:dykes
125:fault
920:ISBN
900:ISBN
873:ISBN
859:ISBN
845:ISBN
802:gold
787:ions
352:and
277:The
259:and
189:(or
181:(or
82:dyke
768:in
744:in
736:in
728:in
720:in
712:in
457:),
437:,
197:to
193:),
118:or
42:to
937::
772:.
341:,
330:.
234:,
687:.
670:.
656:.
619:(
540:.
534:.
528:.
495:.
489:.
385:.
363:.
322:ā
263:.
253:.
65:)
59:(
54:)
50:(
36:.
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
