Geology 3120 - Introduction to Shear Zonesgeode.colorado.edu/~structure/teaching_GEOL3120/... · Formation of Shear Zones “All faults are shear zones but not all shear zones are

Geology 3120 Geology 3120 --Introduction to Shear ZonesIntroduction to Shear Zones

OutlineOutline

•• What’s Going On With Shear Zones?What’s Going On With Shear Zones?

•• Brittle Shear ZoneBrittle Shear Zone

•• Ductile Shear ZoneDuctile Shear Zone

•• SemiSemi--Brittle Shear ZoneBrittle Shear Zone

•• BrittleBrittle--Ductile Shear Zone Ductile Shear Zone

Formation of Shear ZonesFormation of Shear Zones

““All faults are shear zones but not all shear zones are faults”All faults are shear zones but not all shear zones are faults”

•• Brittle DeformationBrittle Deformation

•• Ductile DeformationDuctile Deformation

•• Zone of DeformationZone of Deformation

•• DiscontinuousDiscontinuous

•• ContinuousContinuous

••Brittle shear zones tend to form in the upper 5 Brittle shear zones tend to form in the upper 5 -- 10 km 10 km

••Ductile shear zones are located in the midDuctile shear zones are located in the mid--lower crust andlower crust and aesthenosphereaesthenosphere

Controlling FactorsControlling Factors

BrittleBrittle DuctileDuctile

TemperatureTemperature CoolerCooler HotterHotter

PressurePressure LowerLower HigherHigher

Strain RateStrain Rate HigherHigher LowerLower

Fluid Fluid PressurePressure

LowerLower HigherHigher

•• Also mineralogy and grain sizeAlso mineralogy and grain size

Brittle Shear ZonesBrittle Shear Zones

P

A

E


A

E


E


Example Example -- Volcanic TablelandsVolcanic Tablelands

2 km

N

Example Example -- Volcanic TablelandsVolcanic Tablelands

Volcanic Tablelands (horsetail splay)Volcanic Tablelands (horsetail splay)

Ductile Shear ZonesDuctile Shear Zones

Markers Xenoliths/Plutons





Example Example -- Sheared XenolithsSheared Xenoliths

Semibrittle Semibrittle Shear ZonesShear Zones

V

E

F

ss11 -- axis of maximum instantaneous stretchingaxis of maximum instantaneous stretching

ss33 -- axis of maximum instantaneous shorteningaxis of maximum instantaneous shortening


E

F




F






En Echelon Vein ArraysEn Echelon Vein Arrays

Where is sigma 1?Where is sigma 1?

BrittleBrittle--Ductile Shear ZonesDuctile Shear Zones

Example Example -- BrittleBrittle--Ductile Shear ZoneDuctile Shear Zone

•• Brittle deformation Brittle deformation -- feldspar crystalsfeldspar crystals

•• Ductile deformation Ductile deformation -- groundmassgroundmass

ReferencesReferences

Slide 1Slide 1

http://earth.leeds.ac.uk/http://earth.leeds.ac.uk/shearzonesshearzones/gallery/kohi2./gallery/kohi2.htmhtm

Slides 3, 5 Slides 3, 5 -- 8, 12 8, 12 -- 14, 16 14, 16 -- 19, 2119, 21Davis. G. H. and S. J. Reynolds, Structural Geology of Rocks andDavis. G. H. and S. J. Reynolds, Structural Geology of Rocks and Regions, 2nd ed., John Regions, 2nd ed., John Wiley & Sons, New York, 776 p., 1996.Wiley & Sons, New York, 776 p., 1996.

Slide 9Slide 9http://terraserver.homeadvisor.msn.com/image.http://terraserver.homeadvisor.msn.com/image.aspxaspx?t=1&s=14&x=116&y=1296&z=11&w=2?t=1&s=14&x=116&y=1296&z=11&w=2

Slide 15Slide 15http://earth.leeds.ac.uk/shearzones/gallery/laghetti.http://earth.leeds.ac.uk/shearzones/gallery/laghetti.htmhtm

Slide 20Slide 20http://earth.leeds.ac.uk/minor/tensiongash/index.http://earth.leeds.ac.uk/minor/tensiongash/index.htmhtm

Slide 22Slide 22www.www.brunelbrunel.ac.uk/.ac.uk/deptsdepts/geo//geo/iainsubiainsub// ShearKinShearKin//ShearKinShearKin.PPT.PPT

http://earth.leeds.ac.uk/shearzones/gallery/laghetti.htm

http://earth.leeds.ac.uk/minor/tensiongash/index.htm

Geology 3120 - Introduction to Shear Zonesgeode.colorado.edu/~structure/teaching_GEOL3120/... · Formation of Shear Zones “All faults are shear zones but not all shear zones are

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