Geology 360 - Geology 360 - Kinematic Indicators in Shear ZonesKinematic Indicators in Shear Zones

OutlineOutline

• What to look forWhat to look for

• Kinematic indicatorsKinematic indicators

What to look forWhat to look for

• Lines or PlanesLines or Planes

• RotationRotation

• Mineral GrowthMineral Growth

• Cross-Cutting RelationshipsCross-Cutting Relationships

Offset and Deflected MarkersOffset and Deflected Markers

How would you describe the kinematics of this system?How would you describe the kinematics of this system?

Offset and Deflected MarkersOffset and Deflected Markers

Right-lateral offsetRight-lateral offset

Foliation

How would you describe

the kinematics of this

system?

FoliationFoliation

Left-lateral offsetLeft-lateral offset

Foliations (S-C fabric)Foliations (S-C fabric) Foliation develops as a result of simple shearFoliation develops as a result of simple shear Dominant simple-shear parallel to walls of shear zone Dominant simple-shear parallel to walls of shear zone

develops a S-surface foliationdevelops a S-surface foliation New foliation develops (C-surface) at an angle (18°-25° to the New foliation develops (C-surface) at an angle (18°-25° to the

shear-zone walls)shear-zone walls)

S-C FabricS-C Fabric

S-surfaces - foliation planesS-surfaces - foliation planes

C-surfaces - shear bands (C-surfaces - shear bands (useful for determining the sense of shearuseful for determining the sense of shear))

S-C FabricS-C Fabric

Right-lateral motionRight-lateral motion

Foliation (mica) fishFoliation (mica) fish• A layer with oblique internal fabric becomes A layer with oblique internal fabric becomes

detached along S-fabric to form an asymmetric detached along S-fabric to form an asymmetric ‘fish’.‘fish’.

•Fish “flash” - oriented sample reflected sunlight…Fish “flash” - oriented sample reflected sunlight…

Mica FishMica Fish

• If you can stack the fish up, you’ve got the sense of shear If you can stack the fish up, you’ve got the sense of shear

correct. Or, think of the fish as thrust surfaces…correct. Or, think of the fish as thrust surfaces…

Mica FishMica Fish

Right-lateral movementRight-lateral movement

Porphyoblasts / PorphyroclastsPorphyoblasts / Porphyroclasts• PorphyroblastsPorphyroblasts = large grains that have grown in the rock = large grains that have grown in the rock

mass during deformation and metamorphismmass during deformation and metamorphism• PorphyroclastsPorphyroclasts = relict earlier large grains that have = relict earlier large grains that have

survived ductile deformation (e.g. feldspar)survived ductile deformation (e.g. feldspar)

Blast: grain size Blast: grain size

increaseincrease

Clast: grain size Clast: grain size

decreasedecrease

• TailsTails on on porphyroclasts porphyroclasts are asymmetric in are asymmetric in the direction of the direction of ductile flow. ductile flow.

RotationRotation

• Works for inclusions, porphyroblasts, and porphyroclastsWorks for inclusions, porphyroblasts, and porphyroclasts

• Tails and fabric in the rotated grains are usefulTails and fabric in the rotated grains are useful

?

?

RotationRotation

• Right-lateral shear (dextral)Right-lateral shear (dextral)

CW

Mylonitic orthogneiss

•K-spar porphyroclasts with asymmetric tails •What is sense of shear here?

Nash Fork Shear Zone, Medicine Bow Mtns, S. Wyoming.

Pressure Shadows with Fiber Pressure Shadows with Fiber

GrowthGrowth

• Minerals will grow in the direction of maximum extensionMinerals will grow in the direction of maximum extension

Pressure Shadows with Fiber Pressure Shadows with Fiber

GrowthGrowth

Maximum extensionMaximum shortening

Fractured GrainsFractured Grains

How would you describe the kinematics of this system?How would you describe the kinematics of this system?

How would you describe the kinematics of this system?How would you describe the kinematics of this system?

Fractured GrainsFractured Grains• Large grains of some Large grains of some

minerals (e.g. feldspars, minerals (e.g. feldspars, pyroxenes) continue to pyroxenes) continue to deform brittley within a deform brittley within a ductile matrix.ductile matrix.

• The fractures indicate a The fractures indicate a sense of motion opposite sense of motion opposite that of actual shear sensethat of actual shear sense

Fractured GrainsFractured Grains

Right-lateral offsetRight-lateral offset

VeinsVeins

• Calcite extensional veinCalcite extensional vein

Tension GashesTension Gashes

• Initial shearingInitial shearing

Tension GashesTension Gashes

• Directions of maximum extension and maximum shorteningDirections of maximum extension and maximum shortening

Tension GashesTension Gashes

• Mode I (opening) fracture developsMode I (opening) fracture develops

Tension GashesTension Gashes

• Continued development of a tension gashContinued development of a tension gash

Tension GashesTension Gashes

How would you describe the kinematics of this system?How would you describe the kinematics of this system?

Tension GashesTension Gashes

Finally, left-lateral motion!!!Finally, left-lateral motion!!!

Tension GashesTension Gashes

Tension GashesTension Gashes

How would you describe the kinematics of this system?How would you describe the kinematics of this system?

Tension GashesTension Gashes

Left-lateral motionLeft-lateral motion

ReferencesReferences

Slide 1Slide 1: : http://earth.leeds.ac.uk/shearzones/gallery/odhair.htm

Slides 4, 5Slides 4, 5: : http://earth.leeds.ac.uk/shearzones/gallery/apliteshear.htm

Slides 6, 7:Slides 6, 7: http://earth.leeds.ac.uk/shearzones/gallery/kohi1.htm http://earth.leeds.ac.uk/shearzones/gallery/kohi1.htm

Slides 8, 9:Slides 8, 9: http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html

Slides 10,11:Slides 10,11: Image 30 at http://talc.geo.umn.edu/orgs/struct/microstructure/sec2.html Image 30 at http://talc.geo.umn.edu/orgs/struct/microstructure/sec2.html

Slides 12, 13:Slides 12, 13: http://www.uni-mainz.de/~mezger/Text/Mezger_metamorphic_geology.html http://www.uni-mainz.de/~mezger/Text/Mezger_metamorphic_geology.html

Slides 14, 15:Slides 14, 15: http://earth.leeds.ac.uk/strain/gallery/presshadbig.html http://earth.leeds.ac.uk/strain/gallery/presshadbig.html

Slides 16, 17:Slides 16, 17: http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html

Slide 18:Slide 18: http://earth.leeds.ac.uk/minor/dmt/reprecipitation.htm http://earth.leeds.ac.uk/minor/dmt/reprecipitation.htm

Slides 19 - 22:Slides 19 - 22: http://earth.leeds.ac.uk/minor/tensiongash/gash1.htm http://earth.leeds.ac.uk/minor/tensiongash/gash1.htm

Slides 23 - 24:Slides 23 - 24: http://earth.leeds.ac.uk/minor/tensiongash/tgash.htm http://earth.leeds.ac.uk/minor/tensiongash/tgash.htm

Slide 25:Slide 25: http://earth.leeds.ac.uk/minor/tensiongash/gash3.htm http://earth.leeds.ac.uk/minor/tensiongash/gash3.htm

Slides 26 - 27:Slides 26 - 27: http://earth.leeds.ac.uk/minor/tensiongash/gashpic2.htm http://earth.leeds.ac.uk/minor/tensiongash/gashpic2.htm