Geology 360 - Kinematic Indicators in Shear Zones

Outline What to look for Kinematic indicators

What to look for Lines or Planes Rotation Mineral Growth Cross-Cutting Relationships

Offset and Deflected Markers How would you describe the kinematics of this system?

Offset and Deflected Markers Right-lateral offset

Foliation How would you describe the kinematics of this system?

Foliation Left-lateral offset

Foliations (S-C fabric) Foliation develops as a result of simple shear Dominant simple-shear parallel to walls of shear zone develops a S-surface foliation New foliation develops (C-surface) at an angle (18°-25° to the shear-zone walls)

S-C Fabric S-surfaces - foliation planes C-surfaces - shear bands (useful for determining the sense of shear)

S-C Fabric Right-lateral motion

Foliation (mica) fish A layer with oblique internal fabric becomes detached along S-fabric to form an asymmetric ‘fish’. Fish “flash” - oriented sample reflected sunlight…

Mica Fish If you can stack the fish up, you’ve got the sense of shear correct. Or, think of the fish as thrust surfaces…

Mica Fish Right-lateral movement

Porphyoblasts / Porphyroclasts Porphyroblasts = large grains that have grown in the rock mass during deformation and metamorphism Porphyroclasts = relict earlier large grains that have survived ductile deformation (e.g. feldspar) Blast: grain size increase Clast: grain size decrease

Tails on porphyroclasts are asymmetric in the direction of ductile flow.

Rotation ? ? Works for inclusions, porphyroblasts, and porphyroclasts Tails and fabric in the rotated grains are useful

Rotation CW Right-lateral shear (dextral)

Mylonitic orthogneiss Nash Fork Shear Zone, Medicine Bow Mtns, S. Wyoming. K-spar porphyroclasts with asymmetric tails What is sense of shear here?

Pressure Shadows with Fiber Growth Minerals will grow in the direction of maximum extension

Pressure Shadows with Fiber Growth Maximum shortening Maximum extension

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

Fractured Grains Large grains of some minerals (e.g. feldspars, pyroxenes) continue to deform brittley within a ductile matrix. The fractures indicate a sense of motion opposite that of actual shear sense How would you describe the kinematics of this system?

Fractured Grains Right-lateral offset

Veins Calcite extensional vein

Tension Gashes Initial shearing

Tension Gashes Directions of maximum extension and maximum shortening

Tension Gashes Mode I (opening) fracture develops

Tension Gashes Continued development of a tension gash

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

Tension Gashes Finally, left-lateral motion!!!

Tension Gashes

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

Tension Gashes Left-lateral motion

References Slide 1: http://earth.leeds.ac.uk/shearzones/gallery/odhair.htm Slides 4, 5: http://earth.leeds.ac.uk/shearzones/gallery/apliteshear.htm Slides 6, 7: http://earth.leeds.ac.uk/shearzones/gallery/kohi1.htm Slides 8, 9: http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html Slides 10,11: Image 30 at http://talc.geo.umn.edu/orgs/struct/microstructure/sec2.html Slides 12, 13: http://www.uni-mainz.de/~mezger/Text/Mezger_metamorphic_geology.html Slides 14, 15: http://earth.leeds.ac.uk/strain/gallery/presshadbig.html Slides 16, 17: http://www.earth.monash.edu.au/Teaching/mscourse/lectures/lec4b.html Slide 18: http://earth.leeds.ac.uk/minor/dmt/reprecipitation.htm Slides 19 - 22: http://earth.leeds.ac.uk/minor/tensiongash/gash1.htm Slides 23 - 24: http://earth.leeds.ac.uk/minor/tensiongash/tgash.htm Slide 25: http://earth.leeds.ac.uk/minor/tensiongash/gash3.htm Slides 26 - 27: http://earth.leeds.ac.uk/minor/tensiongash/gashpic2.htm