Week 10: Microtectonics and Metamorphism

• Deformational Structure changes through the mantle

  • Upper crust - frictional forces and deform with brittle behaviour
  • The brittle-ductile transition zone- variability in its location, usually around 10-15km deep but it can be affected by strain rate, fluids, temperature
  • Middle crust- Where there are high levels of shear zones, mylonitic rocks and greenschist metamorphic rocks
  • Lower crust, where there are still levels of shearing but not as common, also in this region is Amphibolite and Eclogite metamorphic rocks
• Geothermal Gradient and Metamorphic Rocks

  • Different resulting structures can be due to pressure or temperature. Temperature is impacted by the geothermal gradient evident in the region. As a rough rule, every km into the crust is a 20-30 degree increase in temperature. And pressure roughly increases by 3kbar every 9km
  • Cold gradient: 5-10 degrees
  • Hot gradient: 60-100 degrees
  • The geothermal gradient experienced can differ greatly based on the tectonic setting of the locality
  • Cold – Subduction
  • Average – Continental crust
  • Hot – Mid Ocean Ridge
• Metamorphism

  • Depending on the depth and temperature/pressure conditions, rocks will undergo various degrees of metamorphism.
  • When rocks reach 200 degrees they reach the lowest end of metamorphism – diagenesis
  • The upper limit of metamorphism is 750 degrees – above this the rocks begin to melt
  • Classified into low grade, medium grade and high grade
  • Typically, the grade of metamorphism increases with depth
  • In the Himalayas the metamorphic grade experienced is reflected by an increase in grain size from slate to gneiss
• Structural Differences in the Crust

  • Upper crustal rocks are subject to faulting with evidence of cataclastic rocks
  • Lower crust - rocks behave plastically and flow. This zone typically experiences shear zones
  • Fault structures are most common in the upper crust, they are brittle deformation structures due to built up stresses. Brittle faults may contain cohesive cataclasite material
  • Shear zones occur when the rocks have a certain rheology and are able to flow, often with metamorphosed rocks included. Theses ductile faults may include large shear zones with striped gneiss at great depths
• Minerals and shear direction

  • Rock behaviour and mineral assemblages change throughout a mountain belt due to the changes in the way that individual minerals react to stresses and changes
  • In thin sections of rocks taken from mountain belts, the shape of mineral grains can be used to determine the direction of shear fault movement
• Mica in thin sections

  • By using minerals such as micas, as markers of shear sense in a ductile regime, micas can appear in a ‘fish’ shape. This shape helps determine the S and C plane of movement as well as the shear zone boundary
• Mylonites

  • Recrystallisation of porphyroclasts (mylonite’s) can help determine the original minerology of the rock before its metamorphism and how it’s journey through the crust would have changed
• Porphyroblasts

  Porphyroblasts can also be used to determine the tectonic setting and timing of a metamorphic rock, depending on the timing of growth of the inclusion with relation to the matrix of the rock