1 / 44

Metamorphic Fabric

Chapter 13A. Metamorphic Fabric. Solid-state Crystal Growth. Nucleation Crystallization of new phases Crystal growth Modification of existing grain boundaries. Nucleation. Homogeneous nucleation Heterogeneous nucleation. Homogeneous Nucleation.

monteith
Télécharger la présentation

Metamorphic Fabric

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 13A Metamorphic Fabric

  2. Solid-state Crystal Growth • Nucleation • Crystallization of new phases • Crystal growth • Modification of existing grain boundaries

  3. Nucleation • Homogeneous nucleation • Heterogeneous nucleation

  4. Homogeneous Nucleation • Formation of new minerals within another mineral • Free energy of new phase is greater than its surface energy • Rate of homogeneous nucleation is very slow

  5. Heterogeneous Nucleation • New minerals form along existing grain boundaries or other discontinuities • This mechanism is common

  6. Crystal Growth • Nucleation • Independent growth • Some contacts • Granoblastic texture

  7. Granoblastic Texture • Triple points • Straight grain boundaries • Interfacial angle controlled by surface energy of crystals in contact

  8. Interfacial Angles • Dihedral angles () • Controlled by facial energy • Sine relationship applies

  9. Stress Categories • Tension • Compression • Shear

  10. Hydrostatic Pressure • Defined as a uniform stress on a point regardless of direction • Hydrostatic pressure increases with depth in the earth • Its value equals gz

  11. Pressure Solution • Areas under high stress dissolve • Material moves to regions of low stress • Migration facilitated by an intergranular fluid • Driving mechanism is a chemical potential • Evidenced by growth into pressure shadows

  12. Formation of Porphyroblasts • Controlled by nucleation phenomena • Megacrysts have a high surface energy • If only a few nuclei may form • They may grow to a very large size

  13. Stress • Stress is measured by F/A • Units are Newtons/m2, MPa, bars, etc. •  is the symbol for stress •  = lim  F/ A as  A becomes infinitely small

  14. Directed Stress • Tectonism produces non-uniform stress • This causes: • Rock deformation • Preferred orientation of mineral grains • Development of large-scale structures

  15. Strain • Strain is the response to stress •  is the symbol for strain •  = lim l/lo as l approaches zero  l is the change in length in a line element l0 is the original length of the same line element

  16. Strain Measurements • Units of strain are given as a fraction of the initial dimension • Length strain • l = l/lo • Volume strain • v = V/Vo

  17. Timing of Porphyroblast Growth Pre-tectonic, Syn-tectonic, Post-tectonic

  18. Pre-tectonic Textures

  19. Post-tectonic Textures

  20. Types of Rock Behavior • Elastic (like a spring) • All deformation recoverable • Linear deformation with applied stress • Viscous (honey) • No deformation recoverable • No threshold for deformation

  21. Types of Rock Behavior • Plastic (clay) • Some deformation recoverable • Yield strength must be overcome • Brittle (halite) • Yields by fracturing • Generally elastic behavior prior to rupture

  22. Rheology • The study of the flow of materials • Strength describes the condition of materials when they fail • Soft materials begin to yield at their yield strength • Brittle materials will rupture at their fracture strength

  23. Linear Rheological Models • Hookean model elastic • Newtonian model viscous • Saint Venant model plastic

  24. Elastic Model

  25. Viscous Model

  26. Sliding Block Model

  27. Complex Rheological Models • Bingham model • Has both a yield strength (kb)and a viscosity (b) • Common behavior of natural materials

  28. Plastic Model

  29. Metamorphic Tectonites • Undulatory extinction • Wavy extinction in quartz • Bent twin planes in crystals • Deformation bands • Deformation lamellae

  30. Low Temperatures & High Strain Rates • Undulatory extinction • Kink bands • Deformation lamellae

  31. High Temperatures& Slow Strain Rates • Recovery and recrystallization occur • Sutured grain boundaries • Small new grains form

  32. Diffusive Flow • Thermally activated • Stress induced • Diffusive recrystallization • Sometimes called pressure solution

  33. Role of Fluids in Deformation • Hydraulic weakening of non-hydrous silicates • Prograde dehydration reduces ductility • High pore PH20 may cause rock to be brittle

  34. Anisotropic Fabric • Results from syntectonic flow under stress • Causes include: • Applied nonhydrostatic stress • Magnitude of strain • Strain rate • T & P

  35. Fabric Geometry • Very complicated • Non-homogeneous rock bodies • Linked chain of events • Unambiguous answers

  36. Grain Orientation • Foliation commonly parallel to axial plane of folds • Noted by orientation of platy minerals • Lineation commonly parallels hinge line • Given by alignment of elongate minerals

  37. Orientation Mechanism • Not easy to determine • Nucleation and growth? • Rotation of grains? • Pressure solution?

  38. Segregation Layering • Alternating bands of different minerals • Relict beds? • Mechanical transportation processes? • Easier to determine in lower grade rocks • Uncertain origin in higher grade rocks

  39. a. type of cleavage?b. how many S?

  40. c. texture?

  41. d. rock name?

  42. e. names? (1=L, 2=R)

More Related