Presentation on theme: "Announcements Field trip to Tanque Verde this Saturday! 8:00 AM departure; loading dock."— Presentation transcript:
Announcements Field trip to Tanque Verde this Saturday! 8:00 AM departure; loading dock
In brittle regime: joints, tensile fractures, shear fractures (faults!), pressure solution (cleavage development)- deformation mechanisms depend on pressure! What about deformation in the deeper crust?
Today: Foliations, stretching lineations, and tectonites- deformation in the deeper crust (D&R, pp ; )
"General" definition: Foliation: Penetrative (at outcrop and microscopic scale) and parallel planar fabric elements in a rock. "Structural geologist's" definition: Planar fabric is secondary and due to mineral recrystallization and/or plastic behavior during deformation at elevated temperatures
phyllitic structure is a type of foliation
Schistosity: coarser grained fabric- also a foliation
Gneissic structure: Compositional banding produced during deformation.
Migmatite: Compositional banding due to in- situ partial melting. Swirly appearance
In a conglomerate, flattened pebbles may define a foliation- "flattening fabric"
Flattening of strong layers surrounded by weak layers may cause strong layers to "neck" and form boudins.
Boudins in 3-D
Mylonitic foliation: Forms due to grain-size reduction by a mix of brittle and plastic deformation in shear zones brittle deformation of feldspar porphyroclasts plastic deformation of quartz "ribbons" and mica
Coarse-grained mylonitic augen gneiss. The large porphyroclasts are called augen ("eyes")
A strongly mylonitized rock- note the extremely fine grain size due to "pulverization" during shearing
Lineation: penetrative linear fabric. We will focus on those that are related to deformation. How does it differ from other linear structures we have talked about, like slickenlines on a fault surface? Types of lineations: 1) Intersection 2) Crenulation 3) Mineral 4) Stretching
Intersection lineation: Intersection of two planar features- an "apparent" lineation in that there is no fabric that is linear. e.g., intersection between cleavage and planar surface
Crenulation lineation: Intersection between fold hinges and foliation
Mineral lineation: preferred alignment of minerals due to deformation and/or recrystallization during deformation
Stretching lineation: elongation of minerals due to "stretching" deformation
Lineation defined by stretched pebbles in a conglomerate
Tectonites: Rocks that are pervaded by foliation and/or lineation- flowed in solid state S: Schistosity (foliation) only due to flattening- no lineation L: Lineation only, due to unidirectional stretching/ constriction LS: Foliation and Lineation, related to noncoaxial strain- shearing
What is it? L-tectoniteWhat kind of tectonite is this? Coaxial (pure shear) or non- coaxial strain? Coaxial
What kind of tectonite is this? Coaxial vs. non-coaxial? L-S tectonite non-coaxial
S-C fabrics- occur in L-S tectonites and serve as excellent sense-of-shear indicators S-Surfaces- planes of schistosity/foliation (flattening) C-Surfaces- planes of maximum shear "shear bands" C comes from cisaillement, French for shear
When studying S-C fabrics- must look perpendicular to lineation!
S-C fabrics and the strain ellipse
First step- find shear bands (C-surfaces) Second step- find flattening planes (S-surfaces) Third step- sense of shear from strain ellipse! left-lateral sense of shear
Practice! C S top to right sense of shear
For fine-grained mylonites- S-C fabrics can be studied using a microscope
Saturday's field trip to Tanque Verde Wash (Redington Pass area) A look at deformation in a shear zone related to the Catalina detachment
Hypothesis for evolution of metamorphic core complexes including the Catalina-Rincon core complex and detachment system.
Next Lecture: Shear zones and shear sense indicators Please read (D&R, pp )
Important terminology/concepts foliation gneissic structure migmatite boudins mylonites augen gneiss lineation (intersection, crenulation, mineral, stretching) tectonites (L, S, L-S) tectonites and strain ellipsoid S-C fabrics and sense-of-shear Structural evolution of metamorphic core complexes