Announcements Field trip to Tanque Verde this Saturday Announcements Field trip to Tanque Verde this Saturday! 8:00 AM departure; loading dock

In brittle regime: joints, tensile fractures, shear fractures (faults 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. 456-479; 485-492)

"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 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.

more 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 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 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

Stretched calcite

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

Strain ellipse and tectonites S-tect = S1= S2 > S3 (coaxial) L-tect = S1 > S2 = S3 (coaxial) LS-tect = S1 > S2 > S3 (non-coaxial)

What kind of tectonite is this? Coaxial (pure shear) or non-coaxial strain? L-tectonite Coaxial What is it?

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! top to right sense of shear S C

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. 493-551)

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