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

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. 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. 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 porphyroclastsplastic 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) Mineral4) 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

Stretchedcalcite

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 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 tectonitenon-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!

CS

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. 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