Transcript Summary and review

Announcements:
Final Exam
Monday, Dec. 16, 11-1
this room
Recap of Monday's whirlwind tour:
1) Archean to Middle Proterozoic growth and
deformation of the N.A. craton
2) Late Proterozoic – Cambrian rifting of the W. NA
margin
3) Paleozoic passive margin sedimentation
4) ~350 Ma Antler orogen to west
5) 300-250 Ma Ancestral Rockies to east
6) Permo-Triassic Sonoma orogeny to west
Bigger picture: formation of the Cordilleras due to longlived oceanic subduction
The Sevier orogeny:
E-directed thrusting
behind a magmatic
arc–
"retro-arc" fold-thrust
belt and associated
foreland basin system
What's going on at this
time (~60 myr ago?)
Eastward sweep in
magmatism
Basement-involved
deformation
"Laramide" orogeny
Magmatism starts
sweeping back to west
– 45 Myr ago
More "Laramide"
deformation
More westward sweep
in magmatism-
Major change in style
of deformation!
Rifting and
development of
Cordilleran
metamorphic core
complexes related to
major crustal extension
Basin and Range
Extension
Eruption of Columbia
River basalts
Magmatism back to
west
Growth of Basin and
Range Province
Today!
Relating plate
tectonics to
continental
deformation
Concept of "Suspect"
terranes- possibly fartraveled and torn off of
other continents, rafted
across large tracts of
oceans and finally
accreted to W. N.A.
Crustal "immigration"
Review for:
Final Exam
Monday, Dec. 16, 11-1
this room
Format: Short essay questions (~1 paragraph),
SKETCHES!, some short answer questions
Recall Coulomb's
Law of Failure
In compression,
what is the
observed angle
between the
fracture surface
and s1 (q)?
~30
degrees!
sc = critical shear stress required for failure
s0 = cohesive strength
tanf = coefficient of internal friction
sN = normal stress
Anderson's theory of faulting
Summary
Thrust systems:
1. Accommodate significant crustal shortening
2. Basal detachment; decoupling within the crust
3. Faults have ramp and flat geometries
4. Fault place older/higher grade rocks over younger/lower grade rocks
5. Faults cut up-section
6. Faults generally propagate (get younger) toward the foreland
7. Younger and structurally deeper faults rotate older faults to steeper
angles
Colorado Plateau
monoclines may
be related to
thick-skinned
deformationbasementinvolved
thrusting
Fault-bend folds
Fault-propagation
folds
monoclines as "drape" folds
Mechanical "paradox" of large thrust sheets
Possible explanation- water pressure plays a big role
sc = tanf(*sN), where tanf is the coefficient of
sliding friction and *sN = sN – fluid pressure
Critical Taper
Thrusts belts are
wedge shapedcharacterized by a
topographic slope
(a) and a
decollement dip (b)
Only at some
critical angle (a+b),
will the thrust belt
propagate
Sequence of events:
summary of
strike-slip-related
deformation
Tectonic significance of strike-slip faults
(1) oceanic transform faults
(2) trench-trench transform faults (Alpine fault)
(3) trench-ridge transform fault (San Andreas)
(4) Oblique convergence (Denali fault)
(5) transfer fault between thrust systems (Altyn Tagh?)
(6) continental extrusion (Altyn Tagh?)
Cleavage: closely spaced, aligned, planar surfaces;
associated with folds- impart a splitting property to
the rock- often cuts bedding
continuous cleavage under a microscope, showing
domainal nature
Origin of cleavage + passive folds
Pressure solution
(dominant!!)
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?
Flattening of strong layers surrounded by
weak layers may cause strong layers to "neck"
and form boudins.
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
Stretching
lineation
Deformation mechanisms: Processes that permit
rocks to deform at the microscopic and atomic scale
How much and when were rocks buried to depth?
When were rocks deformed?
When were rocks metamorphosed?
When were rocks brought up from depth (exhumed)?
How fast?
How did this all happen?
To get at displacement on BIG structures- need to
know depths/temperatures from which rocks were
brought up- thermobarometry
To get at timing- need geochronology and
thermochronology
Thermobarometry: Quantitative determination
of temperature (T) and pressure (P) using
equilibrium reactions
THERMOCHRONOLOGY: determining the time when
a rock was at a certain temperature
(1) aseismic movements that occur in
between earthquakes
Real-time action; Real-time measurementwhat can we learn from seismology about
structural geology??
1) Location and depth of faulting (brittle-ductile
transition)
2) fault plane solutions- orientation of fault and sense of
slip- geometry and kinematics
3) Energy release- size of fault, rupture
characteristics- unidirectional, bidirectional, chaotic?
What we are learning from GPS
1) Plate tectonic assumptions OK- but only to first
order- Within plate deformation can be huge
2) How continents deform during orogenesisdiffuse? plate like?
3) What parts of faults are slipping vs. what parts
are "locked" up- important for EQ predictions
4) unprecedented knowledge of recent movements
on Earth
Landers Earthquake
What can we learn from
InSAR?
Study the earthquake cycle- recurrence intervals
of major events through neotectonic studies
1) study deformed historic sites of known ages
(Great Wall of China)
2) Paleoseismology
3) offset geomorphologic surfaces + surface
dating
Continental deformation- what we are learning from
Tibet
Active deformation in Tibet- normal and
strike-slip faulting- WHY?