Folds and Foliation

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Transcript Folds and Foliation

FOLDS and thrust
Which deformation
and stresses
cause theses structures ?
Folding of Shale-Sandstone sequence,
Kings Canyon,
California
… Compression
…Shortening
The fold is like a
Thrust defined
by its
orientation
Parts of a fold:
Hinge
–axial plane
– limbs
–“hinge”
Types of Folds
• Anticline = arch
• Syncline = trough
• Monocline = stair step
• Dome or Basin
Chevron Fold, Laurel Mt., CA
Syncline
Youngest rocks
in middle
syncline, Israel
Anticline
Oldest rocks
in middle
Virgin anticline, southern Utah
Domes &
Basins
Chernicoff and Whitney
Think of an Egg Carton!
Map from Topozone.com
Grenville Dome: Sinclair, WY
SynclineAnticline
Pairs +
Domes:
Zagros
Mts, Iran
NASA “Earth as Art” web page
Monocline
Fold
Classification
Fold in Glacier, Antarctica
Folds axis are perpendicular to the
main direction of compression
David Rogers
Casey Moore, UCSC
Little Shuteye, Sierra Nevada, CA
Domes are not produced by horizontal
but vertical compression
Minor Folds and Foliation Are
Clues to Much Larger
Structures
Foliation
– pressure flattens
and/or aligns
minerals in a rock
1 mm
– platy or sheet-like
structure reflects
the direction in
which pressure was
applied
– Slate, schist, and
gneiss foliated
Microscope Image of Foliated Garnet Schist, VT
Foliation
In every case, the
foliation is:
In the direction of
least resistance
at right angles to
the direction of
greatest
compression.
Fold And Foliation
Folds and Foliation
How Geologists Use These Clues
• Here's an outcrop that might be seen in the
field.
Minor Folds and
foliations can be
used to determine
the axe of the fold
Pay attention
it migth look like folds
•
•
•
•
K = Cretaceous
J = Jurassic
Tr = triassic
At X, we have
younger rocks
surrounded by
older rock
But it is produced by differential
errosion on a thrust
• Dark green is older rock, thrust over the younger yellow rock
• The Klippe (K)
• W is a Fenster or Window
Folds and and Thrust are often
associated
The overturned fold in the
upper diagram may
break, yielding an
overthrust fold or thrust
fault
Folds and thrust
have the same
origin
Reverse
Fault
Small thrust fault, Las Vegas, NV,
Source: M. Miller, U. of Oregon
Thrust Faults in Snow
Folds and thrust are both responsible
for the orogens
R.W.H. Butler
fold-thrust complex developed in Upper Jurassic limestones in the Haut
Giffre area of the Subalpine thrust belt (Morcles nappe in France)
Folded Appalachians
Near Harrisburg, PA, Source: NASA
Folding in Malaspina Glacier, AK, 1969
Oil and Gas Concentrate in Domes
Chernicoff and Whitney
Growth of Minerals
Small Scale Structures Mimic
Large Scale Structures!!
Bohlen et al., 1987
Folded Amphibolite in Marble, Warrensburg
(K. Hollocher, NYSGA field photo)
Foliation Macroscopic and Microscopic
Feldspar (strong)
Quartz (weak)
Foliated Slate, Shelburne, VT, UVM
Foliated Gneiss, Nunavut,S. Tella Photo.
Mulwaree fault zone, Australia
Mulwaree fault zone, Australia, Tullis et al.
Pressure
Cararra Marble
Deformation
Experiments
K. Hamblin
A. Kronenberg
Brittle
Ductile
Continental Extension
Chernicoff and Whitney
Basin
and Range
Nevada Bureau of Mines and Geology
Extension
– Crust Thins
Ductile
– Faulting
Brittle
Shearing
– lateral slip
creates faults
– common at
transform
boundaries
– result from
brittle
deformation
Faults
Hanging Wall
– rocks offset
across fault
– Sides referred
to as “hanging
wall” and
“footwall”
– 3 types of fault
Footwall
Strike & Dip
– Describe fault
orientation
– Direction of slip
determines kind
of fault: “dip-slip”
or “strike-slip”
Chernicoff and Whitney
Normal
Fault
Normal Fault, Lamb Canyon, CA
Strike-Slip
Fault
Strike-slip fault near Las Vegas, NV,
Source: M. Miller, U. of Oregon
Strike-slip fault displacement in orchard
Joints
•
•
•
•
Brittle “cracks” in rocks
Form near surface
Regular spatial distribution
No offset
Preferential
weathering
of joints in
Sandstone;
Calcite veins in
joints of marble,
Laurel Mt., CA