Transcript Faults and Faulting 2

Structural Geology
Faults and Faulting 2 – Lecture 17 –
Spring 2016
Anaverde Cut, Western Mojave Desert, showing folds and a fault
1
- Pliocene Anaverde Formation adjacent to the San Andreas Fault
Fault Gouge
Photo 1
• Photo: Garry Hayes,
Modesto Junior College
• Near Natural Bridge,
Death Valley N.P.
• Such rocks have random fabrics,
with no distinctive foliation
• However, continued movement
along the fault may shear the
gouge, and produce foliation
• Clay minerals, formed by
weathering of silicate minerals,
may closely resemble fault gouge
2
Fold in Gouge Photo 2
• Folded Gouge, San
Gregorio Fault Zone,
Moss Beach CA
• Soft smectite-rich clays
form a gouge zone 10-40?
m wide that has enjoyed
10 km to perhaps 100 km
displacement
• Huge strains result in
banding and folding seen
here
• Photo by Meredith Lohr
3
Banded Clay Gouge
• Lewis thrust,
Alberta, Canada
Figure 8.16b in text
4
Fault Breccia
• Buckskin
detachment,
Battleship Peak,
Arizona
Figure 8.16a in text
5
Sango Bay Fault
Breccia
• Located along the
northern part of the
Moine Thrust,
Great Britain
6
Indurated Breccia
Photomicrograph
• Note the angular fragments
(fr) of quartz sandstone in a
matrix of fine-grained iron
oxide cement (ic)
• Field of View 4 x 2.7 mm,
Cross Polarized Light
• Photomicrograph of fault
breccia in the Antietam
Formation, Blue Ridge
province
• Breccias form when
rocks are extensively
fractured in fault zones
and are cemented
together when minerals
precipitate in the cracks
7
and fractures
Pseudotachylyte Photo
• Silicate rocks are excellent
insulators, and heat generated
by friction does not escape
• Temperatures in excess of
1000ºC are possible
• Tachylyte is a type of
• Newer pseudotachylyte
volcanic glass, and the prefix
injection vein cuts the older one
pseudo means false, so the
• Shimanto accretionary
name literally means false
complex
volcanic glass
8
Pseudotachylyte, Nova Scotia
• Pseudotachylyte
• Frictional melt,
Nova Scotia
• Note conchodial
fracture, strongly
suggestive of glassy
nature, plus
intrusive
relationships
9
Argille
Scagliose
Photos
• Argille scagliose
melange associated
with obducted
ophiolite of the
Alpine system of
Macedonia
10
Cataclasite photo
• Foliated cataclasite in the core of the San
Gabriel fault, San Andreas System,
California
• Photo: Frederick M. Chester
11
Formation of
Scaly Fabrics
• Serpentinite
is California's state
rock
• 1-1/2 meter boulder in the roof
garden of the Oakland Museum
of California, its polished surface
gleaming blue and jade-green
• Once scraped off on
the continent, the
fragmented
ultramafic rocks
quickly weathered
• Scaly fabrics can
develop in
serpentinite, just as
they did in clay 12
Slickensides
Figure 8.18a in text
• Shiny slickensided surface in Paleozoic strata
from the Appalachians (Maryland)
13
Slickenside
Photo
• They may form on the
original wall of the
fault, or on a thin layer
of gouge
Slickensides, beautifully
preserved atop a bedding plane
in the Manlius Formation of the
Helderberg Group, Hudson River
Valley, NY
14
Slip Fibers
• Slip fibers on fault
surface
• Note Brunton
compass for scale
• Steps indicate sense
of shear
• Figure 8.18b in text
15
Quartz Fibers
• Quartz fibers in ductile shear zone, Alpine
basement, Switzerland
16
Formation of Pits
• Any step on the fault surface
subjected to pressure solution
experiences more pressure than
the areas around them
Figure 8.19a in text
17
Slickolites
Figure 8.19b in text
“Restraining steps become
pitted by pressure solution,
releasing steps become the
locus of grain growth, and
oblique restraining
steps become slickensides”
• The pits then resemble
a cross between a
styolite and a slip
lineation, leading some
geologists to call them
slickolites
18
Subsidiary Fault and Fractures
Geometries
• In major fault zones, it is common to have an array of
subsidiary faults develop
 Some are discrete, smaller faults
 These may or may not anastomose with each other
 Fault splays may split off the major fault and some of the
subsidiary faults
• Subsidiary faults may initiate when the primary rupture
splits into more than one surface during formation in intact
rock
• They may also occur when numerous faults initiate
simultaneously in a fault zone
19
San Andreas and Subsidiary Faults
• San Andreas Fault to left; Hayward Fault to right of SF Bay
20
Clay Experiment
Figure 8.20a in text
• We can model the situation by placing a clay
layer over two wooden blocks, and then moving
one block opposite the other, as shown in the
figure
• The clay will accommodate some of the strain,
but will then rupture
21
Formation of
Reidel Shears
Figure 8.20b in text
• The first fractures are short, shear fractures inclined to the
trace of the through-going fault
 They are called Reidel shears, and generally occur as a conjugate pair
 The acute bisectrix of the Reidel shears gives the local orientation of
σ1
 A third set of shears, the P shears develops, and links the Reidel
shears
 Eventually the linked P and Reidel shears form a large, throughgoing
fault
22
Reidel Shear Example
• Map shows
location of
Reidel Shears
in central
Washington
23
Washington
State Reidel
Shears
• Lines in the deformed
ellipsoids represent right
lateral strike-slip faults that
allow movement to take
place in the brittle crust
• Movement of two terrains (western
Washington vs. eastern
Washington) with a weak block
trapped between (central
Washington)
• Right lateral transverse faults form at
30° to σ1 ( the big arrow) and then
the terrain slowly rotates over time
• Note how the blocks are deformed
and that the lower circles are rotated
more than the upper circles
24
Fault-Related Folding
• Faults and folds are commonly seen together in the same
outcrop
 Since faults represent brittle behavior and folds ductile behavior,
this appears strange
 The simple explanation is that the two structures formed at
different times, under much different conditions
 Folds formed 10-15 kilometers below the surface are locked in
place, and uplifted near the surface
 Brittle behavior now dominates, and accumulated stress causes the
rock to break and move
 Faults formed this way usually bear no geometric relationship to
the folds
25
Development of Folds
• Stages in development of
folds, leading to a fault
• This might reflect simply an
increase in the regional strain
rate, or it might reelect a
“lock-up”
• Lock-up means that the folds
reach a point where continued
folding is very difficult
Figure 8.21 in text
26
Fault-Propagation Fold Photo
• Fault propagation
fold in Mesozoic
sedimentary rocks
in the Salt Range,
northern Pakistan
• Photo by Kevin
Pogue, Whitman
College
27
Fault-Bend
Folds
Figure 8.22a in text
• A bend in the fault surface may cause folding of strata that
moves past the bend
• The moving layers must accommodate the bend, without gaps
or overlaps
• Folds that form in this manner are called fault-bend folds
• They develop in association with all kinds of faults, but have
been most studied in dip-slip faults
28
Diagram of Fault-Bend Fold
Development
• Steps in the formation of a fault-bend fold
29
Fault-Bend Fold Photo
• Kink-style fault-bend
fold
• Front Range,
Canadian Rockies,
Alberta
• Photo by Tekla Harms,
Amherst College
30
Folding Between Shears
Figure 8.22b in text
• Figure shows what happens when such a sequence is trapped
between opposing walls in a fault zone
• Shear on the opposing walls causes the trapped strata to fold
• Asymmetric folding is quite common
31
Folding with Detachment Faults
• Figure 8.22c in text
• Above a detachment fault,
the sheet of rock may
deform independently of
the underlying rock
• Shear on the detachment
fault accommodates the
difference in strain
between the folded
hanging wall and the
unfolded footwall
32
Drape Folds
Figure 8.22d in text
• The overlying sedimentary rocks often bends passively
into a drape over the displaced blocks
• Such folds are called drape folds or forced folds
• They are a type of fault propagation fold, but their name
emphasizes how they form
33
Drape Fold Photo
• Drape fold structure
at Rattlesnake
Mountain. near the
Buffalo Bill Cody
Dam, Wyoming
34