Transcript fault

DIASTROPHISM –
WARPING,FOLDING, AND
FAULTING
FORCES OF PRESSURE THAT
SHAPE THE EARTH’S SURFACE
WARPING
• Large portions of the Earth’s crust are
subjected to uplift or depression.
• Uplift possibly due to tectonic as well as
erosion processes.
• Depression usually due to glacial weight
added to crust
– Isostasy: rebound of the Earth’s crust as
glacial weight is removed through melting and
global warming
COLORADO PLATEAU: WARPING
•Compression
forces over last 20
million years
uplifted Colorado
Plateau
DOMES AND BASINS: WARPING
ISOSTASY: POST-GLACIAL CRUST
REBOUND
FORCES THAT SHAPE THE
EARTH’S SURFACE
• There are three main types of forces of
pressure that work to shape (and re-shape)
the Earth’s surface:
– Compression forces (‘squeezing’)
– Extension (or tension) forces (‘stretching’)
– Shearing forces (‘ripping’)
FORCES
COMPRESSION FORCES: FOLDS
• Folding
– A fold is formed by the bending or buckling of rock layers, as a result of
great force and pressure over extremely long periods of geologic time
–
There are two primary types of folding:
– Synclines and Anticlines
•
Syncline: Rock layers bend downward in the folding process to form a
trough-like physical feature called a syncline. This physical feature
often shows itself in the form of valleys and lakes.
•
Anticline: Rock layers buckle upward during folding to form an archlike structure called an anticline. This physical features often shows
itself in the form of mountains or ridges.
Structure of Folds
force
force
FOLDS RESPOND TO TECTONICS
A. No compression forces and no folds
B. Compression forces create
somewhat symmetrical upfolds
(anticlines) and downfolds
(synclines)
C. Continued compression pushes
symmetrical upfold over into an
‘overturned fold’
D. Compression forces cause a fault to
form and pushes one limb of the
‘overturned fold’ onto the other limb
E. A Recumbent fold along a fault has
developed
Anticline and Syncline
OVERTURNED FOLD
Recumbent Fold
FOLDED STRATA ALONG SAN
ANDREAS FAULT – HWY 14
FOLDED MOUNTAINS –
COMPRESSION FORCES
•
Folded Mountains form as the
edges of two adjacent rock layers
are pushed together
– The layers buckle like a wrinkled
rug
– Mountains form from multiple
parallel synclines and anticlines
• Under great pressure and steady
force, rocks can actually bend
rather than breaking.
• The Appalachian Mountains in the
North America, the Himalayan
Mountains in India, the Atlas
Mountains in Northwest Africa, and
Swiss Alps in Europe are examples
of folded mountains.
Applachians
Swiss Alps
Himalayas
Atlas
Mountains
FOLDED MOUNTAINS ERODE
OVER TIME
Initial
uplift
Erosion
features
FAULTING – COMPRESSION, EXTENSION
AND SHEARING FORCES
• When enormous stresses build and push large intact
rock masses beyond their yield limit, faulting of the
surface is likely to occur.
• A fault is a fracture in the rock layers along which
movement occurs
Movement is the displacement of once connected blocks of rock
along a fault plane. Displacement can occur in any direction with
the broken blocks moving along the fault in opposite directions
from each other.
Measuring Displacement along a
Fault
• Some faults have vertical displacement,
while others have horizontal displacement
• The measure of displacement is referred
to as either “dip-slip” or “strike-slip”.
– Strike: The compass direction of a line of
strata
– Dip: The angle in degrees between a
horizontal surface and an inclined surface –
measured as perpendicular to strike
Dip versus Strike
UNDERSTANDING FAULT
TERMINOLOGY
• Faults are identified by their patterns of
displacement:
– Vertical (dip slip)
• The movement is along the line of the dip
– Horizontal (strike slip)
• The movement is along the line of the strike
Dip-slip versus Strike-Slip
TOPOGRAPHIC FEATURES OF
DIP SLIP FAULTS
• Fault scarp: steep cliff that represents
edge of vertically displaced rock
– Can be 100s of meters in height
– Can extend 100s of kilometers in straight lines
– Sharp rise in terrain and steep slopes
Fault scarp
Fault scarp
Identifying Dip Slip Fault Structures
NORMAL FAULTS: DIP SLIP
FAULTS
• Normal faults are the result of tensional (or
extensional) forces acting to pull apart the
surface.
• The hanging wall drops relative to the
footwall.
• Normal faults can occur across vast areas
due to lithospheric stretching.
– Basin and Range in Western USA
NORMAL FAULT: DIP SLIP
Tension forces
Footwall
Tension forces
Hanging wall
HORSTS AND GRABENS
Tension forces
Tension forces
Mountains and Basins created by a series of parallel Normal
Faults – The Basin and Range Province in Western North
America is a topographic example of normal faulting:
Grabens: downdropped basins
Horsts:
Uplifted mountains and ranges
Basin and Range
Western USA exhibits
‘horst and graben’
structures due to
extensional tectonics.
The Western edge of the
Basin and Range
includes the Sierra
Nevada in California.
The Eastern edge of the
Basin and Range
includes the Wasatch
Range in Utah
Faults across Basin Range Province
TENSION FORCES ARE PULLING THIS AREA APART
REVERSE FAULT – DIP SLIP
• Reverse faults are the result of
compression forces
• The footwall drops relative to the hanging
wall
Reverse Fault – Dip Slip
Hanging wall
Footwall
REVERSE THRUST FAULT
• Reverse thrust faults
are the result of very
low angle faults,
pushing the hanging
wall up and over the
foot wall
Hanging
wall
Footwall
Compression forces
BLIND REVERSE THRUST FAULT
• A blind reverse
thrust fault does not
extend to the
surface – we only
know of their
existence because
of earthquakes and
surface deformation
• Hanging wall lifts up
and over footwall
Hanging wall
footwall
TRANSFORM FAULTS:
SHEARING FORCES
• Transform faults can be found at plate
boundaries as one plate slides horizontally
past another.
– Strike-slip faults
• Most transform faults are found on the
ocean floor as part of the active offset
along divergent plate boundaries.
TRANSFORM FAULT – SEA
FLOOR SPREADING
TRANSFORM FAULTS: PLATE
BOUNDARIES
• At plate boundaries, when two tectonic
plates grind past each other, there is
usually no volcanism or mountain building
occurring.
• One of the largest transform faults in the
world is the San Andreas Fault
– Separating the North American Plate from the
Pacific Plate in southern California.
TRANSFORM FAULTS: SAN ANDREAS
San Andreas Fault
FORMATION OF SAN ANDREAS
FAULT
• The northwest-southeast trending fault
zone extends from the East Pacific rise in
the Gulf of California (between Baja
California and the Mexican mainland) to
the Mendocino fracture zone offshore of
northern California - approximately 800
miles
San Andreas Fault Zone
• The San Andreas fault zone includes the
main fault trace and many other major and
minor fault strands.
• The relative rate of motion between the
North American plate and the Pacific plate
is approximately 3.5 to 4.6 cm per year,
most of which (2.0 to 3.5 cm per year) is
accounted for by horizontal displacement
along the San Andreas fault zone.
Evolution of San Andreas Fault
• Before 30 million years ago, the western edge of
the North American plate met the eastern
Farallon Plate in a convergent plate boundary –
complete with subduction.
• The western edge of the Farallon Plate was
diverging from the Pacific Plate
– “East Pacific Rise” spreading center
• The rate of convergence was greater than the
rate of divergence and the spreading center
moved towards the subduction zone.
Evolution of San Andreas Fault
• Approximately 30 million years ago, the
spreading center (East Pacific Rise) came
into contact with the active subduction
zone.
• The Farallon plate was split into two
pieces which are still being subducted
beneath the North American plate
– Juan de Fuca (northern plate)
– Cocos (southern plate
Evolution of San Andreas Fault
• The relative motion between the Pacific
plate and the North American plate was
altered to become a transform boundary.
• Subduction along the transform boundary
stopped.
– New motion of this portion of Pacific Plate is
to the northwest, parallel to the North
American plate
CONSEQUENCES OF SAN
ANDREAS FAULT
• Along the San Andreas Fault, the Pacific plate slowly
grinds to the north.
• Los Angeles lies on the Pacific plate side of the fault,
while San Francisco is on the North American side.
– About 25 million years in the future, if movement continues in the
same direction, Los Angeles will be a suburb of San Francisco
(or vice versa)
• The San Andreas is a right-lateral transform fault, which
means that if you imagine standing on either side of the
fault and looking across to the opposite side, it seems to
you that the people and objects on the opposite side are
moving to your right.
Features of San Andreas Fault
• Linearity: This fault exhibits an almost
‘straight line’ in appearance on the Earth’s
surface.
• Beheaded streams: Streams that cross
the San Andreas are displaced as the
Pacific Plate slowly moves along
• Sag Ponds: Groundwater, under pressure
from the two plates grinding together, is
forced to the surface.
San Andreas Fault in the
Carrizo Plain
View is looking south. Fault
is in the center of the folded
ridge area
Wallace Creek
Sag Pond Along San Andreas Fault
FOLDS, FAULTS AND FOSSIL FUELS
• Fossil fuels such as oil and natural gas are produced
through decomposition and heating of organic materials
in marine sediments.
• Oil and natural gas are collected in ‘reservoir rocks’.
• Folding and faulting of reservoir rocks aids in the hunt for
fossil fuels.
• Anticlines offer the best prospect for finding ‘pools’ of oil
or natural gas that have migrated upward (they are less
dense that surrounding rocks).
• Faulting moves impermeable surfaces against
permeable surfaces – allowing oil to collect along the
fault plane.