FORCES OF PRESSURE THAT
SHAPE THE EARTH’S SURFACE

• 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.

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

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
Himalayas
Swiss Alps
Atlas
Mountains

Initial uplift
FOLDED MOUNTAINS ERODE
OVER TIME
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

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

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

Tension forces
Footwall
Hanging wall
Tension forces

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 faults are the result of compression forces
• The footwall drops relative to the hanging wall

Hanging wall
Footwall

• 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

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

• 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.

• 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.

• 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

• 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.

• 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.