Download Structural Geology Stress and Strain

Structural Geology
Stress and Strain
• Structural Geology – the study of crustal
deformation and basin/mountain development
• Stress – force applied to a rock
• Strain – change in shape and/or volume induced
by stress (deformation)
Types of Stress
• Compression – convergent plate boundaries
– Crumpled, thickening vertically and shortening laterally
– Creates folds, reverse and thrust faults
– Himalayas, NW coast of N.A., Appalachians, western coast of S.A.
• Tension – divergent plate boundaries
– Extends crust, thins vertically and lengthens laterally
– Creates basins, normal faults, grabens
– Mid-Atlantic Ridge, East Pacific Rise (Gulf of California), Red Sea Rift
• Shear – opposing forces along a plane
– Forms parallel blocks, pull-apart basins, transform faults, folds and rotational
structures
– Gulf of California, San Andreas fault system, East Anatolian fault system,
Enriquillo-Plantain Garden fault system (Caribbean and North America plates –
Haiti)
Types of Deformation
• Elastic – returns to original state
– Temporary, not permanent
– Yield point – point of deformation beyond which change is permanent
• Plastic – irreversible change in shape or volume that
occurs without the rock breaking
– Usually under conditions of high temp and press
– Usually a slow process giving atoms time to shift in response to force applied
• Brittle – irreversible change that penetrates mineral
bonding
– Usually under conditions of low temp and press
– Force applied suddenly not allowing atoms time to shift or move in response
to force
Deformation Factors
• Heat – Allows “fluid” behavior, > heat = > plastic deformation
• Time – Great amount of time allows plastic deformation to occur
if force is applied continually. Little time doesn’t allow atoms to
adjust so brittle failure is the result
• Composition – hard vs soft minerals and rocks (e.g. qtz vs
micas or granite vs shale)
Folds
• Monoclines – a single draped fold bending in only one
direction (Fig. 10.18)
• Synclines – Concave up, trough-like, youngest rocks at
center (Fig. 10.9)
• Anticlines – Convex up, arch-like, oldest rocks at center
(Fig. 10.9)
– Axial plane – divides fold into two “equal” parts along it’s crest
or line of maximum curvature
– Limbs – the sides of the fold
– Symmetrical – near vertical axis, limbs are equal
– Asymmetrical – rotated axial plane, limbs are not equal
•Monoclinal folds
form when deepseated faults occur
at depth and are not
propagated through
overlying
sedimentary rocks
to the surface
•More pliant
sedimentary rocks
are able to
accommodate strain
by folding instead
of fracturing in a
brittle manner
Monocline
with associated fault
Monocline
A classic monocline near Mexican Hat, Utah is Comb Ridge. Mesozoic strata are bowed
down along the fold. they are horizontal on the plateau at left and in the foreground but dip
45 degrees or more along the fold.
Monocline Comb
Ridge, AZ-UT
• Flat-irons & hog-backs
• Tensional fracturing
causes preferential
erosion along fold axis
• Differential erosion of
units
Monocline
Waterpocket Fold
southern Utah
Echo Cliffs Monocline
Hwy. 89 near turnoff to Tuba City
Syncline
Ellesmere Island, Canadian Arctic (below left), Barstow syncline, a fold in Miocene shales and
sandstones, Rainbow Basin, Mojave Desert, California (below top) Saint-Godard-de-Lejeune,
Canada (below right)
Anticline
Anticlinal folds near : Calico ghost town Mojave Desert, CA (below top), Zagros Mountains,
Iran/Iraq, Northcott Mouth, Cornwall (below left)
Anticlinal Fold
Bradshaw Mtns. west of I-17
Granitic intrusion to right deformed overlying
sedimentary and metamorphic rocks on left to form large
anticlinal structure (only western limb of fold is visible in
this image)
Folded Mtn. Belts
Appalachians Mountains
• Complex folds broken by reverse faulting
• Low-High grade metamorphic rx
• Convergent plate boundaries create orogenic events
– Taconic ~440mya lasting for >250mys
Himalayan
Mountains
• Suture Zone
• Faulted, folded mtn.
belt
• Crustal thickening
• Cont-cont collision
closes Tethys Sea
• Young mtn range
– 40-50mya
Domes and Basins
• Form in mid-continent/mid-plate locations
• Result from vertical rather than lateral forces
• Domes form when low-density or heated materials rise
– Salt domes
– Magma intrusions
• Basins form when influx of material is enough weight to
force subsidence
Type of Faults
• Normal – dip-slip, more vertical than horizontal,
hanging wall moves down relative to footwall
(tensional) (Fig. 10.21)
• Reverse – dip-slip, hanging wall moves upward
relative to footwall (compressional) (Fig. 10.23)
• Strike-slip – Horizontal slip of adjacent blocks, right
and left (Fig. 10.27)
• Thrust – special type of reverse, block moves at low
angle (~45 degrees) (compressional)
Faults
Hanging-wall/Foot-wall
Normal Fault
A small normal fault cutting
carbonaceous silt and
mudstones
Reverse Faults
Reverse faults in
Entrada Fm., Grand
Staircase-Escalante
Nat. Mon., Utah
(Note hanging walls
move upward relative
to foot walls)
Strike-slip fault
Right-lateral SS
along the Las
Vegas Shear Zone,
Nevada
Part of a largescale
displacement
feature trending
NW-SE
Transform Fault
•San Andreas fault
and Pt. Reyes
Peninsula, California
•The San Andreas
trends northwestward
up the narrow
Tomales Bay
•Elongated transform
fault between
spreading centers in
the Gulf of California
(East Pacific Rise)
and of the coast of
NW U.S. (Juan De
Fuca Ridge
Fault Scarps
Strike and Dip
• Strike -Horizontal direction or orientation of a
fault plane or dipping bed (azimuth)
• Dip – the vertical plunge or angle of a fault
plane or bed measured perpendicular to the
strike
Strike & Dip
When a bed is not horizontal, it has dip and strike. Strike of a bed is the direction of the line of intersection of the bed
with an imaginary horizontal plane.
Dip of a bed is the angle a bed makes with a horizontal line in a vertical plane. The dip has two attributes - amount and
direction.