Chapter 11 - Crustal Deformation & Mt. Building

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Kirsty Camba

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Crustal Deformation & Mountain Building
Crustal Deformation
What causes rock deformation?
tectonic forces can cause rocks to move tilt and/or change type.
colliding plates can uplift a flat lying bed of marine limestone so that it's exposed at the surface, rotate the ruck at a steep angle and crumple it into folds
all these types of change are called deformation
deformation occurs mostly along plate boundaries-the plates where lithospheric plates push together pull apart or scrape past each other.
Crustal Deformation & Mountain Building?
Crustal Deformation
Geologists use the terms tectonic structures/geologic structures for the structural features that can be observed and that reflects Earth's history.
Crustal Deformation
Stress cause of Deformation
more precisely, rocks responds to stress.
deformation is caused by differential stress in w/c the force is stronger in 1 direction and weaker in another.
a rock strength is its resistance to being deformed permanently by stress.
a rock will resist deformation until the stress exceeds its strength.
3 types: compressional, tensional and shear
Crustal Deformation 
Stress: Cause of Deformation
Compressional
differential stress that squeezes a rock mass as if it in a vise.
most often associated w/ convergent plate boundes, collide and earth's crust is shortened horizontally and thickened vertically.
this stress produce mountain belts.
Crustal Deformation
Stress: Cause of Deformation
Tensional
differentional stress that pulls apart rock bodies
often associated w/ divergent plate boundaries where plates move apart.
this stress stretch and lengthen rocks horizontally and thin them vertically.
e.g. Basin and Range provence, tensional fones have fractured and stretched crust as much as twice its original width.
Crustal Deformation
Stress: Cause of Deformation
Shear
differential stress that can cause rock to shear w/c involves the movement of one part of a rock body past another.
e.g. San Andreas Fault, shear is important at transform faults where large segments of Earth's crust slip horizontally past one another.
Crustal Deformation 
2. Strain: Change in shape due to stress
differential stress that changes a ruck's shape.
on a microscopic scale, this shape change is accomplished thru shearing.
deformation on a large scale often involves shearing on a smaller scale.
Crustal Deformation 
2. Strain: Change in shape due to stress
2 main ways in w/c a solid rock can change its shape by undergoing small-scale shearing.
a rock can undergo slippage along parallel surfaces of weaknesses such as foliation surfaces or microscopic fractures,
Shearing can take place w/ intact mineral grains.
Crustal Deformation
2. Strain: Change in shape due to stress
Shearing
occurs because mineral crystals are often far from perfect; the crystal structure may be missing atoms or the atoms of one element may substitute for those of another.
when a crystal undergoes differential stress these defects are where bonds can easily break and reform.
result is a slippage along microscopic surfaces within the crystal structure and the mineral grain as a whole changes shape.
Crustal Deformation
2. Strain: Change in shape due to stress
Mineral grains can also shape in response to differential stress that does not slippage along zones of weaknesses.
this is called recrystallization where it involves the movements of atoms from a location that is highly stressed to a less stressed position on the same grain.
Crustal Deformation
Types of Deformation
3 types: elastic, brittle and ductile
Elastic
deforms temporarily and return to its original configuration once stress is removed.
chemical bonds in a mineral grain acts like a spring.
the energy released by most strong earth quakes comes from stored energy that is suddenly released as ruck elastically snaps back lelastic rebound) to its original shape.
Crustal Deformation
Types of Deformation
2. Brittle
when a rock's strength is exceeded
when it is deformed beyond its ability to respond elastically it will either break or bend.
rocks that break into smaller pieces exhibit this.
occurs also when stress breaks chemical bonds that hold a material together.
Crustal Deformation

Types of Deformation
3. Ductile
when an object changes shape w/out breaking.
takes place in rocks through slippage along surfaces of weakness within the rock and the gradual reshaping of mineral grains.
These processes enable rock to flow very slowly, even though it remains in a solid state.
eg. Intricate folds -example of ductile deformation.
e.g. Clay or Taffy
Crustal Deformation
Factors that affect rock deformation:
temperature, confining pressure, type of rock, time
1. Temperature
plays a major role
when temperatures are high (deep in Earth's crust or adjacent to a heat source suit. as magma chamber), rocks are nearer their melting temperatures and are there fore weaker and more capable of ductile deformation.
near the surface or in a cooler environment such as the subduction zone, rucks are more brittle to fracture.
Crustal Deformation
Factors that affect rock deformation:
temperature, confining pressure, type of rock, time
2. Confining pressure
confining pressure on rocks increases w/ depth as the thickness of the overlying rock increases.
since confining pressure squeezes rocks equally from all directions, it tends to make them harder to break and less brittle.
increase in pressure keeps the rock intact and more likely to bend than fracture.
Crustal Deformation?
Factors that affect rock deformation:
temperature, confining pressure, type of rock, time
3. Rock type
mineral composition and texture greatly influence the responce of rocks to stress.
granite basalt and well-cemented quartz sandstones are strong, brittle rocks that tend to fail by breaking 1 brittle deformation) when subjected to stress.
by contrast, day-rich or weakly cemented sedimentary rocks and foliated metamont wcks exhibit ductile deformation. This include halite, shale limestone & schist.
Crustal Deformation
4. Time
if stress is applied to a rock unit relatively quickly, the rock will deform elastically until its strength is exceeded and then it will fracture.
stress has to be applied slowly enough that the sluggish process of deformation can keep up.
in practice, rocks that are near Earth's surface tend to accommodate even gradual strain by fracturing; ductile behavior happens mainly at depth.
e.g. Taffy - if you hit a bar of taffy against the edge of a table, it will break. But if you put weight on it and leave i foremight, it will gradually spread & flatten.
Folds 
rock structures formed by ductile deformation
along convergent plate boundaries, rock strata are often bent into these series of wavelike undulations.
most folds result from compressional stresses that result in a lateral shortening and vertical thickening of the crust.
described by their axial plane: a surface that connects all the hinge lines of the folded strata.
in simple folds, the axial plane often leans to one side so that one limb is steeper than the other.
the axial plane is also vertical and divides the fold into 2 roughly symmetrical limbs.
Folds 
Anticlines & Synclines
most common types of Folds
Anticlines
usually form by the upfolding, or arching of sedimentary layers.
typically found in association w/ anticlines are down folds or troughs, called synclines.
the limb of an anticline is also a limb of an adjacent syncline.

Depending on their orientation, these basic folds are symmetrical when the limbs are mirror images of each other and asymmetrical when they are not.
Limbs of symmetrical fold
dip in opposite directions but at the same angle.
Limbs of asymmetrical fold
dip in opposite directions but at different angles
Folds 
Anticlines & Synclines
also said to be overturned if both limbs dip in same direction with one limb tilted beyond the vertical.
overturned fold can also lie on its side so plane is horizontal
Recumbent Folds
common in highly deformed mountain belts such as The Alps.
Folds can also be tilted by tectonic Forces so their hinge lines slope downward.
these are plunge folds because the hinge lines penetrate Earth's surface.
e.g. Sheep mountain, Wyoming
example of plunging anticline
tip of V points in the direction of plunge
Folds 
2. Domes & Basins
a circular or slightly elongated bulge that was produced by a broad upwarping of basement rock that deforms the overlying cover of sedimentary stratac is called dome.
e.g. Black Hills of Western South Dakota represent a large structural dome generated by upwarping. (erosion has stripped away the highest portions of the overlying sedimentary beds, exposing older igneous and metamorphic ncks in the center)
Folds? 
2. Domes & Basins
Structural domes can also be formed by the intrusion of magma
the upward migration of buried salt deposits can produce salt domes
Salt domes are economically important because when salt migrates upward, the surrounding oil-bearing sedimentary strata deform to form oil reservoirs.
Folds? 
2. Domes & Basins
Inverse of a dome is a down warped structure called basin.
Structural basins usually contain sedimentary beds sloping at low angles
With this, they are identified by the age of the rocks composing them.
The youngest rocks are found near the center and the oldest rocks are at the flank.
By contrast, in a dome oldest rocks form the core.
Folds
3. Monoclines
Folds and Fault often occur together as a result of the same tectonic stresses.
These can be found in broad, regional features called monoclines.
are large, steplike folds in otherwise horizontal sedimentary strata.
appear to have resulted from the reactivation of ancient, steep - dipping reverse faults located in basement rocks beneath the plateau.
as large blocks of basement rocks were displaced upward, the ductile sediment strata above responded by draping over the fault.
displacement along these reactivated Faults can exceed 1 km (0.4 miles)
Faults & Joints? 
both structures formed by brittle deformation and leads to fracturing of Earth's crust
Joint fracture
fault fracture along w/c motion has occured so that rocks on either side are offset from each other.
Fault zones
displacements of hundreds of kilometers and consist of many interconnecting Fault surfaces.
Faults & Joints 
Dip Slip Faults
faults in which movement is parallel to the slope of the fault surface
"dip" is the angle at which fault surface is inclined relative to the horizontal.
rock surface above the fault is hanging wall block.
below the fault is footwall block
Fault scarps
vertical displacements along dip-slip faults tend to produce these long, low cliffs.
2 Kinds: normal and reverse faults.
Faults & Joints
Dip Slip Faults
Normal Faults
hanging wall moves down relative to the footwall.
associated w/ tensional stresses that pull rock units apart, thereby lengthening the crust laterally and thinning it vertically.
this pulling apart can be accomplished either by uplift that causes the surface to stretch and break or by horizontal forces that have opposing directions.
these faults are called normal faults because it is "normal" for gravity to pull a block of rock down an inclined plane.
Faults & Joints 
Dip Slip Faults
Normal Faults
occur in a variety of sizes:
small, with displacements of only a meter or so.
large, extends for tens of kilometers but tend to flatten out with depth
Faults & Joints 
Dip Slip Faults
large normal faults are associated w/ fault-block mountains.
crust has been elongated and broken to create more than 200 relatively small mountain ranges.
eg. Basin and Range province
topography of Basin and Range province evolved in association w/ a system of normal faults trending roughly N-S.
Movements along these faults produced altemating uplifted fault blocks called horsts (hill) and down-dropped blocks called grabens (graben = ditch)
Faults & Joints 
Dip Slip Faults
Horst form the ranges and are the source of sediments that have accumulated In the topographic basins created by grabens.
Structures called half-grabens which are tilted fault blocks also contributed to the alternating topographic highs and lows in Basin and Range Province
The slopes of many of the large, normal faults in the Basin and Range province decrease w/ depth and forms a low angle, nearly horizontal fault called detachment Fault.
Faults & Joints
Dip Slip Faults
these faults represent a major boundary between the rocks below which exhibit ductile deformation and the rocks above which exhibit brittle deformation.
Faults & Joints
Normal Faults
Reverse & Thrust Faults
hanging wall block moves up relative to the footwall block are reverse fault.
a thrust fault is a type of reverse fault in which the dip is less than 45°.
both result from compressional stresses that produce honzontal shortening of the crust.
most high angle reverse faults are small and accommodate local displacements in regions dominated by other types of faulting.
Faults & Joints
Normal Faults
Reverse & Thrust Faults
Thrust faults, on the other hand, exist at all scales with some large thrusts faults having displacements ranging from tens to hundreds of kilometers.
Thrust faulting is most common along convergent plate boundaries.
compressional forces associated with colliding plates generally create folds as well as thrust faults that thicken and shorten the crust to produce mountainous topography.
eg. Alps, Northern Rockies, Himalayas and Appalachians
Faults & Joints
2. Strike-slip Faults
fault in we the dominant displacement is horizontal and parallel to the trend (direction) of the fault surface.
e.g. San Francw Earthquake, 1904
2 types night lateral and left lateral
Transform Faults
unique class of strike-slip faults that slice through Earth's lithosphere and accommodate motion between 2 tectonic plates.
Faults & Joints
2. Strike-slip Faults
Transform faults
most cut the oceanic lithosphere and Iink spreading oceanic ridges while others accommodate displacement between continental blocks that slip horizontally past each other.
most continental transform faults consist of a zone of roughly parallel fractures.
this zone may be up to several kilometers wide but the most recent movement is often along a strand only a few meters wide, which may offset features such as stream channels.
Faults & Joints
3. Joints
fractures which differ from faults because no appreciable displacement has occurred on the fracture.
some joints have a random orientation, most occur in parallel groups.
2 types:
columnar joints - form when igneous rocks cool and develop shrinkage fractures that produce elongated, pillar-like columns.
sheeting - produced a pattern of gently curved joints that develop more/less parallel to the surface of large exposed bodies such as batholiths.
Faults & Joints
3. Joints
most joints are produced when rocks in the outermost crust are deformed, causing the rock to fall by brittle fracture.
develop in response to regional upwarping and downwarping of the crust.
many rocks are broken by 2 or even 3 sets of intersecting joints that slice the rock into numerous regularly shaped blocks.
these joints sets often exert a strong influence on other geological processes.
eg. chemical weathering tends to be concentrated along joints and joint patterns influence how ground water moves thru the crust.
Faults & Joints
3. Joints
Joint rocks can be beneficial to humans.
Hydrothermal solutions (mineralized fluids) can migrate into fractured rocks and precipitate economically significant amounts of Cu, Au, pb and Zn.
Mountain Building 
Orogenesis
processes that collectively produce a mountain belt
Orogeny
episode of mountain building
Collisional mountains
result from the collision of one or more small crustal fragments to a continental margin or from the closure of an ocean basin that results in the collision of 2 major landmasses.
contain large quantities of preexisting rediments and sedimentary rocks that at one time lay along the margin of a continent and were faulted and contorted into a series of folds.
Mountain Building 
Young mountain belts - American cordillera
runs along the wester margin of the Americas from Southem most S. America to Alaska and includes both the Rockies and the Andes; the Alpine Himalaya chain, which extends along the margin of the Mediterranean thru Iran to northem India and into Indochina and the mountainous terrain of western pacific which include volcanic island arcs such as Japan, Philippines, and much of Indonesia.