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11-Deformation criteria in earthquake study

11-Deformation criteria in earthquake study

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Crustal Deformation & Mountain Building Crustal Deformation & Mountain Building • The process of forming a The process of forming a mountain not only uplifts the mountain not only uplifts the surface of the Earth, it causes surface of the Earth, it causes rocks to undergo rocks to undergo Deformation. Deformation. • Deformation: Deformation: The process by The process by which rocks are deformed which rocks are deformed (squashed, stretched, sheared, (squashed, stretched, sheared, etc…) in response to squeezing, etc…) in response to squeezing, stretching, shearing etc (i.e. stretching, shearing etc (i.e. differential stress). differential stress). • Deformation produces a variety Deformation produces a variety of geologic structures including of geologic structures including – Joints Joints – Faults Faults – Folds Folds – Foliation Foliation Mt. Cook, New Zealand Mt. Cook, New Zealand
Mountain Belts and Orogens Mountain Belts and Orogens • Except for volcanoes, mountains do not occur in isolation; they occur in linear Except for volcanoes, mountains do not occur in isolation; they occur in linear ranges called ranges called Mountain Belts Mountain Belts, or , or Orogenic Belts Orogenic Belts • Orogeny: Orogeny: A mountain building event; tends to last ~10 million years (varies a lot) A mountain building event; tends to last ~10 million years (varies a lot) • Erosion counteracts Erosion counteracts Orogens Orogens so most mountains that we see today are young (most so most mountains that we see today are young (most are < 100 Ma) compared to the Earth. are < 100 Ma) compared to the Earth.
Deformed vs. Undeformed Deformed vs. Undeformed • In an undeformed sequence, strata occurs in horizontal layers, just like it was deposited. • No metamorphic rocks, no foliation, no large faults, maybe some joints • Grains are round, just like when they were deposited, clay minerals are horizontally-aligned from compaction • In a deformed sequence (e.g. mountain belt) rocks are folded, and possibly metamorphosed • Faults with large offsets may be present, juxtaposing different rocks side by side • Rocks may be highly folded and squashed grains may create strong foliations. Road Cut in Indiana Cliff Exposure in The Swiss Alps
Deformation Deformation • In general you can say that a rock has been deformed if it has: – translated (moved) from its original position – changed in orientation (folding, rotation and/or tilting) – changed in shape (distortion)
Types of Strain Types of Strain • Strain: A change in size and/or shape due to the application of stress – Descriptive terms: shortening/contraction, stretching/extension, shear – Strain = change in length / original length • Stress: A force exerted over some area that causes rocks to undergo strain – Descriptive terms: compression, tension, shear – Stress = force / area
Strain Ellipse Strain Ellipse • Geologists can quantify strain by looking at changes in angles or areas of certain shapes called strain ellipses • For example if you draw a circle on a deck of cards and then shear it, the circle undergoes strain to becomes an ellipse. • Remember that shear is a term that applies to both stress and strain. Shear stress causes shear strain.
Brittle and Ductile Deformation Brittle and Ductile Deformation • Brittle deformation (lithosphere) occurs when temp and pressure are low (shallow depth) and strain rates are high – Forms faults, joints • Ductile deformation (asthenosphere) occurs when temps and pressures are high (deep depth) and strain rates are low – Forms folds and foliations • Rocks can undergo temporary deformation such as elastic strain in between earthquakes • But eventually permanent strain may occur where deformation is not recoverable (all the features listed up top are types of permanent deformation) • The line between brittle and ductile deformation also depends on composition • In some cases brittle and ductile features can form in the same rock
Stress Stress ≠ Force ≠ Force • Newton gave us: force = mass * acceleration • Geologists think in terms of: stress = force / area • Forces in the Earth are distributed over entire plate boundaries, so the area over which the force is applied is important. • So it is stress, not force that is important for determining if deformation will occur.
Types of Stress Types of Stress • Pressure: When stress is the same in all directions. Causes volumetric change, not shape change. E.g. water or air pressure. • Compression: A stress that causes contraction.
Types of Stress Types of Stress • Tension: A stress that causes extension. • Shear: A stress that causes shearing
Measuring Orientation: Strike and Dip Measuring Orientation: Strike and Dip • In order to characterize geologic structures, geologists must be able to quantify the orientation of structures. For Planar features we use: • Strike: The orientation of the intersection line between a horizontal surface and the feature of interest. Measured with a compass. – E.g. north, N45W, 285, etc… • Dip: The acute angle between the feature of interest and a horizontal plane. – E.g. 0° = horizontal 90° = vertical For linear features we use: • Trend: the trend of the line if you were looking down on the feature from above – E.g. north, NW, 320, 090, etc… • Plunge: Acute angle between the line and a horizontal – E.g. 46°, 75°, etc…
Joints Joints • Joints Joints are fractures in rock that have are fractures in rock that have not accommodated sliding. The two not accommodated sliding. The two walls simply spread apart (open). walls simply spread apart (open). • They commonly occur in sedimentary They commonly occur in sedimentary rocks as vertical cracks rocks as vertical cracks • May occur in regional sets May occur in regional sets • Form due to cooling, or stresses from Form due to cooling, or stresses from overlying rocks overlying rocks • If the joints are filled with If the joints are filled with minerals, then we call them minerals, then we call them veins. veins. • Veins are commonly non- Veins are commonly non- planar while joints are usually planar while joints are usually very planar. very planar. • Joints are very important for Joints are very important for the flow of fluids through the the flow of fluids through the ground. E.g. oil, water, etc… ground. E.g. oil, water, etc… • Joints are also important to Joints are also important to consider when building things consider when building things such as roads. such as roads.
Faults Faults • Fault – a fracture on which sliding has occurred. • 3 Main Types of Faults – Normal Fault – Reverse Fault – Strike-Slip Fault • Many faults are mixed in slip type, called oblique-slip faults.
Types of Faults Types of Faults • In general, faults come in three different types: Normal, Reverse, and Strike-Slip • Shallow angle (< 30° ) reverse faults are called thrust faults • Faults that have a mix of slip styles are called oblique slip faults Fault animations
• Normal Faults: from stretching of or extending rock; points on Normal Faults: from stretching of or extending rock; points on opposite sides of a fault are father apart after an earthquake opposite sides of a fault are father apart after an earthquake • Reverse Faults: from contraction or squishing rock; points on Reverse Faults: from contraction or squishing rock; points on opposite sides of the fault are closer together after an earthquake opposite sides of the fault are closer together after an earthquake • Strike-Slip: can form in either areas of stretching or squishing, Strike-Slip: can form in either areas of stretching or squishing, material slides laterally past each side of the fault. material slides laterally past each side of the fault. – Described by sense of motion: Described by sense of motion: • Right-lateral (Dextral) Right-lateral (Dextral) • Left Lateral (Sinistral) Left Lateral (Sinistral) Why are there different types of faults? Why are there different types of faults?
Measuring Motion Across a Fault Measuring Motion Across a Fault M7.8 1906 Great San Francisco Earthquake
Motion Across a Fault Motion Across a Fault • The amount of The amount of motion along any motion along any fault is called the fault is called the slip slip, , offset offset, or , or displacement displacement • Fault trace (line): Fault trace (line): where the fault plane where the fault plane intersects the surface intersects the surface of the Earth of the Earth • Active/Inactive Active/Inactive Faults Faults: not all faults : not all faults are likely to produce are likely to produce an earthquake an earthquake • Fault scarp: Fault scarp: vertical vertical motion on a fault motion on a fault produces a small produces a small escarpment escarpment • What was the sense of What was the sense of slip here? slip here? • Offset stream animation Offset stream animation Fault Trace Fault Trace Offset road from the Mw7.1 1999 Hector Mine earthquake Offset road from the Mw7.1 1999 Hector Mine earthquake
Thrust Faults – Crustal Shortening Thrust Faults – Crustal Shortening • Thrust fault – A shallow-dipping reverse fault (<30°) • Capable of moving rocks 100’s of km horizontally • Builds large collisional mountain belts (Appalachians, Himalaya) • If the hanging wall block gets eroded away and a piece remains, it is called a klippe. • Since thrusts can move rocks 100’s of km, we call rocks that have been moved great distances, allochthonous. • Rocks that are where they formed are called autochthonous.
Chief Mountain, MT Chief Mountain, MT • Chief Mt. is an Chief Mt. is an allochthonous klippe allochthonous klippe GoogleEarth view…
Normal Faults – Crustal Extension Normal Faults – Crustal Extension • Recently active normal Recently active normal faults leave behind fault faults leave behind fault scarps – cliffs or scarps – cliffs or escarpments that are due to escarpments that are due to motion along the fault. motion along the fault. • Reverse faults usually Reverse faults usually don’t make such nice don’t make such nice scarps because they tend to scarps because they tend to act like a bulldozer and act like a bulldozer and leave a pile of rubble leave a pile of rubble instead of a nice scarp. instead of a nice scarp. • Normal faults typically dip Normal faults typically dip about 60 about 60° °, so they, alone, , so they, alone, are not able to move rocks are not able to move rocks large horizontal distances. large horizontal distances. A Normal Fault Scarp after an Earthquake in Nevada A Normal Fault Scarp after an Earthquake in Nevada
Normal Fault Structure Normal Fault Structure • Normal faults commonly come in antithetic (dip opposite directions from each other) pairs forming horsts and grabens. • Horst – The part that went up (footwall blocks) • Graben – The part that went down (hanging wall blocks). • Half grabens - occur when only one normal fault is present
Detachment Faults / Décollements Detachment Faults / Décollements • Normal faults, alone, can’t move rocks very far horizontally, yet we find allochthonous terrain in some extensional environments. • In these environments we find normal faults and tilted blocks of rock • Normal or Reverse faults may connect into nearly horizontal faults called detachment faults (if extensional) or décollements (if contractional) • Detachments / Décollements may have a very low friction due to the presence of fluids
Recognizing Faults Recognizing Faults • The easiest way to recognize a fault would be to look for offset layers or different layers juxtaposed side by side that do not belong together.
Moab Fault, UT Moab Fault, UT Different Different color color Different Different color color Fault Fault Fault Fault Youngest Youngest Rocks Rocks Young Young Rocks Rocks Oldest Oldest Rocks Rocks Faults can sometimes be recognized by the presence of a zone of discolored, broken rocks. Faults can sometimes be recognized by the presence of a zone of discolored, broken rocks.
Deep Faults – Shear Zones Deep Faults – Shear Zones
Fault Surface Fabric - Slickensides Fault Surface Fabric - Slickensides • Rocks on both sides of a fault grind past each other and may scratch the surface of the fault. These striations are called slickensides or slickenlines or simply “slicks” • They show the direction of movement (finger is pointing in along the slip vector. • Faults may also develop corrugations or mullions, which are basically large-scale slickensides.
Fault Breccia & Cataclasis Fault Breccia & Cataclasis • During an earthquake the ground may violently shake, so it stands to reason that rocks near large faults are going to have been subjected to lots of shaking • Coseismic (during an earthquake) shaking may cause rocks near the fault to shatter or break into pieces that may rotate in place. This is called cataclasis • cataclasis may form a fault breccia. • Alternatively, rocks along the fault surface may get ground down to a fine powder from slip, this is called fault gouge. A fault breccia formed from cataclasis along a fault
Ductile Deformation - Folds Ductile Deformation - Folds • Anticline: An arch-shaped fold • Hinge Line: An imaginary line that shows the location of maximum curvature on a fold • Axial Plane: The plane through the hinge lines on all of the layers • Monocline: A fold shaped like a carpet draped over a stair step • Syncline: A trough-shaped fold • Limb: The sides of a fold that show less curvature.
Descriptive Fold Terms Descriptive Fold Terms In order for geologists to describe folds we need to have terms to describe them • Open Fold: A fold that is broad, i.e. the angle between the limbs is large • Tight Fold: A fold that is narrow, i.e. the angle between limbs is small • Plunging Fold: The hinge line is non-horizontal • Non-plunging Fold: The hinge line is horizontal Doubly Plunging Folds form domes and basins • Dome: An upside-down bowl-shaped fold • Basin: A right-side-up bowl-shaped fold
• An Anticline in a road cut An Anticline in a road cut
• A Syncline in a road cut A Syncline in a road cut – Note that the current-day topography does not Note that the current-day topography does not necessarily follow the fold pattern necessarily follow the fold pattern
Fold Trains Fold Trains • Anticlines and synclines are Anticlines and synclines are commonly found together in trains commonly found together in trains of folds. of folds. • Note that in these particular folds, Note that in these particular folds, the axial planes are not vertical. the axial planes are not vertical. • Folds exposed along a cliff Folds exposed along a cliff in eastern Ireland in eastern Ireland
Fold Formation Fold Formation • Folds are formed in two general ways: 1- When the rocks are brittle and layered, slip between the layers allows folds to form. This is called flexural slip folding. Since there is sliding on the bedding planes, they are act as faults. Sometimes they are called flexural slip faults or bedding plane slip. 2- When rocks are ductile and can flow, flow folds form, which form because different parts of the rock flow at different rates. Flexural slip folding Flow folding
Flow Folding Flow Folding • This rock unit was ductile This rock unit was ductile during deformation and during deformation and was able to flow was able to flow • Note the thinning of layers Note the thinning of layers along the limbs of folds along the limbs of folds and thickening along and thickening along hinges. hinges. • This rock was deformed in This rock was deformed in the asthenosphere. the asthenosphere.
Flexural Slip Flexural Slip Folding Folding • Layers are folded with Layers are folded with significant slip on the significant slip on the bedding planes. bedding planes. • Layer thickness stays Layer thickness stays constant throughout. constant throughout. • May see slickensides on May see slickensides on bedding planes bedding planes
What Causes Folding? What Causes Folding? • Folds can form due to tectonic compression or shear. • Folds can also form near buried or curved faults.
Tectonic Foliation Tectonic Foliation • When a differential stress is applied to a rock, e.g. during an orogeny, grains in the rock may change shape or develop a permanent strain. • Foliation forms as a result of strain, so when you see a foliated rock, you know that it has developed some amount of permanent strain.
An Orogeny in Cross Section An Orogeny in Cross Section
Uplift and Crustal Roots of Mountain Ranges Uplift and Crustal Roots of Mountain Ranges • Leonardo da Vinci noted in his journals that marine fossils exposed in rocks high in Leonardo da Vinci noted in his journals that marine fossils exposed in rocks high in the mountains suggested that there had been significant uplift. the mountains suggested that there had been significant uplift. • Mt. Everest is 8.85 km above sea level, but this is very small compared to the size Mt. Everest is 8.85 km above sea level, but this is very small compared to the size of the Earth. of the Earth. • If the Earth was the size of a billiard ball, even with its large mountains, it would be If the Earth was the size of a billiard ball, even with its large mountains, it would be smoother than the average billiard ball. smoother than the average billiard ball. • In the mid 1800’s Sir George In the mid 1800’s Sir George Everest surveyed India and Everest surveyed India and discovered that the immense discovered that the immense mass of the Himalayas was mass of the Himalayas was enough to deflect his plumb enough to deflect his plumb bob (lead weight at the end of bob (lead weight at the end of a string) from horizontal. a string) from horizontal. • But when people did the But when people did the calculation of the mass, the calculation of the mass, the deflection of the bob was deflection of the bob was smaller than they expected, smaller than they expected, suggesting that the mountains suggesting that the mountains had a low density root. had a low density root.
Roots of Collisional Mountain Belts Roots of Collisional Mountain Belts • Typical continental crust = 35-40 km thick • Continental crust beneath mountain belts = 50-70 km thick • When the crust contracts it tends to vertically thicken (think of squishing silly putty) • Since the lithosphere effectively floats on the more ductile material below (asthenosphere), the crust must be thicker below mountains to compensate for their extra weight. • Isostacy / Isostatic Equilibrium: When the gravitational force pulling rock downwards equals the buoyancy force pushing the lithosphere upward • So when fault motion pushes a piece of the Earth upwards, isostatic compensation will pull the area back down to isostatic equilibrium. • Also applies to glaciers
How Tall Can Mountains Be? How Tall Can Mountains Be? • Mountains that are much higher than Mt. Everest cannot exist • The height of mountains is limited by two main factors 1- Erosion - removes topography 2- Rocks have finite strength. -As mountains rise, they push down on the rocks below. Eventually, the root of the mountain will become hot and will flow outwards. This is called orogenic collapse. For a mountain range to exist, it must be uplifted faster than erosion removes it. As mountains are eroded, for every km that is eroded, they isostatically rebound by ~1/3 km.
Accretionary Orogens Accretionary Orogens • Subduction of island arcs can cause so-called exotic terranes to be accreted onto continents. Once the exotic terrane is attached, it is called an accreted terrane. • Fold and thrust belts form on continental crust where the land is undergoing regional contraction.
Accreted Terranes in the Western U.S. Accreted Terranes in the Western U.S. • Much of the current-day western U.S. did not exist as part of the continent at the end of the Precambrian • These accreted terranes are now part of the North American Continent and the current-day subduction zone is now offshore Washington/Oregon/N. California • How do geologists recognize accreted terranes? – Oceanic rocks within the continent – Locating and dating of faults
Other Ways to Form Other Ways to Form Mountain Belts Mountain Belts • Mountain belts can also form due to continent- continent collisions • Continental rifts can also form fault block mountain belts
Measuring Measuring Modern Modern Orogens Orogens • Today, geologists and geophysicists can measure motions in active orogens, such as the Andes Mountains in South America • Using satellites-based techniques (GPS, InSAR, LIDAR) we can watch these modern mountain belts move horizontally and vertically by millimeters each year.

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11-Deformation criteria in earthquake study

  • 1.
    Crustal Deformation &Mountain Building Crustal Deformation & Mountain Building • The process of forming a The process of forming a mountain not only uplifts the mountain not only uplifts the surface of the Earth, it causes surface of the Earth, it causes rocks to undergo rocks to undergo Deformation. Deformation. • Deformation: Deformation: The process by The process by which rocks are deformed which rocks are deformed (squashed, stretched, sheared, (squashed, stretched, sheared, etc…) in response to squeezing, etc…) in response to squeezing, stretching, shearing etc (i.e. stretching, shearing etc (i.e. differential stress). differential stress). • Deformation produces a variety Deformation produces a variety of geologic structures including of geologic structures including – Joints Joints – Faults Faults – Folds Folds – Foliation Foliation Mt. Cook, New Zealand Mt. Cook, New Zealand
  • 2.
    Mountain Belts andOrogens Mountain Belts and Orogens • Except for volcanoes, mountains do not occur in isolation; they occur in linear Except for volcanoes, mountains do not occur in isolation; they occur in linear ranges called ranges called Mountain Belts Mountain Belts, or , or Orogenic Belts Orogenic Belts • Orogeny: Orogeny: A mountain building event; tends to last ~10 million years (varies a lot) A mountain building event; tends to last ~10 million years (varies a lot) • Erosion counteracts Erosion counteracts Orogens Orogens so most mountains that we see today are young (most so most mountains that we see today are young (most are < 100 Ma) compared to the Earth. are < 100 Ma) compared to the Earth.
  • 3.
    Deformed vs. Undeformed Deformedvs. Undeformed • In an undeformed sequence, strata occurs in horizontal layers, just like it was deposited. • No metamorphic rocks, no foliation, no large faults, maybe some joints • Grains are round, just like when they were deposited, clay minerals are horizontally-aligned from compaction • In a deformed sequence (e.g. mountain belt) rocks are folded, and possibly metamorphosed • Faults with large offsets may be present, juxtaposing different rocks side by side • Rocks may be highly folded and squashed grains may create strong foliations. Road Cut in Indiana Cliff Exposure in The Swiss Alps
  • 4.
    Deformation Deformation • In generalyou can say that a rock has been deformed if it has: – translated (moved) from its original position – changed in orientation (folding, rotation and/or tilting) – changed in shape (distortion)
  • 5.
    Types of Strain Typesof Strain • Strain: A change in size and/or shape due to the application of stress – Descriptive terms: shortening/contraction, stretching/extension, shear – Strain = change in length / original length • Stress: A force exerted over some area that causes rocks to undergo strain – Descriptive terms: compression, tension, shear – Stress = force / area
  • 6.
    Strain Ellipse Strain Ellipse •Geologists can quantify strain by looking at changes in angles or areas of certain shapes called strain ellipses • For example if you draw a circle on a deck of cards and then shear it, the circle undergoes strain to becomes an ellipse. • Remember that shear is a term that applies to both stress and strain. Shear stress causes shear strain.
  • 7.
    Brittle and DuctileDeformation Brittle and Ductile Deformation • Brittle deformation (lithosphere) occurs when temp and pressure are low (shallow depth) and strain rates are high – Forms faults, joints • Ductile deformation (asthenosphere) occurs when temps and pressures are high (deep depth) and strain rates are low – Forms folds and foliations • Rocks can undergo temporary deformation such as elastic strain in between earthquakes • But eventually permanent strain may occur where deformation is not recoverable (all the features listed up top are types of permanent deformation) • The line between brittle and ductile deformation also depends on composition • In some cases brittle and ductile features can form in the same rock
  • 8.
    Stress Stress ≠ Force ≠Force • Newton gave us: force = mass * acceleration • Geologists think in terms of: stress = force / area • Forces in the Earth are distributed over entire plate boundaries, so the area over which the force is applied is important. • So it is stress, not force that is important for determining if deformation will occur.
  • 9.
    Types of Stress Typesof Stress • Pressure: When stress is the same in all directions. Causes volumetric change, not shape change. E.g. water or air pressure. • Compression: A stress that causes contraction.
  • 10.
    Types of Stress Typesof Stress • Tension: A stress that causes extension. • Shear: A stress that causes shearing
  • 11.
    Measuring Orientation: Strikeand Dip Measuring Orientation: Strike and Dip • In order to characterize geologic structures, geologists must be able to quantify the orientation of structures. For Planar features we use: • Strike: The orientation of the intersection line between a horizontal surface and the feature of interest. Measured with a compass. – E.g. north, N45W, 285, etc… • Dip: The acute angle between the feature of interest and a horizontal plane. – E.g. 0° = horizontal 90° = vertical For linear features we use: • Trend: the trend of the line if you were looking down on the feature from above – E.g. north, NW, 320, 090, etc… • Plunge: Acute angle between the line and a horizontal – E.g. 46°, 75°, etc…
  • 12.
    Joints Joints • Joints Joints arefractures in rock that have are fractures in rock that have not accommodated sliding. The two not accommodated sliding. The two walls simply spread apart (open). walls simply spread apart (open). • They commonly occur in sedimentary They commonly occur in sedimentary rocks as vertical cracks rocks as vertical cracks • May occur in regional sets May occur in regional sets • Form due to cooling, or stresses from Form due to cooling, or stresses from overlying rocks overlying rocks • If the joints are filled with If the joints are filled with minerals, then we call them minerals, then we call them veins. veins. • Veins are commonly non- Veins are commonly non- planar while joints are usually planar while joints are usually very planar. very planar. • Joints are very important for Joints are very important for the flow of fluids through the the flow of fluids through the ground. E.g. oil, water, etc… ground. E.g. oil, water, etc… • Joints are also important to Joints are also important to consider when building things consider when building things such as roads. such as roads.
  • 13.
    Faults Faults • Fault –a fracture on which sliding has occurred. • 3 Main Types of Faults – Normal Fault – Reverse Fault – Strike-Slip Fault • Many faults are mixed in slip type, called oblique-slip faults.
  • 14.
    Types of Faults Typesof Faults • In general, faults come in three different types: Normal, Reverse, and Strike-Slip • Shallow angle (< 30° ) reverse faults are called thrust faults • Faults that have a mix of slip styles are called oblique slip faults Fault animations
  • 15.
    • Normal Faults:from stretching of or extending rock; points on Normal Faults: from stretching of or extending rock; points on opposite sides of a fault are father apart after an earthquake opposite sides of a fault are father apart after an earthquake • Reverse Faults: from contraction or squishing rock; points on Reverse Faults: from contraction or squishing rock; points on opposite sides of the fault are closer together after an earthquake opposite sides of the fault are closer together after an earthquake • Strike-Slip: can form in either areas of stretching or squishing, Strike-Slip: can form in either areas of stretching or squishing, material slides laterally past each side of the fault. material slides laterally past each side of the fault. – Described by sense of motion: Described by sense of motion: • Right-lateral (Dextral) Right-lateral (Dextral) • Left Lateral (Sinistral) Left Lateral (Sinistral) Why are there different types of faults? Why are there different types of faults?
  • 16.
    Measuring Motion Acrossa Fault Measuring Motion Across a Fault M7.8 1906 Great San Francisco Earthquake
  • 17.
    Motion Across aFault Motion Across a Fault • The amount of The amount of motion along any motion along any fault is called the fault is called the slip slip, , offset offset, or , or displacement displacement • Fault trace (line): Fault trace (line): where the fault plane where the fault plane intersects the surface intersects the surface of the Earth of the Earth • Active/Inactive Active/Inactive Faults Faults: not all faults : not all faults are likely to produce are likely to produce an earthquake an earthquake • Fault scarp: Fault scarp: vertical vertical motion on a fault motion on a fault produces a small produces a small escarpment escarpment • What was the sense of What was the sense of slip here? slip here? • Offset stream animation Offset stream animation Fault Trace Fault Trace Offset road from the Mw7.1 1999 Hector Mine earthquake Offset road from the Mw7.1 1999 Hector Mine earthquake
  • 18.
    Thrust Faults –Crustal Shortening Thrust Faults – Crustal Shortening • Thrust fault – A shallow-dipping reverse fault (<30°) • Capable of moving rocks 100’s of km horizontally • Builds large collisional mountain belts (Appalachians, Himalaya) • If the hanging wall block gets eroded away and a piece remains, it is called a klippe. • Since thrusts can move rocks 100’s of km, we call rocks that have been moved great distances, allochthonous. • Rocks that are where they formed are called autochthonous.
  • 19.
    Chief Mountain, MT ChiefMountain, MT • Chief Mt. is an Chief Mt. is an allochthonous klippe allochthonous klippe GoogleEarth view…
  • 20.
    Normal Faults –Crustal Extension Normal Faults – Crustal Extension • Recently active normal Recently active normal faults leave behind fault faults leave behind fault scarps – cliffs or scarps – cliffs or escarpments that are due to escarpments that are due to motion along the fault. motion along the fault. • Reverse faults usually Reverse faults usually don’t make such nice don’t make such nice scarps because they tend to scarps because they tend to act like a bulldozer and act like a bulldozer and leave a pile of rubble leave a pile of rubble instead of a nice scarp. instead of a nice scarp. • Normal faults typically dip Normal faults typically dip about 60 about 60° °, so they, alone, , so they, alone, are not able to move rocks are not able to move rocks large horizontal distances. large horizontal distances. A Normal Fault Scarp after an Earthquake in Nevada A Normal Fault Scarp after an Earthquake in Nevada
  • 21.
    Normal Fault Structure NormalFault Structure • Normal faults commonly come in antithetic (dip opposite directions from each other) pairs forming horsts and grabens. • Horst – The part that went up (footwall blocks) • Graben – The part that went down (hanging wall blocks). • Half grabens - occur when only one normal fault is present
  • 22.
    Detachment Faults /Décollements Detachment Faults / Décollements • Normal faults, alone, can’t move rocks very far horizontally, yet we find allochthonous terrain in some extensional environments. • In these environments we find normal faults and tilted blocks of rock • Normal or Reverse faults may connect into nearly horizontal faults called detachment faults (if extensional) or décollements (if contractional) • Detachments / Décollements may have a very low friction due to the presence of fluids
  • 23.
    Recognizing Faults Recognizing Faults •The easiest way to recognize a fault would be to look for offset layers or different layers juxtaposed side by side that do not belong together.
  • 24.
    Moab Fault, UT MoabFault, UT Different Different color color Different Different color color Fault Fault Fault Fault Youngest Youngest Rocks Rocks Young Young Rocks Rocks Oldest Oldest Rocks Rocks Faults can sometimes be recognized by the presence of a zone of discolored, broken rocks. Faults can sometimes be recognized by the presence of a zone of discolored, broken rocks.
  • 25.
    Deep Faults –Shear Zones Deep Faults – Shear Zones
  • 26.
    Fault Surface Fabric- Slickensides Fault Surface Fabric - Slickensides • Rocks on both sides of a fault grind past each other and may scratch the surface of the fault. These striations are called slickensides or slickenlines or simply “slicks” • They show the direction of movement (finger is pointing in along the slip vector. • Faults may also develop corrugations or mullions, which are basically large-scale slickensides.
  • 27.
    Fault Breccia &Cataclasis Fault Breccia & Cataclasis • During an earthquake the ground may violently shake, so it stands to reason that rocks near large faults are going to have been subjected to lots of shaking • Coseismic (during an earthquake) shaking may cause rocks near the fault to shatter or break into pieces that may rotate in place. This is called cataclasis • cataclasis may form a fault breccia. • Alternatively, rocks along the fault surface may get ground down to a fine powder from slip, this is called fault gouge. A fault breccia formed from cataclasis along a fault
  • 28.
    Ductile Deformation -Folds Ductile Deformation - Folds • Anticline: An arch-shaped fold • Hinge Line: An imaginary line that shows the location of maximum curvature on a fold • Axial Plane: The plane through the hinge lines on all of the layers • Monocline: A fold shaped like a carpet draped over a stair step • Syncline: A trough-shaped fold • Limb: The sides of a fold that show less curvature.
  • 29.
    Descriptive Fold Terms DescriptiveFold Terms In order for geologists to describe folds we need to have terms to describe them • Open Fold: A fold that is broad, i.e. the angle between the limbs is large • Tight Fold: A fold that is narrow, i.e. the angle between limbs is small • Plunging Fold: The hinge line is non-horizontal • Non-plunging Fold: The hinge line is horizontal Doubly Plunging Folds form domes and basins • Dome: An upside-down bowl-shaped fold • Basin: A right-side-up bowl-shaped fold
  • 30.
    • An Anticlinein a road cut An Anticline in a road cut
  • 31.
    • A Synclinein a road cut A Syncline in a road cut – Note that the current-day topography does not Note that the current-day topography does not necessarily follow the fold pattern necessarily follow the fold pattern
  • 32.
    Fold Trains Fold Trains •Anticlines and synclines are Anticlines and synclines are commonly found together in trains commonly found together in trains of folds. of folds. • Note that in these particular folds, Note that in these particular folds, the axial planes are not vertical. the axial planes are not vertical. • Folds exposed along a cliff Folds exposed along a cliff in eastern Ireland in eastern Ireland
  • 33.
    Fold Formation Fold Formation •Folds are formed in two general ways: 1- When the rocks are brittle and layered, slip between the layers allows folds to form. This is called flexural slip folding. Since there is sliding on the bedding planes, they are act as faults. Sometimes they are called flexural slip faults or bedding plane slip. 2- When rocks are ductile and can flow, flow folds form, which form because different parts of the rock flow at different rates. Flexural slip folding Flow folding
  • 34.
    Flow Folding Flow Folding •This rock unit was ductile This rock unit was ductile during deformation and during deformation and was able to flow was able to flow • Note the thinning of layers Note the thinning of layers along the limbs of folds along the limbs of folds and thickening along and thickening along hinges. hinges. • This rock was deformed in This rock was deformed in the asthenosphere. the asthenosphere.
  • 35.
    Flexural Slip Flexural Slip Folding Folding •Layers are folded with Layers are folded with significant slip on the significant slip on the bedding planes. bedding planes. • Layer thickness stays Layer thickness stays constant throughout. constant throughout. • May see slickensides on May see slickensides on bedding planes bedding planes
  • 36.
    What Causes Folding? WhatCauses Folding? • Folds can form due to tectonic compression or shear. • Folds can also form near buried or curved faults.
  • 37.
    Tectonic Foliation Tectonic Foliation •When a differential stress is applied to a rock, e.g. during an orogeny, grains in the rock may change shape or develop a permanent strain. • Foliation forms as a result of strain, so when you see a foliated rock, you know that it has developed some amount of permanent strain.
  • 38.
    An Orogeny inCross Section An Orogeny in Cross Section
  • 39.
    Uplift and CrustalRoots of Mountain Ranges Uplift and Crustal Roots of Mountain Ranges • Leonardo da Vinci noted in his journals that marine fossils exposed in rocks high in Leonardo da Vinci noted in his journals that marine fossils exposed in rocks high in the mountains suggested that there had been significant uplift. the mountains suggested that there had been significant uplift. • Mt. Everest is 8.85 km above sea level, but this is very small compared to the size Mt. Everest is 8.85 km above sea level, but this is very small compared to the size of the Earth. of the Earth. • If the Earth was the size of a billiard ball, even with its large mountains, it would be If the Earth was the size of a billiard ball, even with its large mountains, it would be smoother than the average billiard ball. smoother than the average billiard ball. • In the mid 1800’s Sir George In the mid 1800’s Sir George Everest surveyed India and Everest surveyed India and discovered that the immense discovered that the immense mass of the Himalayas was mass of the Himalayas was enough to deflect his plumb enough to deflect his plumb bob (lead weight at the end of bob (lead weight at the end of a string) from horizontal. a string) from horizontal. • But when people did the But when people did the calculation of the mass, the calculation of the mass, the deflection of the bob was deflection of the bob was smaller than they expected, smaller than they expected, suggesting that the mountains suggesting that the mountains had a low density root. had a low density root.
  • 40.
    Roots of CollisionalMountain Belts Roots of Collisional Mountain Belts • Typical continental crust = 35-40 km thick • Continental crust beneath mountain belts = 50-70 km thick • When the crust contracts it tends to vertically thicken (think of squishing silly putty) • Since the lithosphere effectively floats on the more ductile material below (asthenosphere), the crust must be thicker below mountains to compensate for their extra weight. • Isostacy / Isostatic Equilibrium: When the gravitational force pulling rock downwards equals the buoyancy force pushing the lithosphere upward • So when fault motion pushes a piece of the Earth upwards, isostatic compensation will pull the area back down to isostatic equilibrium. • Also applies to glaciers
  • 41.
    How Tall CanMountains Be? How Tall Can Mountains Be? • Mountains that are much higher than Mt. Everest cannot exist • The height of mountains is limited by two main factors 1- Erosion - removes topography 2- Rocks have finite strength. -As mountains rise, they push down on the rocks below. Eventually, the root of the mountain will become hot and will flow outwards. This is called orogenic collapse. For a mountain range to exist, it must be uplifted faster than erosion removes it. As mountains are eroded, for every km that is eroded, they isostatically rebound by ~1/3 km.
  • 42.
    Accretionary Orogens Accretionary Orogens •Subduction of island arcs can cause so-called exotic terranes to be accreted onto continents. Once the exotic terrane is attached, it is called an accreted terrane. • Fold and thrust belts form on continental crust where the land is undergoing regional contraction.
  • 43.
    Accreted Terranes inthe Western U.S. Accreted Terranes in the Western U.S. • Much of the current-day western U.S. did not exist as part of the continent at the end of the Precambrian • These accreted terranes are now part of the North American Continent and the current-day subduction zone is now offshore Washington/Oregon/N. California • How do geologists recognize accreted terranes? – Oceanic rocks within the continent – Locating and dating of faults
  • 44.
    Other Ways toForm Other Ways to Form Mountain Belts Mountain Belts • Mountain belts can also form due to continent- continent collisions • Continental rifts can also form fault block mountain belts
  • 45.
    Measuring Measuring Modern Modern Orogens Orogens • Today, geologistsand geophysicists can measure motions in active orogens, such as the Andes Mountains in South America • Using satellites-based techniques (GPS, InSAR, LIDAR) we can watch these modern mountain belts move horizontally and vertically by millimeters each year.