Structure An Introduction to Deformation

Standards Describe the composition and structure of Earth’s materials

A Brief Intro to Mapping Geologic maps – represent the rock formations exposed at Earth’s surface. Maps have special symbols to indicate strike and dip of rock formations, and lines to mark faults

Geologic Map of New Mexico

Strike and Dip Strike – the compass direction of a rock layer as it intersects with a horizontal surface Dip – is measured at right angles to strike and is the amount of tilting of the formation (angle at which the bed is inclined from the horizontal)

Geologic Cross Sections Diagrams that show the features that would be visible if vertical slices were made through part of the crust Shows the rocks and structures under the surface

Plate Tectonics Steady motion between plates causes deformation at plate boundaries. Rocks are deformed by faulting and folding. Layers of rock can be pushed into folds. Or, rock formations can break and slip on both sides of a fracture, forming a fault.

Review Types of Plate Boundaries Convergent Plates push together Divergent Plates pull apart Transform Plates slide horizontally past each other

Types of Plate Tectonic Force Tensional forces – stretch and pull formations apart. Dominate at divergent boundaries. Compressive forces – squeeze and shorten rock formations. Dominate at convergent boundaries. Shearing forces – push two sides of a formation in opposite directions. Dominate at transform boundaries.

Ductile vs. Brittle Brittle material – a material that undergoes little deformation under increasing force until it suddenly breaks Ductile material – a material that undergoes smooth and continuous plastic deformation under increasing force and does not spring back to its original shape when the force is removed.

Brittle & Ductile Behavior in the Crust Some rocks are brittle and others ductile. The same rock can be brittle at shallow depths and ductile deep in the crust A rock formation that would flow as a ductile material if deformed slowly may break as a brittle material if deformed more rapidly. Exs: silly putty and gak (in-class & video) Rocks break more easily under tension than compression

Basic Deformation Structures Faults Folds Circular Structures Joints

Fault Surface across which rock formations have been displaced Offset can be centimeters to hundreds of kilometers Faults are classified by their slip direction. 2 Main types: Dip-slip fault Strike-slip fault

Dip-Slip Fault There has been relative movement of the blocks (rock to either side of the fault) up or down the dip of the fault plane. Dip = tilt of the fault Caused by compressive or tensional forces.

Dip-Slip Fault 3 types: 1.Normal fault 2.Reverse fault 3.Thrust fault

Fault Parts Fault – plane along which rocks have been displaced Hanging wall – rocks above fault plane Foot wall – rocks below fault plane Draw & label this diagram in your notes Hanging Wall Foot Wall Cross-Section View Fault Plane

Normal Fault Dip-slip fault formed when rocks above the fault plane (on the top side of the dip) move down relative to rocks below the fault plane. This extends the structure horizontally. Formed by tensional forces

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Reverse Fault Rocks above fault plane move upward in relation to the rocks below. Causes shortening of the structure. Formed by compressive forces

Photo: IH html

Thrust Fault Low-angle reverse fault – the angle of the fault plane is less than 45  This causes the overlying block to move horizontally

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Strike Slip Fault Movement is horizontal (just like movement at transform boundaries) Caused by shearing forces Two types: 1.Right-lateral fault 2.Left-lateral fault

Right-Lateral Fault An observer on one side of the fault sees the block on the opposite side move to the right.

Photo: Fence along San Andreas Fault

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Left-Lateral Fault The block on the opposite side of the fault moves to the left.

Recognizing Faults in the Field Faults may form a small cliff called a scarp

Recognizing Faults in the Field Formations that have large offsets (like on the San Andreas) differ in age and rock type Small amounts of offset can be observed and measured

Folds A bend in rocks Are often observed in layered rocks Have a range of sizes Two types: 1.Anticlines 2.Synclines Photo: Images/Many%20chev%20folds%20crop.jpg

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Anticline Layered rocks that fold upward into arches Oldest rocks are in center of fold

Photo: J. T. Daniels. foldImages/index.html

Syncline Layered rocks that fold downward into troughs. Youngest rocks are in center of fold Synclines look like Smiles

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Photo: cf/images/syncline440.gif

Parts of Folds Anticlines and synclines have limbs and an axial plane: Limbs – the two sides of the fold Axial plane – an imaginary surface that divides a fold as symmetrically as possible, with one limb on either side of the plane

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Overturned Fold Forms when deformation is intense and one limb has been tilted beyond the vertical. Both limbs dip in the same direction, but the order of layers in the bottom limb is the reverse of their original sequence – that is, older rocks are on top of younger rocks

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Circular Structures In many cases, these form from upward force of rising material or downward force of sinking material Two types: 1.Dome 2.Basin

Domes Broad, circular or oval upward bulge of rock layers Type of anticline

Basins Bowl-shaped depression of rock layers Type of syncline Photo: public/images/maliau/satellite_image2.jpg

Joints A type of fracture, or crack, along which there has been no movement. Found in almost every outcrop Formed by: Tectonic forces Expansion & contraction of rocks

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Deformation textures As rocks along a fault plane move past each other, they grind and fragment Brittle deformation will form fault breccias Ductile deformation deeper in the crust will form mylonites

Fault Breccias Rocks with a broken appearance Photo:

Mylonites Minerals recrystallize and string out in bands or streaks. Photo: