Crustal Deformation and Structural Geology Flashcards
(53 cards)
1
Q
Mountains
A
- Vivid evidence of tectonic activity
- Manifestations of geologic processes
○ Uplift
○ Deformation
○ Metamorphism - Frequently occur in elongate, linear belts
- Orogenesis: mountain building
2
Q
Orogenesis Involves:
A
○ Uplift ○ Deformation ○ Jointing ○ Faulting ○ Folding ○ Foliation ○ Metamorphism ○ Igneous activity ○ Erosion ○ Sedimentation - Constructive process build mountains up - Destructive processes tear them back down again
3
Q
Orogenic Belts
A
- Mountains have a finite life span
○ Young mountains are high, steep, and still growing
○ Middle-aged mountains are lowered by erosion
○ Old-age mountains are deeply eroded remnants
4
Q
Deformation
A
- Changes the character of rocks
○ Undeformed (unstrained): horizontal beds, spherical sand grains, no folds or faults
○ Deformed (strained): tilted beds, metamorphic alteration, folding and faulting - Results in one or all of the following:
○ Displacement - change in location
○ Rotation - change in orientation
○ Distortion - change in shape
5
Q
Strain
A
- Change in shape as a result of deformation
- Several types:
○ Stretching - pulling apart
○ Shortening - squeezing together
○ Shear - sliding past
6
Q
Brittle Vs. Ductile Deformation
A
- Brittle deformation: rocks break by fracturing, occurs in the shallow crust
- Ductile deformation: rocks deform by flowing and folding, occurs at higher T and P deeper in the crust
- Transition between the two types occurs ~10-15km depth
- Earthquakes do not appear in the deep crust b/c breakage does not occur in the deep crust
7
Q
Type of deformation depends on:
A
- Temperature
- Pressure
- Deformation rate
- Composition
8
Q
Causes of Deformation
A
- Strain is caused by force acting on rock (stress)
- Stress is applied across a unit area
○ Large force per area results in much deformation
○ Small force per area results in little deformation
9
Q
Stress
A
- Compression takes place when an object is squeezed
○ Shortens and thickens the material - Tensions occur when the ends of an object are pulled apart
○ Horizontal tension drives crustal rifting, stretches and thins the material - Shear develops when surfaces slide past one another
○ Shear stress neither thickens nor thins the crust
10
Q
Geologic Structures
A
- Geometric features are created during rock deformation
- The 3D orientation of a plane is described by strike and dip
○ Strike - horizontal intersection with a tilted surface
○ Dip - the angle of the surface down from the horizontal
11
Q
Measuring Structures
A
- Dip is perpendicular to strike and measured downward
- Linear structures can be similarly measured
○ Bearing - compass direction
○ Plunge - angle from the horizontal
12
Q
Joints and Veins
A
- Joints: planar rock fractures without any offset, develop from tensile stress in brittle rock (systematic joints occur in parallel sets), often control weathering of rock
○ Groundwater often flows through joints - Dissolved minerals precipitate = veins
- Chemical weathering and weathering from streams in joints
13
Q
Faults
A
- Planar fractures showing displacement
○ Abundant in the crust and occur at all scales
○ Sudden movements along faults cause earthquakes
○ Can be active or inactive
14
Q
Fault Orientation
A
- On a dipping fault, the blocks are classified as the:
○ Hanging-wall block (above the fault) = picture walking on it, you can’t = hanging
○ Footwall block (below the fault) = picture walking on it successfully = footwall
15
Q
Dip slip
A
blocks move parallel to the dip of the fault (vertical movement = normal faults or reverse faults)
16
Q
Strike slip
A
blocks move parallel to fault plane strike (lateral movement)
17
Q
Oblique slip
A
components of both dip slip and strike slip (both lateral and vertical, almost all faults are oblique, but identify by major vectors)
18
Q
Dip-Slip Faults
A
- Sliding is parallel to the dip of the fault
- Blocks move up or down the slope of the fault
19
Q
Reverse fault
A
hanging wall moves up fault slope, accommodate crustal shortening = compression
(COMPRESSIVE STRESS, shortening system, only for reverse/thrust)
20
Q
Thrust fault
A
special type of reverse fault, lower angle <35º than reverse = gentle dip, often result of continental collisions
21
Q
Normal fault
A
hanging wall moves down fault slope, accommodate crustal extension = pulling apart (TENSILE STRESS, lengthening the system)
22
Q
Strike-Slip Faults
A
- Motion is parallel to strike of fault
- Usually vertical, no hanging wall or footwall
- Classified by relative sense of motion
○ Right lateral - opposite block moves to observer’s right
○Left lateral - opposite block move to observer’s left
23
Q
Amount of offset =
A
displacement
24
Q
Fault Recognition
A
- Every new fault must be individually assessed
- Most obvious indicator of faulting is displacement
○ Interrupts and offsets layers in the rock - Brittle faulting results in shattered and crushed rock
- Scarps are visible when faults intersect the surface
- Fault zones with breccia and gouge preferentially erode
- Fault zones may be mineralized by fluid flow
25
Fault Breccia
Consists of rock fragments along a fault
26
Fault gorge
Made of pulverized, powdered rock
27
Slickensides and linear grooves
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slip lineations
(smoothly polished surface caused by frictional movement between rocks along the two sides of a fault)
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28
Ductile Deformation
- Layered rock may be deformed into complex folds
| - Orogenic settings produce large volumes of folded rock
29
Fold Geometry imagine:
LAYERS OF SHEET CAKE OR BOOK PAGES CURVED OVER
30
Hinge
line along which curvature is greatest (like spine of the bend of a book)
31
Limbs
less curved "sides" of fold (on sides of axial plane that like go down)
32
Axial plane
connects hinges of successive layers = cuts plane in half!
33
Anticline
fold that looks like an arch, limbs dip out and away from hinge, older rocks on inside of A (shaped like rounded A for anticline)
34
Syncline
fold that opens upward like a trough, limbs dip inward and toward the hinge, younger rocks on inside of y (shape like y for syn)
35
Monocline
fold-like carpet draped over a stair step, faults do not cut through to the surface, displacement folds the overlying sedimentary cover
36
Folds are described by geometry of the hinge
- Plunging fold has a hinge that is tilted
| - Non-plunging fold has a horizontal hinge
37
Large plunging folds create prominent landforms
Resistant sandstones form high; eroded shales are low
38
Dome
fold that looks like overturned bowl
39
Basin
fold shaped like an upright bowl
40
Domes expose:
older rocks in the centre
41
Basins expose:
younger rocks in the centre
42
Folds develop in two ways:
- Flexural slip: layers slide past one another
○ Like the movement when a deck of cards is bent
- Passive flow: form in hot, soft, ductile rock at high T
43
Forming Folds
- Horizontal compression causes rock to buckle
- Shear causes rocks to fold over on themselves
- When layers move over step-shaped faults, they fold
- Deep faulting may create a monocline in overlying beds
44
Mountain Building
- Mountain uplift is driven by plate tectonics
○ Convergent plate boundaries
○ Continental collisions
○ Rifting
- Linear plate boundaries make linear mountain belts
45
Causes of Mountain Building
- Subduction
- Exotic terranes may be added to subduction margins
- Continental collision follows ocean basin closure
- Buoyant continental crust shuts down subduction
- Crustal thickening results from continental collisions
- Continental rifting creates mountains
46
Subduction (convergent) boundaries create mountains
- Compression shortens and uplifts overriding plate
| - A fold-thrust belt develops landward of the orogen
47
Exotic terranes may be added to subduction margins
- Consist of island fragments of continental crust
- Too buoyant to subduct; sutured onto the upper plate
- Terrane geology is very different from that of the surroundings
- Western North America has numerous exotic terranes
48
Continental collision follows ocean basin closure
Buoyant continental crust shuts down subduction
49
Crustal thickening results from continental collisions
- Crust in collision zone may be twice its normal thickness
| - Thrusting brings metamorphic rocks up to shallow depths
50
Continental rifting creates mountains
- Normal faulting creates fault-block mountains and basins
| - Decompression melting adds volcanic mountains
51
Forming Rocks In and Near Mountains
- Orogenies lead to the formation of all three rock types
○ Igneous activity beneath collisions and rift zones
○ Erosion of uplifted rocks and sedimentation in basins
○ Metamorphism associated with continental collisions
52
What Goes Up…
- Mountains are steep and jagged due to erosion
- Rock characteristics control erosion
○ Resistant layers form cliffs
○ Easily eroded rocks form slopes
53
Cratons
crust that hasn’t been deformed in 1 Ga (1bil)
- Low-geothermal gradient; cool, strong, and stable crust
- Two cratonic provinces
○ Shields - Precambrian and metamorphic and igneous rocks
○ Platforms - shields covered by layers of Phanerozoic strata
