Crustal Deformation and Structural Geology Flashcards
Mountains
- Vivid evidence of tectonic activity
- Manifestations of geologic processes
○ Uplift
○ Deformation
○ Metamorphism - Frequently occur in elongate, linear belts
- Orogenesis: mountain building
Orogenesis Involves:
○ Uplift ○ Deformation ○ Jointing ○ Faulting ○ Folding ○ Foliation ○ Metamorphism ○ Igneous activity ○ Erosion ○ Sedimentation - Constructive process build mountains up - Destructive processes tear them back down again
Orogenic Belts
- 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
Deformation
- 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
Strain
- Change in shape as a result of deformation
- Several types:
○ Stretching - pulling apart
○ Shortening - squeezing together
○ Shear - sliding past
Brittle Vs. Ductile Deformation
- 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
Type of deformation depends on:
- Temperature
- Pressure
- Deformation rate
- Composition
Causes of Deformation
- 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
Stress
- 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
Geologic Structures
- 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
Measuring Structures
- Dip is perpendicular to strike and measured downward
- Linear structures can be similarly measured
○ Bearing - compass direction
○ Plunge - angle from the horizontal
Joints and Veins
- 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
Faults
- Planar fractures showing displacement
○ Abundant in the crust and occur at all scales
○ Sudden movements along faults cause earthquakes
○ Can be active or inactive
Fault Orientation
- 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
Dip slip
blocks move parallel to the dip of the fault (vertical movement = normal faults or reverse faults)
Strike slip
blocks move parallel to fault plane strike (lateral movement)
Oblique slip
components of both dip slip and strike slip (both lateral and vertical, almost all faults are oblique, but identify by major vectors)
Dip-Slip Faults
- Sliding is parallel to the dip of the fault
- Blocks move up or down the slope of the fault
Reverse fault
hanging wall moves up fault slope, accommodate crustal shortening = compression
(COMPRESSIVE STRESS, shortening system, only for reverse/thrust)
Thrust fault
special type of reverse fault, lower angle <35º than reverse = gentle dip, often result of continental collisions
Normal fault
hanging wall moves down fault slope, accommodate crustal extension = pulling apart (TENSILE STRESS, lengthening the system)
Strike-Slip Faults
- 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
Amount of offset =
displacement
Fault Recognition
- 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
Fault Breccia
Consists of rock fragments along a fault
Fault gorge
Made of pulverized, powdered rock
Slickensides and linear grooves
slip lineations (smoothly polished surface caused by frictional movement between rocks along the two sides of a fault)
Ductile Deformation
- Layered rock may be deformed into complex folds
- Orogenic settings produce large volumes of folded rock
Fold Geometry imagine:
LAYERS OF SHEET CAKE OR BOOK PAGES CURVED OVER
Hinge
line along which curvature is greatest (like spine of the bend of a book)
Limbs
less curved “sides” of fold (on sides of axial plane that like go down)
Axial plane
connects hinges of successive layers = cuts plane in half!
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)
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)
Monocline
fold-like carpet draped over a stair step, faults do not cut through to the surface, displacement folds the overlying sedimentary cover
Folds are described by geometry of the hinge
- Plunging fold has a hinge that is tilted
- Non-plunging fold has a horizontal hinge
Large plunging folds create prominent landforms
Resistant sandstones form high; eroded shales are low
Dome
fold that looks like overturned bowl
Basin
fold shaped like an upright bowl
Domes expose:
older rocks in the centre
Basins expose:
younger rocks in the centre
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
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
Mountain Building
- Mountain uplift is driven by plate tectonics
○ Convergent plate boundaries
○ Continental collisions
○ Rifting - Linear plate boundaries make linear mountain belts
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
Subduction (convergent) boundaries create mountains
- Compression shortens and uplifts overriding plate
- A fold-thrust belt develops landward of the orogen
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
Continental collision follows ocean basin closure
Buoyant continental crust shuts down subduction
Crustal thickening results from continental collisions
- Crust in collision zone may be twice its normal thickness
- Thrusting brings metamorphic rocks up to shallow depths
Continental rifting creates mountains
- Normal faulting creates fault-block mountains and basins
- Decompression melting adds volcanic mountains
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
What Goes Up…
- Mountains are steep and jagged due to erosion
- Rock characteristics control erosion
○ Resistant layers form cliffs
○ Easily eroded rocks form slopes
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
