Crust made thicker and shorter = deformation
Deformation = structures
Mountains
Mountain Building => orogeny or orogenesis
Isostacy - crust floating in gravitational balance
Orogeny = uplift
Crustal Loading => subsidence
Rock Deformation and Geologic Structures
Deformation=>rocks subjected to stresses that exceed their strength......
controlling factors ->1) Pressure and Temperature & 2) Time/Rate
Brittle versus Ductile Deformation
Geologic Structures
types of deformed rock
1) Faults, 2) Folds, and 3) Joints
Faults => fractures along which there has been movement.....
parts => fault plane, upthrown block, downthrown block, hanging wall and footwall
Faults
Dip-slip = vertical movement
a. Normal Fault => hanging wall moves down relative to the footwall
b. Reverse Fault =>hanging wall moves up relative to the footwall
low-angle reverse fault = thrust fault
Faults
Strike-slip = horizontal movement
Transform Fault => the result of shear forces
Folds
Folds => rocks bend due to compression
parts = axis and limbs
Anticline => convex upward fold
=> oldest beds towards the middle
Syncline => concave upward fold
=> youngest beds towards middle
Nonplunging Folds
Plunging folds.....
Basins and Domes
What are these folds?
Joints => fractures in the crust with no movement
Types of Mountains
1) Fault Block Mountains
2) Upwarped Mountains
Faulblock and Upwarped mountains
Mountains at Convergent Boundaries
Subduction=> magmatic/Andean Arc The Andes
1) 200 my ago-> South America and Africa part of the same plate: Western edge of South America = passive margin
2) 140 my ago-> South America and Africa begin to break apart
-> South America moves west....
The Andes
3) Oceanic crust fractures to accommodate movement => subduction.....
4) partial melting of subducted crust => Andean Arc
- forearc basin, - accretionary wedge, and - backarc basin
5) Subduction Angle Decreases
-> volcanic activity moves westward => Andean Arc
-> old volcanoes eroded
Continental Collision the Himalayas......
the Himalayas
1) 120 my ago -> plate with India breaks away from Antarctica
=> moves north at ~20 cm/yr
the Himalayas
2) Leading oceanic part of the plate subducted along southern Tibet
-> partial melting => Andean Arc
the Himalayas
3) 50 my ago -> oceanic crust entirely subducted => continental collision
-> igneous activity stops...
=> thrust faulting and folding => crustal shortening and thickening.
=> suture zone -> plates welded together
Thrust Faults
The Himalayas
* Results in mountain chain of highly deformed sedimentary rocks plus
associated igneous and metamorphic rocks...
=> compression still active = continued uplift
Appalachians
formed the same way as the Himalayas, but compression not active
for over 250 my = erosion........
Appalachians
Step 1 = subduction (Taconic Orogeny)
Appalachians
Step 2 = continental collision (Acadian and Alleganian Orogenies)
=> erosion begins to outpace uplift ........ Especially when uplift stops
Influence of Orogeny on the interior of North America
Compression - subtle features
Basins & Domes