Tectonics

mountain building and crustal deformation

What is a mountain?
• Geologically speaking = where the crust is thicker….

• 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