Ch. 14 - Mountain Building

Class: GEOL-101

Notes:

Fact:
According to GPS data, the Sierra Nevada Mountains grew in response to loss of water during extreme drought in California.

Between 2012 and 2017, California experienced its driest period since officials began recording rainfall levels back in 1840. A recent NASA study found that during this same period, the Sierra Nevada range rose nearly an inch in height, mostly due to the drought.

14.1 Mountain Building

Name and locate Earth’s major mountain belts on a world map.

How do we categorize mountains

Presentation content:

• Mountain building has occurred during the recent geologic past
  • American Cordillera
  • The Alpine–Himalaya chain
  • The mountainous terrains of the western Pacific
• Several other chains are Paleozoic in age

Notes from the lecture:

• Young mountain belts
• Old mountain belts
• Shields
• Stable platforms

Orogenesis

Presentation content:

• The process that produces a mountain belt
• Mountains that display faulted and folded rocks are compressional mountains
  • Display visual evidence of compressional forces
  • Including metamorphism and some igneous activity
• Plate tectonics provides a model for orogenesis
  • Earth's major mountains have formed along convergent plate boundaries

14.2 Subduction Zones

List and describe the four major features associated with subduction zones.

Major Features of Subduction Zones

Presentation content:

• Volcanic arc

• Deep-ocean trench

• Forearc region

• Back-arc region

• Volcanic Arcs

  • The subducting slab partially melts the overlying mantle wedge
  • Melt migrates upward through the overlying oceanic lithosphere and forms a growth called a volcanic island arc or island arc
  • When the melt migrates through continental lithosphere, a continental volcanic arc is created

• Deep Ocean Trenches

  • Created when oceanic lithosphere bends as it descends into the mantle
  • Trench depth is related to the age of the subducting lithosphere
    • Old lithosphere is cold and dense
      • Plates subduct at a steep angle, producing a deep trench
    • Young lithosphere is warm and buoyant
      • Plates subduct at a shallower angle and produce shallower trenches (if at all)

• Forearc and Back-Arc Regions

  • The forearc region is the area between the trench and the volcanic arc
    • Example: Great Valley, CA
  • The back-arc region is located on the side of the volcanic arc opposite the trench
    • Both regions consist of accumulated pyroclastic material and eroded sediments
    • Tensional forces prevalent in these regions, causing stretching

Notes from the lecture:

• Most of our mountain belts are at convergent plate boundaries
• Angle impacts the amount of partial melting -> difference in surface area being exposed

Extension and Back-Arc Spreading

Presentation content:

• Two plates converging, but not necessarily dominated by compressional forces
• When the subducting plate is cold, the plate sinks vertically as it descends along an angled path
  • This causes the trench to “roll back” away from the overlying plate
    • Consequently, the overlying plate is stretched
• Tension and thinning may initiate seafloor spreading, enlarging the back-arc basin

Notes from the lecture:

• Stretch enough to create a divergent plate boundary
• Over time because this is less dense basaltic material it will start to sink and accumulate as sediment

14.3 Subduction and Mountain Building

List and describe the four major features associated with subduction zones.

Island Arc-Type Mountain Building

Presentation content:

• Results from the steady subduction of oceanic lithosphere
• Continued growth can result in topography consisting of parallel belts of igneous and metamorphic rocks
• Just one phase in the development of mountain belts

Notes from the lecture:

• More sediment -> the greater the amount of drag

Andean-Type Mountain Building

Presentation content:

• Subduction beneath a continent rather than oceanic lithosphere
  • Associated with long-lasting magmatic activity and crustal thickening
• Exemplified by the Andes Mountains
  • Starts with a passive continental margin
    • Thick platform of shallow-water sedimentary rocks
  • Eventually, the forces that drive plate tectonics change direction and a subduction zone forms
    • Oceanic lithosphere must be dense enough to sink
• Building volcanic arcs
  • As crustal rocks descend, water and volatiles are driven from the crustal rocks into the overlying mantle wedge
  • These volatiles trigger the partial melting of ultramafic peridotite
  • Generates mafic primary magmas which rise through the mantle wedge
  • Magmas pool at the base on the continental crust
  • Magmatic differentiation creates less dense magmas that rise through the crust
• Emplacement of batholiths
  • Thick continental crust impedes the ascent of magma
  • Most magma crystallizes underground as massive plutons called batholiths
  • Eventually, uplift and erosion expose the batholiths
    • Example: The Sierra Nevada in California
  • Batholiths typically range from diorites to granites
• Development of an accretionary wedge
  • An accretionary wedge is the accumulated sediments and scraped upper crust of the subducting plate plastered against the edge of the overriding plates
• Forearc basin
  • The region of relatively undeformed layers of sediment and sedimentary rock

Notes from the lecture:

• As it continues to subduct away it forms a forearc basin -> some of the most agriculturally rich regions of the world.
• Western portion of North America where we see a lot of our batholists and chemical rich minerals
• Subduction of the Farallon plate is an example

The Sierra Nevada, Coast Ranges, and Great Valley

Presentation content:

• One of the best examples of features associated with an Andean-type subduction zone
  • Features produced by the subduction of the Farallon Plate (part of the Pacific basin) under the western margin of California

14.4 Collisional Mountain Belts

Compare and contrast the formation of an Alpine-type mountain belt with that of a Cordilleran-type mountain belt.

Cordilleran-Type Mountain Building

Presentation content:

• Associated with the Pacific Ocean

  • Highly likely that subduction zones will form island arcs which will eventually collide with a continental crust

• The collision and accretion of small slivers of continental crust form the mountainous regions that rim the Pacific

• Terranes (crustal fragments of exotic material) make up much of the western United States

  • Prior to accretion onto the continent, some terranes were microcontinents (similar to Madagascar)
  • Crustal fragments that have been accreted, and have a geologic history distinct from those of the adjoining fragments
  • Other terranes were island arcs (similar to Japan)

• Accretion and orogenesis

  • Large, buoyant features do not subduct
  • These features are peeled off the subducting plate and accreted onto the continental crust

• The North American Cordillera

  • Many terranes that make up the North American Cordillera were scattered through the eastern Pacific
  • During the breakup of Pangaea, the Farallon plate began to subduct under North America
    • Resulted in the piecemeal addition of crustal fragments to the western side of North America

Notes from the lecture:

• Eventually collide at a convergent plate boundary and as they reach the subduction they are more buoyant and less dense so they accretes to the plate

Alpine-Type Mountain Building: Continental Collisions

Presentation content:

• Named for the Alps—two continental masses collide
• The zone where two continents collide is called a suture
  • Typically contains slivers of oceanic lithosphere
  • May also include accreted terrane
• Most compressional mountains exhibit the deformation of a thick sequence of sedimentary rocks called a fold-and-thrust belt

Notes from the lecture:

• Colliding: fusing together
• Example: Convergence of the Indian plate and Asia
• The himalays -> you can find limestone on one of the highest places on earth.
• This pressure is referred to as "shortening"
• The appalachians are an example

The Himalayas

Presentation content:

• Collision began about 50 million years ago
• India collided with Asia following the subduction of oceanic lithosphere
• Precambrian rocks of India resisted deformation while the younger crustal fragments of southeast Asia were highly deformed
• Followed by a period of uplift that raised the Tibetan Plateau
• India is still moving northward
• Crust is shortening and thickening, accommodating some of this movement
• Much of the remaining penetration into Asia caused lateral displacement of large blocks of the Asian crust by continental escape
• The thick, cold slab of India has stayed essentially intact

Notes from the lecture:

• All oceanic crust has either subducted away or accreted to the plate
• Highly deformed
• Displacement is so significant that rocks cannot go any higher vertically so actually rocks start moving to another direction

The Appalachians

Presentation content:

• Of a similar origin to the mountains in the British Isles, Scandinavia, northwest Africa, and Greenland

• Formed from three main orogenic events that cumulated with the formation of Pangaea

• Taconic Orogeny

  • Volcanic arc located east of North America was thrust over the continental block 450 million years ago
  • The volcanic rocks and marine sedimentary rocks were metamorphosed and are exposed in New York

• Acadian Orogeny
  • Continued closing of the ocean basin resulted in a micro-continent colliding with North America 350 million years ago
  • Thrust faults, metamorphism, and granite intrusions are associated with this event
  • Substantially added to the width of North America

• Alleghanian Orogeny
  • Africa collided with North America 250–300 million years ago
  • Material was displaced 250 km inland on North America
  • Pangaea began rifting 180 million years ago
  • Rift was eastward of the suture, leaving a remnant of Africa welded to North America

Notes from the lecture:

• Very old mountain belts, consisted of multiple mountain building events
• When Pangea was starting to form.
• 450M after Avalonia is still moving
• This is an Alpine type orogeny
  • Very large mountain belt
  • They are old -> they have been weathered and eroted
  • At least 14000 ft in elevation
• Extends from Pensilvannia all the way back to Georgia

• Look at the different points of elevation where rocks have resisted erosion

14.5 Fault Block Mountains

Summarize the stages in the formation of a fault-block mountain range.

• Continental rifting can produce uplift and the formation of mountains known as fault-block mountains
  • Example: The Tetons of Wyoming
  • Mountains formed through crustal extension and normal faulting

The Basin and Range Province

Presentation content:

• One of the largest regions of fault-block mountains on Earth

  • Located between the Sierra Nevada and the Rocky Mountains
  • Extends N–S roughly 3000 km, encompasses all of Nevada, portions of surrounding states, and a large part of New Mexico

• Tilting of faulted structures, called half-grabens, has produced nearly parallel mountain ranges that average 80 km in length

• Extension beginning 20 million years ago has stretched the crust twice its original width

• Prevailing hypotheses of formation

  • Following the subduction of the Farallon plate, the northwest movement of the Pacific plate produced tensional forces that have stretched the region
  • 20 million years ago, the lower lithospheric mantle decoupled from the crust beneath the region
    • This delamination resulted in the upwelling and lateral spreading of hot mantle rocks, producing tensional forces in the crust

Notes from the lecture:

• Once pression is removed it goes back to its original shape, which created that large ragion of extension
• Other prevalent hypothesis: there is an upwelling of magma creating extension

14.6 Vertical Motions of the Crust

Explain the principle of isostasy and how it contributes to the elevated topography of mountain belts.

The Principle of Isostasy

Presentation content:

• Less dense crust floats on top of the denser rocks of the mantle

• Isostasy is the concept of floating crust in gravitational balance

• Envision a series of different-sized floating blocks on water

• How is isostasy related to changes in elevation?

  • If weight is added or removed from the crust, isostatic adjustment will take place as the crust subsides or rebounds
    • Crustal rebound is present in Canada's Hudson Bay region following the melting of ice sheets in that region

• How high is too high?

  • As mountains grow, gravity acts on the warm and weak rocks inside the mountains
    • Eventually, the gravitational forces are so large that these rocks will flow laterally
    • This ductile spreading and consequential subsidence is called gravitational collapse

Notes from the lecture:

• "If you uplift the mountain, gravitational forces will push it back down"

• Extending vertically but also into the mantle
• As you remove it, it is going back to the top of the surface

Mantle Convection: A Cause of Vertical Crustal Movement

Presentation content:

• Uplifting whole continents
  • Mantle plumes (superplumes) can elevate a region on continental crust
    • Southern Africa has large-scale vertical motion
    • Elevation is nearly 1500 m higher than would be expected for a stable craton
• Crustal subsidence
  • Extensive areas of downwarping
  • The slabs of oceanic lithosphere will detach from the trailing lithosphere
  • A downward flow is created as the detached slab continues to sink, pulling down the crust into a basin structure
    • Example: nearly circular basins in Michigan and Illinois

Notes from the lecture:

• Opposite direction: Crustal subsidence
  • As it cools, it starts to subduct, the same process happens to our continents
  • Over time they cool and this happens.
  • Example: Michigan Basin
    • Basalt cools and overtime it starts to sink and pulls down a region
    • This is why you have certain kind of rocks in the middle different to the borders of a region.

End of Chapter 14 - Concept Checks

14.1 Mountain Building

1. Define orogenesis.
2. Which type of plate boundary is most directly associated with Earth’s major mountain belts?

14.2 Subduction Zones

1. List the four major features of subduction zones.
2. Briefly describe how back-arc basins form.

14.3 Subduction and Mountain Building

1. In what ways are the Sierra Nevada and the Andes ranges similar?
2. What is an accretionary wedge? Briefly describe its formation.
3. What is a batholith? In what tectonic setting are batholiths generated?

14.4 Collisional Mountain Belts

1. Explain why the continental crust of Asia was deformed more than that of the Indian subcontinent during the formation of the Himalayas.
2. How does the plate tectonics theory help explain the existence of fossil marine life in rocks atop collisional mountains?
3. Differentiate between terrane and terrain.

14.5 Fault-Block Mountains

1. How does formation of fault-block mountains differ from the processes that generate most other major mountain belts?
2. Briefly describe the basic structure of the Basin and Range Province and identify its geographic extent.

14.6 Vertical Motions of the Crust

1. Define isostasy. What happens to a floating object when weight is added? Removed?
2. Give one example of evidence that supports the concept of crustal uplift.
3. Explain the process whereby mountainous regions experience gravitational collapse.