Instructor’s Notes: Chapter 20 Mountains and Mountain Building
Major Mountain Ranges:
North American Cordillera
Appalachians
Andes
Caledonian Belt
Alps
Urals
Himalayans
New Zealand Alps (Tasman Belt)
Earth’s major mountain belts
Terms:
Mountain Ranges- group of closely spaced mountains or parallel ridges
Mountain Belts – Cordillera- mountain chains thousands of km long containing numerous
mountain ranges
Craton- region of continent that has been structurally stable for a long period of time; in US
the interior craton is covered in a blanket of sediment 1000-2000 meters thick.
Precambrian Shield- eroded remnant of Precambrian mountain range.
Characteristic Features of Mountain Belts
Longer than wider
Higher mountains tend to be geologically younger
Mountains are associated with convergent plate boundaries
Oceanic-oceanic
Oceanic- continental
Continental- continental
Folding Faulting in Mountain Ranges:
Tighter folds indicate greater stress, recumbent of overturned folds= more intense
deformation reverse faults are also common
Fold and fault belts including Trust faults – Detachment faults
Fold and Trust Belts = crustal shortening and crustal thickening
Example: The Alps are made of rock units that once covered an ocean floor of 500 km
compressed into current width of 200 km.
Sedimentary Rocks:
Sedimentary rocks on the craton are 1000-2000 meters thick little deformation may show
gentle warping into basin and domes, and tend to be continental in origin.
Example: Colorado Plateau, Central United States and Canada.
In contrast, sedimentary rocks in mountain belts are deformed and thick and most likely of
marine origin mixed with volcanic rocks.
Why?
Crustal Thickness:
Continental crust is thicker and less dense than oceanic crust.
Crust is thicker beneath mountain belts
(an exception is the Basin and Range Providence).
Crust is thicker under younger mountain belt than older ones.
Remember : Seismic velocities indicate an average continental crustal density equivalent
to granite where the average oceanic crust density is equivalent to basalt.
Thematic cross-section across the United States- note the crust is thicker beneath the younger (Cenozoic) North
American Cordillera and thinner under the older (Paleozoic Mesozoic) Appalachian Mountains. The Appalachians
were once as tall as the North American Cordillera but have been eroding for a much longer period of time ( over
200 my).
Mountain Building:
Young Mountain Ranges (past 100 million yrs)
American Cordillera,
Alpine –Himalayan (45 mya)
Japan, Philippines, Sumatra- island arcs
Older Mountain Ranges
Appalachians
Urals
Caledonians
Three Stages of Mountain Building:
1) Accumulation Stage
2) Orogenic Stage
3) Uplift and Block-Faulting Stage
Stage 1: Accumulation Stage:
Deposition of sedimentary and volcanic rocks – sedimentary rock generally in a marine
environment
Passive Continental Margin – modern examples: North American Atlantic Coastmostly sedimentary and metamorphic rock - little or no volcanic activity
Active Continental Margin- modern examples: North America Pacific Coast-andesitic
volcanoes, sedimentary rocks= greywacke, deposited on the seaward side of the
convergent plates
Stage 2: Orogenic Stage
Orogeny- a period of intense deformation of rocks in a region usually associated with
metamorphism and igneous activities.
Associated with Convergent Plate Boundaries
Oceanic –Continent Convergent Plate Boundaries
Oceanic –Oceanic Convergent Plate Boundaries
Continental-Continental Convergent Plate Boundaries
Oceanic Continental Convergent Lithospheric Plate Boundary:
Also called Andean -Type Mountain Building
Formation of accretion wedges- marine sediment that is folded and faulted as they are
pushed off the subduction oceanic plate
Sedimentary rock caught and pushed down in the subduction zone will be subjected to
intense shear stress and metamorphosed
Folding and trust belt on the back arc of the side of the mountain belt- trusting away from the
magmatic arc towards the craton.
Mountain building is the result of:
Crustal shortening
Gravity collapse and spreading
Expanding magmatic arc
Gravitational Collapse and Spreading
Gravity Collapse and Spreading
The rate of uplift of rock is so great during orogenies (3 km above sea level) that the
underlying rock cannot support the overburden and gravitational collapse occurs, resulting
in increased pressure for metamorphism. Deep rocks behave plastically and flow, shallower
rocks fracture- rock is pushed outward and help to form fold and thrust belts.
Oceanic-Continental Convergence - Accretion of new continent when an island arc collides
with a continent and a new subduction zone is created on the seaward side of the island arc.
Accretion of accretionary wedge material
Examples: Mesozoic – Sierra Nevada Complex, California. – Western North America
Accreted Terrane
Oceanic-Continental Convergence (figure 19.32)
Oceanic-Oceanic Lithospheric Plate Convergence
Also called Aleutian-Type Mountain Building
shrinking ocean basins (Pacific)
partial melting of mantle- volcanic emplacement
sediment deposited on the shore faces and in trench
Formation of a volcanic island arc
Oceanic-Oceanic Convergence (figure 19.28)
Continental-Continental Convergence
Also called Himalayan-Type Mountain Building
Ural Mountains -Collision of Asia and Africa-Paleozoic
Alps – African and European Plate -Triassic
Himalayas – Miocene - India with Asia plates
Appalachian Mountains – more complex
Continental-Continental Convergence – the formation of the Himalayan Mountains
(from Lutgens and Tarbuck, 2006)
Continental-Continental Convergence (figure 19.33)
Stage 3: Uplift and Block Faulting:
Late stage uplift after convergence stops and compressional forces relax. The mountain
range moves into a period of uplift and erosion.
After orogeny stage structural adjustments are considered isosatic adjustments of thickened
crust
The present Rocky Mountains position is the result of Cenozoic period up-warping after
earlier period of folding and intense erosion. The region has risen 5000-7000 feet in the past
15-20 million years.
Normal Faulting –is characteristic of this stage-
Fault Block Mountains- product of extensional forces
Examples: Sierra Nevada and the Tetons are tilted fault Block Mountains
Isostasy (figure 20.14)
Less dense continental crust floats higher on the mantle than denser oceanic crust. Craton is
at equilibrium and mountains are not. Mountains float higher because they are thicker.
Erosion removes material from the mountains and the mountains adjust to reach equilibrium.
The principle of isostasy- “the thicker the crust the higher the mountains”
Appalachians- eroding for 250 million years eastern coastal plain has been a lowland for 70
million years.
Crustal thickness varies:
Tibet Plateau 75 km
Kansan 44 km
Denver 50 km
Some adjustment is due to sub-regional variations in the mantle temperature (less dense).
Lithospheric Detachment
Basin and Range providence due to block faulting an extension feature. This is thought to
be caused by a hot relatively shallow mantle causing extension and block faulting.
Characteristic of mountain ranges with valleys full of erosion debris
Lithospheric delamination- detachment of the mantle portion of the lithosphere beneath a
mountain belt
Location Basin and Range Providence ( from Lutgens and Tarbuck, 2006) It extends into west Texas.
Lithospheric Delamination in the Basin and Range Providence: lithosphere is cooler and denser than
Asthenosphere breaks off and sinkshot Asthenosphere heats crust to meltingcrust is heated and
thinned Extension tectonics and tensional stress production normal- block faulting
Delamination theory explains the volcanic activities (rhyolitic to basaltic volcanic eruptions
with no apparent pattern) after the orogeny stage - Fossil evidence indicates the area was
once 3 km higher than present
Accreted terrane:
Technostratigraphic terranes or exotic terranes – geologic terrane that is found in a
location that is different from where it was formed.
Accreted terrane regions with geologic continuity are named after major geographic features.
Example: Wrangellia Terrane- Wrangell Mountains, West Coast of North America
Accreted materials could be: island arc, submarine deposits, ancient ocean floor, displaced
continental fragments
Accreted Terrane on the western boundary of North America
Modern submerged fragments of continental crust (from Lutgens and Tarbuck, 2006)
Geologists study fossil assemblages and paleo-magnetic data to determine the “travel
history” and reconstruct the Plate movement over time.
Reconstructions of the Geologic History of the material that makes up the current Alaskan Peninsula
Complex History for the Formations of the Appalachian Mountains
West Coast of the United States - Crustal Deformation:
San Andres System
Everything from the Sierra Nevada to the modern coastline has been accreted at a convergent plate
boundary- the transform plate boundary was see today has only been around for the last 15-20 million
years.
C’
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Geologic Map and Cross Section Las Trampas Ridge, California Coastal Range- these rocks are highly
folded and cut by high angle reverse faults- typical of mountain building at a convergent plate boundary.
Mt Diablo Ron Crane and Craig Lyon
Geologic Map and Cross-sections (Ron Crane)
A little bit of a review: The Formation of Oceanic and Continental Crust
In the asthenosphere ultramafic rocks are partially melted to form mafic magmas that solidify to form
oceanic crust (basaltic in compositions. At convergent plate boundaries oceanic crust basaltic in
composition is subducted, and at a depth of approximately 100 km is partially melted to produce a more
silica rich magma. The resulting magma, as it moves up through the continental crust, assimilates more
silica rich continental rocks and crystallizes to form intermediate and felsic continental rocks.
Major Precambrian Mountain Belts:
Shows the making of a continent- material is added at the edges of the continents by compressional
tectonics (crustal shortening) and mountain building (from Lutgens and Tarbucks, 2006)
Presentation Acknowledgements:
Stan Hatfield and Ken Pinzke
(Southwestern Illinois College)
Tarburk and Lutgens
USGS
Ron Crane and Craig Lyon
Deborah R. Harden
Animation and Figures: Plummer Mc Geary and Carson