Download Lecture 23: Orogenesis: Mountain Belts and the Continental Crust

mountains, mountain building, & growth of continents
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mountains and mountain building
mountain belts
• are chains of mountains 1,000’s of km’s long
• sit at or near edges of continents
• form from tectonic or volcanic processes
over millions of years--geosphere
• erode as they grow higher and steeper--hydrosphere
• cause precipitation as air rises above them--atmosphere
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mountains and mountain building
mountain belts
are very long
compared to
their width
North American
Cordillera
extends from
Alaska
to
Panama
mountains and mountain building
mountain belts
height is related to age:
old mountains (100’s of millions of years) (Appalachians)
have lower elevations (due to erosion)
than young mountains (a few million years) (Himalayas)
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mountains and mountain building
ancient mountain belts
eroded flat:
form stable, interior of continents: craton
-- oldest parts are shields -- e.g. Canadian shield
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mountains and mountain building
mountain belts
• thick sequences of folded and faulted rocks
--typically marine (formed in ocean) sedimentary rocks
• metamorphic rocks locally common
mountains and mountain building
mountain belts
• fold and thrust belts
--crust shortened and thickened
(remember: thrust faults indicate shortening)
• common at convergent boundaries (compression)
mountains and mountain building
mountain belts
fold-thrust mountains
Himalayas, Alps, Urals, northern and Canadian Rockies
green arrow shows Lewis Thrust, Glacier National Park
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mountains and mountain building
evolution of mountain belts
accumulation stage
rocks (sedimentary) that will
later be uplifted, faulted, and
folded into mountains are
deposited in opening ocean
(sea floor spreading)
orogenic stage
mountain building episode
from plate convergence
(provides compression necessary for thrusting)
Mt. Everest
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Himalayan foothills
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Swiss Alps
mountains and mountain building
evolution of mountain belts
when mountains get very tall (e.g. Himalayas),
gravitational collapse and spreading may occur
--normal faulting and extension/thinning of crust
--uplift of metamorphic rocks from depth as crust thins and spreads
mountains and mountain building
evolution of mountain belts
after convergence stops, erosion and uplift occur
--isostatic adjustment-to thin continental root
mountains and mountain building
evolution of mountain belts
uplifting crust spreads and results in tension (extension)
that produces normal faulting and creates
fault-block mountains
(horsts and grabens from normal faulting)
mountains and mountain building
evolution of mountain belts
topography
fault-block mountains
Basin and Range, western US
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Basin and Range
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mountains and mountain building
evolution of Basin and Range
crustal extension dominates today
--accompanied by high heat flow-older period of thrusting and
formation of mountains when
continental root developed
uplift and erosion yield extension
but must explain high heat flow
delamination of mantle?
mantle lithosphere detaches
and sinks into asthenosphere
warm asthenosphere
fills space and results in
stretching of crust
mountains and mountain building
modern Basin and Range
extension at surface; upwelling asthenosphere at depth
mountains and mountain building
continents grow as
mountain belts evolve at
active continental margins
igneous activity adds
new crust
sedimentary rocks
originally deposited
in ocean
are uplifted, folded,
and faulted to
form new terranes
that are “accreted”
or added to
continent along its edge
accreted terranes in
western US
continental growth
continents decrease in age toward their margins
growth of continents: US example
geologic map of the United States
on-going subduction
(Cascadia)
Basin and Range
(rifting)
Canadian shield
transform
boundary
(San Andreas)
Paleozoic
to Recent
active
margin
Paleozoic to Recent
orogenic belts
from: http://pubs.usgs.gov/publications/text
Paleozoic
orogenic belts
(Appalachians)
craton
Mesozoic to
Recent
passive margin
Paleozoic
orogenic belts
(Ouachitas, Marathons)