Evolution of Sevier Orogeny and Cretaceous Extension in North American Cordillera

 
Atticus Proctor, November 2021
 
Key Points
Cretaceous extension took place in the North
American Cordillera during the development of
the Sevier foreland fold and thrust belt.
Gravitational collapse led to extension in the
Sevier fold and thrust belt
Mid-crustal decoupling of an intraplate allochthon
moved westward, leading to extension
 
Map of metamorphic core complexes within the Sevier Internal Zone 
 
Geologic Setting
 
Sevier Orogeny, spanned the Cretaceous
(160-50 Ma)
“Internal Zone” consists of:
Metamorphic core complexes
Mesozoic-Paleogene plutons
Bounding features are complex due to
complex structural geology
Little to no surface expression of Sevier-age
extension
 
Evidence for Cretaceous Extension
16 metamorphic core complexes in the Internal
Zone show evidence of Cretaceous extension.
 
Rheological Conditions for Basement-Involved
Shortening
 
     Key consideration to the model:
Shorten the basement to get mid-crustal
extension!
Analogous structures and geophysical data
show the crust was thickened in the
basement rocks, not by upper crust fold
and thrust belts
Basement thickening allows for thrusting
at depth that places high-strength rocks at
intermediate depths
 
Model for Cretaceous
Extension
 
Mid-Crustal Extensional Allochthon is
created by gravitational collapse due to
eastward verging thrusting and crustal
thickening
New detachment surfaces form in response
to gravitational collapse
Extensional allochthon moves west
relative to the east verging upper and
lower crust
Mid-crustal extension is
contemporaneous with upper-crustal
shortening
This model allows for extension without
surface-breaking normal faults
Tertiary normal faulting finishes
exhuming metamorphic core complexes
to the present-day surface
 
Important points pertaining to the evolution of the
Great Basin
 
During the Sevier orogeny the
dominant movement was
convergent, but there was also
some extension
This Sevier/Cretaceous extension
partially exhumed many of the
metamorphic core complexes
that stretch from southern
Canada to Arizona in the
present-day
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The Sevier Orogeny in the Cretaceous period led to extension in the North American Cordillera, with gravitational collapse playing a key role in the development of the Sevier fold and thrust belt. The internal zone consisted of metamorphic core complexes and Mesozoic-Paleogene plutons, showcasing evidence of Cretaceous extension. Basement-involved shortening and mid-crustal extension were crucial in this geological evolution, allowing for thrusting at depth. The Cretaceous extension model illustrated how extensional allochthon moved westward, emphasizing the importance of gravitational collapse. The Sevier/Cretaceous extension partially exhumed metamorphic core complexes, highlighting the complex structural geology of the region.


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  1. 1992 Atticus Proctor, November 2021 Key Points Cretaceous extension took place in the North American Cordillera during the development of the Sevier foreland fold and thrust belt. Gravitational collapse led to extension in the Sevier fold and thrust belt Mid-crustal decoupling of an intraplate allochthon moved westward, leading to extension Map of metamorphic core complexes within the Sevier Internal Zone

  2. Geologic Setting Sevier Orogeny, spanned the Cretaceous (160-50 Ma) Internal Zone consists of: Metamorphic core complexes Mesozoic-Paleogene plutons Bounding features are complex due to complex structural geology Little to no surface expression of Sevier-age extension Evidence for Cretaceous Extension 16 metamorphic core complexes in the Internal Zone show evidence of Cretaceous extension.

  3. Rheological Conditions for Basement-Involved Shortening Key consideration to the model: Shorten the basement to get mid-crustal extension! Analogous structures and geophysical data show the crust was thickened in the basement rocks, not by upper crust fold and thrust belts Basement thickening allows for thrusting at depth that places high-strength rocks at intermediate depths

  4. Model for Cretaceous Extension Mid-Crustal Extensional Allochthon is created by gravitational collapse due to eastward verging thrusting and crustal thickening New detachment surfaces form in response to gravitational collapse Extensional allochthon moves west relative to the east verging upper and lower crust Mid-crustal extension is contemporaneous with upper-crustal shortening This model allows for extension without surface-breaking normal faults Tertiary normal faulting finishes exhuming metamorphic core complexes to the present-day surface W E J-K time K time K time Tertiary time

  5. Important points pertaining to the evolution of the Great Basin During the Sevier orogeny the dominant movement was convergent, but there was also some extension This Sevier/Cretaceous extension partially exhumed many of the metamorphic core complexes that stretch from southern Canada to Arizona in the present-day W E

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