Extensional Tectonics: Rifting and Divergence Processes in Structural Geology & Tectonics Ben van der Pluijm © WW Norton+Authors, unless noted otherwise 4/5/2017 15:26
Extensional Tectonics:
Rifting and Divergence
Processes in Structural Geology & Tectonics
Ben van der Pluijm
© WW Norton+Authors, unless noted otherwise
4/5/2017 15:26
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We Discuss …
Extensional Tectonics• Today’s divergent plate boundaries
• Ocean Ridges
• Lithologies
• Failed Rifts
• Structural Styles
• Pure shear systems
• Simple shear systems
• Stretching factor
• Metamorphic core complexes
• Isostasy
• Rift evolution
• Passive margins
• Causes of rifting and extension
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Today’s Plates and Divergent Boundaries
Divergent
Convergent
Strike-slip
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Continental Rift Systems: East African Rift
Gregory Rift
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Africa’s Gregory Rift
Mt Kenya
Ancient Continental Rift Systems
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Stein et al., 2011
Midcontinent
Gravity High
Rifts and Ocean Ridges
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Morphology of Ocean Ridges and Spreading Rate
Fast: East Pacific
Rise (EPR);
10+cm/y
Slow: Mid-
Atlantic Ridge
(MAR); few cm/y
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Extra: Petrology of Ocean Ridges (“ophiolite”)
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Structural Styles of Rift Systems
Pure-shear model
Simple-shear (or
detachment) model
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Examples of Rift Systems
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Extension and Stretching Factor
Stretch and extension:
s= l/lo = 2
e = (l-lo)/lo = 1 (=100% extension,
so 50% thinning)
(recall: s = 1 + e)
e is typically 1-3
(s = 2-4)
100-400% extension, 50-75%
thinning)
Extension and Isostasy
Application of Archimedes’ Law of Buoyancy to
Earth:
“the mass of water displaced by a block
is equal to the mass of the block”
So, surface thinning of “block” (lithosphere)
results in subsidence.
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PA = PB is called
isostatic equilibrium
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Extra: Extension and Sedimentation
Viking Graben
(North Sea)
(a)Thinning and basin formation
(b)Cooling and further
subsidence, forming broad
basin fill (“steerhead basin”)
Continental Extension: Metamorphic Core Complexes (MCCs)
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MCC: idealized cross section
Whipple Mnts, CA
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Evolution of Metamorphic Core Complexes
Whipple Mnts, CA
Extension and Isostasy
Application of Archimedes’ Law of Buoyancy to Earth:
“the mass of water displaced by a block
is equal to the mass of the block”
So, surface thinning of “block” (upper crust) results in
upward flow of “water” (lower crust).
In MCCs, thinning by normal faulting results in
exhumation of deeper detachment and
basement rock.
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PA = PB is called
isostatic equilibrium
Rift Evolution and Lithologic Assemblages
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(a)Rift stage with non-marine basins
(b) rift–drift transition with evaporate
deposition
(c) drift stage, with seafloor
spreading occurring and passive-
margin basins evolving, and
marine deposition in the basins.
Passive Margins (US Gulf Coast)
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Note:
listric growth faultsUS Gulf Coast region
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Causes of Rifting
a) Rifting above thermal plume.
b) Outer-arc extension of bending slab
at a subduction zone.
c) Gravitationally driven extensional
collapse of thickened crust in an
orogen.
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Causes of Rifting, cnt’d
d) Backarc extension associated with
convergence.
e) As plates move apart, continent is
stretched and broken apart.
f) Pull-apart basin at a releasing bend
along a lateral(=strike)-slip fault.