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Extensional Tectonics:
Rifting and Divergence
Processes in Structural Geology & Tectonics
Ben van der Pluijm
© WW Norton+Authors, unless noted otherwise
3/28/2016 7:45 PM
We Discuss …Expressions of 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
Serra do Mar, along the east coast of Brazil. Mountains
exposing Precambrian granite and gneiss rise directly
out of the ocean along South America’s passive margin.
Basalt flow on top of a rhyolitic
ignimbrite in the Basin and Range
Rift of the western United States.
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Today’s Plates and Divergent Boundaries
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Continental Rift Systems: East African Rift
Gregory Rift
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Gregory Rift
Mt Kenya
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Ancient Continental Rift Systems
Stein et al., 2011
Midcontinent
Gravity High
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Rifts and Ocean Ridges
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Morphology of Ocean Ridges and Spreading Rate
Fast: East Pacific
Rise (EPR);
10+cm/y
Slow: MidAtlantic Ridge
(MAR); few cm/y
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Extra: Petrology of Ocean Ridges (“ophiolites”)
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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)
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Continental Extension:
Metamorphic Core Complexes (MCCs)
MCC: idealized cross section
Whipple Mnts, CA
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Evolution of Metamorphic Core Complexes
Whipple Mnts, CA
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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.
PA = PB is
isostatic equilibrium
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Extra: Extension and Sedimentation
Small basin with sediment fill
(a) Thinning and basin formation
(b) Cooling and further subsidence,
forming broad basin fill
(“steerhead basin”)
Viking Graben
(North Sea)
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Rift Evolution and Lithologic Assemblages
(a) Rift stage with nonmarine 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.
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Passive Margins (US Gulf Coast)
Note: listric growth faults
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Causes of Rifting
a)
b)
c)
Rifting above thermal plume.
Outer-arc extension of bending slab
at a subduction zone.
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.
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