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Wadati-Benioff zone: zone
of seismicity created by
slip between upper surface
of slab and lithosphere of
the upper plate
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Reminder / change in schedule
Friday is a discussion of the paper “How
Erosion Builds Mountains” by Pinter and
Brandon. Paper is on electronic reserve
 On Wednesday, we will talk about tectonic
basins
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Rift to drift: a story of extension
Four main stages related to
Wilson cycle
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RIFT VALLEY stage, prior to continental
splitting; caused by upwelling of hot mantle
material? Example: East African Rift
YOUTHFUL stage: thermal effects dominate for
about 50 my after the onset of seafloor
spreading. Example: Red Sea
MATURE stage: subdued regional subsidence.
Example: most of the present Atlantic
continental margins
FRACTURE stage: subduction starts
Fates of continental rift
elements
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Development of
ocean basin and
passive margins
aulocogen (failed rift
arm)
interior rifts (prone
to reactivation)
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Currently active continental
rifts
East African rift zone
 Rio Grande rift
 Lake Baikal, Russia
 Salton trough, California (transtensional)
 Dead sea rift (transtensional)

Some former continental rifts
Rhine Graben, northern Europe
 Triassic grabens, East coast U.S.
 Keweenawan rift - northern midwest, U.S.
 base of all passive margins

Elements of extensional
system
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High- and low-angle
normal faults
Brittle-ductile
transition
Dikes
lower crustal intrusion
Lower crustal and
lithospheric
attenuation
Thermal and uplift history
Advection: heat transfer through
magmatism
 Higher heat flow due to thinner crust and
lithosphere, asthenosphere nearer
surface
 thermal uplift components
 isostatic uplift components

ISOSTASY
Isostasy: the state of
gravitational equilibrium
between the Earth's
lithosphere (analogous to
iceberg) and asthenosphere
(analogous to seawater).
Tectonic plates ‘float’ at an
elevation which depends on
their thickness and relative
density; thus high areas will
have large lithospheric
‘roots’. Where a balance is
achieved between
topography and size of
roots, lithosphere is said to
be in isostatic equilibrium.
Igneous activity
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Extension typically
involves ‘bimodal’
volcanism,
characterized by
rhyolite and basalt
Contrasts with
andesitic volcanism
of arcs
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Rift basin sedimentation
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sediment depocenters: accommodation space
may be 10s of km deep
immature sediments (lots of feldspar, lithic
fragments)
half grabens common
Fault-controlled patterns of sedimentation:
alluvial fans and debris flows
Along-strike changes = segmentation of
depocenters
Depocenter symmetry will
reflect structural symmetry
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Pure Shear
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Simple Shear
Conceptual models of rift
evolution

passive vs. active rifting
Passive - in response to far-field stress
 Active - in response to upwelling of
asthenospheric mantle

rift tip propagation: Gulf of California
 influence of previous structure: Tertiary
formation of the Atlantic Ocean
 hot spots, triple junctions and aulocogens
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Rift to drift: a story of extension
http://www.mines.utah.edu/geo/courses/UOnline/slideshow/contrift_1.html
Establishing a passive margin