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QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Wadati-Benioff zone: zone
of seismicity created by
slip between upper surface
of slab and lithosphere of
the upper plate
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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
Rift to drift: a story of extension
Four main stages related to
Wilson cycle
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
Development of
ocean basin and
passive margins
aulocogen (failed rift
interior rifts (prone
to reactivation)
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Currently active continental
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
High- and low-angle
normal faults
lower crustal intrusion
Lower crustal and
Thermal and uplift history
Advection: heat transfer through
 Higher heat flow due to thinner crust and
lithosphere, asthenosphere nearer
 thermal uplift components
 isostatic uplift components
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
Extension typically
involves ‘bimodal’
characterized by
rhyolite and basalt
Contrasts with
andesitic volcanism
of arcs
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Rift basin sedimentation
sediment depocenters: accommodation space
may be 10s of km deep
immature sediments (lots of feldspar, lithic
half grabens common
Fault-controlled patterns of sedimentation:
alluvial fans and debris flows
Along-strike changes = segmentation of
Depocenter symmetry will
reflect structural symmetry
Pure Shear
Simple Shear
Conceptual models of rift
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
Rift to drift: a story of extension
Establishing a passive margin