Wadati benioff zone zone of seismicity created by slip between upper surface of slab and lithosphere of the uppe

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Wadati-Benioff zone: zone

of seismicity created by slip between upper surface

of slab and lithosphere of the upper plate

Reminder / change in schedule

Erosion Builds Mountains” by Pinter and Brandon Paper is on electronic reserve

tectonic basins

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 elements

 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

U.S

 base of all passive margins

Elements of extensional system

 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

gravitational equilibrium

between the Earth 's

lithosphere (analogous to

iceberg) and

asthenosphere (analogous

to seawater) T ectonic

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

Igneous activity

 Extension typically

involves ‘bimodal’

volcanism,

characterized by

rhyolite and basalt

 Contrasts with

andesitic volcanism

of arcs

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Rift basin sedimentation

 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

 Pure Shear

 Simple Shear

Depocenter symmetry will reflect structural symmetry

Conceptual models of rift

evolution

 passive vs active rifting

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 http://www.mines.utah.edu/geo/courses/UOnline/slideshow/contrift_1.html