PowerPoint Presentation - HISTORY

1
Wadati-Benioff zone zone of seismicity created
by slip between upper surface of slab and
lithosphere of the upper plate
2
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

3
Rift to drift a story of extension
4
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

5
Fates of continental rift elements

• Development of ocean basin and passive margins
• aulocogen (failed rift arm)
• interior rifts (prone to reactivation)

6
Currently active continental rifts

• East African rift zone
• Rio Grande rift
• Lake Baikal, Russia
• Salton trough, California (transtensional)
• Dead sea rift (transtensional)

7
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

8
Elements of extensional system

• High- and low-angle normal faults
• Brittle-ductile transition
• Dikes
• lower crustal intrusion
• Lower crustal and lithospheric attenuation

9
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

10
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.
11
Igneous activity

• Extension typically involves bimodal volcanism,
  characterized by rhyolite and basalt
• Contrasts with andesitic volcanism of arcs

12
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

13
Depocenter symmetry will reflect structural
symmetry

• Pure Shear
• Simple Shear

14
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
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Establishing a passive margin
http//www.mines.utah.edu/geo/courses/UOnline/slid
eshow/contrift_1.html