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

Wadati-Benioff zone: zone of seismicity created by slip between upper surface of slab and lithosphere of the upper plate. Reminder / change in schedule. Friday is a discussion of the paper “How Erosion Builds Mountains” by Pinter and Brandon. Paper is on electronic reserve

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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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  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

  15. Rift to drift: a story of extension

  16. http://www.mines.utah.edu/geo/courses/UOnline/slideshow/contrift_1.htmlhttp://www.mines.utah.edu/geo/courses/UOnline/slideshow/contrift_1.html Establishing a passive margin

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