Intro comment: On either side of an ocean basin, and the ocean crust within, are eventually continental rocks (if you go far enough). Running seafloor spreading backwards also brings the continents back together, and so it is clear that continents come apart to make new oceanic basins. In this section we will study what happens when continents come apart. Sometimes they come apart in a continued and big way and an oceanic basin is born, but it turns out that the behavior is much more complex, and sometimes the rifting aborts, sometimes it is concentrated, sometimes it is more diffuse, and localized rifting can occur in what may initially seem unlikely positions - on top of convergent mountain belts for example. So we will look at continental rifting more broadly here.

Wilson cycle concept.

The basic idea in the Wilson cycle is that rifting tends to occur along old orogenic axis, so that these become the edges of the rifted continental fragments, which eventually collide to form mountain belts, which are preferential weakness zones, setting up the axis for the next rifting episode. The classic case example is the Appalachian orogen (see adjacent diagram). There are plenty of examples where the Wilson cycle is not followed faithfully - it is much more of schema.

Continental rifting is a convenient starting point (but not a simple one). Continental rifts are locations of continental crustal extension/divergence, crustal thinning, sedimentary basin formation, and often thermal and igneous activity. As indicated, they are diverse, complex and polygenetic.

Eventual fates of continental rifts:

• ocean basin, and passive margin development.
• aulocogen (failed rift arms).
• interior rift basin (prone to reactivation of multiple types).

Presently active continental rifts: Hanna map.

• East African Rift zone. USGS site.
• Rio Grande Rift.
• Death Valley.
  • Satellite photo (false color image) from NASA's Visible Earth.
  • More info on image from NASA's Visible Earth.
  • More info on rift from USGS.
• Lake Baikal Russia.
  • USGS site.
  • Satellite photo from NASA Visible Earth. More information at Visible Earth.
• Salton trough, California (transtensive).
• Dead sea rift (transtensive).

Some better known past continental rifts:

• Triassic grabens along U.S. east coast.
  • Formed during the rifting of Africa and North America leading to the formation of the North Atlantic Oceanic basin.
  • Note the faults and classic half-graben geometry, with a major curved (listric) fault on one side, and an overall wedge shape of the basin fill (yellow colore here).
  • Cross sections to right show the evolution of a major basin centered on New Jersey. Image and additional information source: http://3dparks.wr.usgs.gov/nyc/mesozoic/mesozoicbasins.htm
• Rhine Graben, in northern Europe.
• Basin and Range province of SW U.S..
  • AGU site. Map below is from the USGS tapestry site. The striped pattern is due to the Basin and Range province. The map to the right shows the larger extent of the Basin and Range province and is from http://geomaps.wr.usgs.gov/parks/deva/ftdan1.html . Note the overall map distribution form of the province - it is not a linear parting of the ways. This very much speaks to the origin of the rifting.
  
• underpinning of all passive margins.
• Keweenawan rift and mid-continent gravity high - interior U.S.. UNO field trip.
  • Below is a map taken from the USGS site: http://pubs.usgs.gov/info/mwni_cu/ . where the interest is in possible nickel-copper deposits. Rifts can not only be the sites of ore mineralization, but can also harbor fossil fuel deposits, and so rifts are a common exploration target - a hot spot.
• 

Extensional accommodation mechanisms:

• normal faulting.
  • high angle normal faulting (component 1).
  • low angle normal faulting (component 6).
  • brittle ductile transition.
• dike swarms (component 2).
• lower crustal attenuation (component 3).
• lower crustal intrusion (component 4).
• lithospheric attenuation is observed with modern rifts. What are the responsible mechanisms?

Rift-related igneous activity:

• bimodal signature.
• trace element geochemistry: the template/signature 'game'. What are all factors that determine the chemical composition of a melt?
• contintental rift basalts enriched in alkalis (K, Ba, Rb), incompatible elements, LIL.
  • deep mantle plume contribution.
  • mantle fluids and metasomatism.
  • lithospheric mantle contribution.

Rift basin sedimentation:

• sediment traps, accomodation space (miles deep).
• often arkoses, immature sediments.
• half grabens.
• fault driven sedimentation: alluvial fans and debris flows.
• along strike changes = segmentation and depocenters.
• every basin unique: Carboniferous rift on Bjørnøya.
• Cross section of rift sediments in the Newark Basin from the USGS. Note the relationship between the sediment type and its position in the half graben.
• 

Thermal and uplift history:

• higher heat flow due to advective/magmatic fluid transfer.
• higher heat flow due to thinner crust and lithosphere.
• thermal uplift components
• isostatic uplift components.
• off axis thermal and igneous activity, and its relationship to a thin-skinnned tectonic style.
• Geothermal map source: http://www1.eere.energy.gov/geothermal/geomap.html . Given what we have discussed so far, which areas can be attributed to ongoing or recent rifting and which to something else?
• 

Map pattern evolution and models of continental rifting:

• hot spots, triple junctions and aulocogens (Burke & Dewey model)
  • stationary plumes -> doming, igneous activity and thinning and weakening of lithosphere -> development of triple junction pattern of rifts -> propagation and continued spreading and rift linkage -> domination of igneous processes over faulting and development of seafloor spreading center for some rifts, while other rifts cease activity.
  • application to EAR, and south Atlantic.
  • some questions:
    • inadequacy of doming stresses.
    • validity of triple junction pattern.
    • transition from triple junction to orthogonal seafloor spreading geometry.
  • map from USGS showing a simplified the EAR rift zone:
    • 
• Influence of previous structure on rift patterns? One broad example of this is how the Norwegian-Greenland oceanic basin follows the axis of the Norwegian-Greenland Caledonides.
  • Ordovician-Silurian major contractional mountain building.
  • Devonian collapse basins.
  • limited Carboniferous rifting.
  • Jurassic-Cretaceous rifting.
  • Tertiary formation of the oceanic basin.
  • Episodic rifting along the Caledonide axis in this area over a course of 300 Ma.
• rift tip propagation, Vink and Courtillot model.
  • propogating rift tip with seafloor spreading on one side, and continental rifting on other.
  • application to Gulf of Aden and Gulf of California (key trait, slight obliquity between seafloor magnetic anomalies and continental margins)
  • locked zones
  • implications for fits of continental margins.
  • beyond pole-of-rotation compression.
  • studies in wax of rift propagation/evolution.
• passive vs. active rifting.

What might be factors that determine the pattern of continental rifting?

Other models to explain continental rifting:

• orogenic topographic collapse
  • main questions is why localized rifting within orogenic welts is coeval and spatially related to crustal thickening in convergent settings.
  • best example is the Himalayas.
• Rigid indentor models.
  • introduction to oroclines.
  • usually used in application to the Rhine graben. Link to satellite image of Alps and Rhine graben, from http://veimages.gsfc.nasa.gov/5188/France.A2003078.1050.500m.jpg.
• Transtensive releasing bends: will take about much more later.
• Intraplate tectonism: good model for Carboniferous basins.

Some general references on continental extensional tectonics:

• Baker, B. H. & others, 1972, Geology of the Eastern Rift System of Africa; Geological Society of America Special Paper 136, 67 p. Mainly a thorough descriptive effort.
• Burke, K. & J. Dewey, 1973, Plume-generated triple junctions: Key indicators in applying plate tectonics to old rocks; Journal of Geology, vol. 81, p. 406-433. A classic in plate tectonics describing a mechanical-historical model for continental rifting with many testable aspects (developed in the context of the EAR).
• Craddock, C., 1973, Structural evolution of the Keweenawan Province; Geology, vol. 1, # 4, p. 190. Describes the history of this Precambrian intracontinental rift.
• Courtillot, V. & Vink, G. E., 1983, How continents break-up: Scientific American, p. 43-49.
• Lister, G. & others, 1986, Detachment faulting and the evolution of passive continental margins; Geology, vol. 14, p. 246-250.
  Describes a variant on the Wernicke model and applies it to continental margins, discussing the effect of the inherent asymmetry of Wernicke's and their model on margin development.
• Mohr, P., 1987, Structural Style of Continental Rifting in Ethiopia: Reverse decollements: EOS, p. 721, 729-730.
• Rosendahl, B. R., 1987, Architecture of continental rifts with special reference to East Africa; Annual Review of Earth and Planetary Sciences, 15, 445-503. Discusses segmentation, and the relation to deeper underlying processes.
• Ruppel, C., 1996, Extensional Processes in Continental Lithosphere; Journal of Geophysical Research, v. 100, p. 24,187-24,215. An excellent summary of continental rifting - a very good place to start, and to look for additional references.
• Scholz, C. H. & Contreras, J. C., 1998, Mechanics of continental rift architecture; Geology, v. 26, p. 967-970. Elegant paper with very testable assertions as to the 3-D geometry of rifts.
• Serpa, L. & others, 1984, Structure of the southern Keweenawan rift from COCORP surveys across the midcontinent geophysical anomaly in northeastern Kansas; Tectonics, vol. 3, # 3, 367-384. Demonstrates the listric, low-angle character of this rift.
• Illies, J. H. (ed.), 1981, Mechanism of Graben Formation - Development in Geotectonics 17; Elsevier press, New York, 226 p.
• Wernicke, B., 1981, Low-angle normal faults in the basin and range province: nappe tectonics in an extending orogen; Nature, vol. 291, p. 645-647. A crucial paper reinforcing the idea of thin-skinned extension, and applying it to the entire plate thickness.

References on the Basin and Range province:

• Allmendinger & others, 1983, Cenozoic and Mesozoic structure of the eastern Basin and Range province, Utah from COCORP seismic reflection data; Geology, v. 11, 532-536.
• Davis, G. & Lister, G., 1988, Detachment faulting in continental extension; Perspectives from the southwestern U. S. Cordillera; GSA Special Paper 218, p. 133-161.
• Gans, P. B., Mahood, G. A., & Schermer, E., 1989, Synextensional magmatism in the Basin and Range Province: A case study from the eastern Great Basin: GSA Special Paper # 233, P. 53.
• Lister, G. & Davis, G., 1989, The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A.
• Mayer, L. (ed.), 1986, Extensional Tectonics of the Southwestern United States: A perspective on Processes and Kinematics: GSA Special Paper # 208, 122 p.
• Wernicke, B., 1981, Low-angle normal faults in the Basin and Range province: Nappe tectonics in an extended orogen; Nature, v. 291, p. 645-648.