Prsentation PowerPoint

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Continental Rifting
Continental Rifting
Chris Goldfinger Burt 282 7-5214 gold_at_coas.ore
gonstate.edu http//activetectonics.coas.oregonsta
te.edu readings Abers et al., 2002 Heimgartner
et al., 2006 Zandt et al., 2004 Baldwin et al.,
2002 Suggested Supplements
Plate tectonics is a little known exclusion to
car insurance policies.
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Rifting continental crust. The variations in
structural style are endless
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Pure shear rifting
Simple shear rifting
But two basic models prevail for crustal
extension. Many more exist for driving forces.

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Low relief accomodation zone
High relief accomodation zone
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Tectonic Environments -Hotspot plumes beneath
continents -Continental Rifts East Africa, Gulf
of California, Woodlark Basin, Basin and Range.
-Rifted (Now passive) continental margins
Continental margins around Atlantic for example
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Why rifts form -Upwelling of hot mantle
material, hotspot plumes, overriding of active
ridge, leaky transform. -Change in plate
geometry -Extension and reactivation of weak
zones (ancient rifts, orogenic suture
zones) -Possibly gravitational response to a
number of buoyancy effects.
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Change in plate geometry
One that is little disputed is slab roll-back and
back arc rifting/spreading.
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So lets start with the east African Rift system,
arguably a no brainer rift.
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• Tomographic inversion of East African teleseismic
  residuals in terms of undulating mantle layers of
  perturbed velocity relative to a background
  mantle velocity of 8 km/sec. The inner lobe is
  thought to contain partial melt by virtue of the
  magnitude of the velocity perturbation and
  corroborative evidence from Q estimates and S
  wave delays and petrology

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GPS determined spreading rates, Afar Triangle
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GPS determined spreading rates, Afar Triangle
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Kilimandjaro volcano 5895 m
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From continental rifting to ocean
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Metal-bearing Basalts
C (wood) O2 (melt) CO2 (gas)
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Continental Magmatism (Mantle-Derived)

• Usually associated with continental rifting
• Some in stable cratonic areas

• Magma-types
• Qtz Tholeiites, Tholeiites Alkali Basalts
• Alkaline (Na2OK2OCaOgtSiO2) ,
  SiO2-undersaturated. Basanites, phonolites,
  tephrites.
• Peralkaline (Na2OK2OgtAl2O3). These can be
  SiO2-undersaturated (nephelinites, leucitites,
  phonolites) or oversaturated (e.g. Peralkaline
  rhyolites, alkali granites)
• Ultrabasic (e.g. kimberlites, lamproites)

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Continental Magmatism (Mantle-Derived)
Doming and lithospheric thinning by ductile
flow Large scale melting of lithosphere producing
flood basalts
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Flood Basalts

• Mg-contents 5-8 are too-low to be directly
  derived from the mantle
• Probably stored in magma chambers
• Also contain few phenocrysts

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Flood Basalts
Columnar jointing in flood basalt
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Flood Basalts

• Enrichments similar to E-morb
• Mainly fertile mantle.
• Some distinct depletions due to crystal
  fractionation.
• Not Mg-rich enough to be direct melts of the
  mantle.
• Accumulation in large magma chambers.

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Flood Basalts
Flood basalts
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Continental Magmatism Rift Stage
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• Extends 3,700 km
• Rifting began in the early Tertiary around the
  Ethiopian dome.
• Rifting in the Miocene in Kenya and Tanzania.

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• Lies on a series of crustal domes.
• Rift triple junctions occur on the domes

rrr-junction
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Continental Magmatism Rift Stage
Motnick Nephelinite/Carbonatite
Gelia Shield Volcano (alkali basalt)
Shombole Nephelinite/Carbonatite
Oldoinyo Lengai (Nephelinite/Carbonatite)
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Continental Magmatism Rift Stage

• Two main alkaline series
• Silica-undersaturated
• Silica-saturated

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Carbonatite-Nephelinite Association

• Most commonly in rift settings but occasionally
  cratonic
• Carbonatites have the highest REE concentrations
  of any rock type.

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Carbonatite-Nephelinite Association
Intrusive carbonatites are always emplaced after
alkaline silicate magmas.
Fenite generated by metasomatism and contains
aegirine-augite
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Carbonatite-Nephelinite Association
Ca-carbonatite lava
Kerimasi
Oldoinyo Lengai
Natrocarbonatite lavas
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• Carbonatites plot on the mantle array.
• Carbon isotopes are also mantle values.

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Continental Magmatism Rift Stage
Some melting in lithosphere
Some contamination by crust
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Continental Magmatism Afar Stage
Qtz tholeiites, alkali basalts further from
plume. N-MORB-like signatures
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Pre-rifting Flood basalts
Rifting-stage Alkaline volcanics
Afar Alkali basalts and quartz tholeiites
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Cratons Kimberlites
Potassic ultrabasic rocks dominated by xenocrysts
of olivine, Cr-diopside, phlogopite etc. Matrices
consist of serpentine, phlogopite(mica),
carbonates, perovskite. More a clan of related
rocks rather than specific rock types. Carbonate
rich basaltic kimberlites Mica rich - Orangeites
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Cratons Kimberlites
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Cratons Kimberlites
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Cratons Kimberlites
Basaltic kimberlites CO2-rich
Micaceous Kimberlites (Orangeites)
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Cratons Kimberlites
Group 1 basaltic
Group 2 micaceous
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Kimberlites and Mantle Metasomatism?
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Map based on Landsat Thematic Mapper imagery.
Complex linear and arcuate NE-trending and
transverse faulting is apparent along the rift
margin. The broad rift margin and rift floor are
shown by darker and lighter shades, respectively.
Other hominid sites within the Middle Awash study
area are located at Aramis (4.4 Myr Ardipithecus
ramidus), Maka (3.4 Myr Australopithecus
afarensis), Bouri (2.5 Myr Australopithecus
garhi) and Bodo (0.64 Myr Homo).
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One of the most significant strat sections on
earth, well to humanoids anyway
The four defined members are shown. Named and
unnamed basaltic and silicic tephra stratigraphic
markers are labelled. Locations of the measured
sections are shown in Fig. 1. SIMA, Stable
isotope samples ANBT, Ankarara Basaltic Tuff
BABT, Bakella Basaltic Tuff DOBT, Dobaado
Basaltic Tuff HABT, Hantuuta Basaltic Tuff
LABT, Ladina Basaltic Tuff WMMT, Witti Mixed
Magmatic Tuff.
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The Woodlark Basin rift of New Guinea is a more
complex beast. On one end, the rifted basin is
subducting. On the other, its rifting a
continental block!
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Crustal scale detachment faulting, as in the Bsin
and Range has been proposed for the Woodlark
basin, but evidence is sketchy at this stage as
shown by the speculative velocity model below.
Author names deleted to protect the perps
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