Download Wilson cycle

Wilson cycle
1. Intracontinental rift
2. From rifting to drifting
Wednesday, March 31, 2010
Stages of the Wilson Cycle
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Intracontinental Rifts
1.
Contemporary examples (EAR, RGR, Baikal,
Rhine graben)
2.
Mechanical aspects. Characteristics (Regional
uplift, Volcanism, Extension, Seismic activity,
High heat flow,…)
3.
Models of rift formation (active vs passive)
4.
Failed rifts
5.
Passive continental margins
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Definitions
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New information
Continental rift
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Rift system
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Modern rift
Paleo-rift
Failed Arm
Aulacogen
Impactogen Passive rifting
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Seismic tomography
Geochemistry
Deformation rates
Active vs. Passive Rifting
adiabatic
decompression
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East African Rift
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Volcanic activity
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Afar Triple junction
Plate
Boundaries(Nubia,
Somalia)
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2 Arms of the EAR in Kenya
N
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The Afar Triangle
N
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East African Rift in Kenya
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Gravity and seismic profiles
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Bouguer Gravity profiles
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Long wavelength
low -> isostatic
compensation of the
uplift.
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Short wavelength
low -> low density
sedimentary
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Short wavelength
high -> magmatic
intrusions
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Shallow crustal structure
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Seismic profile and shallow crustal
structure
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Lithosphere Structure-Gravity
interpretation
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Long wavelength
Bouguer gravity
low
compensation of
topography in
the mantle
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Short
wavelengths
highs: intrusions
in the crust
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East African rift heatflow
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Earthquake depths in Kenya
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earthquakes are
shallower under the
rift axis :
higher temperatures
-->
shallower brittle ductile
transition
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Tanzania craton: seismic tomography
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Afar triple junction: EAR-Red sea-Gulf of Aden
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Continental flood basalts
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Evolution of the Afar triple junction
Remember
paleomagnetism shows
rotation of Danakil block
relative to Africa
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Western US: Basin and Range,
Colorado Plateau, Rio Grande Rift
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B&R Colorado Plateau RGR
N
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Rio Grande Rift
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Seismicity of the western US
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B&R, RGR magmatism
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(a) Mesozoic
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(b) Miocene
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(c) Pliocene
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(d)
Quaternary
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Note: little
activity in
CP
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NW
(Yellowston
e hotspot)
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Western US Normal Faults
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B & R extension (GPS)
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(a) GPS velocity across the Basin and Range, western United States with respect to North
America (blue vectors) with 95% confidence ellipses superimposed on topography
(Lambert conic projection). Confidence ellipses include uncertainty in the North America
reference frame. (b) Expanded view of faults around the Central Nevada Seismic Zone.
Faulting is shown with colored lines: cyan (historic), magenta (Holocene), and purple (Late
Quaternary).
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North America
Heat flow map
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RGR seismic travel time residuals
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Map of the southwestern
United States showing the
location of the La Ristra array
seismic stations. Dashed lines
show boundaries of two
Proterozoic provinces from
Karlstrom and Humphreys
[1998].
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RGR travel time residuals
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The P and S models obtained
through inversion of travel time
residuals.
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Slower regions are shown in
red colors and fast regions in
blue.
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RGR mantle
seismic
tomography and
receiver function
showing velocity
discontinuities
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RGR deep structure
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Chronometry of EAR and RGR
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Rio Grande Rift
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40-30 Ma calc-alkaline volcanism
Subduction of Farallon plate.
32 Ma rhyolitic volcanism
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30-20 Ma Bimodal volcanism
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(rhyolites-basalts)
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~20 Ma Ridge off California is
subducted.
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San-Andreas transform fault
begins.
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East African Rift
50-30 Ma basalts
45 Ma Flood basalts
21-14 Ma Flood basalts
16 Ma basalts
15-11 Ma Regional Uplift (500m)
12 Ma rhyolites
10 Ma Western shoulder uplift
8 Ma Faulting
7-5 Ma Ma Alkaline basalts
6-2 Ma Flood lavas
8-0.5 Ma Flood basalts (2 episodes)
4 Ma Main uplift (1,500m)
3 Ma Dykes
2 Ma Normal faulting (graben)
1 Ma Plateau uplift
1 Ma present Volcanism-Faulting
Baikal rift (<20 Ma)
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Baikal seismicity
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Heat flow
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Baikal mantle structure
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Two-dimensional teleseismic
tomography image: (a) averaged
residual times (P and PKP waves),
(b) target area with blocks, and (c)
velocity cross-section.
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The low-velocity area in the
mantle beneath stations 11, 12, 24,
21, 22, and 23 corresponds to the
asthenospheric upwarp beneath
the Baikal rift zone. The low
velocities in the southeastern part
of the cross-section (from stations
84 to 89) can correspond to a
plume head beneath the Hentey
dome
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Rhine Graben
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European Cenozoic (~60
Ma) rift system and
tectonic setting of the
Upper Rhine Graben in
the northern Alpine
foreland. Cross hatched,
Variscan basement
outcrops on the
European platform;
shaded, Cenozoic rift
deposits; dotted, Alpine
Molasse; dark, Cenozoic
volcanics. BG, Bresse
Graben; BTZ, Burgundy
Transform Zone; EG,
Eger Graben; HG,
Hessian Grabens; LG,
Limagne Graben; LRG,
Lower Rhine Graben
(Roer Valley Graben);
RG, Rhône Graben;
URG, Upper Rhine
Graben; VB,Vogelsberg
volcano. Insert rectangle
shows study area.
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Moho depth
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Rhine graben heatflow
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Gulf of Corynth is just another
example
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General remarks
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Rhine graben and Baikal rift are near collision
zone.
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Stresses are induced by collision and by
crustal and lithospheric thickening
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BR and RGR development coincide with the
termination of the Farallon plate subduction
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Only EAR is a “pure” rift?
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General characteristics of all rifts
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Broad uplift
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Seismic tomography (low velocity anomaly in upper
mantle localized)
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High heat flow localized (with short wavelengths
variations)
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Seismicity + volcanic activity
Crustal thinning
Bouguer anomaly (long wavelength low, +short
wavelengths)
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Rifting
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Continental extension, breakup
Sedimentary Basins
Continental Margins
Stages of the Wilson Cycle
1. Intracontinental rift:
continental extension,
graben formation,
volcanic activity
2. From rifting to drifting:
oceanic crust formation,
central rift formation
3. Evolution of oceanic basin:
Creation of new ocean floor at ridge
4. Initiation of subduction:
Ocean-ocean subduction,
Formation of insular arcs,
Ocean-continent subduction
5. Closure of the ocean basin
Continental Collision:
Building of mountain belts
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Example : East African Rift
Afar
Kenya/Tanzanie
East African Rift,
Afar Triple Junction
(RRR type)
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Mantle seismic structure of Rifts :
example : EAR
P-wave velocity
Continental Rift
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Afar Transitional
Zone
S-wave velocity
Bastow et al., 2005, Geophys. J. Int.
Mantle seismic structure of Rifts :
example : Baikal
Zhao et al., 2006, Earth Planet. Sci. Lett. 243
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Passive or Active Mechanism ?
Remontee
adiabatique
Passive : tectonic forces drive extension; then asthenospheric upwelling
Active : asthenospheric upwelling occurs first, this drives extension
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Passive vs Active Rifting
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Passive
Plate stresses ->
extension
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Crustal and
lithospheric thinning > uplift?
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Adiabatic
decompression ->
volcanism and possible
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Active
Hot spot activity ->
volcanism
Asthenosphere penetrates
lithosphere -> Uplift
Uplift -> Extension
Mechanisms of Rifting
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Active
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Thermal anomaly in mantle (hot spot, plume)
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Magmatism
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Uplift of the region
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Extension
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Rift Formation
Passive
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Extension due to stresses in the plates
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Thinning of crust and lithosphere
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Proposed rifting mechanisms
East African: probably active
mechanism
Rio Grande: délamination
mechanism ?
Baikal/Rhine: associated with
continental collision
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Active rift model
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Delamination
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Delamination involves
the removal and
sinking of lithospheric
mantle and
replacement by
asthenosphere
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It could have
happened beneath
B&R
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Graben valley formation
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In rifts, grabens are
surrounded by symmetric
normal faults
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Why graben depressed
relative to horst?
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Why central graben often
symmetric with uplifted
shoulders?
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How much extension in the rift zone?
Basin & Range
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In rifts, displacement on normal faults (for 60 deg, vertical /
horizontal ~ 1.7 => a few km at most (10-20 %).
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In rifts, crustal thickness (30km vs 40km) => at most 25%.
In Basin and Range, extension is often on listric or low
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Extension much larger in B & R
Estimates are
100% extension
for B&R.
Note this implies
crust was 60km
before extension.
(Post orogenic
collapse?)
Metamorphic « core complex » formation – faulting
and isotstatic uplift  lower crustal outcrops. Typical
of Basin & Range.
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Extension Models
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Pure Shear: décollement produced at interface between
brittle and ductile zones.
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Above the décollement, extension is controlled by normal
listric faults normales
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Below, extension causes flow of lower crust
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Simple Shear – décollement persists throughout the
crust, then is transformed to shear ductile zone
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Rift is generally asymmetric, possibly due to
disequilibrium between surface processes and deeper
structure
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Temperature and lithosphere extension
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Rifting to Drifting : When Extension
continues
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Plumes and rift
Hotpots/plumes often initiate extension -> continental
fragmentation (ex. separation of modern continents
from Gondwana supercontinent)
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Opening of ocean: link with hot spots
and rifts.
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Effects of temperature and extension factor on the formation of oceanic crust along
continental margins and ocean basins. Oceanic crust represents infinite stretching.
(White & McKenzie, 1995, J. Geophys. Res. 100)
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Melting at MOR
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Mantle T determines % melting
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Oceanic crustal thickness is
uniform because it is fixed by
mantle temperature.
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Exception at hotspots on
MOR.Very thick crust in
Iceland.
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Mantle temperature was higher
during the Archean.
Thick crust for the Iceland hot spot
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And if Rifting fails ?
Atlantic failed arms and triple junctions
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Text
Failed arm =
Aulacogen
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Hypothesis: continental rupture
initiates a series of triple
junctions RRR (trois zones de
rift).
Oceanic basin is created by 2
boundaries, the other fails.
Many examples in the Atlantic
margins, and also in the
continental interior, from the
Precambrian to present day.
Failed rifts in North America
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Filtered Gravity map (40km -> 200km)
Failed Rift : Keweenawan
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v
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Keweenawan rift system
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-Keweenanwan is the deepest
rift on Earth
- Seismics show very thick
volcanics and sediments
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Aulacogens in eastern Canada
Saint-Laurence
Valley, Great Lakes,
Ottawa-Bonnechere
Graben, and the
continuation of the
Outaouais Valley of
Lake Témiscamingue
are examples of
failed rift zones.
These zones are
associated with the
rupture of the
Laurentia
supercontinent.
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Adams & Basham,
1991; « The seismicity
and seismotectonics of
eastern Canada »)
shows emplacements of
primary aulacogens in
eastern Canada. Note:
region labelled « Extinct
ridge and transforms »
probably continues in
Baffin Bay.
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