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Patrice Rey
Room: 407
[email protected]
Tel: 12067
Lecture 6:
Structural geology of divergent plate boundaries
Aims: To characterize
deformation at divergent
plate boundaries
The Wilson cycle: Rifting and the formation of continental margins
As the concept of sea floor spreading gained acceptance in the late 60's, the consequences for geology gradually began
to dawn. One of the first to recognise how plate tectonics could be applied to the geological record was J. Tuzo Wilson.
If continents rift apart to form ocean basins, other oceans must close. This may be repeated throughout Earth history.
This cycle is known as the Wilson Cycle:
(1) Rifting of continents by mantle diapirism
(2) Continental drift, seafloor spreading & formation of ocean basins
(3) Progressive closure of ocean basins by subduction of ocean lithosphere
(4) Continental collision and final closure of ocean basin
The two diagrams below illustrate some simple concepts of continental rifting (e.g. the Gondwana continent) at the start
of the Wilson Cycle. Uprising plume causes doming of crust with magma chamber developing underneath. As extension
continues, an ocean basin forms, and thick sedimentary sequences develop at continental margins as rivers dump
sediments in deep water. However in reality may be a bit more complex . . .
From Tarney, Leicester Uni.
Driving force for extension
Passive rifting: Plate boundary forces drive the
stretching and thinning of the continental lithosphere.
Active rifting: Body forces related to the
thermal thinning of the lithospheric
mantle drive thinning.
Development of Continental Rifts...
Early ideas on the development of rifts are conceptualised in the diagram below. This is based on the African rift system,
where there is significant rift magmatism. There is notable extension, shown by the widening of the diagram block by at
least 50 km. At the same time there is uplift or ascent of the more ductile mantle, especially the asthenosphere.
The crust, and particularly the upper crust, is assumed to act in
a brittle fashion. Progressive formation of a rift valley through
extension of the lithosphere and continental crust (by about 50
km). Note that uprise and decompression of the underlying
asthenosphere results in magma formation. The crust responds
by brittle fracture. Early rift sediments are faulted and move
down into the developing rift (graben). Erosion takes place on
the sides of the rift valley.
•The first stage assumes that symmetric normal faults begin to
form in the brittle crust.
•The second stage shows simultaneous necking of the
lithosphere with uprise of an asthenosphere diapir. The
decompression associated with the latter causes melting of
the mantle to give basaltic magmas. Pre-existing sediments are
downfaulted into the graben.
•The third stage is accompanied by significant extension and
by more uprise of the asthenosphere. The latter causes doming
of the crust. New sediments are deposited within the graben as
a result of erosion of the uplifting sides of the graben. So there
are both pre-rift and syn-rift sediments within the developing
rift valley, but sediments on the flanks are progressively
eroded away.
Development of Continental Margins
•The fourth stage shows the actually rifting-apart of the continent, so the asthenosphere rises towards the surface,
causing decompression and extensive melting. New basaltic oceanic crust is formed.
•Finally, sea-floor spreading takes over as the ocean basin widens. The rift sedimentary sequence is buried beneath
younger marine sediments.
Evolution of a passive continental margin: An example
Rift basins bounded by moderately dipping normal and oblique-slip faults
Rifting: Middle Triassic (210Ma) to Early Jurassic (~190Ma)
Inactive rift basins
Seafloor-spreading centers
Producing oceanic crust
Drifting: Beginning Early (~190Ma) continuing today.
Continental Shelf Sediments
The figure below is representative of a number of crustal cross-sections across the continental shelf of the eastern
Atlantic seaboard of North America.
The critical point is the huge thicknesses of Mesozoic and Tertiary sediments, here shown as almost 15 km, but in
other cross-sections this can be even thicker. Note that at the bottom of this pile are volcanics and volcanogenic
sediments, and evaporites, which most likely are shallow water. Also, massive carbonate reef structures, which must
also be shallow water, but also must indicate progressive subsidence slow enough that shallow water sedimentation
can keep pace with it.
Structural Geology of Rift Basins
Rift basins are simply normal-fault bounded
sedimentary basins. Rift basins are a fundamental
manifestation of continental extension, they are prime
repositories for sediment accumulation, and account
for significant accumulations of hydrocarbons.
Asymmetric basin subsidence
Subsidence accelerates
Antithetic fault
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Contrasted models of extensional basin.
A
B1
A/ Domino fault block model in which both the
faults and the intervening fault blocks rotate
during extension. B1/ Planar kink fault
geometry and B2/ Listric fault-subhorizontal
décollement. In both B1 and B2 horizontal
displacement (h) on the décollement fault
creates a potential void bewteen the hanging
wall and footwall which is erased by the
collapse of the hanging wall along antithetic
faults dipping at 45º in B1, and vertical faults
in B2.
B2
The formation of a roll-over anticline in a sand box experiment...
D. Jarmusewski. 1998, Wuersburg
Normal faults in sand-box experiments
Physical models (=analogue
experiments) allow to investigate
simplified geological systems.
Using colored layers of sand, as an
analogue for the upper crust, and
applying various boundaries
conditions, McClay’s team (Univ.
London) can successfully create, in
their laboratory, structures observed
in nature.
Pictures on the right represent
various geometry of sedimentary
basins, from symmetrical extension
(top), to highly asymmetrical
extension (bottom).
More at: http://www.gl.rhbnc.ac.uk/FDP/fdp.html
McClay et al.
Some structural and sedimentary features of extensional basin.
Roll over fold, onlap, and pinchout
The development of extensional basin above a
décollement is associated with the formation of
a roll over anticline. The progressive rotation
of the surface of the basin is responsible for the
“onlap” deposition of layer. In contrast drop of
sea level creates “pinchout”.
1
3
2
1
Onlap
3
2
Pinchout
Rift Basin Inversion
It has long been recognized that the many rift
basins in the world contain a wide variety of
post-rift compressional structures. Many of
these features involve reutilization of normal
fault that were active during extension, and
hence they are referred to as inversion
structures. Inversed basin are they are prime
target for oil and gas exploration.
Extension of stable continental lithosphere: Numerical experiments
Generalized model of continental lithosphere. Mechanical and thermal boudary conditions on the left reflect
symmetry. A uniform force is applied to the right boundary (dynamic condition), or alternatively a constant strain
rate (kinematic condition), the top of the lithosphere is at 0ºC, and a constant basal heat flow q b is applied to the
lower boundary. Isostatic rebound forces act on density interfaces. Horizontal components of gravitational forces
associated with the vertical displacement of density interfaces are neglected.
Extension of stable continental lithosphere: Numerical experiments
6cm/y; Extension: 340km; 8Ma.
12cm/y; Extension: 470km; 5Ma.
Extension rate slow relative to thermal reequilibration
Extension rate fast relative to thermal reequilibration
Govers and Wortel, 1995.
Extension of stable continental lithosphere: Conceptual models
Many steeply dipping normal faults are actually listric faults. As the lithosphere is stretched during continental
extension, the deeper crust thins by plastic deformation, while the upper crust is broken up and pulled apart by listric
faults which 'bottom out' in the ductile layer. This is the essence of McKenzie-type models of basin formation. As the
lithospheric mantle is thinned by stretching it is of course partly replaced by hotter asthenosphere. This will gradually
cool on a time scale of the order of 50 - 100 m.y., and as it cools it becomes denser and the shallow basin above
gradually subsides and is progressively filled with shallow-water sediment. The amount of subsidence will depend on the
initial amount of stretching. Note that subsidence occurs in two stages: (1) as a result of tectonic stretching – on a short
time scale, (ca. 10 my), and (2) as a result of thermal subsidence – long time scale, (ca. 50 -100 my).
Further development have been proposed in particular by Wernicke. The important difference is in that stretching is
accommodated by a single low-angle detachments system which produces a strong asymmetry. Basins associated with
the thermal subsidence phase may be offset from the thin-skinned basins associated with the initial rifting. Magmatic
effects (melting resulting from the uprising asthenosphere) may be offset from the main sedimentary basins. Because of
the asymmetry, the continental margins on the two sides of an opening ocean may have very different profiles.
Wernicke simple shear model
Some promising developments in numerical modelling techniques...
Particle in cell finite element code is a new generation of computer code for modelling fluid of history
dependent properties. Applications of the code include smaller-scale geological studies of crustal
deformation, geomechanical problems such as slope stability, engineering-scale models such as the
flow of granular materia, and fluid-dynamics problems where free-surfaces, interfaces and suspended
particles are included. One of the most attractive aspect of this code is its ability to account for both
brittle and plastic deformations. This type of code could possible simulate both McKenzie and
Wernicke type of lithospheric extension.
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
More at: http://www.ned.dem.csiro.au/research/solidMech/Citcom/DEMO/
L. Moresi CSIRO
Structural characteristics of divergent margins
Rifting, sedimentary and oceanic
basins, and the formation of
continental margins result from
processes that tend to thin and extend
the continental crust.
Normal faults in the upper brittle
crust and flat foliations in the ductile
lower crust result from of a stress
regime in tension. The combined
action of normal faulting and
horizontal flattenning is responsible
for thinning and extension of the
continental crust.
As seafloor spreading occur,
stretching and thinning of the
continental margin stops. Thermal
subsidence follows so that the
sedimentary pile deposited on the
margins consist of pre-, syn- and postrift packages.