Continental Extension

1
Continental Extension
GEOL466 - Lect. 22

• Lithospheric extension is responsible for the
  formation of a large number of geological
  structures
• 1. Intra-continental rifts (East African Rift,
  Rhine Graben Rift, Baikal Rift)
• 2. Passive continental margins (Atlantic margins,
  most Australian margins, the Red Sea)
  intra-continental rifts can actually be viewed as
  the initial stage of a passive margin, although
  not all continental rifts lead to continental
  break up and to the formation of passive
  continental margins (it is an important question
  to find out which ones do and why)
• 3. "Diffuse" rifts, or wide areas of diffuse
  extension like the Basin and Range Province in
  the US, or the Tibetan Plateau
• 4. Extension zones in strike-slip environments
  ("Pull-Apart basins" such as the Dead Sea Rift,
  Death Valley)
• 5. Rifting in zones of net compression
  (Mediterranean, Papua New Guinea/Solomon Sea,
  Tibet)

2
East African Rift
GEOL466 - Lect. 22
Mt Kilimandjaro
3
East African Rift
GEOL466 - Lect. 22
4
East African Rift
GEOL466 - Lect. 22
5
The Baïkal Rift
GEOL466 - Lect. 22
6
The Baïkal Rift
GEOL466 - Lect. 22
7
The Baïkal Rift
GEOL466 - Lect. 22
8
The Baïkal Rift
GEOL466 - Lect. 22
9
Continental Extension
GEOL466 - Lect. 22

• Extension of continental lithosphere has produced
  a large variety of tectonic structures.
• Current research focuses on the controls of the
  geometry of lithospheric deformation during
  extension.
• These controls include
• temperature
• rheology
• rate of rifting
• pre-rifting history.

10
Continental Rifts
GEOL466 - Lect. 22
Causes of extensional tectonics and the
structures that form as a result of the shear and
strain exerted on the continental plates. It
also considers how the geotherms and strength of
a continental plate will influence the structures
and what the characteristics of these structures
are in the upper crust. Tectonic extension is
the process where zones within lithospheric
plates are placed in tension and
'stretched'. The initation of this stretching is
a result of many factors a) Lithospheric
erosion from mantle convection or upwelling of
mantle in the asthenosphere (Active Rifting) b)
Tensile forces acting on the plate as a result of
subduction, mantle convection or mantle plumes
(Passive Rifting) c) Lithospheric weight can no
longer be supported causing a release of material
into the lithosphere (Gravitional Collapse)
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Continental Rifts
GEOL466 - Lect. 22
a) Active Rifting b) Passive
Rifting c) Gravitional Collapse
12
Continental Rifts
GEOL466 - Lect. 22
Depending upon the stress and strain direction,
the plate strength and
respective geotherm different features and
structures will result. Active rifting extends
beyond the rift margins resulting in wide rift
model, such as the
Mckenzie (Top), and Wermicke model (Bottom),
diffuse rifts.
13
Continental Rifts
GEOL466 - Lect. 22
Gravitional collapse is responsible for the
formation of core complexs.
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Continental Rifts
GEOL466 - Lect. 22
Passive Rifting
will remain confined to the rift margins creating
narrow rift structures, such as basins.
All of these structures will
have characteristic features, such as,
characteristic faults and fractures, thermal
activity, volcanic activity and basins
15
Continental Rifts
GEOL466 - Lect. 22
Subsidence/Trench rollback
This process is due to the cooling of
lithosphere over time.
During this process, lithosphere will
condense, resulting in an increase of density and
reduce in thickness (assuming no additional
materials are stacked above it).
But in nature, materials are
constantly being deposited.This increases
lithospheric weight and because of it's own
density increase, it will slowly sink through
time. Which in turn
will initiate either a subduction zone (slab pull
effect for oceanic lithosphere) or thermal
gradient increase due to increase in thickness.
Both cases (thermal
increase and subduction zone) would initiate
forces that will exceed and weaken the
lithosphere, thus initiating extensions.
As in the slab pull
situation, trench rollback will result in the
formation of back arc basins.
16
Continental Rifts
GEOL466 - Lect. 22
Strength The
strength of lithosphere is closely related to
its a) Geotherm
b) Crustal thickness
c) Rate of
deformation d)
Shearing models e)
Strain conditions
Lithospheric strength is shown to be primarily
controlled by lithospheric rheology and as a
consequences is critically dependent on
geothermal gradient and
lithospheric composition. The rheologies of
the upper crust, lower crust and mantle are
assumed to be controlled by dislocation creep in
quartz, plagioclase and
olivine respectively.
Using these characteristic materials of
quartz, plagioclase and olivine, it is possible
to model to a close degree of accuracy the
strength of lithosphere.
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Continental Rifts
GEOL466 - Lect. 22
Geotherm The
strength of lithosphere is critically controlled
by the crustal thickness since the
quartzo-feldspathic rheology of the crust is
weaker than the olivine rheology of the
mantle.
A decrease in crustal thickness will
increases the strength of the lithosphere.
However, lithospheric extension also increases
the geothermal gradient, serving to
weaken the lithosphere at the same
instance. Therefore the rate of extension is
also critical in determining which of these
processes predominates.
In general, due to properties of materials
conducting heat, a high geothermal gradient will
favour a shallower detachment horizons and the
reverse is true.
Whenever the temperature of lithosphere
increases, it serves to decrease it's strength by
reducing it's binding force. This enables
extension to occur much readily.
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Continental Rifts
GEOL466 - Lect. 22
Crustal thickness
In any lithospheric deformation, thickness of the
lithosphere would significantly affect how it
would deform.
Logical assumption would consider a thick
lithospheric crust to be stronger than a thinner
section. But due to principles of heat
conservation, a thicker crust would
retain more heat than a thinner
crust. This resulting in a weakening effect on
the crust, which would assist in the speeding up
of the extension, at a constant extension
force.
Therefore without knowing the rate of
extension it is difficult to determine which
process would dominate.
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Continental Rifts
GEOL466 - Lect. 22
Rate of deformation
The rate at which a lithospheric crust undergoes
an extension can counteract the physical
dimension of a lithosphere.
If the rate at which the lithosphere is
extending overcomes the strengthening effect of
the lithosphere due to cooling, than it would
continue to break up. This is also
true in reverse when the
strengthening effect overcomes those extension
tectonic forces.
20
Continental Rifts
GEOL466 - Lect. 22
Shearing models
There are two models of extension that involve
shear. The pure shear
model as proposed by
McKenzie and the simple shear model ,as outlined
by Wernicke.
Pure shear involves no solid
body rotation. The
deformation to the lithosphere is homogeneous.
The lithosphere
responds to extensional stress by
deforming as a continuum rather than
by faulting. This is
a simple model of extension and works best
for a regional scale, but
breaks down at a local
scale.
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Continental Rifts
GEOL466 - Lect. 22
Simple shear is a combination of shearing and
solid body rotation. Simple shear involves the
extension of the lithosphere at different lateral
positions, this gives
heterogeneous lithospheric deformation. This
model of extension emphasizes the role of brittle
deformation, using high angle (gt60 º) and low
angle (lt30º) faults.
The composition of the lithosphere and the
geotherm will determine the style of deformation
and the extensional features that occur with
shearing. For example, a
strong lower lithosphere will produce
different extensional features than a weak lower
lithosphere.
22
Continental Rifts
GEOL466 - Lect. 22
Strain conditions
There are three modes of continental extension,
the core complex mode,
the wide rift mode and
the narrow rift mode.
The different amount of strain can be seen
relative to the depth
for each mode of
extension. The core
complex mode is predicted to be
caused by rapid flow of lower crust
removes crustal
thickness variations required for the
slow strain or gravitational
collapse. Wide
rifts occur due to either slow strain or
gravitational collapse.
The narrow rift mode
occurs from runaway
thinning of the lithosphere, the extension is
always concentrated at
the weakest part of the
lithosphere. Both of these depend on crustal
thinning at a site of
local extension, which
makes it harder to continue extension at that
point causing the
strain to migrate to an
adjacent unextended area.
23
Continental Rifts
GEOL466 - Lect. 22
This is a strain diagram of microstructures, it
shows various sigmas and how they relate to
eachother in an extention situation. (ie sigma 1
gt sigma 2 gt sigma 3).
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Continental Rifts
GEOL466 - Lect. 22
Passive Margins



Passive margin is the remnant of a continental
rift wone that
succeeded in thinning continental lithsphere to
such a degree that
continental breakup occurred and organised sea
spreading began. It
is NOT inherently linked to passive rifting
processes. A
general characteristic for all the passive
margins is the absence of
an identifiable plate boundary. The margin
can be either volcanic or
non-volcanic, and formed after the onset of
sea-floor spreading
adjacent to the rifted continent. A development
of major sedimentary
basins beneath the continental shelf and slope
is possible, mostly
outboard of the hinge zone where basement rocks
deepen rapidly (see
Plate 2, next slide).
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Continental Rifts
GEOL466 - Lect. 22

Plate 2. The development of sedimentary
basin along with normal faults on the continental
shelf. Continental
rifts that are no longer active, but which have
not developed into a passive margin, can also
exhibit post-rift subsidence . e.g. the North
Sea.Examples of early
post-rift phase includes the Red Sea and the Gulf
of California.
Subsidence is gradual and slow, mainly due to
cooling of the lithosphere.
Passive margins are most widespread
manifestation of Mesozoic and Cenozoic
extensional deforamtion in continental
lithosphere. It provides a complete record of
extensional deformation
from earliest drifting. It is distinguished from
developing rifts in their combined subaerial and
submarine nature.
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Continental Rifts
GEOL466 - Lect. 22
Discrete Intracontinental Rifts
In this kind of terrain, extensional
deformation is localised in narrow zones less
than 100km wide (narrow model from Buck 1991).
Grabens and large normal faults are common,
and valley sides are of
opposite polarity due to spreading. Broad
topographic swells pleataux is a common
phenomenon, as well as high heat flow and
volcanic / geothermal
activities. Seismicity occurs in shallow level,
and magmatism is extremely variable. Crustal
thinning occurs at local level. A good example of
the terrain is the African Rift System.
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Continental Rifts
GEOL466 - Lect. 22
Diffuse Rifts Diffuse rifts are rift systems
with widespread extentions over a large area, and
is particularly favoured

in previously thickened crust. This
results in a high average topography of the
terrain. Highly

localised extension patterns maybe superimposed
on general tectonics trends. High heat flow is a

characteristics
of active rifting in this area (plate 4), as well
as widespread pre- and syn-rifting

magmatism. The resultant
landform has an extreme (0-400) asymmetric
extension, which

sometimes results in a metamorphic core complex
(plate 5). However, the mechanisms of the

formation of this type
of terrain remains hotly debatable.

Plate 4. Heat flow
distribution in BRP.
Plate 3. Aerial view of the parallel ridges from
the Basin and Range
Province, US.
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Continental Rifts
GEOL466 - Lect. 22
Plate 4. Heat flow distribution in BRP.
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Continental Rifts
GEOL466 - Lect. 22
Plate 5. Metamorphic core complex in BRP.
Examples of this type of terrain includes the
Basin and range Province (BRP) in western US, the
Tibetan plateau rift system and some backarc
systems within oceanic crust, e.g.
Lau Basin.
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Continental Rifts
GEOL466 - Lect. 22
Compressional and extensional phases of New
Caledonia.
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Continental Rifts
GEOL466 - Lect. 22
Shearing