Download Lecture10 File

LECTURE 10 THE PLATE TECTONIC SETTING OF
STRUCTURES AND DEFORMATION
LECTURE PLAN
Arc volcanoes
1) SITES OF CRUSTAL DEFORMATION
2) STRUCTURES AT CONSTRUCTIVE PLATE BOUNDARIES
3) STRUCTURES AT DESTRUCTIVE PLATE BOUNDARIES
4) STRUCTURES AT CONSERVATIVE PLATE BOUNDARIES
intra-plate
thrusting
trench
indent-linked
strike-slip
faults
tear faults
(transfer)
trench-linked
strike-slip fault
c
c
c
back-arc basin
c
c
c
c
c
c
o
c
o
o
c
continental
crust
arc
o
o
o
o
boundary
strike-slip
fault
continentcontinent
collision
1) SITES OF CRUSTAL DEFORMATION
o
c
ridge transform
ridge
oceanic crust
o = oceanic crust and lithosphere
c = continental crust and lithosphere
Global seismicity is generally confined to very thin zones
known as plate boundaries. The seismicity tells us where
deformation and the formation of new structures is occurring
today. The zones of seismicity occur within four main areas:-
Back to
text
a) the centres of oceans (ocean ridges) marked by shallow
earthquakes down to around 65km. Vulcanicity produces new
crustal material which forms "new magnetic stripes".
b) between convergent plates such as:- areas of continental collision which have distributed
seismicity with shallow earthquakes from detachment faults
and deep earthquakes from basement culminations.
- subduction zones. Deep earthquakes of over 0-600km. Zone
of earthquakes defines a zone known as a Benioff Zone.
Course Homepage
Lecture
1
2
3
4
Contact Staff
5
6
7
8
9 10
Practical 1 2 3 4 5 6 71 8 9 10
c) s ome s eis micity occurs within continental areas in the
world's intra-continental rifts . S hallow earthquakes (<15-20 km)
and s ome volcanic activity is common.
We know that tectonic activity has not res ulted in intens e
deformation of the s table areas as thes e areas contain longlived geological features which have maintained their s hape
s ince Trias s ic times .
- T he link between s eis micity and pres ent-day tectonic activity
s ugges ts that s eis mic zones mus t repres ent the boundaries to
the s table, s lowly-deforming areas . T he areas bounded by the
zones of s eis micity and vulcanicity are plates . T he zones of
s eis micity and vulcanicity are termed plate boundaries .
Pacific
Plate
So
uth
Cocos Plate
East
African
Rift
Somali
Plate
Nazca
Plate
South
American
Plate
T
eas
t In
Arab
ian
Plate
African
Plate
Tre
nch
Indian-Austalian
Plate
dian
hw
ut
So
dia
t In
es
n
Antarctic Plate
Spreading centre with divergence
directions
Subduction zone with
convergence directions
Schematic
earthquake
epicentres
Transform fault
This is a cartoon and is a guide to approximate positions of earthquakes. All intraplate areas occasionally experience earthquakes and
we have shown a few schematically. Refer to specialised web-sites to get accurate information on earthquake locations. For example:
http://wwwneic.cr.usgs.gov/neis/
Back to
text
Eurasian Plate
Aleutian Trench
Pacific
Plate
Philippine
Plate
va
So
uth
Tre
nc
h
African
Plate
Cocos Plate
Nazca
Plate
Indian-Austalian Plate
eas
t In
Anatolian
Plate
Caribbean
Plate
Gorda
Plate
Marianas Trench
Eurasian
Plate
North
American
Plate
Juan de
Fuca Plate
Ja
b) T he magnetic s tripes in the oceans have maintained their
s hape for upto 180Ma.
Philippine
Plate
Anatolian
Plate
Caribbean
Plate
Gorda
Plate
Marianas Trench
Mid dian
-In
Examples:a) T he oppos ing coas tlines of Africa and the America s till fit
together after 180Ma and 4000km of drift, s o that the interiors
of the continents mus t als o be undeformed.
Juan de
Fuca Plate
va
Eurasian Plate
North
American
Plate
Aleutian Trench
Ja
Away from thes e zones , s table areas (cratons ) exis t with
s pars e tectonic activity.
Eurasian Plate
Mid dian
-In
d) along cons ervative plate boundaries s uch as ,
- oceanic trans form faults ; thes e are really part of the midocean ridge s ys tem.
- continental s trike-s lip faults at plate boundaries or within
plates . T hes e are marked by s hallow earthquakes which may
include normal, revers e and s trike-s lip focal mechanis ms .
Back to
text
East
African
Rift
South
American
Plate
Arab
ian
Plate
Somali
Plate
T
hw
ut
So
dian
Antarctic Plate
Subduction zone with
convergence directions
Spreading centre with divergence
directions
Start of this Lecture
Transform fault
dia
t In
es
n
Eurasian Plate
Juan de
Fuca Plate
Philippine
Plate
Pacific
Plate
Cocos Plate
Ja
East
African
Rift
Somali
Plate
Nazca
Plate
South
American
Plate
T
eas
t In
Arab
ian
Plate
African
Plate
Tre
nch
Indian-Austalian
Plate
So
uth
Anatolian
Plate
Caribbean
Plate
Gorda
Plate
Marianas Trench
va
Eurasian Plate
North
American
Plate
Aleutian Trench
dian
hw
ut
So
dia
t In
es
n
Antarctic Plate
Spreading centre with divergence
directions
Subduction zone with
convergence directions
Schematic
earthquake
epicentres
Transform fault
This is a cartoon and is a guide to approximate positions of earthquakes. All intraplate areas occasionally experience earthquakes and
we have shown a few schematically. Refer to specialised web-sites to get accurate information on earthquake locations. For example:
http://wwwneic.cr.usgs.gov/neis/
Back to
text
Eurasian Plate
Aleutian Trench
Pacific
Plate
Philippine
Plate
Ja
va
So
uth
Tre
nch
Indian-Austalian Plate
eas
t In
Anatolian
Plate
Caribbean
Plate
Gorda
Plate
Marianas Trench
Eurasian
Plate
North
American
Plate
Juan de
Fuca Plate
Mid dian
-In
However, the edges of the continents are not the edges of the
Earth’s Plates. The global distribution of earthquakes shows
where the the crust is being deformed. Areas of rapid
deformation are marked by concentrations of earthquakes,
which reveal the edges of the plates. Thus, the mid-ocean
ridges are plate boundaries, as are the deep ocean trenches
around the Pacific. The areas between the zones of
earthquakes are rigid, lacking deformation related to
earthquakes. These are the Earth’s tectonic plates. For
example, the African plate is composed of African continental
crust plus ocean crust from the eastern Atlantic and western
Indian Oceans.
Back to
text
Mid dian
-In
Surface features: Maps showing global topography of the
Earth show that elevations fall into two main areas (see next
page). The oceans are generally low in elevation whilst the
continents are higher in elevation. The reason for this is that the
crust has a different composition and thickness in these 2 areas.
The ocean crust is basic in composition.This ocean crust is quite
thin, ranging between 5-15 km in thickness. The continental
crust is thicker, with a normal thickness of 35 km, but having
greater thickness in mountain ranges (e.g. 70 km under the
Tibetan Plateau). It is composed predominantly of the minerals
quartz and feldspar, so that it is said to be acid in composition.
Acid crust is less dense than basic crust. The lower density and
greater thickness of continental crust, and the fact that it is
floating on the mantle, means the continental crust floats higher
than ocean crust, explaining the higher topography of the
former. (Note, as an aside, the high, and smooth topography of
Greenland and Antarctica which are to due to the presence of
thick icesheets).
African
Plate
Cocos Plate
Nazca
Plate
East
African
Rift
South
American
Plate
Arab
ian
Plate
Somali
Plate
T
hw
ut
So
dian
dia
t In
es
n
Antarctic Plate
Subduction zone with
convergence directions
Spreading centre with divergence
directions
Transform fault
Start of this Lecture
Uniformitarianism suggests that the styles of structures that
characterise modern plate margins may also characterise
ancient plate margins. Thus, by comparison, we may be able
to characterise the ancient tectonic setting of areas by
examining their structures. This is one of the major goals of
structural geology.
Plate
boundary
Constructive plate margins:
Mid-Ocean Ridges
Pillow lavas
with overlying
ocean sediments
Sheeted
Dolerite dykes
Magma
chamber
Gabbros
Cumulates
Moho
Mantle
Peridotite
er
e
Mantle
Peridotite
Lit
ho
sp
h
Asthenosphere
So we will now examine the structures found at modern-day
plate margins and compare them with ancient examples.
Back to
text
2) CONSTRUCTIVE PLATE BOUNDARIESDead Sea
Transform
Plates are moving apart. Called constructive as new material in
the form of volcanic and igneous rocks infiltrate the boundary
to fill the gap and form new crust. 2 main types:-
Red Sea
ARABIA
a) Mid-Ocean Ridges exist within ocean crust. Basalt dykes,
gabbro intrusions and ocean floor basalt pillow lavas.
AFRICA
Bathymetry of a mid-ocean ridge
Gulf of
Aden
Modern examples- Mid-Atlantic Ridge and the Iceland Hot
Spot. Features include normal faulting and fissuring (dykes),
seismicity, high heat flow (magma chambers) and basaltic lava
flows. The Carlsberg Ridge in the Indian Ocean.
Owen
Transform
INDIA
The Carlsberg Ridge is a mid-ocean ridge
(submarine mountains) which has developed
during the break-up of Gondwanaland, the
southern portion of the super continent
Pangaea. India has moved away from Africa
and Madagascar due to ocean spreading on
the Carlsberg Ridge. The ridge is situated
about half-way between India and Madagascar.
The NW end of the spreading centre is the
Owen Transform Fault which has allowed India
to move c. NE relative to Arabia. This transform
now appears to have been abandoned with the
Carlsberg Ridge propagating west to open the
Gulf of Aden. Arabia has rifted away from Africa
and the extending Red Sea and right-lateral
(dextral) Dead Sea Transform have formed in
response.
Carlsberg
Ridge
Madagascar
Ancient example- Now seen in obducted ophiolites such as the
Lizard Complex or the Semail Ophiolite of the Oman.
Start of this Lecture
Back to
text
Ocean crust is everywhere less than c. 180 Ma due to subduction of older ocean crust.
Gravity
The strike of the ridge can be oblique to the spreading direction. Transform faults form to accommodate these
motions. Plate motion vectors are parallel to transform faults which form small circles about the Euler poles of
rotation of each plate (see earlier lectures).
Oceanic Fracture Zones (Oceanic Transforms)
• offset of the ridge by strike-slip fault movement
• faults penetrate the Moho
• typical of slow-spreading ridges
They form to accommodate lateral variations in
spreading rates.
The East Pacific Rise is an example of a fastspreading centre (upto 80mm/yr). It shows
the typical axial uplift morphology (sonar data).
The Mid-Atlantic Ridge is a slow-spreading centre (<22 mm/yr). It has the typical axial depression (rift) morphology.
Overlapping Spreading Centres (OSCs)
• the geometry of OSCs is unstable, one rift tip will eventually
link with the other …
A.
B.
C.
D.
overlap basin
The old OSC is preserved as a discontinuity in the crust
Hydrothermal Activity
Ocean waters circulate through the oceanic crust due to convection and
become enriched in sulphides and causing metasomatisation of the
ocean crust (serpentinites form). The sulphide enriched waters emerge
as black-smokers.
Vents at Black Smokers on Earth have fauna that derive their energy
from the hydrothermal activity rather than the sun. NASA believes that
such fauna may have developed on other planets/moons where ice blots out
the sun and where large gravitational forces cause tidal heating and volcanism, e.g. on Europa, where they intend to drill through ice to access underlying oceans.
b) Continental rift-zones are incipient constructive boundaries
which are the first stage of continental break-up. They have
varied vulcanicity because rising magmas interact with
continental crust of widely varying composition.
Modern example- Afro-Arabian Rift System. The Gulf of Aden
has been opening over the last 20Ma along a continuation of
the Carlsberg Ridge spreading axis. Gulf of Suez lies at the
NW end of the Red Sea. Tilted fault blocks occur associated
with large normal faults and sedimentary basins. Rifts may be
associated with out-pourings of tholeitic basalts and dyke
swarms situated above "mantle hot spots" e.g. East African
Rift.
An ancient exampleNorth Sea Basin. Periods of rifting
occurred from the Devonian to the Cretaceous. Doming and
uplift in mid-Jurassic times was related to high heat flows and
a volcanic centre. Note:- thinned continental crust
characterises the area.
The structures of continental rifts may become stranded on the
opposing sides of a new ocean. These continent-ocean
junctions are no longer areas of seismicity and therefore
cannot be plate margins. They are known as passive
continental margins.
Modern example- The Atlantic coastlines. Contain tilted faultblocks bound by normal faults.
Ancient examples- These are commonly found on the
telescoped margins of former oceans which are now preserved
in continent-continent collision zones. The former passive
margin on the NW side of Tethys now lies within the
telescoped Western Alps.
Start of this Lecture
The East African Rift System (shown dashed)
is an incipient plate boundary that may
eventually develop into a mid-ocean ridge
between the NE horn of Africa and the rest of
the continent. At present, rifting is extending
and thinning the African Continent along a
series of linear extensional fault systems. The
Lake Malawi Rift is one of a number of rift
segments composed of normal faults.
Gulf of
Suez
Rift
Red Sea
Lava plateau
post-dating
rifting
ARABIA
East African
Rift System
Bathymetry of a mid-ocean ridge
Gulf of
Aden
Owen
Transform
INDIA
Carlsberg
Ridge
Lake
Malawi
Madagascar
Back to
text
There are two models for the structure of rifts (and
sedimentary basins in general).
1) the pure shear model was proposed by McKenzie (1978),
and involves brittle failure of the upper crust and ductile
stretching of the lower crust and lithospheric upper mantle.
The rift takes on a symmetrical geometry. The amount
of stretching in the lithosphere is called the β factor. β factors
of 1.5 - 2.0 are computed for several aulacogens and basins,
e.g. Central Graben of the North Sea.
2) the simple shear model proposed by Wernicke (1985) who
considered that the whole of the lithosphere is stretched by
simple shear along low-angle detachment faults. This model
was applied to the Basin and Range Province (W USA) and to
the East African Rift system.
Symmetical McKenzie Style Extension
Thinning in the green and red layers (lower
crust and lithospheric mantle) occurs through
distributed flow without large shear zones. The
deformation is bulk pure shear. Thinning in the
upper crust is through normal faulting.
Brittle-ductile transition
Upwelling
asthenosphere
Back to
text
Lithosphere-asthenosphere
boundary
Asymmetical Wernicke Style Extension
Back to
text
Brittle upper crust
65km
Crust
35km
Mantle
Half-graben complex
Brittle-ductile transition
Ductile lower crust
Back to
text
MOHO
(a)
Mantle
Flow
(a)
(b)
Flow
(b)
Sediments
Crust-mantle boundary
(a) Venning Meinesz's hypothesis for the formation of a
rift valley. (b) isostatic equilibrium of a wedge and a
parallel sided block of the same thickness.
(C)
Development of a rift valley by faulting in the
upper crust and ductile flow in the lower crust.
Detachment fault
Upwelling
asthenosphere
Lithosphere-asthenosphere
boundary
Start of this Lecture
180°
150°
120°
90°
60°
30°
0°
30°
60°
90°
120°
150°
180°
Back to
overview
160-190
60°
Back to
overview
30°
0-21 0°
30°
60°
The Basin and Range is characterised by normal faulting focal mechanisms
The age of the ocean basins (in millions of years)
0-5
5-21
21-38
PleistocenePliocene
38-52
Eocene
Miocene
52-65
Paleocene
Oligocene
65-140
Cretaceous
140-160
Early Jurassic
The Basin and Range extensional province lies inboard of the San Andreas fault which is a transform
fault. GPS data show rates and directions of extension that indicate that extension is due to the drag
of the Pacific Plate as it passes to the NW past North
America. Rifting is induced by lateral forces from
the transform. The northward continuation of the
East Pacific Rise lies beneath the Basin and Range (a
subducted ridge). Perhaps the heatflow was enough
to weaken the continental lithosphere facilitating
the distributed extension.
(see Bennett et al. 2003)
Back to
overview
Figure 2. GPS velocities of sites in the Sierra Nevada – Great Valley microplate, northern Basin and
Range, and Colorado Plateau regions with respect to North America. Error ellipses represent the 95%
confidence level. Deformation is evident as far east as the WFZ (Figure 1a). Also shown are selected
Quaternary faults (broken black lines). The green square shows the average geographic location of sites
on the Colorado Plateau and represents the origin of the horizontal axes in Figure 4.
The Basin and Range - Shaded digital elevation model from the Space Shuttle (SRTM) data.
Back to
overview
Back to
overview
The c.N-S ridges are active normal faults that are spaced tens of kilometres apart.
Extension is distributed across 600-800 km of the North American Continent.
Active volcanism is relatively minor compared to East Africa.
180°
150°
120°
90°
60°
30°
0°
30°
60°
90°
120°
150°
180°
Back to
text
160-190
60°
30°
0-21 0°
30°
60°
The age of the ocean basins (in millions of years)
0-5
5-21
21-38
PleistocenePliocene
38-52
Eocene
Miocene
52-65
Paleocene
Oligocene
65-140
Cretaceous
140-160
Early Jurassic
The East African Rift System lies where the
Carlsberg Ridge spreading centre comes onshore. It has therefore formed due to upwelling mantle at a constructive plate boundary. It
has caused continental break-up as Arabia is
now a separate plate. Extensive volcanism accompanies the rifting due to upwelling
mantle.
East African Rift System
Back to
overview
The Afar regions lies at the junction of the Gulf of Aden, the Red
Sea, and the East African Rift system. The Gulf of Aden appears to
have propagated into this region to form the triple junction. Note
how localised the rift systems are (especially individual branches
of the East African Rift. The rifting is accompanied by extension
volcanism. The high resolution of this digital elevation model
allows one to see individual rift faults in the coastal region of Djibouti. See next page for more detail.
Offshore, the rift is extremely localised (<40 km across), whilst onshore tens of small-scale faults are spaced only a few kilometres (<10
km) apart. (From Manighetti et al. 1997). The rift has propagated onshore during its evolution, rifting continental crust.
3) DESTRUCTIVE PLATE BOUNDARIESPlates are converging. Termed destructive as crust is
subducted reducing the surface area of the plate. 3 main
types:a) Ocean-Ocean Subduction. A section of oceanic crust
intervenes between the subduction zone and the nearest
continent. Composed of:- a partially submerged volcanic
mountain chain 50-100km wide which a deep ocean trench
around 50 to 250km away on the convex side of the arc.
A
VO
TRE
N
LC
CH
AN
OES
BACK ARC BASIN
Features include:i) Oceanic crust with a cover of pelagic sedimentary rocks
descending into a deep oceanic trench.
RC
JAPAN
FOREARC
BASIN
ii) Accretionary complex or accretionary prism which is offscraped sedimentary cover of the subducted crust which has
compressional thrusts and folds.
iii) Fore-arc basin formed between the topographic highs of the
growing pile of off-scraped sediment in the accretionary
complex and the magmatic arc.
Back to
text
iv) The magmatic arc where magmas derived from the
subducted slab travel upwards to form volcanic islands.
+
+
v) Back-arc basin where subsidence and sedimentation are
controlled by extensional faults which form due to the
gravitational effects of the subducted slab.
+
+
+
+
Extensional
back-arc
basin
+
+
+
+
C o n+ t i n +e n t a l
c ru s t +
+
Trench
Sea-level
+
+
+
+
Volcanic
Arc
Fore-arc basin overlying
off-scraped ocean sediment
of the accretionary complex
+
extension
+
Back to
text
Start of this Lecture
B
The scale of subuction zones (Stern 2002)
Figure 1.Subduction zones and convergent plate margins. (a) Location of convergent plate margins onE
arth (modified afterLallemand[1999]). Active back arc basins are also shown. (b) Schematic section throughth
e upper 140 km of a subduction zone, showing the principal crustal and upper mantle components and theirint
eractions. Note that the location of the“mantle wedge”(unlabeled) is that part of the mantle beneath theoverri
ding plate and between the trench and the most distal part of the arc where subduction-related igneousorfluid
activity is found. MF stands for magmatic front. (c) Schematic section through the center of the Earth,which sh
ows better the scale of subduction zones. Subducted lithosphere is shown both penetrating the 660 kmdisconti
nuity (right) and stagnating above the discontinuity (left). A mantle plume is shown ascending fromthe site o
f an ancient subducted slab. Dashed box shows the approximate dimensions of the shallow subductionzone of
Figure 1b.
The effects of the age of subducted material (Sterns 2002).
Figure 4. End-member types of subduction zones, based on the age of lithosphere being subducted
(modified afterUyeda and Kanamori
[1979]).
The fate of subducted slabs of oceanic crust is largely to
dehydrate and melt, and leave a residue of very dense eclogite
(metamorphosed basalt) and harzburgite (oceanic lithospheric
mantle), which slowly absorbs heat from the surrounding
asthenosphere and heats up until equilibrium with the
surrounding mantle has been achieved. Most earth scientists
think that this occurs above the 670km discontinuity, and that
slabs do not normally penetrate through this level. However,
seismic tomography has shown that in some instances, the
cold slab can be imaged passing through the discontinuity into
the lower mantle.
This debate has profound implications for the nature of
convection in the mantle. Many geochemists consider that
convection is of a "two-layered" type, with the regions above
and below the 670km discontinuity convecting separately. The
only material which passes between the two is considered to
be (in an upwards direction) heat and primordial helium gas,
associated with very large mantle plumes such as Hawaii. The
return flow may be via these very deep subduction zones and
may enrich the lower mantle in an unusual mixture of strongly
depleted mantle and eclogitic-facies basic rocks and even
sediments.
Reading
Kearey, P. and Vine, F. Global Tectonics. Chapter 8.
Wilson, M. Igneous Petrogenesis, Chapters 6 and 7.
Doglioni, C. et al. 1999. Orogens and slabs versus their
direction of subduction. Earth Science Reviews, 45, 167-208.
Start of this Lecture
660km
Enhanced seismic tomographic imaging of a descending slab apparently deflected at the
660km discontinuity. A cooler descending slab (darker colours) approaching the 660km
discontinuity would transform to a more dense structure through phase transformations later
(lower) than the surrounding (hotter) material. Thus at 660km, less dense (untransformed slab
material) is adjacent to denser (transformed) material. The result is buoyancy in the slab and
thus a resistance to penetration of the slab through 660km, which appears to result in a
deflection of the slab on some seismic tomographic sections.
Back to
text
660km
Enhanced seismic tomographic image showing a subducting slab
penetrating the 660 km discontinuity.
Back to
text
Figure 2.Structure of subducted slabs as inferred from mantle tomography (fromKa´rason and van der Hilst[2000]). Red lines show the surface
projection of each section. The base of each section is the core-mantleboundary (CMB); dashed lines show the location of mantle discontinuitie
s at 410, 660, and 1700 km. Red andblue colors in each section denote regions where thePwave velocities are relatively slow and fast, respectively,
compared to average mantle at the same depth. Fast regions are most easily interpreted as relatively cool areascorresponding to the subducted sla
b and its viscous mantle blanket. This allows the cool, subducted slab to betraced well below the deepest earthquake. Note that some slabs pe
netrate the 660 km discontinuity anddescend into the lower mantle (e.g., Central America, central Japan, and Indonesia), while other slabs see
mto stagnate in the upper mantle (such as Izu-Bonin). The subducted slab beneath Tonga seems to stagnate atthe 660 km discontinuity for a w
hile, then cascade into the lower mantle.
(Sterns 2002)
JAMAICA
El Pilar Fault
C rawl
e Riv
er
TRINIDAD
Caval iers
P l a n ta
South Coast Fault
VENEZUELA
i n F a u lt
Yallahs
Basin
(active)
Or i e n t e
transform fault
Transform
Hispaniola
Fold
Thrust
Lesser
Antilles
Subduction
Zone
Puerto Rico
Transform
Jamaica
Swan
Mid-Caymen
spreading centre
Back-arc
extensional
basin
r A n till e s V o l c a n o
CARIBBEAN SEA
Central America
Strike-slip
fault
100 km
Strike-slip fault
Earthquake Epicentre
Thrust
Normal fault
transform fault
Trinidad
Venezuela
PACIFIC OCEAN
Ancient examples are often complexly deformed, have a low
preservation potential and are difficult to recognise. Some
possible examples have been found along the suture zones in
the Himalayas (see below).
TOBAGO
MARGARITA
40 km
Duanv ale
Faul
R io Minho-
se
Les
Modern example- The Lesser Antilles Subduction Zone where
the Atlantic ocean crust passes beneath the small Caribbean
Plate. The Barbados ridge is the accretionary complex
associated with the Lesser Antilles Volcanic Arc and the
Grenada back-arc basin. The subduction zone passes into c.
east-west oriented strike-slip transform faults in the north and
south. Areas close to the transform faults experience strike-slip
deformation.
Back to
text
Motion of North
American Plate
San
Andreas
Fault
Caribbean
Plate
b) Certain subduction zones border a continent
Sub
duc
ti
on
Modern example- Peru Trench associated with the South
American Cordillera exposed within the Andes Mountains, west
side of south America). Volcanic belt situated within the
continent such as in the Andean Granites and volcanoes.
Motion of
Pacific Plate
Motion of Northern
part of Nazca Plate
Back to
text
Ancient example- Southern Uplands Accretionary Prism, U.K.,
is composed of imbricated Lower Palaeozoic Rocks such as
the Moffat Shales. The Lake District is probably an ancient
magmatic Arc.
South American
Plate
Pacific/Nasca
Plate
Start of this Lecture
Back to
text
200 km
42
F
NA
40
38
36
Back to
text
10
12
14
16
18
Neogene Oceanic Crust
Ocean-Continent Subduction
Tethyan Oceanic Crust
Continent-Continent Collision
Approximate limits of thinned or thinning continental crust.
Crustal/lithospheric thickness contours and pre-existing
thrusts are sub-parallel to the limit line.
Continental crust
below sea-level
Motion of African Plate
Transform
Fault
Areas of active
extensional
faulting
Motion of Anatolian Plate
The surface area of the African Plate is being reduced by subduction beneath Eurasia along the
Hellenic and Calabrian Subduction zones. Extension occurs in the over-riding plate.
Back to
text
Back to
text
c) Continental Collision Zones are the result of continued plate
convergence after the subduction of all the oceanic crust which
exists between two continents. This results in the deformation
and shortening of the continents involved. Imbrication of the
two continents involved leads to the development of thickened
crust which can achieve thicknesses of 50-70km e.g. Tibet,
with high mountains.
Lake
Baikal
Tibetan
Plateau
ARABIA
INDIA
Owen
Transform
Modern example- Central Asian Collision Zone which form the
Himalayan Mountain Chain.
Collision between Indian and Eurasian Plates occurred from
the Eocene to Recent. Pre-collision velocities of 1018cm/year existed with movement towards the NNE. At 38Ma
the rate slowed to 5cm/year and rotated towards the north.
This probably indicates the initiation of collision of the two
continents at 38Ma. 1500km of convergence has taken place
since then indicated by the ocean floor magnetic stripes.
Carlsberg
Ridge
Back to
text
Northward motion of India after collision has thickened the crust and produced the high topography
of the Himalayas and the Tibetan Plateau. Several lineaments in the topography demarcate strikeslip faults that transfer motion towards the subduction zones bordering the western Pacific Ocean.
Collision may have produced the extension around Lake Baikal.
Evolution of the Himalayas
Tethys
India
+
+ + +
+
+
+ +
+
+
N. Tibet
+ + + +
+ + + ++
S. Tibet
+
++ +
+ +
+
c. 140 Ma
Tibetan Plateau
Tethys
Plate Boundaries include:-
+ + + +
+
+
+
+ ++
+
+
c. 100 Ma
Back to
text
+
Tibetan Plateau
+ +
+ + + +
+ +
+ +
+
+
Imbrication
of India
(a)End-Jurassic
(c) Mid-Cretacous (Turonian)
(b) End-Cretaceous-Palaeogene
30 N
10 S
10 N
ARABIA
ARABIA
20 S
GRE
AT
E
ARABIA
OB
RI
ND
IA
0
30 S
10 S
AFRICA
M
AFRICA
SB
S
0
of
Bay
MB
30
S
Somali
Basin
10 S
MAD
MAD.
MB
ANTARCTICA
40
S
S
al
Beng
S
MB
60
CR
20 S
INDIA
S
S
INDIA
Arabian
Sea
10 N
AFRICA
INDIA
Somali
Basin
40 S
MAD
50
M
20 N
M
?
Start of this Lecture
Ophiolites
+ + +
+
+ + + +
+
c. 38 Ma
-Recent
DFZ
a) Indus Suture is the southern boundary to the Eurasian
Plate, which marks the former position of the subduction zone
which existed between the converging plates. The ocean
between the continents was called Tethys and remnants of
Tethyan Oceanic Crust exist just south of the Indus Suture
such as basic and ultra-basic igneous complex associated with
pillow lavas and greywackes of the Kohistan Complex.
Remnants of accretionary complexes appear to be located
along these boundaries.
c ti c B a sin
A n tar
ia nInd
CR
CR
20 S
Revised plate-tectonic reconstruction for the south-east margin of Arabia and adjacent areas in the Cretaceous.
(a) At the end of the Jurassic, rifting along the eastern margin of the Arabian and African Plates formed the Owen Basin (OB), Somali Basin (SB) and
Mozambique Basin (MB). The spreading centres were oriented E-W and India and Madagascar (MAD) moved southwards relative to Arabia/Africa
along the Davie Fracture Zone (DFZ). Masirah oceanic crust (M) formed at a spreading centre in the north Somali Basin at approximately 38o S.
(b) In the mid-Cretaceous, initiation of spreading along the Proto-Carlsberg Ridge (CR), caused India to move north away from Madagascar along a series
of left-lateral transform faults.
(c) At the end-Cretaceous the Seychelles (S) micro-continent rifted from India, and the consequent anticlockwise rotation of India, combined with ongoing left-lateral strike-slip along the major transform boundary, caused the emplacement of the Masirah Ophiolite.
Back to
text
b) Indian Plate is bound to the east and west by two major
strike-slip faults, the Quetta-Chaman strike-slip fault and a
major strike-slip fault running through Burma.
Deformation of the Eurasian Plate involves thrust fault systems
and strike-slip fault systems. Local extensional basins occur.
Examples- Himalayan Frontal Thrust, the Red River Strike Slip
Fault and the Baikal Rift System.
Indentation Tectonics:- Deformation of the continental interiors
by strike-slip fault systems due to the collision of a rigid
continental indentor may occur. The strike-slip faults arising in
the Himalayas and continuing into S.E. Asia may be examples
of this.
An ancient example- of a continental collision zone would be
the Moine Thrust Belt of NW Scotland formed during closure of
the Iapetus Ocean.
Start of this Lecture
4) CONSERVATIVE BOUNDARIESMediterranean
Sea
Plates are moving horizontally past each other with a strike-slip
sense of movement. Material is neither created or destroyed
but conserved. Steep strike-slip faults zones. 2 main types:a) Transform Faults- Offset mid-ocean ridges, and therefore
separate neighbouring plates. Modern example- Discovery
Fracture Zone along the East Pacific Rise. Ancient examples
exist within obducted ophiolites.
Restraining
bend
Sea of
Galilee
b) Continental Strike-Slip FaultsExample- The Dead Sea Transform Fault
Example- San Andreas Fault System which separates the
Pacific Plate from the North American Plate. This is a strikeslip fault systems which is composed of anatomising fault
strands which form zone 100km wide zone. Parallel faults exist
such as the Imperial Valley, San Jacinto and Elsinore Faults.
Faults are associated with en echelon folds such as in the
Santa Maria Oilfields. Local bends in strike-slip faults are
associated with localised areas of thrust faulting and growing
highland topography (e.g. Near Santa Barbara) and localised
extensional "pull-apart" basins containing extensional faults
which are the site of subsiding lowland areas.
Dead Sea
Back to
text
Left-lateral motion on the Dead Sea Transform has produced subsiding areas where
extensional jogs occur (Sea of Galilee and the Dead Sea) and uplifted areas where a
restraining bend occurs in the north.
Start of this Lecture
FURTHER READING AVAILABLE
FROM THE ELECTRONIC LIBRARY
Yasuto Itoh, 2001. A Miocene pull-apart
deformation zone at the western margin of
the Japan Sea back-arc basin: implications
for
the
back-arc
opening
mode,
Tectonophysics, 334, 235-244
S. Lallemand, C. -S. Liu, J. Angelier and Y. -B.
Tsai, 2001. Active subduction and collision in
Southeast Asia, , Tectonophysics, 333, 1-7
Marc-André Gutscher, 2001. An Andean
model of interplate coupling and strain
partitioning applied to the flat subduction
zone of SW Japan (Nankai Trough),
Tectonophysics, 333, 95-109
O. Dauteuil, P. Huchon, F. Quemeneur and T.
Souriot, 2001. Propagation of an oblique
spreading centre: the western Gulf of Aden,
Tectonophysics, 333, 423-442
Christoph Gaedicke, Boris Baranov, Nokolay
Seliverstov, Dmitry Alexeiev, Nikolay
Tsukanov and Ralf Freitag 2000, Structure of
an active arc-continent collision area: the
Aleutian¯Kamchatka
junction,
Tectonophysics, 325, 63-85
Y. Rolland, A. Pêcher and C. Picard, 2000,
Middle Cretaceous back-arc formation and
arc evolution along the Asian margin: the
Shyok Suture Zone in northern Ladakh
(NW Himalaya), Tectonophysics, 325, 145173