3. Composition of the Earth
• All bodies in the solar system are believed to have been
formed by the condensation and accretion of the primitive
interstellar material from which the solar nebula was made.
• Heating of proto-Earth differentiated it into a radially
symmetric body made up of a series of shells.
• The density of the Earth increases towards the center.
• It is believed that meteorites are representatives of the
material of solar nebula.
• Thus, estimates of the Earth’s composition can be made from
the study of meteorites.
• From this it is indicated that the Earth is composed of an
iron/nickel core surrounded by light density silicate mantle
and crust.
• Seismic data, knowledge of mass, and moment of inertia of
the Earth has shown its mean atomic weight to be 27.
4. Continued: Earth’s composition
• Mantle and crust contribute 22.4% while core alone contributes
47%.
• Proportions of different meteorites can be mixed to give the above
atomic weight and core/mantle ratio.
• It is apparent that at least 90% of the Earth is made up of
Fe, Si, Mg, and O.
• The bulk of the remainder is comprised of Ca, Al, Ni, Na, and S.
The Crust
The continental Crust
• Only the upper part of the crust is available for direct sampling at
the surface or from the boreholes.
• At greater depth, all information collected is indirect.
• Some metamorphic rocks, some xenoliths, and a knowledge of
variation in the velocities of seismic waves along the depth, and
experimental determination of such velocities at various conditions
of pressure and temperature give much of the information.
5. Continued: Composition of the Earth--The Crust
• Pressure increases with depth at a rate of 30 Mpa/km .
• Temperature increases at a rate of 25 degrees C / km but
decreases to about half this value at the Moho.
• Collectively, observations from both geologic and geophysical
studies show that the continental crust is vertically stratified
in terms of its chemical composition.
• The variation in seismic velocities with depth results from a
number of factors:
-increase of pressure up to top 5km, thereafter none
-Velocities also change due to change in the chemical as well
as mineralogical composition resulting from phase changes.
• Abrupt velocity discontinuities are caused by the change in
chemical composition, while more gradational velocity
boundaries are associated with mineralogical phase changes.
6. Continued: Composition of the Earth--The Crust
• They argue on the basis of the heat flow, that average
continental crust has andesitic or granodioritic composition
with K2O <1.5%.
• This is less silicic than most previous estimates.
• Pressure increases with depth at a rate of 30 Mpa/km .
• Temperature increases at a rate of 25 degrees C / km but
decrease to about half this value at the Moho.
• Collectively, observations from both geologic and geophysical
studies show that the continental crust is vertically stratified
in terms of its chemical composition.
• The variation in seismic velocities with depth results from a
number of factors:
-increase of pressure up to top 5km, thereafter none
-Velocities also change due to change in the chemical as well
as mineralogical composition resulting from phase changes.
7. Continued: The Crust
• Abrupt velocity discontinuities are caused by the change in
chemical composition, while more gradational velocity
boundaries are associated with mineralogical phase changes.
• Models for the bulk chemical composition of the continental
crust vary widely because of the difficulty in making estimates.
• Abundance of heat-producing elements: K, U, Th.
• They argue on the basis of the heat flow, that average
continental crust has andesitic or granodioritic composition
with K2O <1.5% by weight.
• This is less silicic than most previous estimates.
8. Continued: Composition of the EarthUpper Continental Crust
• Past theories suggested granitic composition for the upper
continental crust
• Widespread occurrence of large negative gravity anomalies
over granite plutons showed that this was not the case. These
anomalies showed a little lower density for the granite
plutons.
• Experiments and observations have shown that the average
composition of the upper continental crust corresponds to a
rock type between granodiorite to diorite, and is
characterized by a relatively high concentration of heat
producing elements.
Middle and Lower Continental Crust
• Middle cont. crust is 11km thick (from 12km-23km)
• Lower cont. crust is 17km thick (from 23km-40km)
9. Continued: Composition of Earth-Lower
and Middle cont. crust.
• Thicknesses and depths may vary from setting to
setting, e.g. tectonically active rifts and rifted
margins, Mesozoic –Cenozoic orogenic belts.
• Seismic velocity range (6.8-7.7km/sec.) shows that the lower
crust is made up of denser and more mafic rocks.
• Heat producing elements also decrease with depth indicating
thereby increasing proportions of mafic lithologies.
• In areas of thin continental crust such as rifts and rifted
margins, the middle and lower crust may be composed of low
to medium grade metamorphic rocks.
• In regions of very thick crust such as under orogenic belts, the
middle and lower crust is composed of high grade
metamorphic mineral assemblages.
• The middle crust may be composed of more evolved and less
mafic compositions compared to lower crust.
• Metasedimentary rocks may be present in both layers.
10. Continued: Composition of the Earthmiddle and lower crust
• Thicknesses and depths may vary from setting to setting, e.g.
tectonically active rifts and rifted margins, Mesozoic –
Cenozoic orogenic belts.
• In the overthickened roots of orogens, parts of the lower crust
may record transition to eclogite facies where plagioclase is
unstable.
• Here, mafic rocks may transform into very dense garnet and
pyroxene bearing assemblages.
• If the lower crust is wet, the basaltic rocks would occur in the
form of amphibolites.
• Sudies of exposed ancient lower crust show that both dry and
wet rock types typically are present.
11. Continued: Composition of the EarthLower and Middle crust
• Another indicator of lower crust composition is the elastic
deformation parameter Poisson’s ratio (ratio between P and S
wave velocities for a medium).
• This varies systematically with the composition from0.20 to
0.30. low values represent felsic rocks while high mafic rocks.
• Certainly lower crust is compositionally more complex than
suggested by the above geophysical models.
• Studies of deep crustal xenoliths and crustal contaminated
magmas indicate that there are significant regional variations
in composition, age and thermal history.
• This compositional complexity is matched by a very
heterogenous structure.
• This heterogeneity reflects a wide range of processes that
create and modify the lower crust.
12. THE OCEANIC CRUST
• The oceanic crust is in isostatic equilibrium with the
continental crust according to the Airy mechanism
• Seismic refraction studies have shown that oceanic crust is
typically 6—7 km thick beneath an average water depth of
4.5km
• Thicker than normal oceanic crust exists where more than
normal supply of magma occurs due to high temperatures in
the upper mantle.
• Thinner than normal oceanic crust forms where anomalously
low temperatures occur in the upper mantle.
• Seismic refraction surveys showed the presence of three
principal layers: Table
• According to more recent studies further subdivision of these
main layers is possible.
• There appears to be a progressive increase in velocity with
depth.
13. Continued: Oceanic Crust
• Oceanic Layer 1
• Layer 1 has been extensively sampled by coring and drilling
• Seabed surface materials comprise unconsolidated sediments
carried into the deep oceans by turbidity currents
• Pelagic deposits, zeolites, calcareous and silicic oozes,
manganese nodules
• Contourites
• Layer 1 is 0.4km thick; progressively thickens away from the
ocean ridges
• Within layer 1, there are a number of horizons that show up
as prominent reflectors on seismic reflection records
• Oceanic Layer 2
• This layer varies in thickness from 1.0 to 2.5km.
• Seismic velocity: 3.4—6.2km per second
14. Continued: Oceanic Crust-layer 2
• Igneous origin
• Basalts: Olivine tholeei tes containing calcic palgioclase and
poor in Na, K
• Subdivisions of Oceanic Layer 2
• Sub-layer 2A: Only present on ocean ridges near eruptive
centers near areas of hydrothermal circulation of sea water
indicating porosity
• Sub-layer 2B: Its higher velocity 4.8-5.5km per suggests a
lower porosity
• With time 2A may be converted to 2B
• Sub-layer 2C: 1km thick; velocity 5.8—6.2km per second, may
indicate high proportion of mafic intrusive rocks
• Pillow lavas and dikes throughout 1800m.
15. OPHIOLITES
• Oceanic Layer 3
• It is the main component of the oceanic crust and represents its
plutonic foundation
• It can be subdivided into sub-layers3A and 3B.
• Velocity ranges between 6.5-6.8 for 3A and from7 to 7.7km/sec.
for lower sub-layer 3B.
• According to Hess, this layer was formed from upper mantle
material whose olivine reacted with water to form
serpentinized peridotite
• But the Poisson’s ration for layer 3A is more in accord with a
gabbroic composition.
• It is possible that part of layer 3B is formed of serpentinized
peridotite.
16. • Ophiolites: snake rock
• Ophiolites usually occur in collisional orogens.
• They are associated with deep sea sediments, basalts, gabbros and
ultramafic rocks.
• They originated as oceanic lithosphere that were later obducted upward
into the continental setting as a result of collision.
• Complete ophiolite sequence: Table
• Several evidence to prove their association with oceanic lithosphere—
rocks, metamorphic grades, similar ore minerals, seismic velocities
structure etc. Figure, Table
• Ophiolites represented lithosphere that was obducted while young and
hot.
• Geochemical evidence suggest that the original sites were back-arc
basins, Red sea type ocean basins, or fore-arc regions of subduction zones.
17. Continued: Ophiolites
• Petrology and geochemistry of the fore-arc regions support it.
• Many different mechanisms have been proposed for the
obduction of ophiolites, but none explains all cases
satisfactorily.
• Thus it must be recognized that there may be several
operative mechanisms.
• Also, ophilite sequences may differ considerably in terms of
their detailed geochemistry from lithosphere created at midocean ridge crests in the major ocean basins.
• There are indications that there are more than one type of
ophiolites.
18.
19.
20. The Mantle
Introduction
• The mantle constitutes the largest internal subdivision of the
Earth by both mass and volume.
• It extends from the Moho at a mean depth of 21km, to the
core –mantle boundary at a depth of 2891km.
• On a grass scale it is considered to be chemically
homogeneous (but for minor and trace elements) formed of
silicate minerals.
• The mineralogy and structure of the silicates change with
depth and give rise to a transition zone.
• The transition zone lies between 410 and 660km depth, which
separates the Upper and Lower Mantle.
• Mantle material is only rarely brought to the surface through
ophiolite emplacement, kimberlite pipes, and as xenoliths in
alkali basalts.
21. Continued: The Mantle-Introduction
• Consequently, most of the knowledge about mantle is indirect
based on variation of seismic velocities with depth.
• other sources of information include: studies of mineral
behavior at high temperature and pressure, shock wave
experiments, geochemical studies of meteorites and
ultramafic rocks.
Seismic Structure of the Mantle
• The uppermost part of mantle forms a high velocity lid 80160km thick.
• The seismic velocities remain more or less constant at above
7.9km/sec.
• This part of the mantle makes up the lower portion of the
lithosphere.
22. Continued: The Mantle-Seismic Structure
• Beneath the lithosphere lies the Low velocity zone extending
to a depth of 300km.
• This is present beneath most regions of the earth except
beneath cratonic areas.
• From the base of this zone, velocities begin to increase slowly
until the major discontinuity at a depth of 410 km.
• This marks the upper surface of the transition zone.
• The other velocity discontinuity occurs at a depth of 660 km
which marks the base of the transition zone.
• Within Lower Mantle velocities increase slowly with depth
until the basal 200-300km where low velocities are present.
• This lowermost layer at the core-mantle boundary is Layer D”.
• There is a thin ultra low velocity zone at the base of Layer D”.
23. Continued: The Mantle-Composition
Mantle Composition
• Much of the oceanic crust is made up of basalt derived from the
upper mantle.
• This suggests that upper mantle is composed of either peridotite
or eclogite.
• Peridotite possesses abundant olivine and less than 15% garnet,
whereas eclogite cotntains little or no olivine but at least 30%
garnet.
• Both possess a seismic velocity of about 8km/sec , which
corresponds to the upper mantle value
• Several evidence suggest very strongly that upper mantle is
peridotitic.
• Velocities over 15% higher perpendicular to ocean ridges are due
to preferred orientation of olivine crystals.
• This is not the case in minerals of eclogite.
• Other evidence: Poisson’s ratio; ophiolites; nodules in alk.Basalt.
24. Continued: The Mantle-composition
• The Bulk composition of the mantle can be estimated from:
compositions of various ultramafic rock types, geochemical
computations, various meteorite mixtures, experimental
studies.
• Undepleted mantle and depleted mantle*
• Incompatible elements
• Most of the lower mantle must be more enriched in
incompatible elements than the upper mantle.
• The lower mantle is not involved in producing melts that
reach the surface.
• Estimates of bulk mantle composition vary in detail, however,
it is generally agreed that at least 90% of the mantle by mass
can be represented in terms of the oxides FeO, MgO, and
SiO2. The rest is made up of CaO, Al2O3, and Na2O.
25. Continued: The Mantle-Low Velocity Zone
The Mantle Low Velocity Zone
• This zone is characterized by low seismic velocities, high
seismic attenuation, and a high electrical conductivity
• The effects are more pronounced for S-waves than for P
waves.
• The low seismic velocities could arise from different
mechanisms but partial melting / molten material is the most
important.
• It is at this level that mantle material is most nearly reaches its
melting point.
• Only a small amount of melt is required to lower the seismic
velocity of the mantle.
• A liquid fraction of less than 1% would do the job if
distributed along the network of fractures around grains.
26. Continued: The Mantle-low velocity zone
• The melt may also be responsible for the enhanced electrical
conductivity of this zone.
• A small quantity of water is required to lower the melting
point of silicate phases of mantle to bring about partial
melting.
• This water is supplied by the break-down of hydrous minerals.
• The existence of low velocity zone may be controlled by the
availability of water in the upper mantle.
• The mantle low velocity zone is of major importance to plate
tectonics.
The Mantle Transition Zone
• The major velocity discontinuity at a depth of 410km and at
660km marks the top and base of this zone.
27. Continued: The Mantle-Transition Zone
• The discontinuities are rarely sharp.
• They represent phase changes rather than changes in
chemistry.
• Pressure induced phase changes are considered to be
responsible for the existence of this zone.
• High pressure studies show that it is the olivine that
undergoes transformations to spinel structure at a depth
pressure of 410km and then to provskite at 660km depth.
• The other component of peridotites pyroxene and garnets
also undergoes phase changes, they are gradual and do not
cause discontinuities of seismic velocity with depth.
• Pyroxene transforms into the garnet structure at pressures
corresponding to 350-500km depth.
28. Continued: The Mantle Transition Zone
• At 580 km depth, Ca-perovskite begins to exsolve from garnet.
• At 660-750 km depth the remaining garnet dissolves in
perovskite formed from the transformation of olivine.
• Thus, the lower mantle mostly consists of perovskite
structure.
The Lower Mantle
• The lower mantle represents about 70 % of the mass of the
solid earth and 50 % mass of the entire Earth.
• There is generally a smooth increase of seismic velocity
through this layer with depth.
• This indicates a relatively homogenous mineralogy with
perovskite structure.
• More detailed studies show some heterogeneity .
29. Continued: The Lower Mantle
• The lowest 200-300 km of the mantle, the Layer D” is
characterized by a decrease in seismic velocity.
• This is due to increased temperature gradient above the coremantle boundary.
• This lower Layer shows large lateral changes in velocity.
• Ultra low velocity zone indicate presence of partially molten
material.
• Interaction between the iron from core and silicates derived
from Layer d” has been suggested.
• Layer D” is important because it controls core-mantle
interactions and also it may be the source of deep mantle
plumes.
30. THE CORE
• The core is a spheroid with a mean radius of 3480 km.
• It occurs at a depth of 2891 km. and occupies the center of the
Earth.
• The core-mantle boundary is known as Gutenberg discontinuity
that generates strong seismic reflections indicating change in
composition.
• The outer core at a depth of 2891-5150 km does not transmit Swaves. So it must be fluid.
• This is also proved by the generation of magnetic field in this
region. The velocities of the convective motions responsible for
the geomagnetic field are five times greater than the mantle
convection currents.
• The fluid state is also indicated by the response of the Earth to
the gravitational attraction of the Sun and the Moon.
• The boundary between the outer and inner core is sharp at
5150 km.
• The inner core is believed to be solid for several reasons.
31. Continued: The Core
• A particular seismic wave.
• The amplitude of a phase reflected off the inner core.
• Experiments have shown that both outer and inner core are
made up of elements with atomic number higher than 23.
• These may be Fe, Ni, V or Co.
• Of these, since only Fe is abundantly present in the Solar
system, it must have formed major part of the core.
• It is believed that core contains 4 % Ni.
• The iron-nickel mixture forms the composition of the outer core
plus a small amount of lighter elements, e.g. Si, S, O, and K.
• Oxygen may be the most likely light element as FeO is
sufficiently soluble in iron.
• The inner core has a seismic velocity and density consistent
with a composition of pure iron.
32. LITHOSPHERE AND ASTHENOSPHERE
• For large scale structures to attain isostatic equilibrium the
outermost shell must be underlain by a weak layer that deforms
by flow.
• This means the subdivisions of the Earth controlling plate
tectonics movements must be based on rheology rather than
composition.
• Rheology: It is the study of deformation and flow of materials
under the influence of an applied stress
Lithosphere
• It is defined as the strong, outermost layer of the Earth that
deforms essentially in an elastic manner.
• It is made up of the crust and uppermost mantle.
• The lithosphere is underlain by asthenosphere , which is a much
weaker layer and reacts to stress in a fluid manner.
33. Continued: Lithosphere and asthenosphere
• The lithosphere is divided into plates whose crustal component can
be oceanic or continental or both.
• The relative movements of plates take place on asthenosphere.
• These layers have properties that characterise them, such as
thermal, seismic, elastic, seimogenic and temporal.
• Temperature is the main phenomenon that controls the strength of
subsurface material.
• Melting point of rocks increases with depth as does the pressure
with depth.
• Melting will occur when the temperature curve crosses the solidus
(melting curve) for the material present at depth.
• The asthenosphere is b elieved to represent a location where
mantle most closely approaches the melting point of its material.
• This layer is certainly not molten as it transmits S waves.
34. Continued: Lithosphere and asthenosphere
• It is possible small amount of melt is present in it.
• Depth to the asthenosphere depends upon the geothermal
gradient and the melting point of the mantle material.
• It is possible small amount of melt is present in it.
• Depth to the asthenosphere depends upon the geothermal
gradient and the melting point of the mantle material.
• Beneath ocean ridges, where temperature gradients are
high, the asthenosphere occurs at shallow depth.
• The lithosphere is thin under ocean ridges.
• It is quite thick under deep ocean basins, the increase
correlating with the depth of water.
• The mean thickness of the lithosphere on this basis under
oceans is 60-70 km.
35. • Beneath the continents, a temperature gradient in the
subcrustal lithosphere is much lower than in oceanic areas.
• Thus, the continental lithosphere has a thickness of 100250 km, being at a maximum under cratonic areas.
• The depth of Low Velocity Zone for seismic waves agrees
quite well with the temperature model.
• Beneath the continents, a temperature gradient in the
subcrustal lithosphere is much lower than in oceanic areas.
• Thus, the continental lithosphere has a thickness of 100250 km, being at a maximum under cratonic areas.
• The depth of Low Velocity Zone for seismic waves agrees
quite well with the temperature model.
37. SEA FLOOR SPREADING
Introduction
• By late 1950s much evidence for continental drift had been
assembled. But the theory was not generally accepted.
• The paths by which the continents had attained their present
position were not determined.
• In order to study the kinematics of the continental drift, it was
necessary to study regions that separated the continents once
juxtaposed.
• These were the intervening ocean floors.
• Much of the information available over oceanic areas has
been provided by geophysical surveys undertaken from ship
or aircraft.
Magnetic Anomalies
• One such method involves measuring variations in the
strength of the Earth’s magnetic field.
38. Continued: Sea floor Spreading: Magnetic
Anomalies
• This is accomplished using either fluxgate, proton precession,
or optical absorption magnetometers.
• Magnetic effects of ship or aircraft.
• Accuracy of +- 1 nanotasla (nT) or 1 part per 50,000.
• Magnetic effects of ship or aircraft.
• Accuracy of +- 1 nanotasla (nT) or 1 part per 50,000.
• Magnetometers provide continuous record of the strength of
the geomagnetic field along their travel paths.
• These absolute values are corrected for externally induced
magnetic field variations.
• Resulting magnetic anomalies should then be due solely to
the contrasts in the magnetic properties of the underlying
rocks.
39. Continued: Magnetic Anomalies
• The anomalies originate due to the presence of ferromagnetic
minerals in the rocks.
• Most common among them is magnetite,
• Ultramafic and mafic rocks contain high proportion of
magnetite and thus have large magnetic anomalies.
• Metamorphic rocks are moderately magnetic.
• Acid igneous and sedimentary rocks are only weak.ly
magnetic.
• On land magnetic anomalies reflect the variable geology of
the upper continental crust.
• The oceanic crust is known to be laterally uniform.
Marine Magnetic Anomalies
• Magnetic surveys have been easily accomplished by survey
vessels since 1950s.
• A most significant magnetic anomaly map (Fig. Mason & Raff,
1961).
40. Continued: Marine Magnetic anomalies
• The map revealed a pattern of stripes defined by steep
gradients separating linear regions of high amplitude positive
and negative anomalies.
• These magnetic lineations are persistent and can be traced for
hundreds of kms.
• These are interrupted only at oceanic fracture zones where
individual anomalies are off-set laterally by a distance of up to
1100 km.
• Magnetic lineations are generally 10-20 km wide and are
characterized by peak to peak amplitude of 500-1000 nT.
• They are symmetrical about the ridge axis.
• The source of the layer cannot be in layer 1 and layer 3.
• It must be in oceanic layer 2.
41. Continued: Marine Magnetic Anomalies
• The shape of the anomaly is determined by the geometric
form of the source and the orientation of its magnetization
vector.
• Anomalies must arise because adjacent blocks in the layer 2
have been magnetized in different directions.
• Fig.3 shows interpretation of magnetic anomalies on Juan de
Fuca ridge in northeastern Pacific.
Geomagnetic Reversals
• The possibility of geomagnetic polarity reversals was first
suggested in the early part of 20th century.
• By early 1960, the concept of geomagnetic polarity reversal
was revived.
• By the end of 1960s this was widely accepted following the
work of Cox, et al.
42. Continued: Polarity reversals
• It has been estimated that a magnetic reversal occurs over a
time interval of 5000 years.
• It is accompanied by a reduction in field intensity to about
25% of its normal value.
• The total duration in which this reduction accomplishes itself
is about 10,000 years.
• The rates at which geomagnetic reversals have occurred in the
past are highly variable.
• There has been a gradual increase in the rate of reversals in
the Cenozoic following a period of 35 m.y. of constant normal
polarity in the Cretaceous.
• There has been a similar prolonged period of constant reverse
polarity in the Late carboniferous and Permian.
43. Sea floor Spreading
• In the early 1960s Dietz (1961) and Hess (1962) had proposed
that continental drift might have been accomplished by
process termed as ‘sea floor spreading’.
• The new oceanic lithosphere is created at oceanic ridges by
the upwelling and partial melting of material from the
asthenosphere.
• As the ocean grows wider, the continents marginal to the
ocean move apart.
• The Atlantic-180 m.y.
• Subduction at the same rate in another shrinking ocean.
• The driving mechanism was convection currents in the sublithospheric mantle.
• These were thought to form cells--------------.
44. The Vine Metthews Hypothesis
• It is surprising to note that maps of oceans showing magnetic
lineations had been available much before the true significance
of magnetic anomalies was realized.
• Hypothesis of Vine and Metthews (1963) combined the notion
of seafloor spreading with the phenomenon of geomagnetic
field reversals.
• According to the hypothesis new oceanic crust is created by the
solidification of magma injected and extruded at the crust of an
ocean ridge.
• On further cooling, temperature passes through Curie point
below which ferromagnetic behavior becomes possible.
• The solidified magma then acquires magnetization in the same
direction as the ambient magnetic field.
• The process of lithosphere formation is continuous.
45. Continued: Vine Metthews Hypothesis
• It proceeds symmetrically on both sides.
• If the geomagnetic field reverses polarity as the new lithosphere
forms, the crust on either side of the ridge would consist of a
series of blocks running parallel to the crest.
• These blocks would possess ramanent magnetizations that are
either normal or reversed, storing the reversal history of earth’s
magnetic field registered in the oceanic crust (Vine, 1966).
• The intensity of Remanent magnetization in oceanic basalts is
significantly larger than the induced magnetization.
• Shapes of the magnetic lineations are controlled by the primary
remanent magnetization direction.
• Vector of reversely magnetized material is inclined steeply
upwards towards the south.
• Blocks of normally magnetized crust formed at high northern
latitude possess the magnetization vectors that is inclined steeply
towards north, and the vector of reversely magnetized material
46. Continued: Vine Metthews Hypothesis
is inclined steeply upwards towards south.
• The magnetic profile observed at this portion of the crust
would be characterized by positive anomalies on normally
magnetized blocks and negative anomalies on reversely
magnetized blocks.
• A similar situation pertains in high southern latitudes.
• Crust magnetized at low latitudes also generates positive and
negative anomalies; but on any particular block it is markedly
dipolar.
• This obscures the symmetry of the anomaly about the ridge
crest, as individual blocks are no longer associated with a
single positive or negative anomaly.
• However, at the magnetic equator, where the field is
horizontal, negative anomalies coincide with normally
magnetized blocks and positive anomalies with reversely
magnetized blocks, reverse situation to that at high latitudes.
47. Continued: Vine Matthews Hypothesis
• This is precisely the reverse situation to that at high latitudes.
• In general, the amplitude of magnetic anomalies decreases as
the latitude decreases and as the strike of the ridge
progresses from E-W to N-S.
49. DISTRIBUTION OF EARTHQUAKES
• Majority of the earth’s tectonic activity takes place at the
margin of the plates.
• Thus, locations of earthquake epicenters can be used to mark
plate boundaries.
• Figure showing global distribution of epicenters of large
magnitude earthquakes (1961-1967).
• Significance of 1961---Setting up of the world Standardized
Seismograph Network in1961
• Classification of Earthquakes According to Their Focal
Depths:
• Shallow focus earthquakes: 0-70km.
• Intermediate depth earthquakes: 70-300km.
• Deep focus earthquakes: Greater than 300km depth
50. Continued: Distribution of Earthquakes
• An important belt of shallow focus earthquakes follow the
crests of ocean ridge system
• Tensional events associated with plate accretion and strikeslip events where the ridges are offset by transform faults
• On land, tensional events are associated with rifts, e.g. Basin
and Range Province (USA), East African Rift system, and Baikal
Rift system
• All intermediate and deep events are associated with
destructive plate margins
• Pacific ocean on three sides is ringed by a belt of earthquakes
on a plane dipping at 45 beneath the neighboring plate.
• The deepest event recorded lie at a depth of 670 km
• Collisional mountain belts—Alpine –Himalayan chain have no
Benioff zone.
• Intra-plate areas.
51. Relative Plate Motions
• Measurements of present day plate motions using techniques
of space geodesy
• Relative plate motions using geological and geophysical data
• Euler’s theorem:
• “The movement of a portion of a sphere across its surface is
uniquely defined by a single angular rotation about a pole of
rotation. The pole of rotation and its antipodal point on
opposite diameter of the sphere are the only two points which
remain in a fixed position relative to the moving portion.
Consequently, the movement of a continent across the surface
of the Earth to its pre-drift position can be described by its pole
and angle of rotation.”
• “The relative motion between two plates is uniquely defined by
an angular separation about a pole of relative motion”
• Figurer
• “The Euler pole E is the pivot point for the motion of the two
plates relative to each other.”
52. Finding Euler Poles
• Start by looking for transforms in the form of circles or arcs. At
sea you may notice some long, narrow mountain ranges
across which the ocean floor steps down from a shallower to
a greater depth. These have been termed as Fracture Zones.
• Locally they appear to be linear features whereas over great
distances they are segments of circles. Many fracture zones
mark the trace of present or ancient transforms.
• Draw a best fitting straight line segment along the trend of
each of the fracture zones
• Record the azimuths and coordinates of the midpoints of the
line segments.
• Construct the perpendicular bisectors of the line segments.
• Repeat this at different localities.
53. Fracture Zones
• Fracture zones are long, narrow mountain ranges that were
discovered in the early 1950s. They cut across the major
features of the ocean floor, including both rises and abyssal
plains.
• A typical fracture zone is about 60km wide and consists of
several irregular ridges and valleys aligned with the overall
trend of the fracture zone. The depth of the floor changes
across the fracture zone. On map fracture zones are arcuate
features of great length.
• Figures:
• Fracture zones have many of the characteristics of
faults, particularly those of strike-slip faults. But these are not
simple strike-slip faults.
• Earhquakes observed along fracture zones are curiously
intermittent: they occur along only those parts of fracture
zones that lie between the offset segments of mid-oceanic
rise crests.
•
54. Continued: Fracture Zones
• Figure. Elsewhere, fracture zones are seismically quiet. The
longest fracture zones on earth are those in the Pacific basin,
and these have no seismic activity.
• The fracture zone pattern in the eastern Pacific indicates that
faulting on the fracture zone was older than the continental
crust along the continental margin.
• Why is seismicity confined to the segments of fracture zones
between active mid-oceanic ridges?
55. OCEANIC FRACTURE ZONES
• Long, linear bathymetric depressions and mountain chains
that follow arcs of small circles on the surface of earth
perpendicular to the offset ridges.
• One such fracture zone on Mid-Atlantic Ridge is a zone of
complex swarm of faults.
• Fracture zone marks both the active transform segment and
its fossilized trace.
• Fractures result from thermal contraction in the direction of
ridge axis
• It is possible that fracture zones develop along these lines of
weakness.
• Samples from fracture zones have shown both normal oceanic
rocks and much sheared and metamorphosed rocks.
• Large blocks of serpentinite lie at the bases of fracture zones.
• Large equatorial Atlantic fracture zones have yielded samples
with
56. Wilson’s Model for Plate Tectonic
Explanation of Fracture Zones
• The key to understanding the fracture zones is the
relationship between offset rise crests and fracture zones.
•
• Figure:
•
• Explanation:
•
• Except between the rise crests, fracture zones are not
tectonically active features. They are simply the scars left
behind by the intense shearing that takes place between rise
crests.
•
• Difference of depth of ocean floor across fracture zone—
Tectonic Explanation
57. Continued…………….
Ultramafic, gabbroic, and basaltic rock types and their metamorphosed
and tectonized equivalents.
• Serpentinite intrusion, alkali basalt volcanism, hydrothermal
activity, and metallogenesis are quite common. Mantle peridotites
are usually present.
• Ocean fracture zones bring crust of different ages into
juxtaposition.
• Depth of seafloor is dependent upon its age.
• A dip-slip motion component developed along the fracture zone
causes seismicity.
• Transverse ridges form along fracture zones that are sometimes of
higher elevation than the spreading ridges (6km relief).
• Transverse ridges are uplifted due to compressive and tensional
horizontal stresses across the fracture zone that originate from
small changes in the direction of spreading. Thus, transform
movement is no longer exactly orthogonal to the ridge. Figures
• Leaky Transform fault
58. Three methods of determining
relative motions between two
Plates
• Using Euler poles
• For true tangential motion to occur during relative
motion between two plates, the transform faults along
their common boundary must follow the traces of small
circles centered upon the pole of relative motion.
• The Euler pole can be found out as described above.
• This is the most accurate technique and applies best to
accretive type of boundaries.
• Figure
59. Continued: Three methods of Determining
Euler Poles
Based on variation of spreading rate with angular
distance from the pole of rotation
• Spreading rates are determined from magnetic lineations by
identifying anomalies of the same age on either side of the
oceanic ridge and measuring the distance between them.
• The velocity of spreading is at a maximum at the equator
corresponding to the Euler pole
• It decreases according to the cosine of the Euler pole’s
latitude.
• The determination of the spreading rate at a number of points
along the ridge then allows the pole of rotation to be found.
• Figure
60. Continued: Three methods
• Spreading rates are determined from magnetic lineations by
identifying anomalies of the same age on either side of the
oceanic ridge and measuring the distance between them.
• The velocity of spreading is at a maximum at the equator
corresponding to the Euler pole
• It decreases according to the cosine of the Euler pole’s
latitude.
• The determination of the spreading rate at a number of points
along the ridge then allows the pole of rotation to be found.
• Figure
61. Continued: Three Methods
Using Focal Mechanism Solution of Earthquakes
• This is least reliable method.
• If the inclination and direction of slip along the fault plane are
known, then the horizontal component of the slip vector is
the direction of relative motion.
• Divergent plate boundaries can be studied using spreading
rates and Transform faults.
• Convergent boundaries , however, present more of a problem.
This is possible by making use of information from adjoining
plates and treating the rotations between plate pairs as
vectors.
• Thus if the relative movement between plate A and B, and B
and C is known, the relative movement between A and C can
be found by vector algebra.
• The method can be applied to determine relative motions for
the complete mosaic of plates that make up the Earth’s
surface.
62. ABSOLUTE PLATE MOTIONS
• The absolute motion of plates is much more difficult to
define than the relative motion between plates at plate
boundaries.
• A particular point on the plate will be stationary if the
Euler vector of motion of that plate or plate boundary
passes through that point.
• Absolute plate motion should specify the motion of
lithosphere relative to the lower mantle because this
accounts for 70% of the mass of the solid earth.
• It deforms more slowly than the asthenosphere above
and the outer core below.
63. Hotspots
• Wilson introduced hotspots. Passage of crust over the
hotspot in the mantle beneath.
• Morgan elaborated on the idea—mantle plumes rising
from the lower mantle providing fixed reference frame
with respect to lower mantle
• Figure: Model of Gripp & Gordon (2002). It averages
plate motions over the past 5.8Ma, twice the length of
time over which relative velocities are averaged.
• Some other frames of reference
• African Plate—remained stationary for the past 25 Ma.
• Caribbean plate: opposite polarity of subduction made it
stationary.
64. HOTSPOTS
• Volcanic activity within the interiors of plates, intra-plate
volcanism
• Linear island and sea mount chains such as Hawaiian—
Emperor and Line island chains in the Pacific.
• Aseismic ridges are constructed such as Ninety East ridge in
the Indian Ocean, the Greenland-Scotland Ridge in the
North Atlantic, and Rio Grande and Walvis ridges in the
south Atlantic.
• Composition
• Hawaiian-Emperor chain—6000km long
• Morgan proposed that hotspots presented a fixed
framework of reference for determination of absolute plate
motion.
• 40 to 50 present day hotspots have been suggested.
• Figure
65. Continued: Hotspots
• Many are short-lived, others have persisted for tens of
millions of years, may be 100 millions years.
• Form flood basalt on land or an oceanic plateau under the
sea.
• These remarkable episodes of localized enhanced partial
melting in the mantle are collectively termed Large Igneous
Provinces (LIPs)
• These hotspots (LIPS) are called Primary hotspots
• Seven or 10-12 may be recognized as primary hotspots
including that of India.
• Deccan Traps of western India were extruded 65 million years
ago—Reunion Island
67. DIRECT MEASUREMENT OF RELATIVE
PLATE MOTIONS
• It is now possible to measure the relative motion between
plates using methods of space geodesy.
• Before 1980 standard terrestrial geodetic methods of baseline
measurements using optical techniques or laser ranging
instruments such as geodolite were used.
• These methods are good and precise to measure relative
plate motions of a few tens of mm per year.
• Using space technology, three independent methods of
extraterrestrial surveying are available. These are:
• i) Very long baseline interferometry, ii) satellite laser
ranging, and iii) satellite radio positioning (Global Positioning
System—GPS)
69. Ocean ridge Topography
•
•
•
•
•
•
•
•
•
Ocean ridges are accretive or constructive plate margins.
The longest linear uplifted feature of the Earth’s surface.
Follow belt of Shallow focus earthquakes.
Total length of the ridge system is 55000 km.
Total length of the ridge—ridge transform faults is 30,000 km.
Ridges are between 1000 and 4000 km in width
They are 2-3 km high from the neighboring ocean basin.
Topography is rugged
The gross morphology of ridges is controlled by separation
rates
• Spreading rates at different points along mid-oceanic ridges
vary widely
70. Continued: Ocean Ridge Topography
• Along SW Indian Ocean Ridge it is less than 20mm per year
• On East Pacific rise between Nazca and Pacific plates it ranges
upto 150mm per year.
• Thus, topography, structure and rock type vary as a function
of spreading rate.
• Slow rate: 10—50mm/year, e.g. Midatlantic ridge, Indian ridge
A median rift at axis 30-50km wide, 1500-3000m deep forms;
Intermediate rate: 50-90mm/year, e.g. Galapagos, northern
East Pacific rise; median rift only 50-200m deep; topography
relatively smooth.
Fast rate: above 90mm/year, e.g. East Pacific rise, no median
rift, topography smooth
71.
72. Continued: Ocean Ridge Topography
• Axial zone of volcanic activity flanked by zone of fissuring
• Stable regions bound the “crestal accretion zone”or “plate
boundary zone”.
• Presence of topographic high and inner rift valley.
• Figures
73. Broad Structure of the Upper mantle
Below Ridges
• Gravity measurements: Free air anomalies are broadly zero
over ridges.
• They are in a state of isostatic equilibrium.
• Small scale topographic features are uncompensated and
cause positive and negative free air anomalies.
• Seismic refraction experiments by Talwani, et al over East
Pacific Rise indicate:
•
Crust is slightly thinner than that of main ocean basin
•
Upper mantle velocity beneath the crestal region is
anomalously low
•
Since crust does not thicken beneath the ridges, isostatic
compensation must occur within the upper mantle.
74. Continued: Broad structure of the Upper
Mantle Below Ridges
• Talwani proposed that the anomalously low upper mantle
velocities below ridges correspond with the low density of the
mantle material.
• Densities were determined by using Nafe-Drake relationship
between P-wave velocities and density.
• A number of models were presented.
• Ridges are underlain by large low density bodies in the upper
mantle whose upper surfaces slope away from the ridge crest.
75. Origin of Anomalous Upper Mantle
Three possible sources of the low density regions which underlie
ocean ridges and support them isostatically:
1. Thermal expansion of upper mantle material beneath the
ridge crest, followed by contraction under the flanks
2. The presence of molten material within the anomalous
mantle
3. A temperature-dependent phase change
--Thermal expansion and contraction is the major factor
contributing to the uplift of ridges; the other two factors
may contribute to a lesser extent.
76. PETROLOGY OF OCEAN RIDGES
• Due to high heat flow under oceanic ridges, geothermal
gradient crosses the peridotite solidus at a depth of 50 km
giving rise to magma of oceanic crust.
77. Continued…………….
Ultramafic, gabbroic, and basaltic rock types and their metamorphosed
and tectonized equivalents.
• Serpentinite intrusion, alkali basalt volcanism, hydrothermal
activity, and metallogenesis are quite common. Mantle peridotites
are usually present.
• Ocean fracture zones bring crust of different ages into
juxtaposition.
• Depth of seafloor is dependent upon its age.
• A dip-slip motion component developed along the fracture zone
causes seismicity.
• Transverse ridges form along fracture zones that are sometimes of
higher elevation than the spreading ridges (6km relief).
• Transverse ridges are uplifted due to compressive and tensional
horizontal stresses across the fracture zone that originate from
small changes in the direction of spreading. Thus, transform
movement is no longer exactly orthogonal to the ridge. Figures
• Leaky Transform fault
79. Continental Rifts
Introduction
• Continental rifts are regions of extensional deformation
where the entire lithospheric thickness has deformed.
• The term “rift” thus applies only to major lithospheric
features.
• Rifts represent the initial stage of continental break-up.
• Extension may lead to lithospheric rupture and formation of
new ocean basin.
• Then the continental rift eventually becomes inactive and a
passive or rifted margin forms. These margins then subside.
• Not all rifts rupture to the point where new ocean crust is
generated.
• Aulacogens, failed rifts, become inactive at some stage.
80. Continued: Continental Rifts: Introduction
• Where the lithosphere is thick, cool and strong, narrow rifted
zones of localized strain, 100km wide form, e.g. Baikal rift, the
East African Rift system, and Rhine Graben.
• Where the lithosphere is thin, hot and weak, rifts tend to
form wide zones where strain is delocalized and distributed
across zones several hundreds of km wide; e.g. Basin and
Range Province and Aegean Sea.
• Both varieties of rift may be associated with volcanic activity.
• Some rifts such as those of Kenya, Ethiopia, and Afar are
characterized by voluminous magmatism and the eruption of
continental flood basalts.
• Some others such as western East African Rift system and the
Baikal rifts are magma starved
• Figures
81. Continued: Continental Rifts: Introduction
• General Characteristics of narrow Rifts
• Best studied example of intracontinental rifts occur in East
Africa.
• SW of Afar Triple Junction: Nubian and Somalian plates are
moving apart.
• This divergent plate motion results in extentsional
deformation localized into a series of discrete rift segments of
variable age: the Western rift, the Eastern rift, the Main
Ethopian rift, and the Afar Depression.
82. Continued: Continental Rifts: Key Features
• Key features
1. symmetric rift basins flanked by normal faults
An asymmetric half graben morphology
2. Shallow Seismicity and regional tensional stresses
Under the rifts earthquakes are confined to a depth of 1215km of the crust forming a seismogenic layer that is thin
relative to other regions of the continent. Reason?
• Away from the rift axis, e.q. may occur up to 30 km depth.
• 3. Local crustal thinning modified by magmatic activity
• Rifts are characterized by thinning of the crust beneath the rift
axis, as indicated by geophysical data.
• Crustal thicknesses in the rift basins are variable
83. Continued: Continental Rifts: Key Features
4.
High heat flow and low velocity, low density upper mantle
Heat flow measurements averaging 70-90mW/m2 and low
seismic velocities in many rift basins suggest temperature
gradients 50-100oC /km that are higher than those in the
adjacent rift flanks and nearby cratons.
Volcanic Activity-----large igneous provinces (LIPs)
Petrogenesis of Rift Rocks
Mantle upwelling Beneath Rifts
84. Rift Initiation
• Continental rifting requires the existence of a
horizontal deviatoric tensional stress that is sufficient
to break the lithosphere.
• Sources of such stress may be as follows:
1. Plate motions;
2. thermal buoyancy forces due to asthenospheric
upwellings; 3. tractions at the base of the
lithosphere produced by convecting asthenosphere;
4. buoyancy (gravitational) forces created by
variations in crustal thickness.
• Lithospheric strength is the most important
parameter governs the formation and evolution of
continental rifts and rifted margins.
85. Continued: Rift Initiation
• Lithospheric strength is highly sensitive to geothermal
gradient, crustal composition and crustal thickness.
• Only initially thin lithosphere, high heat flow, magmatic
intrusion, or the addition of water may be required to
sufficiently weaken the lithosphere to allow rifting to
occur.
• At any depth, deviatoric stress can cause yielding by
faulting, ductile flow or dike intrusion
• Finally, the location and distribution of strain at the start
of rifting may be influenced by pre-existing weaknesses
in the lithosphere
86. Strain Localization and Delocalization
Processes
• Localization of strain into narrow zones during extension is
determined by processes that lead to mechanical weakening of
lithosphere.
• Lithospheric weakening is accomplished by elevation of
geotherms, heating by intrusions, interactions between lithosphere
and asthenosphere, and behavior of faults and shear zones during
deformation_ strain softening mechanisms
• Lithospheric strengthening may be accomplished by replacement of
weak crust by the strong upper mantle during crustal thinning and
by crustal thickness variations resulting from extension.
• These strain hardening mechanisms promote delocalization of
strain during rifting.
• Net weakening or a net strengthening of the lithosphere would
control the evolution of deformation patterns within the rifts.
87. Lithospheric Stretching
• During horizontal extension, lithospheric stretching results in
a vertical thinning of the crust and an increase in the
geothermal gradient within the zone of thinning.
• The upward movement of the mantle results in increased
heat flow called Heat advection, within the rift
• Heat advection results in compressed geotherms and higer
heat flow, which in turn results in net weakening of the
lithosphere.
• It is opposed by diffusion of heat away from the zone of
thinning
• The geotherm beneath the rift valley increases and the
Integrated strength of the lithosphere decreases.
88. Continued:Rift Initiation: Lithospheric stretching
• Fast strain rates result in larger increase in geothermal
gradient than the slow rates for the same amount of
stretching.
• High strain rates tend to localize strain because inefficient
cooling keeps the zone of thinning weak allowing deformation
to focus into a narrow zone.
89. Buoyancy Forces and Lower Crustal Flow
• Lithospheric stretching results in: crustal thinning; compression of
geotherms; two types of buoyancy forces that influence strain localization
during rifting.
• Thermal Buoyancy force
Lateral variations in temperature and therefore density between areas inside
and outside the rift create the above force that promotes horizontal
extension. This enhances and promotes localization of strain.
Crustal Buoyancy force
--This is generated by local isostatic effects as the crust thins and high
density material is brought to shallow level beneath the rift.
--Crustal thinning lowers surface elevation in the center of the rift
--The subsidence places the rift into compression that opposes the forces
driving extension.
--This results into delocalization of strain and the deformation migrates into
areas that are more easily deformable.
90. Rifted Continental Margins
• Volcanic margins
• Three components occur:
i) Large igneous provinces composed of thick flood basalts and silicic
volcanic sequences
ii) High velocity lower crust in the continent-ocean transition
zone, and (iii) thick sequences of volcanic and sedimentary strata that
give rise to seaward dipping reflectors on seismic reflection profiles.
• Majority of the rifted continental margins are volcanic with some
notable exeptions.
• The high P-wave velocities suggest that they are composed of thick
accumulations of gabbro that intruded the lower crust during rifting.
• Nonvolcanic margins
• These margins show that extreme thinning and stretching of the crust
is not necessarily accompanied by large scale volcanism and melting.
• They lack large volumes of extrusive and intrusive materials;
• Instead, the crust may include highly faulted and extended
continental lithosphere, oceanic lithosphere formed by very slow
seafloor spreading.
91. Evolution of Rifted Margins
• It is governed by many of the same forces and processes that
affect the formation of intracontinental rifts
• Thermal and crustal buoyancy forces, lithospheric
flexure, rheological contrast and magmatism all may affect
margin benavior during continental break-up.
• Two sets of processes that are especially important during
transition from rifting to seafloor spreading include: (i)postrift subsidence and stretching; and (ii) detachment
faulting, mantle exhumation, and ocean crust formation at
nonvolcanic margins.
• As the rift progresses, the margins of the rift isostatically
subside below sea-leveland eventuallybecome tectonically
inactive.
• This subsidence is governed by mechanical effects of
stretching and by a gradual relaxation of thermal anomaly
associated with rifting.
92. Case Study: The East African Rift System
• system is composed of several discrete rift segments.
• These segments record different stages in the transition from
continental rift to rifted volcanic margin.
• The Eastern rift between Tanzania and Kenya is an example of
youthful rift that initiated in thick, cool, and strong
continental lithosphere.
• Volcanism and sedimentation began 5ma.
• Largest fault escarpment formed 3ma.
• Strain and magmatism are localized within narrow asymmetric
rift basins.
• Earthquake hypocenters occur throughout the entire 35km
thickness of the crust.
• This indicates the crustal heating is at a minimum.
• The basins are shallow (3km) with 100km long border faults.
93. Continued: The East African Rift System
• The border faults have grown from short fault segments.
• Geophysical and geochemical data have shown that the
mantle lithosphere has thinned to about 140km from 200350km.
• These patterns conform to the lithospheric stretching models
in regions of relatively thick lithosphere.
• Cross-sectional geometry and along-axis segmentation is
controlled by the flexural strength of the lithosphere.
• The effect of pre-existing weaknesses on the geometry of
rifting are also illustrated in the southern segment of the
eastern rift in Tanzania.
94. THE WILSON CYCLE
• The transition from intracontinental rift to ocean basin has
occurred repeatedly on earth since Archean.
• Relatively young Mesozoic-Cenozoic age of the current ocean
basins.
• Existence of ancient ocean basins is implied by continental
reconstruction and by ophiolites.
• The periodicity of ocean formation and closure of is called
“wilson cycle”.
• Fig: Schematic illustration of various stages in the Wilson Cycle
• Present day analogues
96. OCEAN TRENCHES
• Ocean trenches are the direct manifestation of underthrusting
oceanic lithosphere
• They develop on the oceanward side of both the island arcs
and Andean-type orogens
• Largest linear depressed features of the earth’s surface, e.g.
Chile-Peru Trench—7-8km deep.
• In western Pacific—10-11km deep Mariana and TongaKermadec trenches
• Age of the ocean floor determines the depth of the trench.
• Trenches are 50-100km in width and form an asymmetric v
shaped feature.
• The sediment fill of trenches—from nothing (e.g. TongaKermadec) to complete (e.g. Lesser Antilles and Alaskan
trenches)
97. Morphology of Island Arc Systems
• Oceanic lithosphere is subducted beneath oceanic lithosphere
• They are typical of the margins of contracting oceans, such as
the Pacific.
• They also exist in the western Atlantic
• These are convex to the underthrusting ocean
• Convexity is the consequence of spherical geometry
• Fig. showing generalized morphology of the island arc system
• Flexural bulge
• Forearc region: Trench, accretionary prism, forearc basin
• The volcnanic substrate may represent initial site of volcanism
• The island arc and remanent arc (backarc ridge)
98. Gravity anomalies of Subduction zones
• Figure for free air gravity anomaly profile
across the Aleutianarc
99. Variations in subduction zone characteristics
• The age and convergence rate of subducting oceanic
lithosphere affect the the following:
• Thermal structure, of the downgoing slab, length of the
seismic zone
• Dip of the Benioff zone varies between two end members:
Marianas (90) and Chile, Peru (10).
• Dip is detrmined by a combination of negative buoyancy of
the subducting slab causing it to sink, and the forces
exerted on it by flow in the asthenosphere, which tend to
uplift the slab
• Young, subducting lithosphere underthrusting at a higher
rate will give rise to shallow dips, e.g. Chile, Peru
• The absolute motion of the overriding plate is also a
contributing factor—
• Shallow dips have stronger coupling with overriding plate
and give rise to high magnitude earthquakes
100. Continued: Variations in Subduction Zone
Characteristics
• Shallow dip restricts flow of asthenosphere in the mantle
wedge
• Suppresses all supra subduction zone magmatism
• Backarc compression rather than extension
• Two end-member type of subduction zones—Chilean and
Mariana types
• Whether accretionary or erosive
• Historically-------• Most of the oceanic crust and pelagic sediments are subducted
into mantle
• Some of the overriding plates are eroded and subducted
• Sediment subduction
• Subduction erosion
• Fig. p.263
101. Accretianary Prisms
• Where present, it develops on the inner wall of an
ocean trench
• Its structure has been deduced from seismic reflection
profiles, drilling, study of ancient subduction
complexes now exposed on land.
• Accretionary prisms develop where trench-fill
turbidites (flysch) and pelagic sediments are scrapped
off the descending ocean plate by the leading edge of
over-riding plate to which they become accreted.
• Nankai trough—Japan
– Large active accretionary prism, thick sedimentary
sequence, decollement, fold and thrust belt, frontal
accretion, imbicate thrusts, out-of-sequence
thrusts, underplating, trench slope break, trenchslope
basin, forearc basin
– Fig. page 265, 266, 268
102. Volcanic and Plutonic Activity
• Where subducting lithosphere reaches a depth of 65130km, volcanic and plutonic activity occurs
• This gives rise to an island arc or an Andean type continental
arc-150-200km from the --trench axis
• Thickness of arc crust—younger part of Mariana,3-4 Ma—
20km
• Mature Japanese arc systems– Neogene-30-50km
• Continental arcs—most complex
• Compressional continental settings---70-80km
• Type of volcanic rocks-three volcanic series:
– -Low potassium tholeiitic series-basaltic lavas associated
with andesites
– Caoc-alkaline series,-andesites, dacites, rholites
– Mature arc system especially continental arcs typically
include large linear belts of plutonic rocks called batholiths
103. Metamorphism at convergent margins
• A series of chemical reactions—release water, increase
density—Low temperature but high pressure metamorphism
• Low temperature metamorphic mineral
assemblage, greenschist facies—chlorite, epidote, actinolite----Hydrothermal type of metamorphism
• Blueschist facies-Low temperature, high temperatre
metamorphism
• Transformation from blueschist facies to eclogite facies--omphacite and garnet
• Miyashiro’s concept of paired metamorphic belts
• Miyashiro identified three pairs of metamorphic belts of
different age that parallel the trend of Japanese subduction
zones
• Outer zone of high pressure-low temperature (blue
schist), inner zone of high temperature, low pressure.
104. Backarc basins (Marginal basins)
• Relatively small basins of either oceanic or continental affinity
• Form behind the volcanic arc on the overriding plate of a
subduction zone
• Oceanic varities---Western Pacific
• On the inner concave side of the arc
• Backarc ridge (remnant arc)
• Extensional tectonics, high heat flow
• Models for the formation of marginal seas
a) Active diapirism generated by Benioff Zone
b)Passive diapirism from regional tensional stresses
c) Subsidiary convective circulation
d) Step-back in underthrusting
Example from Tonga-Kermadec region
Example from the Lesser Antilles subduction zone
Figures.
105. CONTINENTAL STRIKE SLIP FAULTS
• Strike-slip or wrench faults
• Primary motion is horizontal along a vertical fault plane
• Fundamental Faults:
– They penetrate the whole lithosphere
• Only transform faults make active plate boundaries
• Examples of Transform faults:
– Dextral San Andreas Fault of California
– Dextral Alpine Fault of New Zealand
– Sinistral Dead Sea Fault system, which connect the Red Sea to
the Bitlis Mountains of Turkey
Some Fundamental faults which do not make plate
Boundaries:
Great Glen fault of Scotland, North Pyrenean fault zone and
certain faults in the Alps. Faults associated with indentation
tectonics
Large strke-slip faults produce distinctive topographic features
106. Continued------Strike-slip faults
• Plate boundaries on continents are more complex in
contrast to oceanic fracture zones
• Reasons for the above
• It is necessary to distinguish between Transform
fault, Transform zone, and Transform fault system.
• Complex nature of continental transform system is
illustrated by San Andreas Fault System (Fig. )
• The system developed in Oligocene and all confined to
the continent alone.
• There has been 1500km of dextral movement along
fault zone since Oligocene times.
• The San Andreas fault has been the only fault that is
continuous for 1200km but for only 300km horizontally
• There also exist some major sinistral fault striking NE.
107. Continued from--------------San Andreas fault
• Present day seismicity is generated along such oblique
faults that along some sections involve clockwise rotation
of crustal blocks as a result of the regional stress fields.
• Much of the deformation caused by this system changes
from brittle to ductile at 15km depth.
• Stress statement of the region of San Andreas Fault System:
East of Great Central Valley of California much strike-slip
and normal faulting but to the west the deformation is
wholly compressional, dominated by folding, strike-slip and
reverse faulting parallel to the San Andreas Fault.
• The direction of maximum horizontal compression is
orthogonal to the Fault zone.
• Weakness of the fault system almost extends through the
whole lithosphere
• Transpression and Transtension
• Since the ancient fractures may control the locations of
fault planes in many transform fault zones, the strike of the
faults may depart from the simple linear trend.
108. Continued-----Strike-slip faults
• The combination of strike-slip motion and compression
(convergent strike-slip)is known as Transpression.
Transcompressive regions give rise to thrust faulting, folding
and uplift. The curvature of the faults imposed by the existing
lithospheric weaknesses gives rise to alternate convergence
and divergence, where the fault shape causes blocks on either
side of the fault to be compressed or extended (fig. ), e.g.
Transverse Ranges of California developed along a portion of
San Andreas Fault.
• They have formed as a consequence of the compression
experienced between SW and NE California across this part of
the fault by crustal shortening (Figs.)
109.
110. Continued from-------Pull-apart basins
• The combination of strike-slip motion and extension (divergent
strike-slip)is known as Transtension. Transtensile regions give
rise to normal faulting, basin extension and volcanacity.
• Gentle strike-slip curvature gives rise to both extension and
compression
• Pull-apart Basins
• Where the curvature of a strike-slip fault is well pronounced, or
where one fault terminates and sidesteps to another adjacent
parallel fault, the curved zone or area separating the ends of
faults is thrown into tension or compression
• Compression results into uplift of regions due to crustal
shortening, folds, and thrusting.
• Tension gives rise to extensional troughs known as pull-apart
basins
111. Continued----Pull-apart basins
• Examples of Pull-apart basins: the Dead Sea, Salton Trough of
southern California
• Pull-apart basins are excellent targets for petroleum exploration
• Growth of pull-apart basins (fig. ); margins, igneous activity at
floor.
• Presence of +ve gravity anomalies and geothermal areas
• Fault wedge basins
• Strike-slip faults may diverge and converge to form an
anastomosing pattern. Duplexes may form at bends.
• In this environment, pull-apart basins are frequently associated
with lens shaped basins and high standing ridges and banks of
similar shape.
• These features owe their origin to the stress regime resulting
from the confluence of two sub-parallel strike-slip faults
112. Triple junctions
The individual blocks are compressed and uplifted when the
faults converge and stretched and down thrown where the
faults diverge.
• Points where three plates meet are called Triple junctions
(fig. )
• The stability of boundaries between plates is dependent
upon their relative velocity vectors
• An unstable boundary will exist only instantaneously before
quickly devolving into a stable configuration
• Sequence of events occurring in the development of Alpine
Fault of New Zealand, which is a dextral transform fault
linking Tonga-Kermadec trench, beneath which Pacific
lithosphere is underthrusting in a southwest direction, to a
trench where Tasman Sea is consuming in NE direction.
• A more complex situation arises when three plates come in
contact at a triple junction.
113. Continued from--------Triple Junctions
• Quadruple junction is always unstable
• The stability of a triple junction also depends upon the
relative directions of the velocity vectors of the plates in
contact
• Figure of a stable triple junction between a ridge (R), trench
(T), and a transform fault (F)
• A stable triple junction is that in which the junction can
migrate up and down the three boundaries between pairs of
plates.
115. OROGENIC BELTS
• Introduction
• Orogenic belts are long, commonly arcuate tracts of highly
deformed rock that develop during the creation of mountain
ranges on continents.
• The processes of building an orogen (mountain building) is
called orogenesis . It occurs on convergent plate margins and
involves intraplate shortening, crustal thickening, and
topographic uplift.
• Processes controlling the orogenesis vary considerably
depending upon the tectonic setting, the type of lithosphere
involved in the deformation.
• Non collisional or Andean type orogens result from oceancontinent convergence where plate margins and other factors
lead to compression in the over-riding plate
• Collisional orogens develop where continent or island arc
collide with a continental margin as a result of subduction.
116. Continued----------Introduction
• Thickness and positive buoyancy of the colliding material
inhibits-• The Himalayan-Tibetan belt and European Alps represent
orogens that formed as a result of continent-continent
collision following the closure of a major ocean basin
• Where continental lithosphere is cool and strong-------------------• Where the lithosphere is hot and weak------------central
Andes and Himalayan-Tibetan orogens
• Gradual accretion of continental fragments, island arcs, and
oceanic material onto continental margins over million of
years------------• Processes that change the rheology and strength of the
continental lithosphetre during orogenesis include: -----------• Subduction gives rise to two different types of orogenic
belts depending on the nature of overriding plate:
117. Continued---------Introduction
• Subduction beneath ocenaic lithosphere results in the
formation of island arc
• Subduction beneath continental lithosphere results in linear
mountain belts—Andean type mountain ranges—Cordillera
• Figure
• This is a response of the lithosphere to continued, steady
state subduction
• Development of Collisional mountain ranges
• Colliding material may be a continent, old island arc, or a
microcontinent.
• Collision mountains ranges are created by the stacking of
thrust slices of crust.
• This brings to a close of the episode of seafloor spreading.
• Movements in the welded continents may continue for long
time after collision
118. Continued from--------Introduction
•
•
•
•
•
Andean-type Mountain Ranges
Peruvian Andes comprise two subparallel fold belts:
Western Cordillera—Mesozoic-Tertiary age
Eastern Cordillera—Late Paleozoic age
In the southern Peru folds diverge by a thick sequence of
Tertiary molasse
• In the south Paleozoic Eastern cordillera is made up of thick
black shale and quartzite assemblage
• In the north it comprises greenschist facies pelites of
Ordovician age associated with gneisses (may be reworked
basement).
• The Mesozoic-Tertiary age Western cordillera is divided
longitudinally into an eastern sedimentary trough of folded
clastic and carbonate sedimentary rocks and undeformed
volcanosedimentary trough.
119.
120. Continued -------Andean type mountain ranges
• The Phanerozoic rocks are thus underlain by old crystalline
rocks and the Andean mountain range is founded on old
continental crust.
• The volcanosedimentary trough of Western Cordillera
includes the massive coastal batholith.
• This is a multiple intrusion of granite, tonalite and gabbro
extending for 1600km. It includes more than 1000 interlocking
plutons intruded into the andesitic volcanic rocks through
deep seated fractures.
121. Continent-Continent Collision
• Examples of collisional mountain ranges:
Himalayan-Tibetan orogen, the Appalachians, the Caledonides,
the European Alps, the Urals, the southern Alps of New
Zealand, and many of the Proterozoic orogens.
• If the subducting plate also contains continental lithosphere,
continued underthrusting causes convergence of the
continents, and eventually brings them into juxtaposition (Fig. )
• Positive buoyancy
• Collision with the overriding continent; rapid relative movement
is halted; a collisional mountain range is formed by crustal
shortening (fig.)
• Initiation of a subduction zone along the new continental
margin
• Behavior of continents after collision is more complex.
122. Continued------Collisional mountain ranges
• Suture: The plane marking the collision. The suture may preserve
the slivers of the old oceanic crust which formerly separated the
continents, known as ophiolites.
• Himalayas represent the youngest collisional mountain range, in
which collision began in Tertiary times.
• Older are represented by
Appalachians, Calidonides, Alps, Urals, etc.
• Himalayan Geology
• 250—350 km wide, extend for about 3000 km from Afghanistan to
Burma
• They comprise a series of lithologic and tectonic units which run
parallel to the mountain belt
• Geology provides evidence for the previous occurrence of oceanic
lithosphere (preserved as ophiolites), passive continental
margin, an island arc, and Andean type batholiths
• Generalized cross-section fig.
123.
124. Evolution of Himalaya
• Rapid uplift of Himalaya at 0.5-4mm per year; rapid erosion
and deposition of thick terrigenous sequence dating from
Miocene. This comprises Siwalik molasse conglomerate going
southward into the Indus and Ganga basins
• Lesser Himalaya—1500-3000m
• Higher or Great Himalaya—8000m
• This was the last stage of true continental collision and
initiation of a regime of ‘Indentation tectonics’ .
• Magnetic anomalies studies and geological studies together
can provide details to the sequential movement.
• The Himalaya formed in response to the collision of India with
Eurasia.
• Northward migration of Indian plate
125.
126. Continued-----Magnetic anomaly and geological
studies—Evolution of Himalaya
• Tethys ocean was subducted beneath the southern margin of
Eurasian plate.
• Magnetic anomalies in the Indian ocean and plaeomagnetic
studies on the continent confirm the northerly drift of India and
allow the reconstruction of its path
• Initial collision took place about 50 Ma ago, and marks the end
of marine sedimentation, and initiates terrestrial deposition in
the suture zone
• All oceanic lithosphere had disappeared by45Ma.
• By 36Ma, the velocity of India’s northward drift had decreased
from100mm to 50mm per year.
• These indicate that main convergence was preceded by the
collision of two smaller plates (fig. ).
127. Continued----Indentation tectonics
•
•
•
•
Further movement would be halted
New trench to be formed on south of India
This prediction is not realized
The north Tibetan plate became welded to Eurasia by about 140
Ma and was followed by a step-back in the subduction zone to
the south of north Tibet.
• India collided with southern Tibet, which then represented the
southern margin of Eurasian plate, 50 Ma ago.
• This formed the Indus-Zangpo suture
• Continued convergence at a slower rate lead to the progressive
formation of the MCT and MBT, accompanying some 2000 km
of crustal shortening.
128. Continued-----Indentation tectonics
• Indentation Tectonic
• After rifting from Gondwana, India drifted northward,
intervening Tethyan ocean contracted due to subduction
beneath Tibet
• Collision took place 45 Ma ago resulting in the formation of
Himalayan ranges
• The Himalayan region is seismically active, undergoing rapid
uplift
• It is believed that India is still moving northward at a rate of
45mm per year, and has penetrated at least 2000km into Asia.
• Combined interpretation of satellite images and focal
mechanism solutions has revealed the pattern of faulting in
the region (fig. )
• A zone of thrust faulting
• A zone of strike-slip faulting, also extending east into
Indochina
129.
130. Continued
• A region of crustal extension and normal faulting starting from
Baikal region to northern China sea
• An anology
• Indentation, extrusion or escape tectonics
• India as rigid indenter, Asia as plastic medium
• Slip lines—alpha, beta correspond to dextral and sinistral
strike-slip motion respectively.
• The pattern of slip lines is controlled by the shape of the
indenter and the lateral constraints placed on the plastic
medium
