Global tectonics


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Global tectonics

  2. 2. Snider’s Reconstruction (1858)
  3. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 18. 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.
  19. 19. 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.
  20. 20. 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”.
  21. 21. 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.
  22. 22. 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.
  23. 23. 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.
  24. 24. 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.
  25. 25. 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.
  26. 26. 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 .
  27. 27. 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.
  28. 28. 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.
  29. 29. 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.
  30. 30. 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.
  31. 31. 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.
  32. 32. 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.
  33. 33. • 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.
  35. 35. 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.
  36. 36. 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.
  37. 37. 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 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).
  38. 38. 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.
  39. 39. 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.
  40. 40. 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.
  41. 41. 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--------------.
  42. 42. 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.
  43. 43. 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
  44. 44. 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.
  45. 45. 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.
  47. 47. 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
  48. 48. 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.
  49. 49. 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.”
  50. 50. 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.
  51. 51. 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. •
  52. 52. 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?
  53. 53. 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
  54. 54. 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
  55. 55. 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
  56. 56. 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
  57. 57. 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
  58. 58. 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
  59. 59. 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.
  60. 60. 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.
  61. 61. 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.
  62. 62. 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
  63. 63. 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
  64. 64. SUPERPLUMES • Cretaceous superplumes
  65. 65. 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)
  67. 67. 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
  68. 68. 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
  69. 69. 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
  70. 70. 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.
  71. 71. 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.
  72. 72. 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.
  73. 73. 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.
  74. 74. 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
  76. 76. 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.
  77. 77. 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
  78. 78. 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.
  79. 79. 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
  80. 80. 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
  81. 81. 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.
  82. 82. 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
  83. 83. 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.
  84. 84. 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.
  85. 85. 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.
  86. 86. 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.
  87. 87. 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.
  88. 88. 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.
  89. 89. 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.
  90. 90. 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.
  91. 91. 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
  93. 93. 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)
  94. 94. 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)
  95. 95. Gravity anomalies of Subduction zones • Figure for free air gravity anomaly profile across the Aleutianarc
  96. 96. 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
  97. 97. 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
  98. 98. 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
  99. 99. 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
  100. 100. 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.
  101. 101. 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.
  102. 102. 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
  103. 103. 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.
  104. 104. 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.
  105. 105. 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.)
  106. 106. 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
  107. 107. 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
  108. 108. 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.
  109. 109. 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.
  111. 111. 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.
  112. 112. 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:
  113. 113. 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
  114. 114. 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.
  115. 115. 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.
  116. 116. 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.
  117. 117. 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.
  118. 118. 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
  119. 119. 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. ).
  120. 120. 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.
  121. 121. 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
  122. 122. 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