Tectonics on a Sphere - GENERAL GEOPHYSICS - University of Sharjah - PDF

1460223 GENERAL GEOPHYSICS Chapter 2: Tectonics on a Sphere Dr. Hamdan Hamdan Petroleum Geosciences & Remote Sensing 1 TECTONICS ON A SPHERE Summary of Lecture  Earth Structure: the main units  Earthquake distribution  Plate Tectonics: basic Assumptions  Plate Tectonics: Type of plate boundaries  Plate boundaries on flat Earth  Plate Tectonics on spherical Earth:  Determination of rotation poles and rotation vectors:  Plate Boundaries can change with time  Triple Junctions  Absolute Plate tectonics Image Taken from : https://www.pngegg.com/ Based on Text book: The Solid Earth, C. Fowler 2 TECTONICS ON A SPHERE Earth Structure: the main units Compositional:  Crust  Mantle  Core Rheological (physical):  Lithosphere  Asthenosphere  Mesosphere Plate tectonics - a theory that explains the function of the upper most layer of the planet. Layers of the earth based on chemical composition (3:5) https://www.youtube.com/watch?v=6RKoLleyDJ4 3 TECTONICS ON A SPHERE Earth Structure: the main units  Crust versus mantle: The crust is a product of mantle melting. Typical mantle rocks have a higher magnesium to iron ratio, and a smaller portion of silicon and aluminum than the crust.  Lithosphere versus asthenosphere: While the lithosphere behaves as a rigid body over geologic time scales, the asthenosphere deforms in ductile fashion. The lithosphere is fragmented into tectonic plates, which move relative to one another. There are two types of lithosphere: oceanic and continental.  Upper versus lower mantle: Together the lithosphere and the asthenosphere form the upper mantle. The mesosphere, extending between the 660 boundary and the outer core, corresponds to the lower mantle. 4 TECTONICS ON A SPHERE Earth Structure: the main units Asthenosphere Phase Changes:  410 km: Above this depth the Mg, Fe, Si and O are primarily within olivine and pyroxene. Below this depth the olivine is no longer stable and is replaced by a higher density polymorph - spinel. The material has a similar overall composition, but the minerals have a more compact structure. Spinel: a group of minerals composed principally of oxides of magnesium, aluminum, iron, manganese, chromium, etc., characterized by their hardness and octahedral crystals.  660 km: Below this depth the spinel gives way to the minerals Mg- perovskite and Mg-wustite. (In fact, Mg-perovskite is probably the most abundant solid of the earth since it appears to be stable through much of the mantle.) https://www.powershow.com/view2b/47769c- MDU5Z/Plate_tectonics_Earth_structure_and_plate_geometry_I_powerpoint_ppt_pr 5 esentation TECTONICS ON A SPHERE Earthquake distribution: Earthquake Belts  Earthquakes are organized along belts.  The world's greatest earthquake belt, the circum-Pacific seismic belt (ring of fire), is located along the rim of the Pacific Ocean.  The second important belt, the Alpide, extends from Sumatra through the Himalayas, the Mediterranean, and out into Taken from: http://whatsinside.earth/grph.html the Atlantic.  The third prominent belt is the mid-Atlantic Home assignment: belt. The ring of fire: https://www.youtube.com/watch?v=DrwYtGf40hA&t=150s The Alpide belt: https://www.youtube.com/watch?v=nupDfh0DeMU The Mid Atlantic Ridge: https://www.youtube.com/watch?v=sgDM6m0lUGY 6 TECTONICS ON A SPHERE Earthquake distribution: Correlation with volcanic activity: Most of the world’s volcanic activity is concentrated along earthquake belts, i.e. the circu-Pacific and part of the Alpide belts. 7 TECTONICS ON A SPHERE Earthquake distribution: The Tectonic Plates:  The seismic belts divide the earth surface into plates.  While some of the plates are huge, e.g. the Pacific, some are tiny, i.e. the Gorda and the Coccos plates. Plate Tectonics: Plate Tectonics Theory (8 minutes) https://www.youtube.com/watch?v=zbtAXW-2nz0 Home assignment: Plate Tectonics (7 minutes ) https://www.youtube.com/watch?v=RA2-Vc4PIOY 8 TECTONICS ON A SPHERE Theory of Plate Tectonics Seven Major Lithosphere Plates 1. North American 2. South American 3. Pacific 4. African 5. Eurasian 6. Australian Indian 7. Antarctic plates Copyright © 2014 Pearson Education, Inc. All rights reserved. 9 TECTONICS ON A SPHERE Theory of Plate Tectonics The Key Principle: Plate Tectonics Basic Assumptions  Generation of new plate material occurs by seafloor spreading; that is, new material is generated along mid-ocean ridges.  The new oceanic lithosphere, once created, forms part of a rigid plate.  The earth’s surface area remains constant; therefore, seafloor spreading must be balanced by consumption of plate elsewhere.  The relative motion between plates is taken up only along plate boundaries. Based on Text book: The Solid Earth, C. Fowler 10 TECTONICS ON A SPHERE Theory of Plate Tectonics  Divergent boundaries  Convergent boundaries Plates move apart Plates move toward each other Magma rises, cools and forms new lithosphere Mountain belts and volcanoes Typically expressed as mid-oceanic ridges common Oceanic plates may sink into  Transform boundaries mantle along a subduction Plates slide past one another zone, typically marked by a Fault zones, earthquakes mark boundary deep ocean trench San Andreas fault in California Copyright © 2014 Pearson Education, Inc. All rights reserved. 11 TECTONICS ON A SPHERE Theory of Plate Tectonics Divergent Boundaries Mostly Oceanic Ridges  Plates move apart  Magma rises, cools and forms new lithosphere  Typically expressed as mid-oceanic ridges  also called accreting or constructive  At such boundaries new plate material, derived from the mantle, is added to the Most divergent plate boundaries are situated along the crests of oceanic lithosphere. ridges- the sites of seafloor spreading Copyright © 2014 Pearson Education, Inc. All rights reserved. 12 TECTONICS ON A SPHERE Theory of Plate Tectonics Divergent Boundaries New oceanic crust is formed at the crust of an oceanic ridge.  Two or more plates pull apart.  Molten material rises through rift zone, with newest magma on either side of rift.  Like conveyer belts, the newer crust travels from the center on each side.  Oceanic crust records reversed and normal polarity episodes. Copyright © 2014 Pearson Education, Inc. All rights reserved. 13 TECTONICS ON A SPHERE Tectonic Plates Boundaries Divergent Boundaries Seafloor Spreading showing progressive age the Rocks Copyright © 2014 John Wiley & Sons, Inc. All rights reserved. 14 TECTONICS ON A SPHERE The Further, the Tectonic Plates Boundaries rock goes the more older and Divergent Boundaries denser it is and it is buried under marine New oceanic crust is formed at the crust of an sediments oceanic ridge.  Seafloor spreading leads to the formation of new crust.  The seafloor grows older as its distance from the rift zone increases.  It cools and becomes denser and is buried under marine sediments Q: where would you expect more sediments/ sand over the rock ? A : further , because sediments need time so you don’t expect to nd sand or sediments above freshly Young rocks ( along the boundaries) Copyright © 2014 John Wiley & Sons, Inc. All rights reserved. 15 TECTONICS ON A SPHERE Theory of Plate Tectonics Divergent Boundaries Continental Rifting Copyright © 2014 Pearson Education, Inc. All rights reserved. 16 TECTONICS ON A SPHERE Theory of Plate Tectonics Divergent Boundaries Copyright © 2014 Pearson Education, Inc. All rights reserved. 17 TECTONICS ON A SPHERE Theory of Plate Tectonics Convergent Boundaries  Plates move toward each other  Mountain belts and volcanoes common  Oceanic plates may sink into mantle along a subduction zone, typically marked by a deep ocean trench and island arc system  also called consuming or destructive  The downgoing plate often penetrates the mantle to depths of about 700 km. Copyright © 2014 Pearson Education, Inc. All rights reserved. 18 TECTONICS ON A SPHERE The denser will be Subducted , If there both oceanic then the older one will be subducted Theory of Plate Tectonics because the older it is the more denser and more compacted rocks Convergent Boundaries Oceanic + Continental Oceanic + Oceanic Copyright © 2014 Pearson Education, Inc. All rights reserved. 19 TECTONICS ON A SPHERE Theory of Plate Tectonics Convergent Boundaries Continental + Continental Copyright © 2014 Pearson Education, Inc. All rights reserved. 20 TECTONICS ON A SPHERE Tectonic Plates Boundaries Convergent Boundaries Two oceanic crust plates collide. Older, denser plate usually subducts, site of island arc formation. geothermal gradient : how much temperature changes with depth. In lithosphere the temperature changes by 25° per km The melting point of the material changes with pressure (more pressure = higher melting point ) lithosphere and asthenosphere material is solid , it just as act like liquid. Any change in the melting point there means that it will start to melt. The friction creates temperature, so as the oceanic plate is subducted it takes water with it , making it easier for the rocks there to become liquid ( melting point + friction temperature) it turns it into a magma. Magma has low density so it goes up and create volcanoes , magma will cool creating rocks that will crate Islands arc within the sea that follows the oceanic trench. Copyright © 2014 John Wiley & Sons, Inc. All rights reserved. 21 TECTONICS ON A SPHERE Tectonic Plates Boundaries Convergent Boundaries Convergence of Eurasian and Indian–Australian Plates over last 10 million years Copyright © 2014 John Wiley & Sons, Inc. All rights reserved. 22 TECTONICS ON A SPHERE Theory of Plate Tectonics Transform Boundaries  Plates slide past one another  Fault zones, earthquakes mark boundary  San Andreas fault in California  Also called conservative boundaries, Copyright © 2014 Pearson Education, Inc. All rights reserved. 23 TECTONICS ON A SPHERE Theory of Plate Tectonics Transform Boundaries  Plates slide past one another  Fault zones, earthquakes mark boundary  San Andreas fault in California Copyright © 2014 Pearson Education, Inc. All rights reserved. 24 TECTONICS ON A SPHERE Theory of Plate Tectonics Transform Boundaries Home assignment: Plate Boundaries and tectonic Plates (6 minutes) https://www.youtube.com/watch?v=Xzpk9110Lyw Copyright © 2014 Pearson Education, Inc. All rights reserved. 25 TECTONICS ON A SPHERE Theory of Plate Tectonics Copyright © 2014 Pearson Education, Inc. All rights reserved. 26 Divergent TECTONICS ON A SPHERE Q: what will happen to the sizes of the plates ? A: it will increase in size because there is new Plate Boundaries on flat Earth: material ( Baslt rocks ) coming, it will increase by 2cm per year Relative Velocities between plates  The velocity of plate A with respect to plate B is written: BVA.  Conversely, The velocity of plate B with respect to plate A is written: AVB.  If you are an observer on plate B, then BVA is the velocity at which you see plate A moving  Note that for spreading ridges, velocities are typically given relative to the spreading center V BVA A B Based on Text book: The Solid Earth, C. Fowler 27  TECTONICS ON A SPHERE Plate Boundaries on flat Earth: Relative Velocities between plates  The double line is the symbol for the ridge axis  the arrows and numbers indicate the direction of spreading and relative movement of the plates away from the ridge.  In this example the half-spreading rate of the ridge (half-rate) is 2 cm yr−1: plates A and B are moving apart at 4 cm yr−1,  Each plate is growing at 2 cm yr−1 V BVA A B Based on Text book: The Solid Earth, C. Fowler 28  TECTONICS ON A SPHERE Azimuth: the true angle with the true North , Clockwise Plate Boundaries on flat Earth: Relative Velocities between plates  A vector has a magnitude and direction  In this case, BVA has a magnitude of 4 and an azimuth of 270° V BVA A B  Based on Text book: The Solid Earth, C. Fowler 29 Convergent TECTONICS ON A SPHERE Plate Boundaries on flat Earth: Relative Velocities between plates  The barbed line is the symbol for a subduction zone  The barbs are on the side of the overriding plate, pointing away from the subducting or downgoing plate.  The arrow and number indicate the direction and rate of relative motion between the two plates. In this example, plate B is being subducted at 10 cm yr−1. Q: what will happen to the sizes of the plates ? A: Plate A , stay the same Plate B , Decrease 10 cm per year Based on Text book: The Solid Earth, C. Fowler 30 Transform TECTONICS ON A SPHERE Q: what will happen to the sizes of the plates ? Plate Boundaries on flat Earth: A: transform boundaries they don’t add or consume Relative Velocities between plates Plate A , stay the same Plate B , stay the same  The single line is a symbol for a transform (conservative) boundary  The half-arrows and number indicate the direction and rate of relative motion between the plates  in this example, 6 cm yr−1.  The relative velocities AVB and BVA for the transform fault shown in (e). 31 TECTONICS ON A SPHERE Type of Plate Boundaries: Transform Boundaries The six types of dextral (right-handed) transform faults. There are also six sinistral (left-handed) transform faults, mirror images of those shown here. a) Ridge–ridge fault b) and (c) ridge–subduction-zone fault, (d) and (e) and (f) subduction-zone– subduction-zone fault.(After Wilson (1965). Based on Text book: The Solid Earth, C. Fowler 32 TECTONICS ON A SPHERE Continental plates will be subducted but they will reach to a certain level where they Type of Plate Boundaries: can’t be subducted anymore because the density is very high so they can’t go below so it start rising up (always there is a subduction zone )  Usually only the oceanic part of any plate is created or destroyed. Obviously, seaﬂoor spreading at a mid-ocean ridge produces only oceanic lithosphere. At subduction zones, where continental and oceanic materials meet, it is the oceanic plate which is subducted (and thereby destroyed). It is probable that, if the thick, relatively low-density continental material (approximately 2.8 × 103 kg m−3) reaches a subduction zone, it may descend a short way, but, because the mantle density is so much greater (approximately 3.3 × 103 kg m−3), the downwards motion does not continue. Instead, the subduction zone ceases to operate at that place and moves to a more favourable location (mountains). Based on Text book: The Solid Earth, C. Fowler 33 TECTONICS ON A SPHERE Plate Boundaries on flat Earth: Two Plates  In some cases, a known plate margin type and the relative velocity across the margin can be used to deduce the types of other unknown margins  If we know that spreading is orthogonal to the spreading center at a nonzero rate, what kind of margins must exist on the other sides of plate B?  Clearly, we can determine the types of the other plate margins Based on Text book: The Solid Earth, C. Fowler 34 TECTONICS ON A SPHERE Plate Boundaries on flat Earth: Two Plates  The western boundary of plate B is a ridge, which is spreading at a half-rate of 2 cm yr-1.  we can see that its northern and southern boundaries must be transform faults.  Since AVB is equal to 4 cm yr-1, the eastern boundary is a subduction zone. Based on Text book: The Solid Earth, C. Fowler 35 TECTONICS ON A SPHERE Plate Boundaries on flat Earth: Two Plates  On the eastern boundary of plate B: Either plate A is subducting underneath plate B and the length of plate B increases by 2 cm yr- 1, or plate B is subducting underneath plate A and plate B is being destroyed at a rate of 2 cm Sinistral, or left-handed; rocks are offset to the left as you cross the fault yr-1. so eventually plate B will cease to exist on the surface of the planet. dextral,or right-handed; rocks are offset to the right as you cross the fault Based on Text book: The Solid Earth, C. Fowler 36 TECTONICS ON A SPHERE Plate Boundaries on flat Earth: Three Plates Three-plate model on a ﬂat planet. Plate A is unshaded.  If we introduce a third plate into the model, the motions become more complex as different plates will move with different relative velocities  The western boundary of plate B is a ridge spreading at a half-rate of 2 cm yr−1.  The boundary between plates A and C is a subduction zone with plate C overriding plate A at 6cmyr−1.  What is CVB, the velocity of plate B viewed by an observer standing on plate C? Based on Text book: The Solid Earth, C. Fowler 37 TECTONICS ON A SPHERE Plate Boundaries on flat Earth: Three Plates Three-plate model on a ﬂat planet. Plate A is unshaded. The solution to the model :  the northern and southern boundaries of plate B are transform faults,  In this situation, velocity CVB is intuitive, as all vectors are in the same direction, but that isn’t often the case  Vector addition to determine the velocity of plate B with respect to plate C, CVB. cV b = cVa + aVb Based on Text book: The Solid Earth, C. Fowler 38 TECTONICS ON A SPHERE Plate Boundaries on flat Earth: Three Plates  Plates A and B are spreading away at a half-rate of 2 cm yr-1.  Plate A being subducted underneath plate C at a rate of 6 cm yr-1.  To determine the relative rate between plate B and C we use vector addition: cVb = cVa + aVb  Plate B is being subducted beneath plate C at 10 cm yr−1.  This means that the net rate of destruction of plate B is 10 − 2 = 8cmyr−1  However, if plate B were overriding plate C, it would be increasing in width by 2 cm yr−1 cVb = cVa + aVb Based on Text book: The Solid Earth, C. Fowler 39 TECTONICS ON A SPHERE Plate Boundaries on flat Earth: Three Plates Now we consider three plates with a more complex geometry  The western boundary of plate B is a ridge from which seaﬂoor spreads at a half-rate of 2cmyr−1.  The boundary between plates A and C is a transform fault with relative motion of 3cmyr−1.  What is CVB, the velocity of plate B viewed by an observer standing on plate C? Based on Text book: The Solid Earth, C. Fowler 40 TECTONICS ON A SPHERE Plate Boundaries on flat Earth: Three Plates Now let us include motion in the north–south direction The stable solution to the model in  the northern boundary of plate B is a transform fault with a 4cmyr−1 slip rate,  Vector addition to determine the velocity of plate B with respect to plate C, cVb. cVb = cVa + aVb Based on Text book: The Solid Earth, C. Fowler 41 TECTONICS ON A SPHERE Plate Boundaries on flat Earth: Three Plates  Plates A and B are spreading away at a half-rate of 2 cm yr-1.  The boundary plates A and C is a transform fault with a relative motion of 3 cm yr-1.  Again, to determine the relative rate between plate B and C we use vector addition as before.  So either plate B is subducting underneath C (as shown), or C is subducting under B.  Case 1: plate B undergoes oblique subduction beneath plate C at 5 cm yr−1  Case 2: plate C to be subducted beneath plate B at 5 cm yr−1  In case2, the boundary between plates C and B would not remain collinear with the boundary between plates A and C but would cVb = cVa + aVb move steadily to the east Based on Text book: The Solid Earth, C. Fowler 42 TECTONICS ON A SPHERE this axis is used as a reference point to any Plate Tectonics on spherical Earth: displacement happening on the Earth surface Rotation axes and rotation poles  To describe motions on the surface of a sphere we use Euler’s ‘ﬁxed-point’ theorem  Euler’s fixed point theorem describes motion on the surface of a sphere: “the most general displacement of a rigid body with a ﬁxed point is equivalent to a rotation about an axis through that ﬁxed point.”  Taking a plate as a rigid body and the center of the Earth as a ﬁxed point:  Euler’s fixed point theorem: “Every displacement from one position to another on the surface earth can be regarded as a rotation about a suitably chosen axis passing through the center of the earth.” Based on Text book: The Solid Earth, C. Fowler 43 TECTONICS ON A SPHERE Plate Tectonics on spherical Earth: Rotation axes and rotation poles  The axis of rotation is the suitably chosen axis passing through the center of the earth.  The poles of rotation or the Euler’s poles are the two points where the axis of rotation cuts through the earth surface.  These are purely mathematical points and have no physical reality, but their positions describe the directions of motion of all points along the plate boundary Home assignment (angular Velocity-3min): https://www.youtube.com/watch?v=ZzFtX14I9O8 Based on Text book: The Solid Earth, C. Fowler 44 TECTONICS ON A SPHERE Plate Tectonics on spherical Earth: Rotation axes and rotation poles  The sign convention used is that a rotation that is clockwise (or right-handed) when viewed from the center of the Earth along the rotation axis is positive.  Viewed from outside the Earth, a positive rotation is anticlockwise.  Thus, one rotation pole is positive and the other is negative  The magnitude of the angular velocity about the axis then deﬁnes the magnitude of the relative motion between the two plates This is how Earth's poles wobble over time https://www.youtube.com/watch?v=peUrvFFC6Zc Home assignment: The axis of rotation (3 minutes): https://www.youtube.com/watch?v=9n04SEzuvXo Based on Text book: The Solid Earth, C. Fowler 45 TECTONICS ON A SPHERE Linear Velocity ; the rate at which the location changes per second ( how much distance Plate tectonics on spherical Earth: you cover per second) Angular velocity ; the rate at which the angle changes per second ( when you rotate the Angular velocity and relative velocity angle change) Units for angular velocity: degrees per second or Rad  The relative velocity, , of a certain point on the earth surface is a function of the angular velocity, , according to: v  Rsin, where R is the earth radius and  is the angular distance between the pole of rotation and point in question.  The relative motion between two adjacent plates changes with position along the plate boundary  Thus, the relative velocity is equal to zero at the poles, where =0 degrees, and is a   maximum at the equator, where =90 degrees. The relative velocity is constant along small circles defined by =constant.  Note that large angular velocity does not mean large relative velocity. Based on Text book: The Solid Earth, C. Fowler 46 TECTONICS ON A SPHERE Plate tectonics on spherical Earth: Angular velocity and relative velocity Question: is it possible to have a plate who’s relative motion is constant at all directions? Answer: The relative velocity is constant along small circles defined by =constant pole of rotation Tectonics on a sphere (4 minutes): https://www.youtube.com/watch?v=6beUTsVq-6I Based on Text book: The Solid Earth, C. Fowler 47 TECTONICS ON A SPHERE Determination of rotation poles and rotation vectors: To find Present-day instantaneous poles of rotation and relative angular velocities between pairs of plates 1. The strike of active transform faults. 2. The spreading rate along a constructive plate boundary changes 3. The analysis of data from an earthquake 4. surveys of displacements can be used along cross land plate boundaries 5. Satellites laser-ranging system (SLR) Very-long-baseline interferometry (VLBI), Global Positioning System(GPS) Earthquake our major source of destruction and the same time our major source of information about earth interior. ( Deepest man made equipment is 12Km ) Based on Text book: The Solid Earth, C. Fowler 48 TECTONICS ON A SPHERE Determination of rotation poles and rotation vectors: To find Present-day instantaneous poles of rotation and relative angular velocities between pairs of plates 1. A local determination of the direction of relative motion between two plates can be made from the strike of active transform faults. The relative motion at transform faults is parallel to the fault and is of constant value along the fault. This means that the faults are arcs of small circles about the rotation pole. The rotation pole must therefore lie somewhere on the great circle which is perpendicular to that small circle So, if two or more transform faults can be used, the intersectionSan Andreas Fault https://www.youtube.com/watch?v=ZxPTLmg0ZCw of the great circles is the position of the rotation pole Based on Text book: The Solid Earth, C. Fowler 49 TECTONICS ON A SPHERE Determination of rotation poles and rotation vectors: To find Present-day instantaneous poles of rotation and relative angular velocities between pairs of plates 2. The spreading rate along a constructive plate boundary changes as the sine of the angular distance θ from the rotation pole. If the spreading rate at various locations along the ridge can be determined (Oceanic magnetic anomalies) The rotation pole and angular velocity can then be estimated. v  R sin  3. The analysis of data from an earthquake can give the direction of motion and the lane of the fault on which the earthquake occurred (fault plane solution). Plane of the fault on which the earthquake occurred can give the direction of relative motion between the two plates Based on Text book: The Solid Earth, C. Fowler 50 TECTONICS ON A SPHERE Plate Tectonics on spherical Earth: To find Present-day instantaneous poles of rotation and relative angular velocities between pairs of plates  Relative plate motion: direction given by transform directions  Transform faults = small circles about the rotation pole  Conversely, rotation pole located on great circle perpendicular to transform direction  Relative plate motion rate given by sea-floor magnetic isochron.  Relative plate motion direction also given by earthquake slip vectors Based on Text book: The Solid Earth, C. Fowler 51 TECTONICS ON A SPHERE Determination of rotation poles and rotation vectors: To find Present-day instantaneous poles of rotation and relative angular velocities between pairs of plates 4. Where plate boundaries cross land, surveys of displacements can be used (over large distances and periods of time) to determine the local relative motion For example, stream channels and even roads, ﬁeld boundaries and buildings may be displaced. 5. Satellites have made it possible to measure instantaneous plate motions with some accuracy. One method uses a satellite laser-ranging system (SLR) to determine differences in distance between two sites on the Earth’s surface over a period of years. Very-long-baseline interferometry (VLBI), uses quasars for the signal source and terrestrial radio telescopes as the receivers Utilizing the Global Positioning System(GPS) which was developed to provide real-time navigation and positioning using satellites : (International GPS Service for Geodynamics (IGS), and is a permanent global network of receivers) Based on Text book: The Solid Earth, C. Fowler 52 TECTONICS ON A SPHERE Determination of rotation poles and rotation vectors: satellite laser-ranging system (SLR)  Measurement of distance (=range) between a ground station and a satellite = Satellite Laser Ranging (SLR)  Ground station transmits a very short laser pulse from a telescope to a satellite  The laser pulse is retro-reflected by corner cube reflectors on the satellite back to the ground telescope  Very precise clock at the ground station measures the round trip time  Time measurement accuracy < 50 picoseconds, or < 1 Tracking a satellite with a network of SLR stations Based on Text book: The Solid Earth, C. Fowler 53 TECTONICS ON A SPHERE Determination of rotation poles and rotation vectors: satellite laser-ranging system (SLR) SLR at the Goddard Geophysical and  3 stations, 1 satellite => position of the satellite (if Astronomical Observatory. The two laser station position known beams are coming from the network standard  3 satellites, 1 station => position of the station (if SLR station, MOBLAS-7 (MOBile LASer) and the satellite orbit known smaller TLRS-3 (Transportable Laser Ranging System) during a collocation exercise. Starlette, a geodetic satellite Launched in 197548 cm diameter, 47 kg Based on Text book: The Solid Earth, C. Fowler 54 TECTONICS ON A SPHERE Determination of rotation poles and rotation vectors: Very-long-baseline interferometry (VLBI)  Radio-astronomy technique, used to locate and map stars, quasars, etc = “sources”  Wavelength = 1-20cm  Measures the time difference between the arrival at two Earth- based antennas of a radio wavefront emitted by a distant quasar  If the source positions are known => ground baseline => “geodetic” VLBI  Time measurements precise to a few picoseconds, => relative positions of the antennas to a few millimeters Quasares: a massive and extremely remote celestial object, emitting exceptionally large amounts of energy, and typically having a starlike image in a telescope. VLBI antenna at Algonquin, Canada Based on Text book: The Solid Earth, C. Fowler 55 TECTONICS ON A SPHERE Determination of rotation poles and rotation vectors: GPS Measurements  GPS measurements in Antarctica (red arrows)  Used to estimate the rotation of Antarctica w.r.t Africa Based on Text book: The Solid Earth, C. Fowler 56 TECTONICS ON A SPHERE Determination of rotation poles and rotation vectors: To find Present-day instantaneous poles of rotation and relative angular velocities between pairs of plates  Systematic measurement of all transform fault directions  Systematic identification of the most recent magnetic isochron  Plate-circuit closure condition  Global plate motion model = e.g. NUVEL1 Based on Text book: The Solid Earth, C. Fowler 57 TECTONICS ON A SPHERE Determination of rotation poles and rotation vectors: To find Present-day instantaneous poles of rotation and relative angular velocities between pairs of plates NUVEL-1A : An estimate of the present-day plate motions,, made by using 277 measurements of ridge spreading rate, 121 oceanic transform-fault azimuths and 724 earthquake slip vectors is given in Table 2.1 5 methods to help us locate the poles to calculate the angular velocity 1.Faults 2.earthquakes 3.consecutive boundary (divergent) 4. surveys 5.satellite It is important to realize that a rotation with a large angular velocity ω does not necessarily mean that the relative motion along the plate boundary is also large. Based on Text book: The Solid Earth, C. Fowler 58 TECTONICS ON A SPHERE Plate Boundaries can change with time:  plates and plate boundaries do not stay the same for all time  the formation of new plates and destruction of existing plates are the most obvious global reasons why plate boundaries and relative motions change  plate may be lost down a subduction zone  two continental plates may coalesce (become) into one (with resultant mountain building).  If the position of a rotation pole changes, all the relative motions also change.  A drastic change in pole position of say 90◦ would, of course, completely alter the status quo Based on Text book: The Solid Earth, C. Fowler 59 TECTONICS ON A SPHERE Plate Boundaries can change with time:  The formation of new plates and the destruction of existing plate are the most obvious reasons why plate boundaries and relative motion change.  For example, the Farallon and the Kula plates were subducted underneath north America 20-30 million years ago. Based on Text book: The Solid Earth, C. Fowler 60 TECTONICS ON A SPHERE Plate Boundaries can change with time: Q : Are Location of the boundaries between plates xed? A: No, plate size / shape changes Based on Text book: An Introduction to Physical 61 Geology , Tarbuck et al TECTONICS ON A SPHERE Plate Boundaries can change with time: The topography of the Atlantic Ocean bottom is shown as if the ocean had been removed. The blue lines indicate the edges of the ocean tectonic plates. The yellow dots indicate locations of earthquakes that have occurred in the period 1960 to 1985. The red triangles are the locations of volcanic eruptions that have occurred in the period 1980 to 1995. Both the earthquakes and the volcanic eruptions follow the plate boundaries. Based on Text book: The Solid Earth, C. Fowler 62 TECTONICS ON A SPHERE Plate Boundaries can change with time: Parts of plate boundaries can change locally, however, without any major ‘plate’ or ‘pole’ event occurring.  convergent boundary between plates A and B,  Strike–slip faults between plates A and C and plates B and C  Relative velocity vectors for the plates are given  From the point of view of an observer on plate C, part of the boundary of C (circled) will change with time because the plate to which it is adjacent will change from plate A to plate B.  The slip rate will change from 2 cm yr−1 to 6 cmyr−1 Based on Text book: The Solid Earth, C. Fowler 63 TECTONICS ON A SPHERE Plate Boundaries can change with time:  The boundary between plates A and C is a strike–slip fault  Boundary between plates A and B is a ridge  Boundary between plates B and C is a subduction zone  The motions are such that the ridge migrates slowly to the south relative to plate C,  The circled portion of plate boundary will change with time from subduction zone to transform fault. These local changes in the plate boundary are a geometric, consequence of the motions of the three rigid plates rather than being caused by any disturbing outside event. Based on Text book: The Solid Earth, C. Fowler 64 TECTONICS ON A SPHERE Triple Junctions :  Triple junction is a point at which three plates meet.  A triple junction is stable if the relative motion of the three plates and the azimuth of their boundaries do not change in time.  An unstable triple junction exist only momentarily before evolving to a different geometry.  If four or more plates meet at one point, the For example, triple junction between conﬁguration is always unstable, and the three ridges is always stable - why? system will evolve into two or more triple junctions. Any unstable triple junction is temperer cause after a while will have to be stable, by changing the geology but it will be no longer a triple junction Based on Text book: The Solid Earth, C. Fowler 65 TECTONICS ON A SPHERE Triple Junctions Examples:  Plate A overrides plates B and C,  Plate C overrides plate B  AVB, CVB and AVC are the relative velocities of the three plates in the immediate vicinity of the triple junction. Based on Text book: The Solid Earth, C. Fowler 66 TECTONICS ON A SPHERE Triple Junctions :  The geometry of the three subduction zones at some time later than  The dashed lines show where plates B and C would have been had they not been subducted.  The point X in (a) was originally on the boundary between plates A and B; now it is on the boundary between plates A and C  The original triple junction has changed its form. Based on Text book: The Solid Earth, C. Fowler 67 TECTONICS ON A SPHERE Triple Junctions :  A ridge is written as R, a transform fault as F and a subduction zone (or trench) as T.  Thus, a ridge–ridge–ridge junction is RRR, a fault–fault–ridge junction is FFR, and so on.  Sixteen possible types of triple junction. Of these sixteen triple junctions, one is always stable (the RRR junction) if oblique spreading is not allowed  Two are always unstable (the FFF and FFR junctions). Based on Text book: The Solid Earth, C. Fowler 68 TECTONICS ON A SPHERE Triple Junctions : Based on Text book: The Solid Earth, C. Fowler 69 TECTONICS ON A SPHERE Triple Junctions : Based on Text book: The Solid Earth, C. Fowler 70 TECTONICS ON A SPHERE Significance of Triple Junctions-Examples: The Azores triple junction Figure from: www.ija.csic.es/gt/ivone/research_AFEU.html Based on Text book: The Solid Earth, C. Fowler 71 TECTONICS ON A SPHERE Significance of Triple Junctions-Examples: The Afar rift Figure from Hugh Rance site: https://www.geowords.com Africa splitting in two 2:30 https://www.youtube.com/watch?v=_Rxp3HQDUvY Based on Text book: The Solid Earth, C. Fowler 72 TECTONICS ON A SPHERE Absolute Plate Motions: We have 14 hotspots around the world  There is no ﬁxed point on the Earth’s surface.  Absolute plate motions are motions of the plates relative to some imaginary ﬁxed point.  One way of determining absolute motions is to suppose that the Earth’s mantle moves much more slowly than the plates so that it can be regarded as nearly ﬁxed.  Such absolute motions can be calculated from the traces of the oceanic island chains or the traces of continental volcanism which are assumed to have formed as the plate passed over a hotspot with its source ﬁxed in the mantle Hot spots: Hot spot volcanoes and plate movement (2:30): https://www.youtube.com/watch?v=umhBT_k8e5c For home: Hot spots and plate motion (4:30) : https://www.youtube.com/watch?v=8zVqsx3OtiI Based on Text book: The Solid Earth, C. Fowler 73 TECTONICS ON A SPHERE Absolute Plate Motions:  some isolated volcanic island chains occur in the oceans away from plate boundaries  These chains of oceanic islands are unusual because they occur well away from the plate boundaries and the chemistry of the erupted lavas is signiﬁcantly different from that of both mid-ocean-ridge and subduction zone lavas  The active volcano may be at one end of the island chain, with the islands ageing with distance from that active volcano; and the island chains appear to be arcs of small circles.  These features, taken together, are consistent with the volcanic islands having formed as the plate moved over what is called a hotspot, a place where melt rises from deep in the mantle Based on Text book: The Solid Earth, C. Fowler 74 TECTONICS ON A SPHERE Absolute Plate Motions: The global distribution of hotspots (grey squares) and associated volcanic tracks. (After Norton, I. O. Global hotspot reference frames and plate motion. Geophysical Monograph 121, 339–57, 2000 Based on Text book: The Solid Earth, C. Fowler 75 TECTONICS ON A SPHERE Absolute Plate Motions: Four volcanic island chains in the Paciﬁc Ocean. The youngest active volcano is at the southeast end of each chain. (From Dalrymple et al. (1973)) Based on Text book: The Solid Earth, C. Fowler 76 TECTONICS ON A SPHERE Absolute Plate Motions:  A demonstration of the relative motions between the hotspot (ﬁxed in the mantle) and the seamount chain on the overriding plate. The pencil represents the hotspot, which is ﬁxed in the mantle (rectangular grid). The plate moves over the mantle and the pencil marks the line of seamounts (the ‘hotspot track’). The star is the position of a seamount. After formation of the seamount, the plate moves north for one unit and then west for one unit so leaving a solid (pencil) line of seamounts, the hotspot track. The ‘ﬂow-line’, the relative motion of the plate with respect to the hotspot, is the dashed line. (Reprinted with permission from Nature (Stein Nature 387, 345–6) Copyright 1997 Macmillan Magzines Ltd.) Based on Text book: The Solid Earth, C. Fowler 77 TECTONICS ON A SPHERE Absolute Plate Motions:  The hotspot reference frame therefore is the motions of the plates relative to the hotspots, which are assumed to be ﬁxed in the mantle.  The ‘hotspot track’, the linear chain of volcanic islands and seamounts, is the path of the hotspot with respect to the overlying plate.  The slow steady motion of the oceanic plate with respect to the hotspot (the ﬂowline) is not marked by any feature, however.  Plate motions relative to hotspots cannot be estimated as accurately as can relative this is because hotspot tracks have average widths in excess of 100 km, which is orders of magnitude greater than the width of active transform faults (

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