ERT

1. UNIT-5
Lithospheric plate reconstruction
► Evidence for plates in motion.
►plate configuration in the past.
►Eular rotation
►methods of plate reconstruction

2. • Plate reconstruction is the process of reconstructing the
positions of tectonic plates relative to each other (relative
motion) or to other reference frames, such as
the earth's magnetic field or groups of hotspots, in the geological
past. This helps determine the shape and make-up of
ancient supercontinents and provides a basis
for paleogeographic reconstructions.

3. Plate motion -Euler’s
contributions
Leonhard Euler

4. • In the 18th century, Swiss mathematician Leonhard Euler showed that the movement
of a rigid body across the surface of a sphere can be described as a rotation (or
turning) around an axis that goes through the centre of the sphere, known as the axis
of rotation. The location of this axis bears no relationship to Earth’s spin axis. The
point of emergence of the axis through the surface of the sphere is known as the pole
of rotation. This theorem of spherical geometry provides an elegant way to define the
motion of the lithospheric plates across Earth’s surface. Therefore, the relative motion
of two rigid plates may be described as rotations around a common axis, known as the
axis of spreading.
• the plates move along transform faults, whose trace defines circles of latitude
perpendicular to the axis of spreading, and so form small circles around the pole of
rotation. A geometric necessity of this theorem—that lines perpendicular to the
transform faults converge on the pole of rotation—is confirmed by measurements.
• According to this theorem, the rate of plate motion should be slowest near the pole of
rotation and increase progressively to a maximum rate along fractures with a 90°
angle to it. This relationship is also confirmed by accurate measurements of seafloor-
spreading rates.

5. The movement of a rigid body, such as a plate, on the surface of a sphere
can be described as rotation about a fixed axis (relative to the chosen
reference frame). This pole of rotation is known as an Euler pole.
The movement of a plate is completely specified in terms of its Euler
pole and the angular rate of rotation about the pole. Euler poles defined
for current plate motions can be used to reconstruct plates in the recent
past (few million years).At earlier stages of earth's history, new Euler poles
need to be defined.
Euler poles

6. An important part of reconstructing past plate configurations is to define the edges of areas
of the lithosphere that have acted independently at some time in the past.
Defining plate boundaries
Plate re construction

7. Present plate boundaries
Most present plate boundaries are easily identifiable from the pattern of recent seismicity.
This is now backed up by the use of geodetic data, such as GPS/GNSS, to confirm the
presence of significant relative movement between plates.
Past plate boundaries
Identifying past (but now inactive) plate boundaries within current plates is generally based
on evidence for an ocean that has now closed up. The line where the ocean used to be is
normally marked by pieces of the crust from that ocean, included in the collision zone, known
as ophiolites. The line across which two plates became joined to form a single larger plate, is
known as a suture. In many orogenic belts, the collision is not just between two plates, but
involves the sequential accretion of smaller terranes. Terranes are smaller pieces of
continental crust that have been caught up in an orogeny, such as continental
fragments or island arcs.

8. • Past plate movements
• Plate tectonics involves the movements of Earth’s lithospheric plates relative to one another over the
planet’s weak asthenosphere. This activity changes the positions of all plates with respect to Earth’s
spin axis and the Equator. To determine the true geographic positions of the plates in the past,
investigators have to define their motions, not only relative to each other but also relative to this
independent frame of reference. Hotspots, as classically interpreted, provide an example of such a
reference frame, assuming they are the sources of plumes that originate within the deep mantle and
have relatively fixed positions over time.
• If this assumption is valid, the motion of the lithosphere above these plumes can be deduced. The
hotspot island chains serve this purpose, their trends providing the direction of motion of a plate. The
speed of the plate can be inferred from the increase in age of the volcanoes along the chain relative to
the distance between the islands.
• Earth scientists are able to accurately reconstruct the positions and movements of plates for the past
150 million to 200 million years because they have the oceanic crust record to provide them with
plate speeds and direction of movement. However, since older oceanic crust is
continuously consumed to make room for new crust, this kind of evidence is not available for earlier
intervals of geologic time, making it necessary for investigators to turn to other, less-precise
techniques.

9. Ages of oceanic lithosphere
Estimating past plate motions
In order to move plates
backward in time it is
necessary to provide
information on either relative
or absolute positions of the
plates being reconstructed
such that an Euler pole can
be calculated. These are
quantitative methods of
reconstruction.

10. Geometric matching of continental borders
Certain fits between continents, particularly that
between South America and Africa, were known long
before the development of a theory that could
adequately explain them. The reconstruction before
Atlantic rifting by Bullard based on a least-squares
fitting at the 500 fathom contour still provides the
best match to paleomagnetic pole data for the two
sides from the middle of Paleozoic to Late Triassic.

11. Plate motion from magnetic stripes
• Plate reconstructions in the recent geological past mainly use
the pattern of magnetic stripes in oceanic crust to remove the
effects of seafloor spreading. The individual stripes are dated
from magnetostratigraphy so that their time of formation is
known. Each stripe (and its mirror image) represents a plate
boundary at a particular time in the past, allowing the two plates
to be repositioned relative to one another.
• The oldest oceanic crust is Jurassic, providing a lower age limit
of about 175 Ma for the use of such data. Reconstructions
derived in this way are only relative

12. Plate reconstructions from paleomagnetism
Paleomagnetic data: Sampling
• Paleomagnetic data different rock types. In igneous rocks,
magnetic minerals crystallize from the melt, and when the rock
is cooled below their Curie temperature, sedimentary
rocks, Metamorphic rocks, Various rock-magnetic
• Paleomagnetic poles
Curie point, also called Curie Temperature, temperature
at which certain magnetic materials undergo a sharp
change in their magnetic properties. In the case of
rocks and minerals, remanent magnetism appears below
the Curie point—about 570 °C (1,060 °F) for the common
magnetic mineral magnetite.

13. Paleogeographic reconstruction of the Pangea
supercontinent at the Permo-Triassic Boundary (250
Ma). Top panel: Synthetic APWP for Africa (the south
paleomagnetic poles are shown with their 95%
uncertainty ovals). The red dot highlights the 250 Ma
paleomagnetic pole. APWP data are from Torsvik et al.
(2012). Middle panel: All continents are assembled in the
Pangea configuration at 250 Ma using the estimates of
their relative motions, with Africa kept fixed in its present
position. The red triangle shows the position of the Euler
pole and the red arrow indicates the rotation that would
reconstruct the paleomagnetic pole to the south
geographic pole. Bottom panel: The Euler rotation has
been applied to Pangea, which is now reconstructed
paleogeographically. The longitude is arbitrary set to
minimize the longitudinal motion of Africa since 250 Ma.
paleogeographic reconstructions

14. Alfred Wegener and the concept of continental
drift
Wegener came to consider the existence of supercontinent from about 350 million to 245 million years ago,
during the late Paleozoic Era and early Mesozoic Era, and named it Pangea, meaning “all lands.” He searched the
geologic and paleontological literature for evidence supporting the continuity of geologic features across the
Indian and Atlantic oceans during that time period, which he assumed had formed during the Mesozoic
Era (about 252 million to 66 million years ago).

16. • Apparent polar wander paths
• Longitude constraints
• Apparent polar wander paths geometric parameterizations
• Paleomagnetic Euler poles derived by geometrizing apparent polar wander paths
(APWPs) potentially allows constraining paleolongitudes from paleomagnetic data. This
method could extend absolute plate motion reconstructions deeply into the geologic
history as long as there are reliable APWPs.
• Hotspot tracks
• Slab constraints
Once oceanic plates subduct in the lower mantle
(slabs), they are assumed to sink in a near-vertical
manner. With the help of seismic wave tomography,
this can be used to constrain plate reconstructions
at first order back to the Permian

17. isostasy
• ideal theoretical balance of all large portions
of Earth’s lithosphere as though they were floating on the denser
underlying layer, the asthenosphere, a section of the upper
mantle composed of weak, plastic rock that is about 110 km (70
miles) below the surface. Isostasy controls the regional
elevations of continents and ocean floors in accordance with
the densities of their underlying rocks.

18. • Driving forces
• The main stumbling block to the acceptance of Wegener’s hypothesis
was the driving forces he proposed. Another mechanism proposed by
Wegener, tidal forces on Earth’s crust produced by gravitational pull
of the Moon, were also shown to be entirely inadequate.
• Wegener’s proposition was attentively received by many European
geologists, and in England Arthur Holmes pointed out that the lack
of a driving force was insufficient grounds for rejecting the entire
concept. In 1929 Holmes proposed an alternative mechanism—
convection of the mantle—which remains today a serious candidate
for the force driving the plates. After Wegener’s death, Du Toit
continued to amass further evidence in support of continental drift.

19. Plate-driving mechanisms and the role of the mantle
Mantle convection

20. Continental reconstructions

21. • Magnetic anomalies, transform faults, hotspots, and apparent polar wandering paths permit
rigorous geometric reconstructions of past plate positions, shapes, and movements.
• Although some important controversies remain, these paleogeographic reconstructions show the
changing geography of Earth’s past and can be determined with excellent precision for the past 150
million years. A variety of geologic data are used to help determine the proper fit of continents
through time. Some of the methods used to test these reconstructions are based on matching
patterns from one continental block to another and are similar to the approach of Wegener.
• However, modern geoscientists have more precise data that help constrain these reconstructions.
the most significant are the improved analytical techniques for radiometric dating, allowing the
age of geologic events. One of the most common methods used measures the radioactive decay of
uranium to lead in the mineral zircon by comparing the ratio of one to the other in the sample of
zircon. Zircon is a common accessory mineral in igneous, metamorphic, and sedimentary rocks.

22. • Since the 1990s the database has improved so that
reasonably constrained reconstructions can now be made as far back as 1 billion
years.
• For example, the abundance of continental-collisional events about 1.1 billion
years ago is one of the principal lines of evidence suggesting the presence of a
supercontinent that is given the name of Rodinia. about 750 million years ago a
number of continental-rift sequences had developed, suggesting that Rodinia had
begun to break up. Between about 650 million and 550 million years ago,
however, a number of mountain belts formed by continental collision, which
resulted in the amalgamation of Gondwana, the supercontinent originally
identified by Du Toit in 1937. The continental fragment that rifted away from
Laurentia did not return to collide with North America as predicted by a simple
Wilson cycle. Instead, it rotated counterclockwise away from Laurentia until it
collided with eastern Africa.