Download Why the Philippine Sea Plate Moves as It Does

1) How long does it take for ocean lithosphere that formed in the hot (> 1000 oC)
MOR environment to cool to an equilibrium state and sink to its maximum depth
below
sea
level?
One answer only.
approx. 5 m.y.
approx. 50 m.y.
approx. 100 m.y.
approx. 150 m.y.
2)
Which
of
One answer only.
the
following
will
subduct
most
readily?
common
feature?
oceanic plateau
old cold oceanic crust
continental crust
young hot oceanic crust
3) Where would
One answer only.
east Pacific
Indian ocean
Atlantic ocean
west Pacific
you
find
marginal
basins
are
a
4) Which of
One answer only.
the
following
best
describes
a
marginal
basin?
a small ocean basin surrounded by continents
a large ocean basin adjacent to a continental arc system
a large ocean basin separated from a continent by an island arc
a small ocean basin between a continent and an island arc
5) Which of the following was formed by old back arc spreading or represents
normal ocean crust that has been trapped behind a recently developed oceanic
island
arc?
One answer only.
inactive marginal basin with high heat flow
inactive marginal basin with normal heat flow
active marginal basin with high heat flow
6) In the diagram below, what does the ? represent?
Type your answer in the textarea below.
7) Geophysical evidence suggests that there is a low-Q zone behind the arc. This is
compatible
with
which
of
the
following?
One answer only.
extension in the back arc region
magnetic anomalies in the back arc basin
melting in the back arc region
sediments deposited in the back arc region
8) Which of the following models for back arc spreading involves; roll-back of the
slab
and
regional
extensional
stresses
in
the
lithosphere?
One answer only.
passive diapirism
active diapirism
stepwise migration
convection driven
9) Which of the following models for back arc spreading involves; snapping off of
the subducting slab, and initiation of a new subduction zone oceanwards?
One answer only.
passive diapirism
active diapirism
stepwise migration
convection driven
10) The angle of dip of the subducting slab appears to govern whether or not
extension occurs in the back arc region. Which of the following is more likely to
promote
extensional
conditions
in
the
back
arc
region?
One answer only.
steep dip
shallow dip
Structural Geology GEOL 3512 Fall 1998
Mid-term review sheet
Here are some highlights of the things that we've covered so far. Any of this material,
plus anything from the covered text chapters (2,3,4,14,15,16, and 18) and from your class
notes, can and may be on the exam.
Whole Earth and Plate Tectonics
We started off talking about the structure of the whole Earth, which means how many and
how thick are its layers and how we obtain this view of the Earth. Seismology is the main
tool by which this view (or model) is obtained. Seismic discontinuities reveal a mantle
that has layer boundaries at ~400 and 670 km; the origin of these discontinuities is
probably related to a phase change that occurs in Olivine, from Spinel to Perovskite. Both
P and S waves take a jump at these boundaries, the deeper of which is related (it is
thought) to the structure of the convecting mantle.
The mantle convects as either a two-layer system or as one layer; there is no consensus.
Seismic images (ie catscan-like images of the distribution of seismic velocities) of the
mantle show that subducting plates reach at least as deep as 670 km and in places perhaps
deeper. Some plates seem to bottom out at this level,which implies that convection is
limited to the upper mantle. There is some evidence, however, that plates can puncture
the 670 level and move into the lower mantle. It has been suggested (and as far as I know
it seems to a popular idea) that the mantle probably convects as a two-layer system most
of the time, but that there are times when a large turn-over occurs and the entire mantle is
involved.
What sort of arguments might you assemble that would suggest a solution to this issue?
(Perhaps geochemical.) This will not be on the exam!
Lower seismic boundaries include the mantle-core boundary and the inner core boundary.
These are also phase changes; the mantle is literally freezing from the core (and releasing
huge amounts of heat as it does so). The inner core is solid. How do we know this?
The upper mantle, above ~400 km, is the asthenosphere, the upper part of which contains
a small fraction (~8%) of melt (the so-called low-velocity zone). This is the shear
boundary across which the rigid lithosphere moves as a plate - the very plate of plate
tectonics.
The lithosphere is divided too into layers: upper mantle, lower crust, upper crust. There
are essentially two types of lithosphere: oceanic and continental. You should know the
approximate composition and dimensions of each of these.
A hypsometric plot of Earth elevations reveal the two types of lithosphere very nicely.
Could you sketch such a hypsometric curve?
Be familiar with the internal structure of the two types of lithosphere, particularly at the
scale of the crust. What is the name given to the boundary between the crust and the
upper mantle?
The lithosphere is broken into about 8 large plates and a number of smaller ones. You
should know the geography of these plates; where the boundaries are, what types of
boundaries these are, and roughly how the plates are moving with respect to the hot spot
refrence frame (fig 14.12). Remember that transform faults fall on small circles about the
Euler pole and spreading rates are a sine function of the distance to the Euler pole.
Earthquakes reveal the subsurface geometry of subducting plates (or slabs as they are
often called), and show that the configuration of the slab can be quite variable. These
seismic zones are called Wadati-Benioff zones.
We have covered the basics of plate tectonics kinematics. You should know what an
Euler pole is (both conceptually and specifically - what parameters define one) and how
to obtain one for any pair of plates given a plate "model" (such as the one you were given
in lab).
We have covered the basics of plate boundary development and the stability and
evolution of triple junctions. Make sure you review these items. And remember that these
exercises provide very powerful tools in getting a first-order understanding of the
geological evolution of a region.
Extensional Tectonics
Extension of the lithosphere may occur by several means: by a whole-scale pure shear (in
which extension of the whole column occurs, the lower crust and upper mantle
homogeneously) or by various asymmetric means (in which extension of the upper crust
is laterally offset from the lower crust or upper mantle; extension may also be relatively
discrete rather than homogeneous).
Normal faults may be planar or listric, the latter is more commonly shown. Faults in the
upper crust are thought to sole into detachment faults that then transfer extension of the
lower crust to another location. Faults are often considered passive or inert features where
the hanging-wall slides down the fault and does all the deforming. But in fact, the
footwalls of normal faults do a lot of deforming, too. Hence, the margins of rift or
extensional zones are often lifted to form imposing flanks.
You should be familiar with the schematic cross-section of a Steerhead basin. Why do
early draping sediments tend to be salts? Why do later sediments tend to onlap the
adjacent rift flanks? Whare are good examples of Steerhead basins?
Metamorphic core complexes (MCCs) are rather different beasts (perhaps?) than typical
rift basins. They are most commonly found along the western side of the Basin and
Range province in the USA, but they are also found within the core complex of mountain
chains such as the Canadian Rockies. Review the basic elements of MCCs.
What is an accommodation zone? Where have old rifts tended to initiate?
Convergent Tectonics
We covered this topic by starting out with a cross-section of a archetype convergent zone,
starting with the downgoing plate that is flexed at the outer bulge, the trench (and those
turgid turbidites), the accretionary prism (and its thrusts and melange character), the
forearc basin, the volcanic arc, and the back-arc basin.
Remember that the character of the back-arc basin (of more perhaps?) is determined by
the relative motion across the convergent plate boundary. The subducting plate may be
rolling back (falling away), which will initiate back-arc spreading. But back-arcs may
also be convergent (e.g., there is a fold-and-thrust belt along the back-arc position of the
Andes), which suggests that the downgoing plate may be moving hard into the upper
plate.
Convergence of two continental plates inevitably gives rise to high mountains and
significant extensive deformation. The classic continental convergent belt is the
Himalyan belt. The Himalayas are what used to be the Indian subcontinent that is now
emerging from the depths of subduction. Although continental material is too light to be
subducted to great depths, still it is underthrust to depths of tens of km before being
transferred to the hanging-wall and brought back up. Remember that what brings it back
is a combination of erosion and the fact that thrusting has propagated to the foreland.
The Ganges River currently occupies the modern foreland of the Himalayas, and its
alluvial basin is the detritus of the Himalayas, the foreland basin. These sediments will
eventually be incorporated into the next hanging-wall and become the source for future
foreland basin sediments.
The Himalayas and all the deformation to the north is often viewed (and understood) in
terms of the collision with Tibet of a rigid indentor (India). This model explains the
variety of structures that extend will into Asia, including the large strike-slip faults, seafloor spreading in the South China Seas, etc. But remember that there are problems and
limitations on these models that have to do with boundary conditions. Be familiar with
these issues.
Strike-Slip Tectonics
Here we covered the basic development of strike-slip faults, particularly as they evolve
from buried and discontinuous to mature systems that are relatively planar. Deformation
above the nascent, buried strike-slip faults is often characterized by en echelon strike-slip
faults that strike obliquely to the main fault. These eventually join as displacement on the
main fault increases. Generally the fault becomes smoother over time.
Strike-slip fault zones often contain reverse and normal faults that accommodate the
shearing the of the zone.
There are some classic strike-slip faults in the world that you should know about (at least
where they are and why they are): San Andreas fault, Anatolian fault, Semangko (sp?)
fault, etc.
What is a flower structure?
Stress
Make sure you know these things:




The definition of stress
Units of stress
Meaning of mean and deviatoric stress
Lithostatic stress and how to calculate it
Stresses may be derived from body forces or from surface forces.
When we consider stress at a point and in a state of equilibrium, stresses form a tensor
that has 9 independent components. Be familiar with the symmetry and reason for the
symmetry of the stress tensor.
We covered the derivation and use of Mohr's circle for stress.
We also went over the meaning of isostasy and the means to calculate the expected height
of mountains. What important factor(s) determined the height of mountains and what
important assumptions did we have to make to figure this out?
Strain
Make sure you know these things:

Definition of deformation as the sum of strain, rigid-body rotation, translation,
and volume change.


Units and the various measures of strain.
Concept of the strain ellipse, coaxial deformation, pure shear, simple shear,
general deformation, etc.
Plate Tectonics 2
Plate Structure
•The various plate margins leave their mark on the plates they define. This helps
geologists piece together both the present and past geology of the regions.
•Plates bordering a subduction zone have specific features depending on whether both
plates are oceanic or one is a continental plate. Features in the first case include a trench,
a melange of piled sediments (a zone of high-pressure, low-temperature metamorphism),
a volcanic island arc (Japan is one of these), and a back-arc basin (the Sea of Japan). In
the second case, the back-arc basin is absent and the island arc is replaced by a volcanic
mountain range (such as the Cascades).
Ocean-ocean plate convergent boundary.
Structure of a continent-ocean convergent boundary.
Continent-continent collision.
•Plates bordering a mature zone of extension (a divergent boundary) are commonly
oceanic plates. The structure includes the mid-ocean ridge, which is a line of mountains
with a rift valley in the middle where magma extrudes to the surface. The crust slopes
away from the ridge, decreasing in height and increasing in age with distance from the
ridge. Along the ridge are transform faults. These are the most predictable (and likely
least dangerous) volcanoes in the world.
Mid-ocean ridge divergent boundary showing transform faults.
•Plates at a new zone of extension can also be continental plates. Upwelling of magma
underneath a continental plate first causes a bulge, then a rift zone with fault blocks.
Once the ridge sinks below sea level, the zone is mature and a mid-ocean ridge results.
Two stages of an opening rift. The top image shows the second stage of the rift zone,
while the bottom image shows the next stage, in which the rift has sunk below sea level.
•Plates defining a transform fault can be of any type. Typically the features on either
plate consist of a main fault and a network of lesser faults.
•In some areas plate motions can raft small fragments of crust tremendous distances.
Eventually, any fragment that is not consumed by subduction will be added to a larger
continental mass (accreted). These fragments are called terranes, and they are
geologically transient. Their ultimate fate is to be accreted to a larger continental mass.
Portions of accreted terrane on the Pacific plate.
•Examples of this type of tectonics include the islands of Taiwan and the Philippines.
Other fragments form when they are sliced off the margin of a large continent; this is the
case with the western side of the San Andreas Fault.
What Drives Plate Tectonics?
•Although the driving motion of plate tectonics is not known for certain, scientists
believe the mechanism is related to thermal convection in the Earth's mantle.
•Thermal convection is a process in which hot material within the mantle rises toward the
surface due to buoyancy (it is hotter and therefore less dense). As it rises, it cools and
spreads. When it cools enough, it begins to sink.
•This convection forms thermal cells that act as conveyor belts, moving the plates along.
Divergent plates form at the rising portions of these cells, while subduction zones occur
where the cells sink.
•Cells may be confined to the aesthenosphere, may involve the entire mantle, or may be
more complex, with thermal plumes rising from the core-mantle boundary.
Upper mantle convection cells.
Full-mantle convection cells.
•Another force that might be at work is called slab-pull or ridge-push.
•In slab-pull, subducting slabs are pulled down because of their greater density. They take
the rest of the plate along with it. In ridge-push, the upwelling magma pushes the
extending crustal plates out of the way as it extrudes.
•It appears that all three of these forces are at work in plate motion, but the contribution
of each is currently unclear.
Turning Back the Clock: Past Plate Motions
Determining past plate positions
•Past plate positions must be reconstructed from the evidence that remains from those
time periods.
•Magnetic anomalies along the mid-ocean ridges (discussed earlier) can be reconstructed
—essentially pushing the plates back together — to trace past plate motion. However,
this method only works so far, as the plates are not only created but destroyed (at
subduction zones), thus erasing previous magnetic stripes.
•Continental fit can also be used to piece together previous continental (and thus plate)
positions. Regions where the continents do not fit together perfectly can often be
explained through subsequent erosion or other activity (the formation of deltas and deepsea fans, creation of volcanic rocks at hot spots and so on).
•Portions of poor fit can also be the result of erosion of regions that were deposited from
other plates (sedimentary basins and such).
•Paleomagnetism also works in volcanic regions away from the mid-ocean ridge, and can
be very useful when occurring on the lighter, more long-lived continental crust.
Putting it together
•Evidence suggests that the mechanism or mechanisms that drive plate tectonics have
been operating for at least 2 billion years and maybe longer.
•The cycle of supercontinent formation and breakup is known as the Wilson Cycle.
•1 billion years ago — The supercontinent Rodinia broke up along western and eastern N.
America.
•450 million years ago — N. America collided with Europe.
•250 million years ago — Pangaea formed when N. America and Europe collided with
Gondwanaland (S. America, Africa, India, Antarctica, Australia).
The Earth as scientists have reconstructed it 250 million years ago.
•180 million years ago — The beginning of the current state of the continents. A
complicated series of events occurred, in which N. America and Africa separated, and
Africa and India began their collision with Eurasia.
•20 million years ago — the remains of the Tethys Sea (small gaps between Eurasia and
Africa) closed.
•6 million years ago — Mediterranean Sea was cut off from the Atlantic.
•What's next — Assuming, as we must, that the plates continue their current speed and
direction of movement, the next 50 million years will see Australia crush the islands of
Indonesia in its move towards Asia, the disappearance of the Mediterranean and the
opening of a new ocean at the East African Rift Zone. California will continue its move
towards the Aleutian Islands.
Australia moves north in this projection of the state of the continents 110 million years
from now.
Resources
•Subduction zones are regions where hot fluids are percolating through igneous and
metamorphic rock. These are ideal conditions for the deposition of heavy precious metals
such as gold and silver.
•Sedimentary resources like gypsum and salt are most likely deposited in geologically
quiet regions such as the continental interiors.
parental moment:
Gordon-Michael Scallion has predicted that three large earthquakes, each one larger than
the one that preceded it would strike the LA area. The third of these great quakes, which
Scallion
says will measure about 8.3 on the Richter scale (+/- .5), will initiate the breaking up of
America. According to Scallion, the first two of these predicted LA quakes have
already taken place: A 6+ on April 22, 1992 and the 7.5 Landers Earthquake which took
place on June 28, 1992. Scallion is predicting the third one will occur before
December of 1995. When the third earthquake occurs, a three stage land fracture and
break-up of the entire western portion of America will begin.
According to Scallion, the cause of this massive break-up is a very large magma bubble
that is pushing up beneath the United States. This magma bubble is caused by a
build up of ice at the poles. The ice build-up at the poles is causing an instability in the
Earth's rotation which, in turn, is creating instability in the earth's magma and core.
A third fracture will occur. This time vast sections of Utah, Nevada, Arizona, and
Colorado will be swallowed up by the ocean. Phoenix,
now a coastal city, will eventually emerge as a major seaport city. A very large seaport
city will also be established in the Nebraska region. Denver and Sedona will become
coastal regions.
Lecture 12: Sedimentary Basins
Concept of an area of sediment acccumulation or basin, e.g.
sediment thickness contours in Himalaya.
Concept of a Basin




Three dimensional architecture of basin fill.
Affected by spatial and temporal pattern of tectonic subsidence:
o Lithospheric deformation process.
o Three basic causes of subsidence:
 Loading and flexure (like an elastic plate).
 Thermal and density changes - isostasy.
 Faulting - isostasy.
Sea level changes.
Sediment supply rates and source position (drainage basin outlets).
Basin Classification: Plate Tectonic Position
1. Extensional:
o Half graben/graben - Rift Basin, e.g. Basin and Range, U.S.A.
o Mature oceanic speading - e.g. Atlantic margin (Passive Margin).
o Syn-rift, post-rift megasequences.
2. Compressional:
o Foreland basin - flexural loading of the Earth's lithosphere.
o Two types:
 Collisional, e.g. Himalaya.
 Back arc, e.g. Andes/Precordillera.
o Piggy-back basin (thrust sheet top basin).
3. Strike-Slip Basins:
o E.g. Dead Sea, Israel.
4. Passive Margins:
o E.g. Atlantic Margin.
5. Subduction Related:
o Oceanic trench, e.g. Marianas Trench.
o Fore-arc basin, e.g Taiwan or Median Valley in Scotland.
o Back-arc basin, e.g. Sea of Japan.
6. Cratonic "sag" Basins:
o E.g. Chad Basin, Africa.
7. Abyssal Plains.
8. Predictive models of facies distributions:
useful for subsurface exploration of oil or understanding dispersal of pollutants.
Reading:


Leeder, M.R. & Gawthorpe, R.L. (1987) Sedimentary models for extensional tiltblock/half-graben basins: Geological Society of London Special Publication, No.
28, pp. 139-152.
Geomorphology from Space: Compendium of Satellite Images. (Short Loan in
Library).
Why the Philippine Sea Plate Moves as It Does
Tetsuzo Seno
Earthquake Research Institute, University of Tokyo
J. Geol. Soc. Phil.55 105-117 2000
ABSTRACT The Philippine Sea plate (PH) is rotating relative to
Eurasia (or to hot spots) around the pole south of Kamchatka central Kuril. I address a question why this pole position is produced
by available driving forces of the PH. The ridge push and slab pull are
the two major sources of driving forces for oceanic plates. The ridge
push forces in the PH are generated by the age gradient within the
plate; the West Philippine Basin is oldest and the Mariana Trough
youngest. The torque calculated from the ridge push forces gives a
pole of 70.1،N, 326.1،E in Greenland, which produces a westward
motion of the PH. On the other hand, the slab pull, which is operated
at the Kyushu - Ryukyu Trench and the Philippine Trench, produces a
torque pole of 61.7،N, 257.9،E in northern Canada. Both of these
poles are far from the pole of the PH motion relative to hot spots,
implying some other driving forces are necessary. I propose here
viscous drag forces beneath the Kyushu-north Ryukyu arc directing
toward east are one of such driving forces. In the East China Sea
west of Kyushu, an upwelling plume from the deep mantle has been
suggested in previous studies. The lateral flow from this upwelling
can drag the PH toward east. If a magnitude of this torque is
comparable to those of the other driving torques, the desired torque
pole south of Kamchatka-Kuril can be produced.
Figure 1 The Philippine Sea (PH) - Eurasian
(EU) plate Euler poles and the Euler pole of
the PH with respect to hot spots (PH-HS).
The PH is rotating clockwise around these
poles looked from above. Sn77, Seno
(1977); Sn99, Seno et al. (1993); WS, Wei
and Seno (1998); Kt, Kotake et al. (1998).
The PH-EU pole is derived from the sum of
the PH-HS Euler vector of Seno et al. (1993)
and the EU-HS Euler vector of HS2-NUVEL1
(Gripp and Gordon, 1990).
Figure 2
The age
distribution
of the backarc basins
in the
Philippine
Sea: the
west
Philippine
Basin (See
Seno [
1988] for
the
references),
Shikoku
Basin
(Okino et
al., 1994),
Parece
Vela Basin
(Okino et
al., 1998),
Mariana
Trough
(Hussong
and Uyeda,
1981), and
Bonin
back-arc
(Taylor et
al., 1991).
The bottom
picture
shows that
from the
difference
of the age
among
these
basins, the
ridge push
forces are
generated
at the
boundaries
between
the basins.
Figure 3
Distribution
of the ridge
push forces
and the
magnitude
of the
torque at
each
segment.
The total
torque pole
(70.1،N,
326.1،E) is
located in
Greenland.
Figure 4 Distribution
of the slab pull forces
and the magnitude of
the torque at each
segment. The total
torque pole (61.7،N,
257.9،E) is located in
central north
Canada.
Figure 5
The
possible
location
of the
pole of
the
additional
torque
required
to
produce
the
observed
PH-HS
pole,
when it is
added to
the ridge
push-slab
pull
torque. It
should be
located
south of
the PH
and
rotates
the PH
clockwise
looked
from
above.
Figure 6 A
schematic
illustration
showing the
mantle
upwelling in
the East
China Sea
west of
Kyushu
which is
driving
Kyushu and
westernmost
Honshu to
the east by
the viscous
drag forces
(Seno,
1999). The
resulting
compression
in the
forearc in
Kyushu north
Ryukyu
might
provide
driving
forces to the
PH
directing
toward east.
Figure 7
The location
of the
upwelling
plume in the
back-arc of
Kyushu north
Ryukyu.
The
resulting
driving
forces due
to the flow
from the
upwelling
are
indicated by
the arrow.
The
direction of
these forces
is
conformable
to the gap
between the
slab pullridge push
torque pole
and the PHHS pole, if
they act as
the driving
forces of the
PH. The
collision
forces at the
Palau-Yap
Trenches
are also
indicated by
the arrow.
They,
however, do
not help to
reduce the
gap. The
Macolod
Corridor is
another
location
where a
mantle
upwelling is
expected
around the
PH.
Geology 304 Review
(Review Homeworks 1 through 4!!)
I.
History of Plate Tectonics: Fixists versus Mobilists
1. List the Major Scientists First Proposing Continental Drift
2. Give the early problems with the theory of Continental Drift
3. Draw a time line of major advancements in the development of Plate Tectonic
Theory and Continental Drift
4. State Wegner’s arguments for Continental Drift
5. State the arguments against Continental Drift
II.
Earth Structure: From the Crust to the Core
A. Seismology Overview
1. Identify a P-wave and S-wave on a seismogram
2. Know the relation between wavelength, frequency, and velocity of a wave
3. Give the approximate velocities for P-waves and S-waves as a function of
depth in the earth
4. Locate an earthquake using a 3 component seismogram
5. Describe the earthquake location procedure using at least three stations
5. Solve for the optimal focal mechanism given first motion polarity data
6. Describe the motion along a fault given a focal mechanism
7. Be able to draw a strike-slip, thrust, and normal focal mechanism
8. Define the term P and T axes related to earthquake focal mechanisms
9. Describe the relationship between P and T axes and stress changes due to an
slip along a fault
10. Describe the procedure used in seismic travel time tomography (what are the
data and what are model parameters)
11. Interpret a seismic tomography image
12. Describe how a refraction experiment is used in determining crustal structure
B. Crustal and Lithospheric Mantle
1. List the differences in properties and composition between oceanic
and continental crust
2. Give average values of oceanic and continental crustal thickness
3. Define the term Moho
4. Give the seismic velocity variations across the Moho
5. Define the terms mantle lid and lithospheric mantle
6. Define the Lithosphere-Asthenosphere Boundary
7. Define Airy and Pratt Isostasy
8. Be able to use the principle of Isostasy in solving for topography, density, and
root thickness
9. Define Bouguer and Free Air Gravity anomalies
10. Define the Isostatic anomaly
11. Be able to prove that the Free Air is equal to the Isostatic Anomaly for long flat
topographic features
C. Mantle Transition Zone, Lower Mantle, and D"
1. Define the Mantle Transition Zone (depths and velocities)
2. Define of the Claperyon slope for a endothermic and exothermic phase
transition
3. Describe the solid phase transitions at 410 and 660 discontinuities
4. State how temperature anomalies change the thickness of the mantle transition
zone
5. List the predominant minerals in:
Upper Mantle
Mantle Transition Zone
Lower Mantle
6. Define D”
7. Give the Seismic Properties of D”
8. State the difference between the core mantle boundary and D”
D.
1.
2.
3.
4.
5.
III.
Inner Core and Outer Core
List the properties of the Earth’s Outer Core
List the Properites of the Earth’s Inner Core
List the predominant elements in the Outer and Inner Core
Give the seismic evidence for a Liquid Outer Core
Describe the reason for the Geodynamo (Fluctuation of Earth’s Magnetic
Field)
Observations
A. Framework of Plate Tectonics
1. Defining an Euler Poles
2. Know Euler’s Theorem for motion on a sphere
3. Know the necessary parameters (data) to determine an Euler pole
4. Know how to calculate the position of an Euler pole given
two points defining an arc on a sphere
5. Know how to use the law of cosines and law of sines for a sphere
6. Define a great circle
7. Define a small circle
8. Describe a transform fault
9. Understand the relationship between the isochron velocity (Viso) and the true
plate velocity
10. Define sea floor spreading
11. Know the Vine-Mathews Hypothesis
12. Describe what magnetic stripes are
13. Know how to estimate spreading rates from magnetic stripes
14. Describe why there are magnetic stripes on the sea floor
15. Define Isochron
B. Plate Motions
1. Define the terms ferromagnetism and paramagnetism
2. Define the Curie temperature
3. Know the three primary types of remnant magnetization
4. Describe how paleomagnetically derived polar wander can be used to find
relative plate motions
5. Define the Magnetic Declination and Inclination
6. Describe the Hot Spot reference frame
7. Describe how the Global Positioning System can be used to measure plate
motion
8. Describe, briefly, SLR (satellite laser ranging), and VLBI (very long baseline
interferometry)
9. Define the term triple junction
10. Know how to determine if a triple junction is stable (Hint: use vectors)
11. Give an example of an unstable triple junction
11. Know the naming convention used in defining triple junctions
C. Seismicity Patterns
1. Define the term Wadati-Benioff zone
2. List and describe the three basic types of plate boundaries (ridge,trench, and
transform)
3. List the regions where you expect to find deep seismicity
4. Describe seismicity patterns in a subduction zone, transform fault, and diffuse
plate boundary
5. Describe the type of focal mechanisms you would expect see in a rift,
subduction zone, spreading center, and plate boundary strike split fault
D. Ocean Floor and Ocean Ridges
1. Describe fast, medium, and slow spreading mid-oceanic center/ridge and give
examples of each
2. Describe gravity profiles over a mid-oceanic ridge and be able to sketch
examples
3. Describe why trenches are the deepest parts of the world’s oceans
4. Describe the process of ocean floor subsidence and how it relates to the age of
the oceanic lithosphere
5. Describe the Petrology of Mid Ocean Ridge Basalts (MORBs)
IV. Kinematics
1. Know all stages of the Wilson Cycle
2. Describe each of the stages and be able to give examples
A.
1.
2.
3.
4.
5.
Transform Plate Boundaries
Know two primary types of transform plate boundaries
Be able to define restraining bend
Be able to define pull-apart basin
Know the meaning of dextral and sinistral
Be able to list three major strike slip faults (San Andreas, Dead Sea Fault,
Altyn Tagh, North Anatolian, Denali, etc.)
B.
1.
2.
3.
4.
5.
6.
7.
6.
7.
8.
Subduction Zones
Describe an accretionary prism
Define back-arc basin
Describe the formation and origin of back-arc basins
Be able to describe at least two hypothesis for back-arc spreading
(induced convection and retreating subduction)
Be able to list various factors influence the angle subduction
Be able to list the properties of flat slab subduction (e.g. no volcanism, broad
region of deformation, etc.)
Know the typical free air and bouguer gravity signals across subduction zones
Define active and passive margins
Know the location and types of metamorphism occurring within a subduction
zone.
Be able to relate metamorphic bands/facies with parts of a subduction zone
C. Continent Collisional Tectonics
1. Define escape or extrusion/escape tectonics (and know the scientists credit
with developing these theories)
2. List the supporting evidence for escape tectonics
3. Know the properties of the upper mantle beneath the plateaus related to
continent-continent collision
4. Be able to define delamination
5. Know the reasons the theories for the initiation of delamination
6. Describe slab breakoff
7. Be able to list and briefly describe the competing models for the development
of the Tibetan Plateau
8. Define ophiolite, and continental accretion
9. Define decollment and nappe (thrust sheet)
10. Describe the difference between thin- and thick- skin tectonics
11. Know the crustal thickness values beneath Tibet (approximately)
12. Know the major boundaries within the Tibetan Plateau
13. Know the age of the Himalayan Orogen
14. Know the types of volcanism observed in continent-continent collisional belts
D. Rifts
1. Be able to list and describe at three hypotheses for the origins of continental
rifts
2. Describe the origin of high topography in continental rifts (doming and
superswells)
3. Describe the asymmetry of continental rifts
4. Describe the origin of collision related rifts and give an example
5. Describe how a continental rift evolves in a young oceanic lithosphere
(and give an example)
6. Describe the kind of igneous rocks you expect to find in continental rifts
7. Describe the type of crustal and upper mantle structure beneath rifts
8. Know the link between rifts and plumes
9. Describe what a metamorphic core complex is
10. Describe possible causes for continental splitting
11. Define aulacogen
V. Driving Forces
1. Know the proposed driving forces of Plate Tectonics
2. Describe Ridge Push
3. Describe qualitatively how oceanic lithosphere is a function of age
4. Describe Slab Pull
5. Describe Basal Drag
6. Be able to able to list evidence for which force is principle force driving plate
motions.
Crashing Continents
Benioff Zones
XEarthquakes
occur at shallow, intermediate and
deep levels beneath subduction zones
XThe earthquakes define a plane which begins at
the trench and dips at about 45° beneath the arc
XThis dipping plane of earthquake foci is called
the Benioff Zone
XThe Benioff Zone follows the upper part of the
descending oceanic plate
XShallow earthquakes also occur through the arc
33-04
Crashing Continents
Island Arcs
XIsland
arcs are of chains of volcanically active
islands arranged in a curved arc
XAn ocean trench occurs on the oceanwards side
XIsland arcs first develop on oceanic crust
XThe crustal thickness in an arc is intermediate
between oceanic and continental
XVolcanic activity begins abruptly at a Volcanic
Front about 200 - 300 km in from the trench
XThe volcanic front and trench are separated by an
Arc-Trench gap with no volcanism
33-05
Crashing Continents
Island Arc Volcanism
XVolcanic
rocks in island
arcs are mostly of
andesitic composition
XThe magmas originate
mostly by partial
melting of subducted
oceanic crust and
overlying mantle
XMelting begins when the
slab descends to about
100 km depth, forming
the volcanic front Partial melting
of basaltic
ocean crust
Rising magmas Volcanic eruptions
Crashing Continents
Chemical Differentiation
XMid-Ocean
Ridge:
Partial melting of Mantle basalt magma
XSubduction Zone:
Partial melting of Basalt crust andesite magma
XMature Arcs:
Partial melting of Andesite crust rhyolite magma
XAll of this is an irreversible chemical differentiation
of the mantle in several stages
XContinental crust grows by accumulation of
increasingly silica-rich rocks
33-06
Crashing Continents
Ocean trench Sedimentation
XUnconsolidated
sediment from the
ocean floor is scraped off the
descending plate at the trench
XSlices of the oceanic crust may be
included as ophiolite belts
XThese rocks form a complex rock
mass called an Accretionary Wedge
XThe Accretionary Wedge is buckled
upwards as new material is pushed
beneath its base
XThe chaotic jumble of rocks in the
Accretionary wedge is called a
Tectonic Mélange
33-08
33-09
33-07
Accretionary Wedge
Crashing Continents
Metamorphic Rocks and Subduction
XHigh
Temp - Low Pressure
Metamorphism
• Occurs in the core of volcanic arcs
• Abnormal heating of the crust
• thermal effects of
subduction-related
magmatism
XHigh
Pressure-Low Temp
Metamorphism
• occurs in the accretionary wedge
• cold rocks are dragged to great
depths and then upthrust again
33-10
Granites
Basin Analysis Basin Analysis
Geology 355, Sedimentology ©Copyright, 2002, Ron Parker Geology 355, Sedimentology ©Copyright, 2002, Ron Parker
December 11, 2002 December 11, 2002
Geology 355, Sedimentology ©, 2002, Ron Parker
Basins Basins
_Topographically low places where sedimentary
materials accumulate.
_Basins are characterized by accomodation space.
_Accomodation is created by…
_Eustatic sea-level rise
_Subsidence
_Topographically low places where sedimentary
materials accumulate.
_Basins are characterized by accomodation space.
_Accomodation is created by…
_Eustatic sea-level rise
_Subsidence
Geology 355, Sedimentology ©, 2002, Ron Parker
Basin Analysis Basin Analysis
_Basin analysis is the detailed investigation of the processes
that
_Form basins
_Fill basins
_Alter basins
_Uplift (invert) basins
_Destroy basins
_Requires sedimentology, stratigraphy, hydrogeology,
petroleum geology, seismology, geophysics, geochemistry,
paleontology, etc.
_Basin analysis is the detailed investigation of the processes
that
_Form basins
_Fill basins
_Alter basins
_Uplift (invert) basins
_Destroy basins
_Requires sedimentology, stratigraphy, hydrogeology,
petroleum geology, seismology, geophysics, geochemistry,
paleontology, etc.
Geology 355, Sedimentology ©, 2002, Ron Parker
Types of Basins Types of Basins
_Intracratonic
_Rift related
_Strike-Slip related
_Collision / Subduction related
_Intracratonic
_Rift related
_Strike-Slip related
_Collision / Subduction related
Geology 355, Sedimentology ©, 2002, Ron Parker
Intracratonic Basins Intracratonic Basins
_Intracratonic – Basins that form within continental crust _Intracratonic – Basins that form within continental crust
Geology 355, Sedimentology ©, 2002, Ron Parker
Rift Basins Rift Basins
_Rifts – Divergent plate boundaries that eventually develop
into spreading centers.
_Rifts – Divergent plate boundaries that eventually develop
into spreading centers.
_Initial rift sediments are
arkosic sandstones
interbedded with basalts.
_Rifts then flood and deposit
a thick sequence of
evaporites.
_Then marine sedimentation
takes over.
_Initial rift sediments are
arkosic sandstones
interbedded with basalts.
_Rifts then flood and deposit
a thick sequence of
evaporites.
_Then marine sedimentation
takes over.
Basins and stratigraphic successions
I.
II.
Basins & basin types
A.
Basin: a region of depressed crust, typically with greater thicknesses of sediment
accumulation than surrounding regions.
B.
Basins form by tectonic processes that cause the crust to subside, and so to create large
amounts of accommodation space. Rapid A-space growth translates into abundant and
well-preserved organic matter within the sediment.
C.
The abundance of organic matter combined with the low grade burial metamorphism that
sediments in thick accumulation experience translate into major oil and gas
accumulations.
D.
Basins provide the most complete successions of the geological record.
E.
Tectonic setting for the majority of basins falls into four general categories with most
associated in one way or another with a plate boundary:
1.
Constructive (spreading) margins: This setting produces a genetic series of
basins from localized rift-related basins through narrow oceans (Red Sea phase)
to voluminous passive margin basins.
2.
Destructive (convergent) margins: This setting produces basins associated with
subduction zones (foredeep basins), intra-arc spreading basins, back-arc
spreading basins (Sea of Japan, Great Basin), and back-arc thrust (foreland)
basins.
3.
Transform (strike-slip) margins: mostly localized pull-apart basins such as the
Gulf of California and Dead Sea. Grades into rift-type and thrust-type tectonics
where relative plate motion is not fully strike-slip (i.e., transtensional and
transpressional regimes).
4.
Intra-cratonic basins. Fairly mysterious, concentrically subsiding basins formed
on continental crust well away from plate boundaries. The cause of subsidence
is unknown, but is generally considered to involve some combination of densitydriven stress and in-plane stress.
Constructive boundary basins.
A.
Rifting and passive margin formation.
1.
Continental rifting begins with dome-formation. Produced by hot-spot
volcanism (bimodal: dominantly basaltic plus some rhyolite from partial melting
of the crust by basaltic magma and decompression melting).
2.
Much extension by normal-faulting. Individual domes link up to form a more or
less continuous series of rift valleys (grabens, as in the East African Rift zone).
Extension results in thinning of the upper crust. The lower crust thins by ductile
flow.
3.
High heat flow produces thermal uplift; uplift leads to further thinning of the
upper crust by erosion.
4.
Continued extension produces oceanic crust between the newly formed, thinned
continental margins. Eventually (over 107 years), sea floor spreading and
thermal subsidence yields a distinct mid ocean ridge, open communication with
the oceans (Red Sea phase) to form a permanent seaway.
5.
Continued seafloor spreading carries the new continental margins away from the
active tectonic zone. The much-extended crust cools and subsides over the next
108 yrs. as a passive continental margin ("passive margin," for short).
B.
Rift basins.
1.
Smallish volumes of sediment (100’s to a few 1000 m of accommodation space)
in localized, fault-bounded basins.
2.
Dominantly terrestrial (subaerial and lacustrine) and arid as result of uplifted
margins that cast rain-shadows into the basins as well as their small drainage
capture area.
3.
Early rift sediments typically heterogeneous with basement clasts and volcanics
dominant. Grains with low physical and chemical maturity. Late rift sediments
often include extensive evaporites from episodic oceanic invasions (reflects
III.
variable rates of subsidence and sea level rise and fall). Remobilized salt
becomes important in passive margin history later (see below).
C.
Passive margin basins.
1.
Lots of accommodation space (10,000 to 15,000 m or more) from combination
of thermal and sediment load-driven subsidence.
2.
As oceans widen, marine conditions come to dominate. Early phases often still
arid and dominated by carbonate deposition and fringing reefs along the shelf
edge (at low latitude sites)
3.
With continued subsidence the fringing mountains diminish, drainage basin size
grows, and clastic sediment supply rates increase. In humid regions or times of
low sea level, deposition switches to the clastic off-lap suite with formation of
the familiar shelf environments: typical association of deltas, beaches, and subtidal clastic shelf deposits grading outward to deep sea fans and contourites in
continental rise settings. Prograding sediments build a wide continental shelf
upon a voluminous accumulation of clastic or carbonate (or some mix of clastic
and carbonate).
4.
The clastic off-lap suite is well developed along the present Atlantic coast of
North America, South America, Africa, and Europe. The southern passive
margin of Laurentia (what is now the eastern margin of North America) was
dominated in the Cambrian and Ordovician entirely by carbonates. Both are
major source of petroleum.
Destructive (convergent) margin basins.
A.
Foredeep Basins.
1.
Basin forming mechanisms: dominantly a combination of load-related and
density-related subsidence. Loading leads to flexure of the crust. Subsidence
near the load is matched by a compensating (but smaller amplitude) up-bend at
the periphery of the down-bend.
2.
At destructive plate boundaries, dense, old oceanic crust subducts beneath
continental crust or younger, less-dense oceanic crust, forming an elongate, deep
oceanic trench.
3.
Interaction between the down-going plate and the overriding plate generates
thrust slices (slabs of rock and deformed sediment) sheared off of one or the
other plate, which are thrust ocean-ward, overtop of the down going plate.
4.
The subduction zone may also include abundant sediment supplied to the trench
(especially in humid climates; ex. central Andean margin of South America).
5.
Thrust and sediment load causes subsidence of the crust, deepening the basin
and creating large amounts of additional accommodation space. This is a
foredeep basin: a basin that form from a combination of trench (density driven)
and load-driven subsidence in the down-going (lower) plate. It is located oceanward of the volcanic arc and is rimmed distally by an uplifted forebulge. Ex:
north Australian shelf-Banda Arc collision.
B.
Foreland Basins.
1.
Thickened crust of an island arc complex or continental crust at the margin of
the upper plate leads to intense interaction between the down-going plate and the
upper plate. This generates considerable additional thickening and deformation
of the upper plate via thrust slices that stack up to form (together with the
volcanoes) a tall, complex mountain system and its isostatic roots. Ex: Andes
Mountains.
2.
Thrusts migrate away from the collision (subduction) zone toward the
"foreland." Thrust and sediment loads causes subsidence of the crust forming a
deep flexural basin (8-10 km accommodation space) followed by an uplifted
forebulge and a much smaller second order basin (a few 100’s of meters Aspace at most). Ex: Amazon Foreland Basin.
C.
Facies patterns.
1.
Load-driven flexure is rapidly accommodated by subsidence/uplift.
2.
3.
4.
5.
Rapid growth of accommodation space in the foreland or foredeep basin leads to
trapping of sediments in the source area and strong transgression and sediment
starvation as relative water depth increases over a period of 10 4 - 105 years.
Rapid uplift on the foreland or peripheral bulge results in loss of accommodation
space, but facies response varies greatly with local conditions the state of
eustatic sea level. Possibilities range from subaerial exposure to prograding
carbonate or siliciclastic facies. Unlikely to exhibit transgression.
Basin fill shows strong progradation of sediments filling deep basin. Sediments
grade upward and proximally from black shales to flysch (mixed shale and
turbiditic litharenites), to near shore clastics (deltaic and shelf-like sediments) to
alluvial plane deposits (fans & rivers, etc.). Early basin fill with abundant, well
preserved organic material.
As the system ages, the mountains erode and the sediments become less
volcanic, more plutonic and metamorphic (granitic) source-rock dominated, and
become more mature both physically and chemically. These late phase deposits
are sometimes referred to as mollasse.
GEOLOGICAL HISTORY OF THE CENTRAL
MEDITERRANEAN:
OUTLINE OF CENOZOIC EVENTS
Amanda Kolker
GEOLOGICAL HISTORY OF THE CENTRAL MEDITERRANEAN BASIN: AN
OUTLINE OF CENOZOIC EVENTS
Geologic features in the present-day Mediterranean essentially result from two major
processes: the tectonic displacement caused by the subduction of the African plate
underneath the Eurasian plate; and the progressive closure of the Mediterranean sea
involving a series of submarine-insular sills.
The development of the Mediterranean basin begins with the breakup of the
supercontinent Pangea in the Mesozoic. During this time, sea-floor spreading triggered
the development of the Atlantic ocean in the Triassic period, which separated the African
and Eurasian plates from the North American plate. Sea-floor spreading in another
geographical location caused the development of the Tethys ocean, separating the African
plate from the Eurasian. In the late Cretaceous period, these African and Eurasion plates
began to converge, closing the Tethys ocean basin, and the remnants of this ancient ocean
are now called the Mediterranean sea.
There are three major geomorphical settings within the Mediterranean basin; areas with
stable margin characteristics, areas with unstable convergent margin charactericstics, and
areas with extensional margin (rifting) characteristics. Thus the Mediterranean basin is a
location of an intercontinental interplate system; with compressional and extensional
events occurring within close proximity. Geologists have yet to come to a consensus
about which plates in addition to the African and Eurasian ones, if any, are involved in
Mediterranean tectonics. Subsidence-related and other vertical displacements are also
found in compressional and extensional areas. A few notable events occurred during the
Cenozoic which affected the entire Mediterranean; the Messinian "salinity crisis", when
the closing off of the Mediterranean-Atlantic seaway caused complete isolation of the
Mediterranean and thus widespread evaporation; and then the Pliocene "revolution",
when the channel opened back up, causing reestablishment of marine conditions; and the
Quaternary "transgressive raised terraces," of controversial geological origin; among
others.
The Central portion of the Mediterranean basin exemplifies the juxtaposition of
compressional and extensional tectonic activity in the area. The region bordered to the
west by Sicily and to the east by Turkey's west coast (encompassing the Aegean, Ionian,
and Adriatic seas) exhibit a particular set of features and will be the focus of my study.
There were four major periods of extension in this area. The first one occurred in the
Mid-Upper Jurassic; evidence of this phase is seen in the Strepanosa Trough and Ionian
plain. A second one occurred in the Mid-Late Triassic, opening up the Ionian sea and the
Eastern Mediterranean. A third extensional phase occurred in the Mid-Upper Cretaceous,
as evidenced by the stretched features of the Sirte Rise, a monocline with normal faults
and tilted blocks. The fourth one, occurring in the Mid-Upper Miocene through to the
Quaternary period, affected many areas of the Central Mediterranean. This extensional
phase is closely associated with compressive motions; it is part of the reason for a
counter-clockwise rotation of the Southern Appennine area which begins in the upper
Cretaceous. All four of these extensional phases are the cause of geologic features found
in the area, such as volcanic activity and rift-related sedimentary processes. Due to such
extension, the oceanic crusts of the Central Mediterranean are considerably thinned in
some places.
The Mediterranean Ridge or Outer Median Ridge is a sea-floor feature that marks the
ustable (convergent) margin between two or more oceanic plates (geologists know that
the African and Eurasian plates are involved, but which, if any, smaller plates are
involved is a matter of debate). The first stages of the major collision between the North
of the African plate and the South of the Eurasion plate is believed to have occurred in
the lower-middle Miocene. This collision is also associated with the counter-clockwise
rotation of the Appennine area, and both of these associations are exhibited in the
Calabrian (Italy & Sicily) and Hellenic (Greece) orogenic arcs which are situated among
both compressive and extensional dynamics. The ridge extends geographically from
Sicily to Cyprus along a generally E/W strike. It is an extensive fold-fault system
corresponding to recent uplift and folding of past abyssal plains.
The features in the Adriatic sea are results of this duality of compression as well as
extension, and also from deposition-related subsidence on a deeply foundered foreland
(on the shelf). The Sicily Channel Rift area is an example of the Miocene-Quaternary
extensional phase. The Adriatic sea itself is relatively shallow, and almost all of the ocean
floor (a thick carbonitic platform underlain by continental crust) exhibits compressional
deformation structures, except for the Ionian Abyssal Plain, which is thought to be
underlain by Paleoceanic crust. The history of the Alpine orogeny, constituting the
northwestern portion of the Adriatic, really begins in the Mesozoic as well, for the
sedimenary strata which constitutes most of its orogenic elements weas laid down in the
continental margins of the ancient Tethys ocean. The Alpine orogeny and the Calabrian
arc orogeny are both results of convergent plate margin movement between Africa and
Europe, and display some vertical uplift associated with the subsidence of Mediterranean
sea-floor deposits during the Cenozoic.
The Ionian sea perhaps experiences the major amounts of subsidence in the Central
Mediterranean. The Ionian Abyssal Plain in this region is characterized by differentially
sudbsiding areas but generally experiences more than adjacent regions, contributing
greatly to the uplift associated with the Alpine orogeny and the Quaternary coastal
blocks. The Hellenic trench (a thrust fault linked to the convergent activity in the
Mediterranean ridge) began propagation in Miocene and continues today; it constitutes a
major element of Ionian seafloor topography. The extensional features in the Ionian
region are somewhat subdued, the dominant tectonic activity is convergent and/or related
to vertical movement.
The Aegean sea experiences considerable amounts of extensional features as well, related
to the suduction of the African plate underneath the Hellenic Arc. Subsidence in the late
Miocene also had a grand affect on the region, resulting in the fragmentation of an
Aegean landmass from vertical displacement. Extension in the Hellenic arc area runs
generally N/S, and crustal shortening forms an E/W insular platform. Here the oceanic
crust is thinned to almost 1/2 its original thickness The counter-clockwise motion is
further expressed in the area by transcurrent faulting in the Northern Aegean, beginning
in the fourth extensional phase of the Mid-Upper Miocene. The outer regions of the
Hellenic zones, by conrtast, exhibit compressive geology.
All of the volcanic activity in the Central Mediterranean is related to one or more of these
processes; subduction, back-arc extension, and/or other tectonic events throughout the
Cenozoic.
OTHER LINKS OF INTEREST:
Volcanoes of Italy
Eolian and Aeolian island arcs
geothermal activity in Italian volcanoes
BIBLIOGRAPHY & WEB SOURCES
Image of tectonic map from web adress: http://geothermal.marin.org/map/tect_map.gif
Stanley & Wezel, Geological Evolution of the Mediterranean Basin, SpringerVerlag,1985
Berckhemer & Hsu, ed; Alpine-Mediterranean Geodynamics, American Geophysical
Union,1982
Higgins & Higgins, A Geological Companion to Greece and the Aegean, Cornell
University Press, 1996