Week
1:
Strength of the continental
lithosphere
What are the
differences between the conventional view of the continental lithosphere, and
the new view claimed by the author (El-Hussein)?
The conventional concept is based on assuming that the continental lithosphere
is formed of a weak lower crust lying between a strong upper crust and strong
uppermost mantle and the mantle is the strongest part of the lithosphere. This
view was resulted from studying the depth distribution of earthquakes, combined
with an extrapolation of laboratory rock mechanics experiments to geological
conditions. Hence, based on this view, it was know for along time that, in most
places, earthquakes on the continents are confined to the upper half of the
crust. When rare earthquakes occur in the uppermost mantle in few areas, the
conventional view attributes this to a strength contrast between the upper
mantle and the generally aseismic lower crust.
The author proposed view for the continental lithosphere, is that the behavior
of the continental lithosphere is dominated by the strength of the upper
seismogenic layer. Hence, the seismogenic layer may be the only significant
source of strength in the continental lithosphere, and that the upper mantle
beneath the continents is relatively weak. In this view the author claimed that
patterns of surface faulting on the scale of a few hundred kilometers are likely
to be controlled by the anisotropic strength of crustal block and their
intervening faults. In addition, transient lower-crustal flow, of the type
associated with metamorphic core complexes, is likely to be controlled by the
input of igneous melts and fluid into the lower crust.
Explain how the author tried
to prove that the behavior of the continental lithosphere is dominated by the
strength of its upper seismogenic layer (El-Hussein)?.
The author started his argument with highlighting that our current views of
continental tectonics are confused by not knowing what really controls the
pattern of deformation we see at surface. Author argued also with and example of
earthquakes in the foreland of the Himalaya, within the underthrusting Indian
shield. The depths of these earthquakes were estimated by waveform modeling or
by direct identification of the surface reflection phased pP and sP. These
depths were found to lie at or above the estimates of the Moho depth. Since most
of these earthquakes are relatively small, with source dimensions of order 5 km,
this results in uncertainties in Moho and centroid depths, and the author
claimed that these depths may not be all above the Moho depth while we know that
the lower crust is seismically active. The other argument that if the deeper
earthquakes were at the top of a separate strong upper mantle layer, they should
show extensions, not shortening, and the single thick seismogenic layer is
partly responsible for the large fault areas and moments of the biggest
earthquakes in the Indian shield, such as the 1897 earthquake beneath the
Shillong plateau, whose fault plane ruptured between 9 and 45 km depth, and the
2001 Bhuj earthquake in Gujarat. In addition, author claimed that an alternative
interpretation of the gravity could be that all or most of the elastic strength
lies in the mantle, rather than the crust, and that earthquakes are an indicator
of the frictional stability rather than strength and that the continental mantle
could still be strong despite being aseismic.
What are the possible
implications if the author view is correct (El-Hussein)?
In addition to the stated reevaluation of the conventional view, the
implications will be that flexure of the Indian shield is likely to be major
support of the topography in the Himalaya and southern Tibet. In addition, it
will be normal that the regional patterns of active faulting at the surface were
dominated by the strength of the crustal blocks and the interactions between
them. If the new views proposed by the author are correct, the detailed patterns
of faulting on the scales of interest to most tectonic and structural geologists
(say, 100-400 km) are likely to be controlled predominantly by the strength of
the crustal blocks and the faults that bound them. Nevertheless, it will be more
likely that the high elevations in the region are supported by the flexure of
the Indian shield, with the entire overlying region 300-400 km north of
Himalayan front falling towards India, caused arc-normal slip vectors on the
thrusts
and arc-parallel extension behind.
How the depths of earthquakes in India and
Southern Tibet have been determined (Bulaihed)?
By waveform modeling or by
direct identification of the surface reflection phases Pp and Sp.
Presenter's Respond:
I agree, yet, the paper doesn’t
take about the percent error in the data. It just takes it into a fact that
the data are better for modeling.
Why the effective elastic
thickness, Te, is smaller than the seismogenic thickness, Ts (Bulaihed)?
For two reasons
1) The top few kilometers,
especially in sediment thick foreland basins are unlikely to contribute much
to the elastic strength.
2) Te reflects the ability of
the lithosphere to support loads over several million years, whereas the
loading and unloading associated with the earthquakes cycle happen on a much
shorter time scale, over which the lithosphere might appear to be stronger.
Where earthquakes on the
continent are confined, in most places (Bulaihed)?
Earthquakes on the continents
are confined to the upper half of the crust
Presenter's Respond:
I agree: the paper, however,
trying to proof that the lower crust is part of the seismogenic layer
and not the upper mantle.
How will you define the strength
of the lithosphere (Akram)?
It can be defined as the vertical integration of the differential stress
required to trigger either brittle failure or the flow failure of rocks.
Failure can occur by power law creep or Dorn law creep at high temperature,
low strain rate, or by frictional sliding at low temperature and high strain
rate, in which case the differential stress depends of the tectonic regime.
Presenter's Respond:
In this paper differential
stress was associated with wetness which was an indication of the weakness
of the lower mantle. Hence, the lower the stress the weaker the layer, the
unlikely it’s contribution to the strength of the continental lithosphere.
Nevertheless, the paper claims that due to the weakness of the upper mantle,
it isn’t part of the seismogenic layer.
What is the effect of
temperature for moho on lithospheric strength (Akram)?
As any material, the strength of rocks decrease as temperature increases.
The graph* shows the evolution of the integrated strength of the continental
lithosphere as the temperature at the moho (TMoho) increases. This graph
shows that when the temperature increases from 500 to 700ºC,
the strength decreases by a factor of ~20. This suggests that for similar
composition and thickness, the continental lithosphere in the Archaean was
much weaker that its modern counterpart. It shows also that abobe ~700ºC
the strength does not change much as the TMoho increases.The strength of the
continental lithosphere depends so much on the temperature at the Moho
(TMoho) that the Moho temperature can be used as a proxy for the
lithospheric strength
Presenter's Respond:
The mention of temperature was
analogues to the Homologous temperature, the ratio of actual temp to melting
temp. it seemed that the author didn’t put emphasis on temperature when it
comes continental lithosphere. The author associated temperature effects on
the oceanic lithosphere.
Why continents are more
complicated than oceans. In general, continental convergence zones should
have intermediate or deep focus earthquakes or not (Akram)?
Continental crust is much thicker and less dense than oceans with
earthquakes and faults distributed over wide areas and not confined to the
narrow plate boundaries that typify the oceans. That's why continents are
more complicated than oceans.
Continental lithosphere is much less denser than the upper mantle, it is not
subducted and a wadati benioff zone is not formed. As a result, continental
convergence zone does not , in general, have the intermediate or dep focus
earthquakes.
What is the effect of
topography on crustal thickness and depth of moho (Akram)?
According to classical studies of isostacy, the higher the topography, the
thicker the crust and deeper the Moho ( roots of the mountains).
Presenter's Respond:
In this paper the technique used
was modeling of the flexural free-air gravity ignoring topography. Hence, an
assumption was made conveying the idea that the plate is bent only by loads
and couples on its end. Two theories associated with the thickness of the
crust. The first one is that isostacy theory. And the second is the flat
base model. I guess the author adapted the second one. The author then mad
the measurements for the model to fall on a flat datum.
Which other factor than the
temperature is related to the lithospheric strength (Akram)?
Water contents as the presence of its minimum amount reduces the creep
strength.
Presenter's Respond:
I agree, However, remember
that it’s a theoretical experiment and it should not be taken as a proof
of the claim of weak upper mantle.
In the area where the
Indian plate collides with the Eurasian plate, how would you explain the
idea that the deeper earthquakes would have shown extension (normal
faulting) and not shortening (thrust faulting) have these deep
earthquakes had been at the top of a separate strong upper mantle layer
(Busfar)?
I didn’t quite understand this concept, however, the best explanation I
came up with is that if these earthquakes where at the top of a separate
strong upper mantle layer then due to the fact that the continental
lithosphere is thicker towards the north than it is in the south, and
thus, a “heavier” load is applied on the solid upper most mantle as we
move towards the north, and since the fault dips towards the north, then
we would expect the “heavier” part (north) to move down and the lighter
part (south) to move up so that we would end up with a normal fault
(extension).
How do we get normal
faulting at the shallow events (about 20 km deep) (Busfar)?
I am guessing that maybe as we move away (upward) from the subduction zone
we start getting normal faults as a result of the underlain reverse
faults which are triggered by the deeper earthquakes.
In figure 3B, why is
the gravity profile decreasing as we move to the north (Busfar)?
The subsiding Indian plate is dipping towards the north which decreases
its effect on our gravity measurements as we move to the
north because it gets deeper.
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Week 2a:
Imaging the Deep Seismic Structure Beneath a Mid-Ocean Ridge
Week 2b:
Lithospheric strength in related to seismogenic layer thickness
Lithospheric strength and its relationship to Te and Ts!
Abstract by
Al-Omar:
With the possibility of estimating Te and Ts at oceanic and continental
lithosphere, the paper shed some light on the relationship between the elastic
thickness and the seismogenic thickness claiming that Te >>Ts in continental
lithosphere, due to its different rheology, but not in the oceanic lithosphere,
due it relatively simple structure. It shows that Te and Ts are different in the
ways they contribute to strength of earth’s lithosphere
Why Seismic activity in the oceanic lithosphere is
limited to a depth range of around 15km (Al-Omar)?
At such depth range a semi-brittle/semi ductile strain rate dependent plastic
flow takes over. Frictional component doesn’t present an important factor at
such depths. In short, at depth where ductile behavior is dominant earthquakes
are rare, whether it is related to oceanic or continental lithosphere.
Is there a difference in the mechanism that
originates shallow and deep earthquakes (Al-Omar)?
In general shallow earthquakes are related to the absolute rock strength and
deep seismic activity is not related to frictional sliding that follows
Bayerlee’s law. Hence, deep earthquakes are weakly related to absolute rock
strength.
Could Te and Ts follow each other and Te is always
less than Ts (Al-Omar)?
From studies conducted recently, the above is possible with reexamining the
data and trying to determine the, accurately, the depth associated with
earthquakes epicenter and Moho depths. They found that Te and Ts follow each
other in different regions. That entails the strength lies on the uppermost
layers of the continental crust, but it doesn’t say much about the oceanic crust
since it is bound by different factors.
What are the factors on which elastic thickness
depends (Akram)?
Elastic thickness depends on mineralogy, temperature and state of stress of the
lithosphere.
How can you estimate elastic thickness from the
gravity data and topography (Akram)?
There are two approaches, 1. bouger coherence 2. Free air admittance
Bouger coherence measures the correlation of topography and bouger gravity as a
function of wavelength where loads are supported predominantly by stress.
Free-air admittance is the transfer function between free air gravity and
topography.
How'll you interpret (Akram)?
i) Te nearly equals Ts
ii) Te >> Ts
iii) Te < Ts
i and iii)
Effective strength of the lithosphere lies in the seismogenic layer.
ii) Strength of the lithosphere is not limited to the seismogenic layer.
Strength of the lithosphere can also reside in any layer that is aseismic. This
also gives support to a strong mantle concept if we consider it aseismic.
Compare the MEM method, the “no-load” approach and
the topography method for the estimation of Te, and state what author implied
out of comparing the Te values predicted from these methods (El-Hussein)?
(1) MEM (Maximum Entropy Method), uses spectral estimates, which calculated in
boxes that are moved step step-wise a cross the study area.
(2) The “no-load” approach uses only the flexure and gravity anomaly to one side
of a load, in order to derive Te.
(3) Topography method uses topography to define load, the flexure and gravity
anomaly to one side of, and beneath a load is used to estimate Te
The author implied that the Te value can not be exactly estimated. However, an
idea of its extent can be figured out using different methods.
Why the author considered that the sub-crustal
mantle is and important contributor to the support of long term beds in both the
oceans and continents (El-Hussein)?
In oceans, oceanic Te studies suggest that thermal cooling, which strengthens
the lithosphere dominates over that of load induced stress relaxation, which
weakens it such that the mantle becomes increasingly more involved in the
support of loads with thermal age.
In continents, stresses generated by flexure are large enough to cause
earthquakes in the uppermost brittle part of the continental crust. They may not
be sufficient to overcome the brittle strength of the continental sub-crustal
mantle when stresses encounter it. Hence, again the mantle contributes as a
support.
How in contrary to Ts, Te reflects the integrated
strength of the entire lithosphere (El-Hussein)?
In oceanic
lithosphere, the potential brittle zone extends to the brittle-ductile zone
(BDT), which may be as deep as 50 km. This is because there is no intermediate
ductile layer that prevents stresses from being propagated into surrounding
competent layers. As a result, the stresses generated by flexure accumulate
locally and if they exceed the confining pressure, cause earthquakes.
In continents, however, there are more ductile layers which may decouple the
competent parts of lithosphere and cause smaller stresses for the same amount of
flexure. Furthermore, small flexures and long loading times suggest that most
continental lithosphere will deform at rates that are significantly smaller than
oceanic lithosphere, which further reduces stress levels.
What is the elastic thickness of the lithosphere
(Tå) for oceans and continents (Bulaihed)?
The elastic thickness of the lithosphere (Tå) is in the range 2-50 km for oceans
and up to 80 km and higher for continents.
What is the Byerlee’s law of frictional brittle
failure, which characterizes deformation in the uppermost part of lithosphere
(Bulaihed)?
It suggests that strength linearly increases with pressure and depth.
How we can determine the flexural rigidity of the
lithosphere (Bulaihed)?
By the brittle and ductile properties of the constitutive rocks that comprise
it.
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Week 3:
Which forces drive North America?
Which are the forces you think can
be responsible for driving plates (Akram)?
Buoyancy and viscosity
2. Trench suction
3. ridge push
4. slab pull
5. thermal convection
If the north American plate
is driven from below then what should happen to the motion of North America
(Akram)?
It should slow down and finally come to rest.
Presenter's respond:
I agree with slowing of motion and disagree with its stopping.
How would lithospheric thickness
effect degree of coupling with mantle (Akram).
The thicker the lithosphere, the stronger the coupling.
How can we test the assumption that
hold simple shear responsible for lithospheric deformation
(Akram).
If this assumption is true, then fast axes must have a dip angle shallower than
45 degree from the horizontal.
Presenter's respond:
I agree, as explained in Figure (3)
If the plate is driven by
side forces or by below then where do you think orogenesis to occur (Akram)?
Orogenesis will occur on side toward which root is moving (see Fig.1).
Presenter's respond:
I agree, while orogenesis will occur
on the contracting side.
Stations in western United
States have northeast trending fast directions. What does this fact suggest
(Busfar)?
I would believe that the observed variation in fast direction would be more
dependent on the decoupling factor. So I would suggest that western United
States has thinner lithosphere and thus is less influence by the flowing mantle
beneath it, or in other words, it is more weakly coupled with the mantle than
other parts of the United States, and more influenced by gravity pull and drag.
Presenter's respond:
I agree, and the coupling relation with lithosphere thickness is illustrated in
Figure (2).
What came first gravity
drag-pull, or mantle flow (Busfar)?
I think this question supports the idea that mantle flow does indeed play a role
in moving the lithosphere. Because I would assume that during the early stages
of the birth of earth, the whole earth had fairly the same surface. So how did
we get the drag-pull effect if nothing was being subducted (dragged by gravity)
beneath the other? This might bring up the idea that probably the mantle flow
caused the plates to break up and then the effect of drag-pull came in.
Presenter's respond:
I think it’s a logical prediction. However, it is very hard to understand the
whole phenomenon initiation.
Will north America really
stop (Busfar)?
I don’t think that it will completely stop, because simply you will still have
the mantle flow, and buoyancy effects acting on everything on the face of the
earth.
Presenter's respond:
I agree, while plates all over the world will be moving and the driving forces
like mantle convection will still be there.
Why the author used a technique
based on angular variations of P-wave delays (Bulaihed)?
To determines both the azimuth and the dip angles for a set of stations in North
America.
What the anisotropic layers
that the deep Canadian Shield consists of (Bulaihed)?
The shallower one has subvertical foliation plane, and the deeper one has
subhorizontal foliation plane.
Presenter's respond:
I agree, as mentioned by the author referring to his paper with Silver in 2000.
Why North America has slowed
dramatically throughout the past 100 m.y. (Bulaihed)?
Because the south western motion of North America currently places the western
part of the stable continent over the downwelling, and if the motion continues
and the stable continent centers itself over the downwelling, the lateral force
acting on it will be zero, and the motion will stop.
Presenter's respond:
I agree. However I disagree with its stopping, while plates all over the world
will be moving and the driving forces like mantle convection will still be
there.
A brief summary given by Al-Omar on
the above paper as:
Two views attempting to explain the driving forces behind the motion of earth’s
plate, plate tectonics. The one theory relate the motion of the plates to
convection and the other view relate the motion of the plates to a combination
of “ridge push” and “slab pull” by the subducting plates. Through studying the
deformation, and the orientation of the deformation, of the deep continental
roots, the paper endeavors to proof that mantle plays a major role in plate
motion. The technique used was based on angular variations of P-wave delays to
determine the orientation, azimuth and dip angle, of the deformation.
Does the orientation of
minerals supports the mantle role in plate motion (Al-Omar)?
Yes, Studies shoes that in North America the mantle movement is faster than the
overlain plate. It also shows that arrivals of from southwestern direction are
faster than arrivals from other direction. This supports the idea of driving
force from below
How is that related to the thickness of the lithosphere (Al-Omar)?
Variation in the thickness corresponds to the speed of the mantle movement.
Presenter's respond:
I agree, and this is illustrated in
Figure (2).
How is the dip direction
supports the claim (Al-Omar)?
The dip direction depends on the driving force of the plate motion. If the plate
motion was caused by mantle convection than the dip is in the direction of the
of the movement. However, if the plate movement was caused by slab pull and
ridge push thean the dip orientation is the opposite.
Instruction's comment:
The GPS studies are evident on the
present-activity of North America. How a discussion on the past
deformational process (100 m.y.) "slowing" and future form of deformation
"stopping" by current deformation rates may be acceptable?
Authors's respond:
![[Picture of GB]](ego_dept.gif) Dear
Dr. Öncel,
looks like an interesting seminar that you are having there, and a selection
of interesting papers. Now what concerns the motion of North America, I
don't think that the deformation would stop; on the other hand the absolute
motion of the North American plate with respect to the deep mantle probably
will. The case of the North American plate is a rather particular one, since
that plate finds itself above a well-documented downwelling. In that case,
forces driving the plate from below converge toward the downwelling from
both sides. That is why the North American continent cannot escape slowing
down if the plate is indeed driven from below. Relative plate motion (plate
tectonics..) would continue, with surface deformation and earthquakes.
Best regards,
Goetz
webpages: laboratoire
http://www.dstu.univ-montp2.fr/
personell http://www.dstu.univ-montp2.fr/PERSO/bokelmann/index.html
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Week 4:
Earthquake Conversation
A brief abstract is given by Al-Omar as:
The idea in this paper is easy
to follow due its logical progression. An earthquake causes tremors that add
stress to an earthquake prone area close by. Faulty planes respond to any
thing that would increase the stress causing them to grind and slid shaking
the earth above them. The cause of the added stress might as well be another
earthquake “near by”. If the theory is sound then the world could be alerted
of the approximate time and location of the next destroyer.
Is there enough evidence of such
theory other than the ones related to the San Andrea’s fault (Omar)?
The paper seems only to talk
about that part of the world.
Presenter's respond:
Well, the paper does seem to talk about only the San Andreas Fault area, But
the people are working on stress changes for different areas, not only on
strike slip regimes, but also on thrusting and normal faulting regimes.
Would it matter if the earthquake is a long a strike-slip fault formation or
underneath a convergent zone (Omar)?
The paper doesn’t specifically
take about the different kinds of earthquakes. However, it seems that the
author believes of the simple theory that any where in the world if there
two earthquake-prone fault then they’ll eavesdrop on each other.
Presenter's respond:
The stress conditions will vary from regimes to regimes, and also the
changes in coulomb failure will vary, depending on the strike slip, thrust
or normal regimes. Like for the same order magnitude stress changes, that
is, the coulomb failure stress change produced by the source earthquake,
being the order of magnitude 10-2 MPa, if the ambient stress level is high (
as for strike slip earthquakes), there is almost no evident effect produced
by such small perturbation. On the other hand, if the ambient stress level
is low (as for thrust earthquakes), the small stress perturbations play a
mole important role.
Could this production be the next forecast after the daily news, next to
weather forecast (Omar)?
Possibly, nowadays earthquakes are happening all over the world. Although,
the cause of these earthquakes might be follow other reasons, such as the
mantle loosing it viscosity.
What happen when the
shear stress exceeds the frictional resistance on the fault or when the
stress pressing the two sides of the fault (Bulaihed)?
The rocks on either side
will slip past each other suddenly, releasing tremendous energy in the form
of an earthquake.
Presenter's respond:
I agree, as whenever the
shear stress exceed the friction, slip will occur and cause an earthquake.
Explain how come in
Turkey and in southern California that even tiny stress changes can have
momentous effects, both calming and catastrophic (Bulaihed)?
Seismicity never shuts off
completely in the shadow zones, nor does in turn on completely in the
trigger zones .Instead the rate of seismicity merely drops in the shadows or
climbs in the trigger zones relative to the preceding rate in the area.
Presenter's respond:
Up to some extent I agree
with your answer , But what i think is that earthquake from the main shock
can both reduce and enhance the regional stress conditions/. In case if it
raises, the earthquake will happen and when it decreases, there are very
less chances that we'll have an earthquake there. That's the calming and
catastrophic effects of the main earthquake, that either it can cause some
earthquakes on other faults or it can inhibit the seismicity on other
faults.
What the author and his
colleagues observe after mapping the locations of Landers, Big Bear and
hundreds of other California earthquakes (Bulaihed)?
They notice a remarkable pattern
in the distribution not only of true aftershocks but also of other, smaller
earthquakes that follow a main shock by days, weeks or even years.
Are earthquakes the only
mechanism in which stresses are generated and released (Busfar)?
I believe that there are other
factors that contribute to the generation and/or release of stress. One
might be the behavior of the mantle beneath. This includes the convection
regimes and the distribution of mass within the earth.
Could we estimate the stresses
in the earth accurately? In other words, how accurate is the “Coulomd stress
change” map that the author presented in his paper? Are they accurate enough
for the purpose of earthquake forecasting (Busfar)?
I am not sure how accurate these
maps are because I don’t know the method used to generate them, however, the
author did mention that he believes that very small change in stress regime
(as little as 1/8 of the pressure required to inflate a car’s tire!!) could
trigger as earthquake. If that is the case, then could these maps be
reliable for the purpose of earthquake forecasting? I tend to believe
otherwise.
What is the capital of
Turkey (Busfar)?
Ankara! Not Istanbul!
Explain how aftershocks
generated in North America as per the author interpretation (El-Hussein).
Along the San Andreas Fault, for
instance, the plate carrying North America is moving south relative to the
one that underlies the Pacific Ocean. As the two sides move in opposite
directions, shear stress is exerted parallel to the plane of the fault, as
the rocks on opposite sides of the fault press against each other; they form
second stress, perpendicular to the fault plane. When the shear exceeds the
frictional resistance on the fault or when the stress pressing the two sides
of the fault together is eased releasing tremendous energy. But because
stress cannot simply disappear, it must distribute some where along the same
fault or to nearby faults, which causes aftershocks.
What is the renewal
probability forecast (El-Hussein)?
It is a more refined
forecast that predicts that the chances of a damaging shock climb as more
time passes since the last one struck. This based on the assumption that
stress along a fault increases gradually in the wake of a major earthquake.
What is main aspect of
the author’s forecast method (El-Hussein)?
He builds the
probabilities associated with earthquakes interactions on top of the renewal
method by including the effects of stress changes imparted by nearby
earthquakes.
Author's respond:
 Ali,
This is great. Your students did a wonderful job with the paper. The SciAm
editor added that Istanbul was the capital of Turkey, and she is very
embarrassed by this mistake.
If any of your students want to go deeper, they can check my web site below,
which includes the most recent work on this subject and a new paper (Toda et
al, JGR, 2005) with 3 animations included.
Best,
R o s s
Ross S. Stein
U.S. Geological Survey, MS 977
345 Middlefield Road, Menlo Park, CA 94025
Tel: 1 650 329 4840 Fax: 1 650 329 5143
Papers, animations, software, teaching tools:
http://quake.usgs.gov/~ross
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Week 5:
Mantle, topography and rift-flank uplift
of Arabia
Do you
think that the failure of this model in predicting the large peak in the
African residual topography is an indication that the theory is
incorrect (Busfar)?
The
shortcoming of this method might be due to different conditions that
exist in the African plate and thus causes the large peak in the African
residual anomaly. Another possibility is that it might be due to
shortcomings in the model itself caused by using simplified models in
computing the final results.
Why did the
author neglect the effect caused by heterogeneity of the mantle below
670 km depth (Busfar)?
The author
claims that taking into consideration the effect of the upper 670km of
the crust and upper mantle and deleting the effect of heterogeneity
caused by depths larger than 670km improves the result of the calculated
topography compared to the observed topography, however, I am curious to
know the results of neglecting different depths. In other words, what
would the results be if we only included the effect of the upper 500km,
400km, 300km and so on? Maybe this will result in a better
approximation, or give us a better understanding to come up with a
better, or rather, alternative explanation.
Describe the mean features of
models that used to explain rift-flank uplift (El-Hussein).
-
Thermal Models: Uplift can
result from depth dependent stretching or from heating of flanks by
small scale convection.
-
Mechanical Models: Indicate
that upward flexure may occur if the lithosphere maintains finite
strength during rifting.
-
Geometric Models: Explain
the symmetry of uplift in terms of a single low angle detachment
penetrating the entire lithosphere.
-
Melt Process Models:
Extensive flank uplift may also result from magmatic underplating due to
asthenospheric partial melting.
Illustrate the main steps used
by the author in formulation of his model (El-Hussein).
-
Prescribing density field within the
model domain by using results from seismic tomography.
-
The governing equations are
solved for instantaneous flow fields throughout the domain.
-
Dynamic topography is
computed by applying surface normal stresses output from the convection
code to a model of elastic beam deformation.
What the author claimed about the
tilting of the Arabian platform (El-Hussein)?
Author stated that the topography dynamically supported by large scale
viscous flow in the mantle is responsible for the dramatic tilting of
the Arabian platform. The tilting also is enhanced by seismically mantle
beneath northeast Arabia, which acts to dynamically depress the
overlying plate in this area.
What do you mean by seismically fast
mantle (Akram)?
Cold Dense mantle, as it is dense therefore velocities will be fast in
that.
In the mega-plume area, would you
expect high velocities or low velocities (Akram)?
Low velocities, as due to temperature, the material will become less
denser to raise upward and the velocities will be decreased in that
zone.
Why don't the thermal and mechanical
models offer a good explanation for tilting of Arabia (Akram)?
As these models are largely concerned with flank uplift within a few
hundred kilometers of the rift basin, therefore, these models don't
provide a good explanation for Arabian tilt.
What does thermal model say about the
rift-flank uplift (Akram)?
According to thermal model, uplift can result from depth-dependent
stretching or from the heating of flanks by small-scale convection.
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Week 6:
Evolution of the Lithosphere Beneath the Rocky Mountains
Figure 2 shows a large lateral velocity
variation in the upper mantle. The author suggests that these
differences could be a reflection of the temperature variation within
the asthenosphere. Could there be other factors that contribute to the
velocity variation within the Asthenosphere (Busfar)?
I tend to believe that lateral variation in density, which could be
caused by physical compression or simply chemical change of the mantle’s
content, would contribute to the variation of P-wave velocity.
Presenter's respond:
What I think, that these chemical
changes also come from temperature variation like when oceanic
(basaltic) crust tend to subduct beneath some oceanic / continental
crust down to mantle, due to temperature and pressure, it goes under
metamorphism and changes to high density eclogite. So these density
variations in these depths are also because of temperature variations.
The article suggests that western North America (e.g., from the Canadian
shield to the Pacific plate margin) contains the largest mantle-velocity
gradient on Earth. If this is true, what do you think causes this
phenomenon (Akram)?
The article suggests that western North
America (e.g., from the Canadian shield to the Pacific plate margin)
contains the largest mantle-velocity gradient on Earth. If this is true,
what do you think causes this phenomenon (Busfar)?
This is most likely, or at least largely, due to the nature of the
geological setting in the Rocky Mountains area where there is a dramatic
change in the lithosphere density going from fast, cratonic, cold, and
dense lithosphere mainly on the east to slow, orogenic, hotter, and less
dense lithosphere mainly on the west. This abrupt change in the nature
of the lithosphere over a relatively (relative to other parts of the
world) short distance causes this unique situation where the
lithosphere’s velocity varies dramatically.
What do you think is the
origin of the high velocity lower crustal layer?
It could be due to the intrusion of other high density materials during
the Proterozoic Eon. Other possibilities include that it had a more
complex origin than currently assumed, or concentration of refractory
residues of partial melting.
Why authors selected North
America for such study (El-Hussein)?
Because it contains one of the thickest mantle on the planet, and
western North America (e.g., from the Canadian shield to the Pacific
plate margin) contains the largest mantle velocity gradient on Earth. In
addition, Gradation from fast to slow upper mantle velocity structure
occurs over a remarkably short distance in the Rocky Mountains.
How the Proterozoic
lithosphere of Colorado and New Mexico differs from lithosphere beneath
the Archean core of the continent (El-Hussein)?
1. In thickness.
2. The strongly segmented nature.
3. Long term fertility for magnetism.
4. Its relative weakness.
How Moho layer formed at
Rocky Mountains as per the authors' point of view (El-Hussein)?
It has formed diachronously and by combination of processes including
original arc development and subsequent magmatic underplating and to be
the product of progressive evolution of the lithosphere.
What does the combined
geophysical and geologic data from the CD-ROM experiment provide
(Al-Bulaihed)?
They are providing a high-resolution, multiscale image of the
lithosphere of the Rocky Mountain region.
What does the integrated
data set for the Cheyenne belt, the Farwell-Lester Mountain zone, and
the Jemez lineament, and their corresponding velocity anomalies in the
mantle (to >200 km) indicate (Al-Bulaihed) ?
They are controlled by Paleoproterozoic subduction zones that were
active during collisions of juvenile terranes.
What are the two provocative
and testable hypotheses concerning lithospheric evolution (Al-Bulaihed)?
1. the lithospheric mantle in the southern Rocky Mountains preserves old
subduction structures, is thick (>200 km) and has been persistently
weak, and
2. the lowermost crust is a record of progressive evolution of the
lithosphere and has grown through several underplating and/or intrusive
events.
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Week 7:
The crustal structure of the interior
Arabian Platform
What do you suggest that implications of more stations to record earthquake data
in Saudi Arabia will help in imaging the crustal structures? And What do you
think that the crustal model suggested by the author and described in Table2 can
be improved as it already shows a lot of velocity variations in the crustal
layers (Akram)?
Well, definitely by increasing the number of recording stations we mean that
more information we get about earthquakes. By interpreting this valuable
earthquake information, we can better image the earth’s structure. If we
integrate different information obtained through recent techniques like Receiver
function studies, stress distribution, seismic tomography etc. we can improve
our results as well as make them accurate.
When do the Nyquist frequency cause aliasing while digitization? What do you
think (Akram)?
Well I think that when the Nyquist frequency fN will be greater than the highest
frequency in the function, then it’ll cause the aliasing effect.
If we consider the assumptions regarding the vertical velocity gradient and
lateral velocity variations, the solutions to elastodynamic
equation becomes easy or very difficult (Akram)?
Very difficult to solve, if not, then impossible.
Explain briefly Thomson and Haskell matrix method (El-Hussein)?
The method provides calculations of responses of any number of horizontal layers
to incident plane waves ate any angle of incidence, by using products of 4x4
matrices, whose elements are functions of the parameters of each layer and
boundary conditions.
How was the Arabian Platform divided structurally (El-Hussein)?
Interior homocline, which is 400 km wide belt of sedimentary rocks dipping
gently.
Interior platform, 400 km wide where sedimentary rocks dip away from the shield
at low angles.
Intrashelf depressions, found around interior homocline and interior platform.
What are the main results
generated by the author's model (El-Hussein)?
The model claimed that the Arabian crust consists of five distinct layers with
thicknesses 3 km, 10 km, 8 km, 15 km and 10 km, from top to bottom, with P-wave
velocities of 5.6 km/s, 6.3 km/s, 6.6 km/s, 6.9 km/s and 7.6 km/s. For Moho, the
velocity is 8.3 km/s for upper mantle and 46 km depth.)
Why do plates move with
different velocities (Busfar)?
I believe that the two forces, namely slab pull and slab suction that causes
plates to move at subduction zones are responsible for plates moving at
different velocities. At subduction zones plates are being subducted faster and
thus the plates move with relatively larger velocities. Whereas at divergent
boundaries, the slab pull effect is minimal because it is remote and thus we
observe smaller values for the plate velocities.
Don’t you think that the results
obtained for Arabia is less accurate, and this less reliable than other regions
in this study (Busfar)?
Due to the fact that Arabia is
an area on minimal seismic activity relative to the region, it is not as widely
covered by seismic stations and other areas such as Turkey. Therefore I would
tend to be suspicious about the results concluded for Arabia.
Why is the DSF rate higher in
comparison with regional GPS studies (Busfar)?
This could be due to an active opening of the gulf of Suez rift but it could
also be due to miscalculation of the DSF using the available data.
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