BRIEF SUMMARY OF THE PAPER " THE STRENGTH OF THE LITHOSPHERE"

James Jackson in this paper tried to replace the JELLY SANDWICH concept of the lithospheric strength with a SINGLE SEISMOGENIC LAYER concept.Different observations , on the basis of which, jackson concluded that the lithospheric strength lies in a single layer within which there are earthquakes are (1) Maggi's restriction of earthquakes to the upper crust, sometimes to the whole crust.
(2) Focal mechanism solutions of earthquakes
(3)Elastic thickness that are similar to but a little less than the seismogenic thickness
But there are certain other points that seemed to support the strong mantle concept like
(1) gravity interpretation shows that strength lies in mantle rather than in the crust.
(2) Earthquakes are indicators of frictional stability rather than the strength, and that the continental mantle can still be strong despite being aseismic.
So at the moment our views of continental tectonics are confused by not knowing what really controls the pattern of deformation we see at the surface. With the availibility of more reliable data, these views may change.

How will you define the strength of the lithosphere?

Ans. 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.

What is the effect of temperature fo moho on lithospheric strength?

Ans. 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

*= the graph couldnot be sent with this mail but can be sent to you as an attachment (if required)

Why continents are more complicated than oceans. In general, continental convergence zones should have intermediate or deep focus earthquakes or not?

Ans. Continental crust is much thicker and less denser 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?

Ans. According to classical studies of isostacy, the higher the topography, the thicker the crust and deeper the Moho ( roots of the mountains).

Which other factor than the temperature is related to the lithopheric strength?

Water contents as the presence of its minimum amount reduces the creep strength.

Mohammed Al-Bulaihed

1) How the depths of earthquakes in India and Southern Tibet have been determined?

Ans: By waveform modeling or by direct identification of the surface reflection phases Pp and Sp.

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.  

2) Why the effective elastic thickness, Te, is smaller than the seismogenic thickness, Ts?

Ans: 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.

Respond:

I agree: see Q4 from Akbar for more in this issue

3) Where earthquakes on the continent are confined, in most places?

Ans: Earthquakes on the continents are confined to the upper half of the crust

Respond:

Question #1:

What are the differences between the conventional view of the continental lithosphere, and the new view claimed by the author?

Answer:

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.

Respond:

That what the paper claims, yet, it is still just a theory, I believe.

Question #2:

Explain how the author tried to prove that the behavior of the continental lithosphere is dominated by the strength of its upper seismogenic layer.

Answer:

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.

Respond:

True: the question is still which part of the Lithopshper is seismogenic. According to this paper it is the lower and upper crust. But, many papers still believe that it isn’t true

Question #3:

What are the possible implications if the author view is correct?

Answer:

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.

Respond:

Akbar

Q.1 How will you define the strength of the lithosphere?

Ans. 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.

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 sesimogenic layer.

Q.2 What is the effect of temperature for moho on lithospheric strength?

Ans. 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

 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.

 Q.3 Why continents are more complicated than oceans. In general, continental convergence zones should have intermediate or deep focus earthquakes or not?

Ans. 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.

Respond:

  I agree

Q.4 What is the effect of topography on crustal thickness and depth of moho?

Ans. According to classical studies of isostacy, the higher the topography, the thicker the crust and deeper the Moho ( roots of the mountains).

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.

Q.5 Which other factor than the temperature is related to the lithopheric strength?

Water contents as the presence of its minimum amount reduces the creep strength.

Respond: