Plate Tectonics - Science topic

Dear Nima Dolatbadi ,

Excuse me for my poor answer!

I don't have the technical knowledge to answer this question accurately... I The previous responder's writing is more in line with your expectations. But as good as the previous work is, you still won't be able to make breakthroughs in your research...

I think that simple common sense is a better way to solve this problem, and (among other things) proper field observations can solve the problem you want to solve with formulas (true, you should use an earlier method, too, to get better results!)... My solution is that you simply need to know the basic laws of nature that play a fundamental role in the sub-evolution of the face of our Earth.

Regards,

Laszlo

That is a really interesting question. I am sure that these terminologies are all widely used in the literature to kinematically describe complex slip behaviors along the fault. These words are not exclusive and can be the same thing sometimes, such as the aseismic transient creep.

In my understanding, slow slip is mentioned as the counterpart of the fast slip. Slow or fast slip is the kinematic description in terms of the speed of the slip. For example, slow slip corresponds to a velocity of several orders of magnitude less than several meters per second (i.e., seismic ruptures).

Aseismic slip is used as the counterpart of the seismic slip. It means that the slip does not generate seismic waves, like common earthquakes. As a result, aseismic slip can be detected only by geodetic observations or indirectly by the accompanied seismic events, such as tremors.

Transient slip is used as the counterpart of the steady slip. It means the slip is transient regarding the time scale, not a long-term steady slip behavior. It can refer to the postseismic transients, transient creep, or triggered slip.

Creep is an important slip behavior in the whole modes of fault slip, including fault locking, creep, slow earthquakes, usual earthquakes and so on.

Dear Muhammad Waqas ,

It would be good to formulate a good Earth tectonic theory first and after it try to answer such a question...

No subduction:

However, if that's how you want it, so be it:

Regardless, deal with the ice instead, The atmosphere can have 'indirect' effect: only in formation: rain, ice, landslides, lightning, ... Let your imagination go! and you will already have the material...

Regards,

Laszlo

In my article on RG 'The driving forces of tectonic activity' I propose a mechanism that can explain an oscillatory motion of moving plates.

It is well documented that tectonic activity has a seasonal variation. The article gives a mechansm based on the axis tilt of the earth with a different velocity ( and centr@ifugal force) of the earth hemispheres in the rotation around the sun according to the seasons.

How about ASPECT(https://aspect.geodynamics.org/), which is supported by CIG (Computational Infrastructure for Geodynamics). It's easy to get started , but I haven't use ASPECT for subduction initiation. And I think you could have a brief look about it , I hope it's useful to you (https://github.com/geodynamics/aspect).

Please read, "Structural Control of Mesozoic Orogens on SE Asia Basin Opening

Benjamin Sautter and Manuel Pubellier, 2022.

Plate tectonics like on Earth, where several crustal plates are constantly in motion, does not exist on the moon.

The research group around Dr. Thomas Watters, Smithsonian National Air and Space Museum, have identified another cause: the interior of the moon is cooling down. As a result, it lost about 50 meters in diameter over the course of several hundred million years.

Quotation: "Our analysis provides the first evidence that these 'faults' are still active and are likely to continue to cause moon-quakes today as the Moon gradually cools and shrinks."

It's like a bunch of grapes: when they shrink and become a raisin, they also wrinkle. In contrast to the grape skin, however, the lunar surface is solid and therefore breaks as the moon shrinks, according to the researchers. The crust parts are said to have pushed over neighboring parts. They call this "thrust error".

I latch on to it as an event that generates seismic waves -- natural or human-made. Right or wrong, this take put me in a better position to understand Dr Stephen Hicks' discovery that COVID-19 lockdown caused 50% global reduction in human-linked Earth vibrations. My RG question about this highly significant research didn't fetch a single reply or recommendation. https://www.researchgate.net/post/Have_you_heard_that_most_pronounced_silence_in_these_COVID-19_days

Seismic or not, it's the "shaking of the surface of the Earth" that makes it a disaster.

Dears Satyendra Kumar and Rudolf Greku:

Interesting question, interesting answer.

Saturn's moon Enceladus is the smallest body in the Solar System known today to be geologically active. Giant water plumes erupting, icy jets spew 200 kg/sec of water, with no significant variations over the Cassini´s mission. The strongest jets are more variable, though Enceladus eruptions are mostly water vapor, with a variable amount of ice grains [Hedman et al. 2018].

Please, I attached two papers to try to help you:

The composition and structure of Enceladus’ plume from the complete set of Cassini UVIS occultation observations C.J. Hansen, L.W. Esposito, J.E. Colwell, A.R. Hendrix, G. Portyankina, A.I.F. Stewart, R.A. West. Icarus 344 (2020) 113461.

Past, present, and future tectonics of Enceladus by L. Czechowski. 49th Lunar and Planetary Science Conference, 2018.

Best regards, Mario E. Sigismondi

Dear Xin Liu, China University of Petroleum - Beijing.

I recommend using the magnetic anomalies to estimate the Curie boundary, which indicates, direct or by analogy, the distribution of the thermal anomalies, present-day heat-flow, providing a clear marker for the thermodynamic effect in the crust and mantle. Therefore, I recommend first the use of magnetic anomalies, for example, from the EMAG2 datasets, it is free, and it has the wavelength (deep mantle large-scale structure) that you need in your research.

The knowledge of the magnetic anomalies in LIP (large igneous provinces) regions is a geophysical way, due that the mafic and ultramafic intrusions linked to those LIPs and their contrast in magnetic properties, magnetic susceptibility, using modeling and inversion.

Also, high densities contrast mafic/ultramafic rock compatible with serpentinized, also could show you in gravity anomaly inspection of a LIP, particularly regarding Bouguer complete anomaly map / Residual isostatic anomaly map.

I attached Jennifer Blanchard´s Marter of Science thesis, "Geophysical identification and characterization of mafic-ultramafic intrusions in plume centre regions", 2015, Carleton University, Ottawa, Ontario, Canada. I recommend reading in focus the Modeling methodology, it has wonderful examples.

Best regards, Mario E. Sigismondi

Hello dear;

I didn't research on Paleomagnetic studies, but i know 2 methods in order to behavioral study between two things. K-means clustering and Artificial Neural Network (ANN). you can read this combination method in this paper :

good luck

Hi Ananya!

I agree with some of the answers that mention that, for the software you are planning to use the vector layer with, you will need to have the information in a common geographic reference system. In order to do that with you can use the free software QGIS for the georeferencing process (look for Raster menu - Georeferencer) and then in the same software you can perform the conversion from raster to shape using (Raster- conversion-Polygonize).

The algorithm usually works well but the result will depend a lot on the complexity of the raster data.

Hope it helps! regards, Soledad

Dear Ahmed Morad: I understand perfectly what is your proposal. The best way to show how marine sediments get into a subduction zone is to draw a detail of the hinge zone where the oceanic plate bends when foundering inside the mantle. In this hinge zone stretching occurs, and some expansive graben structures are formed (normal faults parallel to the trench), which trap sediments and lead them to subduction. Then those sediments are metamorphosed under high P/T conditions and become phyllites and schists, usually white schists, and even metacherts and fine grained marbles. There is a quite accesible outcrop in a main freeway near the city of Puerto Cabello (north-central Venezuela) of eclogite knockers inside a relatively monotonous micaschist. A close look at thin sections of this schist shows the presence of kyanite, garnet, white mica, and Mg-glaucophane, these are usually called white schists, since the glaucophane is almost colorless and retrogradly altered to talc! Some have carbonate too, and in nearby localities of this same high P terrane there are also metacherts and marbles. So, there's no doubt that some marine sediments are indeed subducted and transformed to high P metamorphic rocks in the subduction complex. Regards, Sebastian.

A wealth of data can be found in most of these items to start with

in french https://www.google.com/search?q=données+gps+tectonique+des+plaques&client=safari&hl=fr-jo&sxsrf=ALeKk02wWD-noM9vo-yyK5uZjEOxyvxDUQ:1608261531837&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiE_72KydbtAhVQ3KQKHYgOAYgQ_AUoAXoECAYQAw&biw=1261&bih=819

or in english

I do not completely agree with Harald, with which I often are in concordance. It is not a matter of distance from the Sun. The energy we receive from our star is incredibly high (about 98.000 TW at sea level) much higher than the heat released from the Earth (around 44 TW). The photons of the Sun are essential for the developing of life and mantaining liquid water, but they have no influence on the deep interior dynamics of the Earth.

All the other rocky planets of our system are chemically and mineralogically stratified, with an iron-rich core and silicatic mantle and even lighter crust. Nearly all of them are (or have been) geologically active, with the presence of volcanoes (e.s., Olympus Mons in Mars) and with evidences of faults, rifts and mountain chains. Subsidence is common in the other rocky planets too, but Earth is the only planet charactherized by subduction. The presence of subduction is strictly related to the presence of liquid water, because this compound allowed partial melting of basaltic lithologies at relatively low temperatures, production of granitic s.l. magmas. These evolved melts led to the development of the first continent nuclei. These infant nuclei had (and have) very different rheological behaviour with respect of basaltic oceanic crust, much denster and thinner. However, the presence of liquid water is not sufficient to generate plate tectonics. Indeed, liquid water was present on Mars and Mercury too, but those planets never saw a real plate tectonics.

Worth nothing, subduction is essential for life and for mantaining liquid water on a planet. See what has happened to Mars. On that planet there was liquid water. It reacted with atmospheric CO2 to produce carbonic acid (H2O + CO2 = H2CO3). The dissociation of this acid led to the availability of anionic groups CO3^2-. These reacted with Ca^2+ ions dissolved in Martian water to produce carbonates. In this way, all the atmospheric CO2 was neutralized in the form of carbonates, freezing the planet (whose average temperature is -63 °C).

The CO2 neutralization in the form of carbonates happens to our planet too, but in our case we are lucky because, during subduction, a substantial amount of carbonates is dissociated at mantle depths to CaO and CO2. The vapour phase enters the structure of mantle wedge mineral assemblages and then is transferred to the melt phase, returning in the Earth's atmosphere to re-create the greenhouse effect.

About half of the heat flow of the Earth is related to the presence of radioactive elements 232Th, 40K, 235U and 238U, but these elements are present in the other rocky planets too, so they cannot be considered a peculiarity of the Earth.

In order to have a planet (or a moon) with plate tectonics it is necessary it is geologically active (with a hot core), it has a rigid lid (not too thin nor too thick), a softer (more plastic) mantle beneath the lid, a cold lid denser than hot stuff (i.e., a icy lid does not work), and a sufficiently high Reynolds number. Below I report a statement from Stern et al. (2018, Geosci. Front., 9, 103-119): "stresses associated with mantle convection must exceed the strength of the lithosphere sufficiently to allow plate rupture but not so much stronger that the plate breaks up into drips or so that the bottom of the lithosphere separates from the top of the lithosphere (delaminates; Gerya et al., 2015). These conditions are sufficiently restrictive that plate tectonics currently operates only on Earth, and mantle convection in most terrestrial planets and moons is probably in a stagnant lid regime, where the entire lithosphere of the planet is a single, globe-encircling plate."

I do not know how to reply to this question. By sure plate tectonics developed on Earth only.

Nobody knows when plate tectonics started, with estimates ranging from 4 to 2 Ga. The Earths started cooling also without plate tectonics. The two concepts (cooling with development of a first peridotitic assemblage, then producing basaltic rocks, and plate tectonics) are not directly related.

The first Earth's crust was basaltic and formed as consequence of peridotite partial melting. The first peridotites appeared on Earth's surface few tens to hundreds Myr after its formation, likely during Adean Eon. Then, the density increase led to their sinking in the magma ocean, allowing their partial melting to form the first veneers of basaltic crust, contemporaneously with the formation of the first oceans. In any case the process of continue and irreversible chemical stratification of upper mantle is continuing.

Crust is continuosly growing. Oceanic crust is formed along oceanic ridges and continental crust is formed at subduction settings. Each gram of matter produced along MOR is recycled in a age range from 10 to 250 Myr in subduction zones.

Cheers,

michele

P.S. I invite you to read "A story of the Earth" by Robert Hazen and "Earth: evolution of a habitable world" by Jonathan Lunine.

Dear Srinjoy Datta,

dating and source identification with U-Pb and Rb-Sr are ratehr well established (yet necessary) approaches. From an analytical point of view, If you are looking for recent advances, I would suggest looking into non-traditional isotopes, e.g., Mg, Fe, Si, Mo, Cr, Cu + Zn... . There are several possibilities to constrain magma pathways, assimilation, metasomatism, temperature, fractionation... you name it. You could look into Young et al. 2015 (10.1016/j.chemgeo.2014.12.013) for ideas.

If you are looking for more ideas, geochemical perspective letters are always a good adress to look for recent advances (https://www.geochemicalperspectivesletters.org/current-issue).

Hope this helps.

Thanks, Rabee, thanks, Sebastian. But the general recognition of a model does not exempt a scientist from the need to think. Can we think together? It is not that difficult. Almost everything is already there. At one time, Morgan, one of the fathers of plate tectonics, prudently created an alternative to it: plume tectonics. If we compare plume tectonics with other tectonic concepts, then it is obvious that it perfectly combines with any tectonic concept (classical geosynclinal, expanding and pulsating Earth, regmagenesis, etc.), except for plate tectonics. Indeed, the idea of ​​mantle jets piercing convective meshes, the idea of ​​"hot chains" too complicate both plate tectonics and plume tectonics, explaining only regional sequences of volcanic ages that are not linked to the global picture. So the question is: are there enough plumes to explain the observed tectonic phenomena? “Long” plumes under the boundaries of the plates, “point plumes” under the “hot spots”. Short spreading (up to 100 km) and short subduction as a process of the same scale of movement with collision and obduction. A periodic change in the intensity of plumes brings the entire system into an oscillatory regime. Isn’t such a picture simpler than the hardly imaginable multi-story convection in a solid?

Dear colleagues, I studied all these arguments about 20-30 years ago, and they all do not satisfy me deeply. All these arguments are discussed in detail in the article:

David Pratt, 2000. Plate Tectonics: A Paradigm Under Threat. Journal of Scientific Exploration, vol. 14, no. 3, pp. 307-352, 2000.

I recommend to read.

I work in the Russian Academy of Sciences, and many of us do not accept plate tectonics. In addition, I remind you that I have seen a lot with my own eyes over the decades of field and marine geological and geophysical works in which I participated. And I recommend everyone not to refer to someone's results and theories, but to open his eyes to the fundamental facts and think with his own head.

By the way, any fossils on the tops of any mountains logically indicate vertical movements, or not?

Based on seismic wave studies the results and knowledge will increases gradually .

Sorry, I don't much on this matter

Dear Dr. Minas

All things in the earth are related to each others by some relation...some majer relations are discoverd...and may be thousand miner relations still not discovered ....to answer your quastion ...the world need hundered years of observation and interpretation to achive the actual answer.... I think the rainfall may be caused trigger of an earthquake but this case depends on the geology, tectonic and structural features in addition to the universal effects.......good luck to reach your goal....with best wishes ....

Dear, Jure Žalohar

Thank You for your offer to share your Elsevier book "The Omega-Theory" I want to read this book carefully and give you my opinion send it to my message inbox .

The Earth is a hot and geodynamically active planet, revealed greatest by widely accepted Plate Tectonics’ theory. If you are thinking that seafloor spreading, and subduction have operated in modern plate tectonic mode only since late Cambrian or early Ordovician time (W.B. Hamilton, 2011), Earth still hot by Plumes.

The three main forces that drive Plate Tectonics on Earth are:

• Convection in the Mantle (heat driven)

• Ridge push (gravitational force at the spreading ridges)

• Slab pull (gravitational force in subduction zones)

Convection in the mantle, unquestionably plays a main role, but doesn’t explain in what manner some plates move faster than the convective flows beneath them. Current dynamic models have plates moving as part of a gravity-driven convection system that pushes young hot plates away from spreading ridges and pulls old cold plates down into subduction zones.

The Moon’s Mantle, too cool to move easily, has no convection and no active tectonic plate motions, due the Moon's smaller size and cooler interior than Earth. The surface-to-volume ratio in thermal physics (Planinsic and Vollmer, 2008) is a central concept related with the size of the Moon if you compare with Earth: the smallest volume has the largest surface-to-volume ratio and hence, the cooling for small volume is more efficient than for large volume. Therefore, on the Moon, the thermal equilibrium is reached much faster, shorter in geological time and no core hot of molten metal to help an active dynamo’s mechanism and plate tectonics.

Best wishes Haithem, I hope my answer can help to you. Mario E. Sigismondi

“Many of the objections are long standing but apparently unknown to many geophysicists, while some are peculiar to the new global tectonics. The conclusions drawn, in order of probability, are (1) the continents have almost certainly not moved with respect to each other; (2) convection is not active throughout the whole mantle; (3) even if convection is active in the upper mantle it cannot account for drift; (4) pole positions derived from paleomagnetism, and results of this method of investigation in its global form generally, are afflicted with an unknown cause of error and are in any case too inexact for drift reconstructions.”

(Wesson, 1972)

Thank you very much dear László Attila Horváth for your important advice

Dear Francesco,

With regard to the second question you may find helpful the folowing article.

Regards,

Guido

FRENCH, Scott W.; ROMANOWICZ, Barbara. Broad plumes rooted at the base of the Earth's mantle beneath major hotspots. Nature, 2015, vol. 525, no 7567, p. 95.

Hi, from my point of view, and some might have mentioned this already in the discussion, continental breakup is not the separation of the crust, but the entire continental lithosphere. This can get complicated and depends on inheritance, magmatism, extension rate and thermal state of the lithosphere. In Petersen & Schiffer (2016) we have explored and discussed this. You can find also a number of relevant references. When your crust/lith is warm, you can create highly thinned (hyperextended) margins, before breakup. Vice versa, a cold crust/lith. will "snap" and create abrupt, only minorly thinned margins. It's the interplay of crustal vs. mantle rheology, leading to depth dependent extension. cheers, Christian

Jorge,

There is a You Tube video here:

which I think answers your questions.

David Pratt in his research article Plate Tectonics: A Paradigm Under Threat on 2000 states the lithosphere has a highly complex and irregular structure. Far from being homogeneous, "plates" are actually "a megabreccia, a 'pudding' of inhomogeneities whose nature, size, and properties vary widely" (Chekunov, Gordienko, and Guterman, 1990, p. 404). The crust and uppermost mantle are divided by faults into a mosaic of separate, jostling blocks of different shapes and sizes, generally a few hundred kilometers across, and of varying internal structure and strength. Pavlenkova (1990, p. 78) concludes: "This means that the movement of lithospheric plates over long distances, as single rigid bodies, is hardly possible.

I totally agree with Yariv Díaz and Xue-Ming Yang. MORB is abbreviation for Mid-Ocean Ridge Basalt. That is it. There are chemical variations in oceanic basalts, but they are different than subduction-related basalts. But some of the early arc basalts may be confusing.

The tectonic transport and emplacement of MORB in alien environments is an all together different matter. Here we are talking about their tectonic locale at the time of origin.

Variation diagrams based on basaltic rocks should not be used indiscriminately for cumulates and plutonic rocks.

Dear Mario, Before determining the origin of any phenomenon, it is good to understand what kind of phenomenon it is. In the case of subduction, everything is complicated due to the fact that it has been tied for a very long time to the unconfirmed idea of ​​the infinite sinking of the oceanic crust, which constantly draws down the descending movement of the mantle convective cell. If we look at "subduction" soberly, we can say that it is most likely a zone of interaction between oceanic and continental slabs with predominant compression, an analog of a collision, or obduction, but with a predominant under-thrust component. Now in the zones of "subduction" compression and underlay predominate, later in the framework of the oscillatory process they can be replaced by stretching and spreading. Infinite podviganie plates on the outer part of the descending convection cell is unlikely by the elementary laws of physics. The geometric impossibility of this is proved among other authors and by me. Below is a reference of my abstracts on EGU-13 and mu presentation on EGU-14 on this topic.

Geophysical Research Abstracts

Vol. 15, EGU2013-6448, 2013

EGU General Assembly 2013

© Author(s) 2013. CC Attribution 3.0 License.

Unfortunately, all these publications are on Russian.

A. Buslovich "Мезозойские трубки взрыва Вологодской области" (Mesozoic explosive pipes of Vologodskaya oblast')

Also, there is another author V. Tret'yachenko "Перспективы мезозойского кимберлитового вулканизма юго-восточной части Архангельской алмазоносной провинции" (Perspectives of mesozoic kimberlite magmatism of the southeastern part of Arkhangel'sk diamond province)

Here is a link on the latest Sablukov's presentation (June 2017) about "highly perspective" Ust'yanskaya area where he propose mesozoic kimberlites:

Regards Alexander

Hi

When we talk about climate we appoint to long time(more than day and month),so we know there is different between weather and climate.

about your question, Climate is affected by large systems,ITCZ belt,Easterly and Westerly winds,jet streams,high pressure systems,and many other synoptical systems, difference of gradient pressure between geographical latitudes is the main factor to produce large scale movements ( easterly winds in low latitudes & westerly winds in mid latitudes) also there is thermal systems in equatorial and polar regions.As long as the earth is in circulation There will be difference in the distribution of the energy of the sun,and there will be forces such as gradient pressure, friction and coriolis,These forces affect the movement of air masses,and we have different climate conditions.so we can say in planetary sclae,climate operate independently of plate tectonics,but in meso and micro scales is affected by tectonic plates,sea levels,continents,vegetation levels and others......

best regards

Homa

Thank you so much, Mr Ruszkowski.

Dear Alois,

I think you already know what I am going to say, nonetheless........... Sutures are linear belts that define collision contacts between continental (+/- island arc) blocks. They commonly from broad zones, but are locally razor sharp contacts. They display some common characteristics with major faults , e.g., plastic to shear deformation, termination of sedimentary beds and lithologies, juxtaposition of contrasting rocks in terms of age and/or deformation style and metamorphism, mixing of lithologies (melage), and so on. In general, the suture zones are characterized by the presence of ophiolites and ophiolitic melages (oceanic mafic-ultramafic rocks, pelagic sediments, including radiolarites, cherts, Mn-bearing sediments, and blueschists), which readily distinguish them from other faults.

Regards,

Qasim

Yes, perhaps, but where do you see the source of this radiogenic heat? At what depth?

Volcanic activity and earthquakes are going to be completely uninfluenced by presence or absence of large ice caps, other than the normal ice loading and unloading effects on local earthquakes in the continental crust (if this is the case) under the ice caps.

I'd be very dubious about a Pleistocene ice accumulation 2km to 4km thick. Even the Laurentide ice sheet estimate of thickness at LGM  in: 

Dyke, A. S., Andrews, J. T., Clark, P. U., England, J. H., Miller, G. H., Shaw, J., & Veillette, J. J. (2002). The Laurentide and Innuitian ice sheets during the last glacial maximum. Quaternary Science Reviews, 21(1), 9-31. Fig.4,  

is probably at least 50% too thick at 914m.

The 3km of ice in the Greenland domes is a special case caused by its saucer shaped topography and proximity to north Atlantic weather systems. The base of ice at the South Pole (also at 3km) is close to the pressure-depth melting profile that is dependant on the underlying geothermal gradient generated heat flow. It is unlikely that ice anywhere ever reaches 4km thick. The geothermal gradient underlying ice varies from place to place, depending on its previous uplift-erosion-deposition history.

I think it could, but the depth is so much, I do not know if any kind of equipment could reach it.

Hi Bo,

With regard to the first part of your question, I recommend you read, in tandem, the first two introductory papers from GSA Memoir 204 (see below).

Also take a look at the single-paper synthesis of both orogens by Coney & Evenchick (1993).

Section 4 of O’Driscoll (2012) contains a short discussion of the differences between the Laramide/Sevier and Andean orogenies from a plate-geodynamic perspective.

Hope this helps

Regards

Graham   

Dickinson, William R. "Anatomy and global context of the North American Cordillera." Geological Society of America Memoirs 204 (2009): 1-29.

Ramos, Víctor A. "Anatomy and global context of the Andes: Main geologic features and the Andean orogenic cycle." Geological Society of America Memoirs 204 (2009): 31-65.

Coney, P. J., and C. A. Evenchick. "Consolidation of the American cordilleras." Journal of South American Earth Sciences 7.3-4 (1994): 241-262.

O'Driscoll, Leland. "Dynamics and imaging of subduction." (2012).

Check this one:

K.

The main difference is that the beachrocks are found exposed and not buried. I correlated with pH, salinity, and temperature, but considering a shallow environment. Being in a marine-continental transition environment, I considered the possibility of interference, but the answers regarding these parameters corroborated the results found.

Dear Lu-Lu: for some srange reason I haven't been able to unload the photomicrographs you attached, a message saying "Netwok error" appears all the time. Perhaps the file is too heavy (21 MB) and doesn't come through. Any way, pyroxenes, both ortho and clino, are silica saturated minerals, so they should be stable in a granitic melt, unlike olivine, for example. The possibility that these minerals could be restites is also quite likely, I've found similar examples in Venezuelan and Colombian granitoid rocks. But eclogites don't contain orthopyroxene, only granulites, so the source rock of the magma should have been granulites, not eclogites. Regards. Sebastian. 

P.D.: try to send the pictures one by one, and not as an attached, and heavy, file...

Dear Sumanta: the question you posed is a very complex one! The supercontinent cycle has been operating since at least the Paleoproterozoic or Neoarchean, assembling and disrupting continental masses, creating -and destroying- orogenic belts and ocean basins, volcanic belts, and the like. So it is unquestionable that such tectonic variarions had a strong influence on past climate, just as they have been doing since the Pangea disruption in the Jurassic. The problem is relatively easy to resolve in Mesozoic terrains, a bit more difficult in Paleozoic, and really a hard one in the Precambrian. Conditions for deposition of evaporitic basins were widespread during the Permian, but also in the Devonian and Proterozoic, where large evaporitic basins are found, also aeolian sandstones are frequent in this very dry period, and in older times too. Climate changes so profound as to be unimaginable to us happened when continental blocks collided to form supercontinents. Pangea was covered possibly by a desert three times larger that the actual Sahara, and by an enormous ice cover in its southern parts. The uplift of the Appalachian-Caledonian belt surely had a profound effect in the climate of this supercontinent and global Earth. Before this, in the Neoproterozoic, the uplift of the world-wide Greenvillian system of orogenic belts, which was probably even higher than the Himalaya and longer than the Andes..., due to the assembly of Rodinia, certainly had a marked influence in atmospheric and oceanic circulation, as to eventually lead he planet to the "Snow-ball Earth" global glaciation, just as the uplift of the Andes and Himalaya has done in more recent times, creating large deserts in South America and Central Asia. The problem in older terrains is the large uncertainties in the relative shape, positions and paleolatitudes adquired by land masses, this is relatively easy to solve during the Jurassic, but increasingly difficult to ascertain in older times. With regards, Sebastian.

You can find in the RG Data Base the following papers reflecting the deep structure of the Eastern Mediterranean:

Ben-Avraham, Z., Ginzburg, A., Makris, J. and Eppelbaum, L., 2002. Crustal structure of the Levant basin, Eastern Mediterranean. Tectonophysics, 346, 23-43.

Eppelbaum, L.V., Katz, Y.I. and Ben-Avraham, Z., 2012. Israel – Petroleum Geology and Prospective Provinces. AAPG European Newsletter, No. 4, 4-9.

Eppelbaum, L.V., Nikolaev, A.V. and Katz, Y.I., 2014. Space location of the Kiama paleomagnetic hyperzone of inverse polarity in the crust of the eastern Mediterranean. Doklady Earth Sciences (Springer), 457, No. 6, 710-714.

Eppelbaum, L.V. and Katz, Yu.I., 2015. Newly Developed Paleomagnetic Map of the Easternmost Mediterranean Unmasks Geodynamic History of this Region. Central European Jour. of Geosciences (Open Geosciences), 7, No. 1, 95-117.

Eppelbaum, L.V. and Katz, Yu.I., 2015. Eastern Mediterranean: Combined geological-geophysical zonation and paleogeodynamics of the Mesozoic and Cenozoic structural-sedimentation stages. Marine and Petroleum Geology, 65, 198-216.

Best regards,

Maybe it could be useful Schettino, A. & Tassi, L., 2012. Trench curvature and deformation of the subduction lithosphere, Geophys. J. Int..

Probably this article DOI: 10.1016/0899-5362(94)90061-2 and the citing articles will be a good start

Dear all,

I would like to add another possible explanation. The strength of the oceanic lithosphere (not just crust) is much greater than that of continental lithosphere, as an average. See e.g. the books by Ranally (Rheology of the Earth, pp.360-365 of 2nd Ed.), Kearey et al. on Global Tectonics (http://geology.lnu.edu.ua/phis_geo/fourman/library-Earth/Global%20tectonics.pdf; pp. 37-40) or Park (Geological Structures and Moving Plates, pp. 41-53). You have only to compare typical rheological profiles (strength envelopes) accross distinct types of oceanic and continental lithospheres. This allows oceanic lithosphere to resist and laterally transmit large slab pull, ridge push, or swell forces, without whole-lithosphere failure and it explains long-lived subduction periods (probably this was not the same during the Archean, with a hotter and weaker lithosphere mantle, when recurrent slab breakoff presumably led to short-term subduction episodes according to Moyen and van Hunen, 2012, Geology, 40, 451-454).

Therefore, rarity of extensional events affecting oceanic lithospere (other than ridge settings, of course, most of which were generated by evolution of successful continental rifting events) in comparison with continental (or arc) lithosphere may well be a consequence of their mechanical differences.

Regards,

Carlos

Hello to Everybody!

If you decided to discuss the subduction zones under the South American plate, including  the Benioff zones and the Nazca plate, please, allow me to pay you attention at one very interesting natural fact  that has no explanation yet.

It is revelad recently (attachment) that the southern boundary of the Nazca plate is magnetically conjugate with the northern boundaries of the Cococ and Carribian  plates. The angle of the subductions here is of about  45 degrees, that is exactly the angle of the geomagnetic  Inclination here.  This gives a hint  that  the earthquakes occur along the geomagnetic lines loaned into the crust, that results in the appearing of the subduction line. Attachment is in Russin, but figure captions are in English, thus, are understandable. 

Galina

Dear Ting Luo,

1. What is the relationship between the Philippine Sea Plate and the East Pacific Ocean crust?

-> The relationship between Philippine Sea Plate(PSP) and the East Pacific Crust(EPC) have not been studied yet. Both plates might have no relationship.

2. What is the relationship between the Philippine Sea Plate and the West Pacific Ocean crust？

->There are two conflicting theories suggesting the Origin of the PSP. The first theory suggested by Uyeda and Ben Avraham(1972) was that the PSP started opening from trapped Kula plate(paleo-pacific plate). The other theory was proposed by Lewis et al. (1982) that  the PSP was a type of back arc basin formed by abrupt and fast subduction of the Kula plate. Many people like the entrapment model. But, these two theories are still controversial. 

3. What is the relationship between the subduction zone in East China and the subduction zone in East Mariana?

-> People think that the East china sea is  a part of Eurasian continental plate overriding the PSP. Right now the PSP undergoes subduction process along the Ryukyu trench. the old Pacific plate pushes PSP. The west Mariana is also overriding plate but,it is  tectonic block originated from the oceanic plate.

Dear Paul, I think as already suggested by Martin Menzies, that the best should be to try to correlate as much as possible proximal and distal tephra. This can allow you at least to make a more confident attribution and dating of the possible source avoiding just one single data set and /or number date. Of coarse there are many possible sources but recently many papers on tephrostratigraphy have been published, especially in quaternary journals (eg.doi:10.1016/j.quascirev.2011.07.012; http://dx.doi.org/10.1016/j.quascirev.2012.09.009; http://dx.doi.org/10.1016/j.quascirev.2015.03.006; http://dx.doi.org/10.1016/j.quascirev.2014.04.002 and others) that could help to direct your effort. Data and literature related to very old volcanism are much less because often the proximal part is poorly known or unknown.  Have a good job.

Thank You very much Marwan Wartes

The temporal link between radiometrically-dated Large Igneous Provinces (LIPs)  and mass extinctions is well proved, exemplified perfectly by the end-Permian ecosystem collapse. However, the causal link between LIPs and global environmental perturbation remains conjectural, even if several relevant models and feedbacks were proposed. Generally, massive volcanic CO₂ degassing is seen as a main triggering mechanism. As summarized lastly by Bond and Wignall (2014): “Warming and marine anoxia feature in many extinction scenarios, indicating that the ability of a large igneous province to induce these proximal killers (from CO₂ emissions and thermogenic greenhouse gases) is the single most important factor governing its lethality. Intriguingly, many voluminous large igneous province eruptions, especially those of the Cretaceous oceanic plateaus, are not associated with significant extinction losses. This suggests that the link between the two phenomena may be controlled by a range of factors, including continental configuration, the latitude, volume, rate, and duration of eruption, its style and setting (continental vs. oceanic), the preexisting climate state, and the resilience of the extant biota to change”.

In fact, paroxysmal emplacement of the Cretaceous plateaus is also thought to be the main cause for global oceanic anoxia and severe acidification, as well as to biotic and climatic changes. For example, Aptian biocalcification crises and recoveries correlate with C, Pb, and Os isotopic anomalies, paired with trace metal anomalies. Erba et al. (2015) indicated: “Massive volcanism occurring at equatorial versus high paleolatitudes and submarine versus subaerial settings triggered very different climate responses but similar disruptions in the marine carbonate system. Excess CO₂ arguably induced episodic ocean acidification that was detrimental to marine calcifiers, regardless of hot or cool conditions. Global anoxia was reached only under extreme warming, whereas cold conditions kept the oceans well oxygenated even at times of intensified fertility”.

Two recent GSA Special Papers are especially recommended (505, 2014; 511, 2015) and the papers are useful in the questions:

Sandie, grat question...

That is a question that raised and still raises great debate among scholars, with no definite answer... I think it all depends on how you define "modern style plate tectonics", both in terms of the tectonic mechanism and elements, and both in the extent to which these phenomena dominate the earth. The latter part is difficult to determine, since ancient outcrops (older than 2.5 Ga) are quite rare, and therefore any information we learn from them, can be suspected as an exotic example of something that wasn't widespread... 

However, evidence does exist. Detrital zircons from the Jack hills metaconglomerate in Western Australia date by U-Pb to 4.404 Ga, with delta 18O values that ensue the presence of water in low temperature. This means that at least to some extent, continents and oceans were present on earth 100 m.y. after its formation (Wilde et al., 2001 linked). Further evidence for the existence of subduction complexes, and other "modern" plate tectonic elements during the precambrian were summerized by Cawood et al. (2006, linked). Zeh et al (2014, linked) reached similar conclusions from analysis of Archean zircons from the Limpopo belt of Africa. 

A different approach, claims that all this evidence represents relatively isolated examples from localities that don't represent a widespread regieme over the entire earth. Stern (2007, linked) for example claims that we need to examine the widespread occurance of ophiolites, blueschist belts, and ultra high pressure (UHP) complexes of earth, as these represent existence of oceanic crust and formation of subduction zones. In addition to other criteria, Stern (2007) suggests that full modern plate tectonic regieme began during the Neoporterozoic.

As i said, this is still in debate and not fully resolved, but i think these 4 papers i refer to can give you a good starter on this issue...

Movement of plate tectonics causes a change in the ocean currents and in turn generates more heat. Shifting of these tectonic plates result in the creation of the volcanic eruptions increases the degree of carbon dioxide (CO2) and sulphur dioxide (SO2) in the atmosphere that also leads to a rise in the temperature levels. On the contrary, if the move is insignificant, the temperature levels record a fall. Conclusively plate tectonic movement result in a change in climate though the rate is slow.

Hi

The vitrinite reflectance shows that the coals may have been formed at a depth and lately uplifted to a shallower depth.

Dear Longgang,

The formation of a foreland basin starts during the continent-continent collision when one of the two continental plates starts to be subducted by the obducted plate. The continent-continent collision starts after closure of the remnant ocean. Accordingly, the foreland basin develops not before or after, but during the collison. The sedimentation into the foreland basin occurs during the rise of the newly forming mountain range. The reasons for intense sedimentation into the forelland basin are the intense erosion in the rising mountains and the increasing relief between the rising mountains and the descending foreland basin. If marin or terrestrial sedimentation occurs in the foreland basin is controlled by the relative sea-level and the tectonic conditions. If the grade of mountain rise is high, the subsidence in the foreland bain is also high and marin transgression into the foreland basin may occur if the relative sea-level is high.

The sedimentation in the closing but stll deep ocean before the collision is dominated by deep-marin deposits ("Flysch") whereas sedimentation into the foreland basin is dominated by shallow marine conditions. Characteristic for marine foreland basin deposist are shallow marine siliciclastic deposits in the basin and deltaic fan conglomerates around the rims of the basin.

All the best,

Elmar

Siddhartha,

The answer to you question can be found at: https://www.researchgate.net/publication/264825668_Plate_Tectonics_Driving_Mechanisms_Some_Simple_Rules_that_Explain_Why_the_Plates_Move_the_Way_They_Do

I would be happy to answer any questions you might have.

- Chris Scotese

you can use FAULTKIN or WINTENSOR, but i think that wintensor is better for many raison, with this softawre you can calculate de R ratio and you can deduce the stress regime.

best regards

Dear Yinbao, the Philippine Sea outlines a complex tectonic history due to interference of Pacific and Australian plate motions with changing subduction zones at its margins, changing convergence and spreading orientations through time, which is too complex to be answered here in comprehensive detail, but there are some very good papers, e.g. Hall (1997) Geol. Soc. London Special Pub. v. 126, p. 11-23.

Hope this will help you

Jens

Dear Parkash

The Euler Pole estimation correspond to the Helmert-type velocity transformation model using only rotational rates. Euler Pole Parameters have one-by-one dependency to rotational rates about the three axes.

In my opinion, use the Helmert type transformation model using shift and rotational rates (not scale).

Regards!

That is the principle of the method!!!!! The farther away from main faults you get, more regional significance you get from the fault slip data inversion!!!!!!!!

Look for authors such as: Angelier, Angelier & Mechler, Etchecopar, Pecoraro, as method developers; even myself as user!!!!!

The heat flow intensity at first depends from contents of radiogenic elements in the Earth crust. By mean contents of U 2.6x10**-6 g/g, Th 1.3x10**-6 g/g, and K 2.5x10**-2 g/g we receive that 1.0 g of rocks can yield the heat quantity about 5.0x10 cal./y. If  radiogenic elements contents will enlarger, the heat flow will also enlarge. The mean value of heat flow for continents is 30-60 μW/m**-2, in majority of rift zones it enlarges up to 80-160 μW/m**-2. The same enlarging characterizes also some modern volcanic centers, but always it depends from  the radiogenic elements quantity.  

Pablo,

Your is a primary question in earth sciences and I think that it is rather difficult, or even impossible, to reply with a more or less composite answer, since, as evidenced also by David, several constraints should be considered before trying to reply to your question.

And, first of all, I guess that you should take into consideration whole lithosphere extension, and not simply “crustal stretching”, and the evidence that slow-ultraslow rift systems are characterized by magmatic activity that is stored into the shallow mantle lithosphere without giving rise to subaerial volcanic activity.

But to obtain appropriated answers, the question should be posed with more precision. In fact, you write of “decompression melting (of the upwelling asthenosphere?) and recall that “such decompression results from the crustal stretching related with rifting”. Are you referring to a mechanism of passive rifting (driven by far field tectonic forces) with almost passive asthenosphere upwelling, without any active upwelling of deep/hot asthenosphere, as it happens in the case of asthenosphere diapirs or mantle plumes?

Your question (“how much of this crustal extension is needed for triggering the magmatic activity observed in rift settings”) is apparently simple, but it is solely posed in an oversimplified way. It needs that the estimates evidenced by David and, moreover, the driving forces of rifting and the geodynamic context are constrained.

Rift formation has long been the focus of attention for researchers, and an enormous number of studies have been carried out in order to understand causes and modes of whole lithospheric extension (e.g., Brun, 1999; Corti et al., 2003; Dewey and Hancock, 1987; Illies, 1981; Keen et al., 1987; Khain, 1992; Morgan and Baker, 1983; Neugebauer, 1983; Neumann and Ramberg, 1978; Palmason,1982; Ruppel, 1995; Whitmarsh et al., 2001; Ziegler, 1992; and many others).

Just as an example suggested by my personal experience, I can recall that in the case of passive rifting (where asthenosphere is considered to play no active role in passive lithosphere extension) experimental investigations evidenced that the lithosphere must be thinned to about half of its original thickness (i.e., thinned by a factor β≃2: Foucher et al., 1982; Corti et al., 2007; Ranalli et al., 2007) (considering a lithosphere away from mid-ocean ridges, mantle plumes and subduction settings), in order that the near-adiabatically upwelling asthenosphere undergo decompression melting under spinel-peridotite facies conditions.

FOUCHER, J.P., LE PICHON, X., SIBUET, J.C., ROBERTS, D.G., CHENET, P.Y., BALLY, A.W., OXBURGH, E.R., KENT, P., DEWEY, J.F., BOTT, M.H.P., JACKSON, J.A., OSMASTON, M.F. & TURCOTTE, D.L., 1982. The Ocean-Continent Transition in the Uniform Lithospheric Stretching Model: Role of Partial Melting in the Mantle [and Discussion]. Philosophical Transactions of the Royal Society A, 305, 27-43.

CORTI, G., BONINI, M., INNOCENTI, F., MANETTI, P., PICCARDO, G.B. & RANALLI, G., 2007. Experimental models of extension of continental lithosphere weakened by percolation of asthenospheric melts. Journal of Geodynamics, 43, 465-483.

RANALLI, G., PICCARDO, G.B. & CORONA-CHAVEZ, P., 2007. Softening of the subcontinental lithospheric mantle by asthenosphere melts and the continental extension/oceanic spreading transition. Journal of Geodynamics, 43, 450-464.

In conclusion, my suggestion is that you have to explore fundamental references on rifting, easily found in the international bibliography, and try to make your own understanding of the primary process you are interested in, on the basis of a good knowledge of the present state of art of researches on rifting.

Ciao

Giovanni

Hi Archana... while this is not my area of expertise, I do know that the Mediterranean Sea is closing between the north moving African plate and the European plate.  The Atlantic Ocean is opening wider at the mid-Atlantic ridge and the Pacific plate is subducting beneath the Asian and Australian Plates. The Australian and African plates are spreading making the Indian Ocean larger.

Martin,

  In Semail, where Kelemen worked, your assertions may be correct.  In the Bay of Islands, cumulate dunites have cryptic mineral chemistry trends that reflect crystallization from an increasingly iron-rich melt.  These rocks also have regular and repeated sequences of dunite-wehrlite, a mineralogic trend that is consistent with olivine crystallization  followed by wehrlite precipitation along a cotectic.  The evidence is clear.

Not the same, but nice quantifications have been done for the magmatic lens at the top of the plutonic crust beneath oceanic spreading centers. How long can they survive (how fast do they crystalize) without being replenished ? Not much more than a few decades, 100 years at the most. See references below. Considering that the mafic lenses you are interested in are thicker, deeper, and probably (?) less affected by cooling by a hydrothermal system (important factor, difficult to evaluate), I would guess they could survive longer, but not over time scales of 10-20 Myrs. So if the volcanoes above are active over such long periods, that is probably because the lenses at depth are still feeded from time to time, eventhough they retreated trenchward.

Ref:Models of hydrothermal heat output from a convecting, crystallizing,

replenished magma chamber beneath an oceanic spreading center

Lei Liu1 and Robert P. Lowel JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, B02102, doi:10.1029/2008JB005846, 2009:"...the simulation results for crystal-suspended models show that heat output and the hydrothermal temperature decrease rapidly and that crystallinity reaches 60% in less than 10 a. In crystal-settling models, magma convection may last for decades, but decreasing heat output and hydrothermal temperatures still occur on decadal timescales...The rate of magma replenishment needed ranges between 5 105 and 5 106 m3 a1,which is somewhat faster than that required for seafloor spreading but less than that

of fluxes to some terrestrial volcanoes on similar timescales".

That same paper has some considerations on how fast an ore deposit can form, which is a sideway to look at the life expectency of the heat source, which could be of interest to you

see also: Mid-Ocean Ridges: Hydrothermal Interactions Between the

Lithosphere and Oceans Geophysical Monograph Series 148

Best wishes

see below -

PLATE TECTONICS: RULES OF THUMB CRScotese 12/19/93; updated 081312 Rule I. Plates Move only if they're pushed or pulled, not dragged. • The mantle plays a passive role. o The pattern of flow in the mantle is largely driven by lithospheric motions. • The motion of the mantle does not drive the plates (by mantle drag). o The idea of organized convection cells upon which the plates ride is wrong (though intuitively attractive - unfortunately!). • Oceanic plates move faster than continental plates. o Oceanic plates tend to have ridges (pushing) and attached subducting slabs (pulling). o At the base of oceanic plates in the LVZ (low velocity zone), a region of partial melting that provides 'lubrication" at the base of the plates. • Plates with a large area of continent move slowly (e.g., Eurasia) because they have a deep continental keel connected to the mantle. o For this reason continental plates more likely to be affected by mantle flow. • Plates (oceanic or continental) that are surrounded by ridges tend to move slowly (e.g Africa, Antarctica). o The push from the encircling ridges "cancels out". • Plates that are not driven by ridge push or slab pull do not move (Caribbean plate, Scotia plate). • In all cases plate motion can be understood and predicted by "balancing the forces" that drive and resist plate motion (driving: slab pull & ridge push, resisting: mantle drag). Rule II. Subduction Rules. • Slab Pull is more important than Ridge Push (80% vs 20%?). • Fast moving plates (>5 cm/yr), are attached to subducting slabs (e.g. Pacific, Nazca, Cocos, Indian) • The Phanerozoic speed limit is ~20 cm/yr (India, 65 Ma) o Convenient yardstick: 10 cm/yr = 1° per million year = 111 km/my • Where there is subduction, there is spreading or rifting. (The converse is not true.) Rule III. Ridges are Passive Features. • Ridges are there because the crust breaks when it is pulled. o Continental crust breaks first, because at the same depth it's closer to its melting temperature. • Oceanic crust rarely breaks; when it does it breaks where it has been weakened by a hot spot or subduction realted volcanism. • Ridges tend to align parallel to trenches. • One ridge is all you need. You'll never see two ridges side-by-side. • Fracture zones point to the trench. Rule IV. Subduction is Forever. • Collision only way to stop it. o You need big, gob-stopping continent-continent collision, otherwise subduction zone will "jump over" the colliding terrane. (e.g Capricorn trench in Central Indian Ocean). • Subduction is hard to start. o Most likely mechanisms: terrane collision or subduction virus.  A subduction zone that enters a new ocean can act as a trigger or focus for the start of subduction along an old, cold, heavily loaded passive margin (e.g. Puerto Rican trench and Atlantic passive margin). Rule V. Pacific versus Tethyan Subduction Systems • Pacific Subduction Systems(PSS) are characterized by a ring of subduction with a spreading ridge in the middle (e.g. Ring of Fire).  PSS are stable and can continue subducting for hundreds of millions of years. (As long as the central ridge doesn't get too close to a trench and gets subducted!)  They generate long-lived Andean-style margins or margin back-arc basins. • Tethyan Subduction Systems (TSS) are asymmetric or "one-sided".  There is only one subducting margin (like Tethys). On the other side of "Tethys" is a passive margin.  The ridge in the middle of Tethys must move towards the trench and is eventually subducted. • Because there is no longer a ridge in the subducting ocean, a new rift must form. • The rift forms in the continental plate that is now being "pulled " towards the Tethyan trench. o These new slab pull forces either tear a chunk of continent away from the margin (India), or break the continent apart (breakup of Pangea).  The latter in more likely to happen if the continent is weakened by old collision zones or new hot spots. Rule VI. Plates Subduct Normally. • Oblique convergence is more work; orthogonal least work. o Displaced terranes originate in areas of oblique convergence. o The highest mountains along Andean-style margins are in areas of oblique convergence (In NA that's Mt. McKinley). Rule VII. The Style of Convergent margin depends of the absolute motions of the plates. • Andean margins - net convergence (~10 cm/yr) • Western Pacific margins - net divergence (roll back 1-2 cm/yr) Rule VIII. Island Arcs don't ride their trenches across oceans. • Back arc basins never evolve into wide (>30,000 km) ocean basins. o You'll never find an continental island arc in the middle of an ocean. o There is always a continent nearby. • 90% of all ophiolites originally form in back-arc basins Rule IX. Slab Rollback Can Create Odd Intracontinental Ocean Basins • Oceanic lithosphere can become trapped (encircled) by continents (e.g. Mediterranean, Arctic, Tethys north of Alps). • Ssmall, short-lived subduction zones can consume this ocean floor creating intracontinental extension and small "odd " ocean basins (e.g. Tyrhennian Sea, Pannonian Basin, Makarov Basin). Rule X. Mantle Plumes (i.e. Hot Spots) are Important (Sort of) • Hot Spots are derived from the core/mantle boundary (mostly). • They provide a "good enough" reference frame for absolute plate motions. o Some Hots are Fixed; Some Aren't  Hot Spots are organized in "proper groups" • The Indian-Atlantic Proper Group has been fixed to spin axis since Triassic. • The Pacific Hotspots (Hawaii) have moved about 200 km in 100 my. • Hot Spots "help" break apart continents. o They create areas of weakness in the continental lithosphere. o Thermal uplift associated with mantle plumes causes uplift which help to give the a little "push" (like ridge push).  Important: Mantle Plumes and hot spots do not break apart continents. They just make the job easier. The heavy lifting is done by slab pull forces. o Hot spots under moving plates speed them up, a bit.  They thin the continent's "mantle keel" that usually slows plate motion.  They "grease the skids" by melting the base of the lithosphere (decreasing mantle drag forces). • If mantle plumes and hot spots did not exist, the plates would still move, continents would still break apart, move across the globe and collide! • But the things would be less interesting. Rule XI. Continental Collisions are Important (Really) • Continental collision zones (sutures) provide long-lived zones of weakness in the crust that are likely to become future sites of continental rifting. • A continent with many sutures will be weak and easily deformed (e.g. Asia following collision with India). Last Rule. Plate Tectonics is a Catastrophic System (not chaotic). • Plate motions are generally gradual; but every once and a while "WHAM"! o "Long periods of boredom interrupted by short moments of terror" o Because plate boundaries are metastable. Geometries can interact in unpredictable ways to produce global plate tectonic "events".  2 important instabilities: continent-continent collision & ridge subduction. • Continental collisions can cause new trenches to form. • India's rapid northward flight & the breakup of Pangea were due to ridge subduction. • both of these events cause rapid, global falls in sea-level o continental collision increases volume of ocean basins because the area of continental lithosphere o ridge subduction increases the volume of ocean basins by decreasing the average age of the oceanic lithosphere • Suprecontinents form and break apart because of the metastable nature of plate evolution. o Ridge subduction both breaks supercontinents apart and brings them back together (Wegener or Wilson Cycle).

No regular world data centers like Spidr, WDC Kyoto or Ktushu publish Tanzania magnetic data.I myself have not seen any magnetic data from Tanzania.```

Hi Longgang

I will try to describe the methodology in a very concise way. There is no VEI for a volcano. The index is attributed to single eruptions. You may consider the maximum VEI of the volcano looking for the most voluminous explosive events; this will give an idea of the larger events that the volcano can produce. To do so, you need to make a detailled study of the volcano stratigraphy of the volcano in order to recognize pyroclastic deposits of individual eruptions. Then you have to describe in detail the cross sections of each deposit in as many locations as possible. Some of the most important parameters are the thickness of the deposits and maximum size of juvenile and lithic fragments in every available location. With the surface distribution of the deposits of each volcanic event and the mentioned parametres you can draw isopachs and isopleths. With the isopachs you can estimate the volume (minimum volume) of pyroclasts produced in each eruption, wich allows you to calculate the VEI for each eruption. 

This is a very time consuming process, and a tricky one when working with old eruptions because the geological record may be incomplete due to erosion, or the deposits may not be totally exposed at the surface. In general many assumptions have to be made; for example, the total original thickness of a given deposit may not be preserved due to partial erosion, so if you have two close locations with very different thicknesses, the thicker one should be closeer to the original thickness.

Best regards

There is a wealth of literature out there. As good starting points for the region in this period, I recommend:

De Paor et al. (1989) The Arctic Eurekan Orogen: A most unusual fold-and-thrust belt. GSA Bull.

Harrison et al. (2006) In Search of the Wegener Fault : Re-Evaluation of Strike-Slip Displacements Along and Bordering Nares Strait. Polar Research

Tessensohn et al. (2000) Eocene Compressive Deformation in Arctic Canada, North Greenland and Svalbard and Its Plate Tectonic Causes. Polar Research

There is a short (9 recs) list of results from EBSCO host:

hi, you can calculate your own densities as a function of depth and/or chemical composition with perplex (http://www.perplex.ethz.ch). you can find even a matlab version of the program and integrate the results directly in your computational models.

The shear surface (slickenside) that has the smaller area, located just above the larger shear surface, appears to have shear striae that are inclined steeply, down to the left of the photo. The orientation of possible of shear striae on the larger surface, just above the open field notebook, is ambiguous to me, given just the information provided by this photo. It appears both surfaces dip steeply toward the photographer, and are sub-parallel to each other.

This might be an old slip surface that has been rotated since displacement occurred along it, in which case I would need more information than this photograph alone can provide.

On the other hand, the slip surfaces might be unrotated since displacement, in which case we can venture further interpretation. I might infer that the sense of slip is more likely to be primarily normal (i.e., the hanging wall moved downward) with a slight right-lateral strike-slip component. (Very steep unrotated reverse faults are far less common than very steep normal faults.) If this inference is correct, these slip surfaces could be related to either high-angle normal faulting or to a landslide. After all, the slip surface of a landslide is a normal fault, except locally along the side and toe of the slide mass. More information about the local topography would be needed to assess whether landsliding might be a reasonable interpretation.