I am glad to see this exchange because this is a classic debate related to the chicken vs egg problem in studies of erosion-tectonics interactions. We ran into this headlong in the STEEP work in Alaska. We have written lots of papers on the subject, but never, in my opinion, has the chicken vs egg problem been resolved. The orogenic system clearly is initially dominated by tectonics, but as the erosion machine becomes stronger with time there is no doubt the tectonic system responds. The way it responds, however, seems highly variable depending on the specific local details. In the STEEP area intensification of erosion turned on out-of-sequence structures, but most of this occurred at supracrustal levels. The Himalaya have certainly been suggested as a large-scale system where intense erosion has promoted the “channel flow” process, but as this discussion shows that conclusion is not universally accepted. The bottom line is the distributed exhumation that occurs in active thrust belts clearly influences interactions of structures, but there a LOT of unanswered questions.
On the Namche Barwa/Nanga Parbat issue, that is a different animal, in my opinion. When big rivers, or glaciers, are involved focused exhumation can dominate the system in remarkable ways. Nanga Parbat is the classic case for this where Zeitler and other have developed the concept of tectonic aneurism. (another controversial concept, but the data are pretty spectacular) The other early example cited for this was the stuff we did in central Asia in the 90’s where a river trapped between two mountain ranges becomes a sort of geomorphic conveyor belt where river power is the primary controlling process rather than climate. (well, indirectly climatic because this wouldn’t work in a desert) In STEEP we looked at a tectonic corner that is analogous to Namche Barwa in kinematics, but dominated by the world’s most erosive process—temperate ice. In Alaska the process has not been going on as long as the Himalaya, nor are we in a system with extreme crustal thickening. Nonetheless, there is still a pattern of exhumation that is remarkably similar to the Himalayan syntaxes (shown by low T thermochronology—just google Eva Enklemann and you’ll find half a dozen papers on this). We can also see huge kinematic shifts in the deformation associated with transport of material into the corner, but again, the problem becomes a hopeless issue of chicken vs egg in what is driving what. Clearly tectonics is the driver, but the response to the system on a large scale is difficult to evaluate. Peter Koons has long emphasized that focused exhumation can lead to localized weakening and positive feedback where deformation gets localized to the rapid exhumation zone—the geodynamic context for the aneurism hypothesis. We really hoped we could test this well during STEEP, but like earlier attempts the results have ambiguities. I would really welcome any thoughts on how one might resolve that ambiguity!
Terry Pavlis
From: Tectonics & structural geology discussion list [mailto:[log in to unmask]] On Behalf Of MIke Searle
Sent: Friday, November 14, 2014 3:20 AM
To: [log in to unmask]
Subject: Re: EGU General Assembly session (TS3.3/CL1.9/GM3.6): “Investigating Tectonism-Erosion-Climate Couplings (iTECC): Himalayan orogenic development and climatic feedbacks from micro- to macro-scale"
Peter,
I am afraid I profoundly disagree with almost all of this.
First the MCT (and STD) are not active faults; they were during the Miocene but are they not active now (likewise Channel flow, the exhumation of a layer of partially melted mid-crust in the Himalaya during the Miocene is probably not active now). The active faults are along the southern boundary of the Himalaya (MBT, MFT).
Second Rainfall most certainly did not control initiation of the MCT (or the STD). Tectonics controlled this, not Rain.
Earthquakes generally initiate faults and most earthquakes initiate from above the brittle-ductile transition and at depths down to ~30-40 km and propagate up through the brittle crust. The MCT and STD initiated well down into the ductile mid-lower crust.
You are surely not saying that rain/climate penetrates down there (I hope!).
The climatic differences between the two Himalayan syntaxes are enormous, both are controlled entirely by tectonic processes with maximum compressive stress in all directions accounting for the rapid vertical exhumation of rocks and the young metamorphic ages in the syntaxes. The big rivers (Bhramaputra in the east; Indus in the west) cut at right angles across both syntaxes, across the areas of high uplift and exhumation. The rivers have nothing whatever to do with the rock uplift (or surface uplift) of either syntaxis. Tectonics control these structures entirely.
It rains like hell in the Amazon (and in Oxford sometimes) but I see no major MCT type fault induced here.
Rainfall has nothing to do with mountain building. Tectonics make mountains and rain etc erodes them away.
Likewise in the Karakoram exhumation of kyanite and sillimanite grade rocks formed at depths of 12 kbar or more are not controlled by glaciation. Glaciers eroded the top 5-10 km of present day topography but they cannot be responsible for exhumation of deep crustal rocks from 12 kbar.
Greenland, Northern North America and Siberia are covered in huge ice sheets and glaciers, theres no active mountain building going on there.
Whatever induced the Asian monsoon (Tibetan plateau uplift?) and when noone really knows, but for sure the Himalaya are the northern barrier to the monsoonal rainfall today. Rock exhumation rates are pretty similar along the length of the Greater Himalaya (and timing too) but in the east it is wet and rainy and in the west (Ladakh, Zanskar, Nanga Parbat) dry. Thus in my opinion Tectonics initiates and controls rock uplift and ductile shear zones, faults and mountain building along the Himalaya, not Rain, Glaciers or Climate.
cheers
Mike Searle
On 14/11/2014 00:57, Peter D Clift wrote:
Hello Mike,
Thanks for continuing the debate. There is no doubt that the MCT is a major crustal shear zone which exhumes deep-buried high grade rocks. For me at least, and I feel I am not alone in the this view, the issue is why is it active at all and what controls its location and rate of exhumation. The MCT is not just any old fault after all. I would argue that it is located where it is because of the relative rapid exhumation of the Greater Himalayan slab compared to the Lesser Himalaya. This rock uplift is modulated by the rate of erosion at the surface. In other words bands of rainfall intensity control where the rapid exhumation occurs and therefore the location of the MCT (credit to Cam Wobus and Kip Hodges if memory serves me well). I tend to agree with you that the STD works together with the MCT to control the exhumation of the Greater Himalaya but if that is the case then the STD is partly climatically controlled as well.
As for Nanga Parbat that could be a different kettle of fish. Its seems clear that the Namche Barwe syntaxis is largely achieving its super fast exhumation because of the denudation allowed by the strong monsoon there and the Brahmaputra River, as evidenced by the huge contribution of Namche Barwe to that river. However, the same is not true in the west. Nanga Parbat is in a pretty dry place and does not particularly contribute to the Indus. Its exhumation is much more dominated by the solid Earth in my view, even if erosion does have to lend a hand. Is there any evidence of detachment faulting at Nanga Parbat? Thrusting alone will not result in exhumation, indeed quite the opposite.
The Karakoram exhumation may not be monsoonally controlled but the action of glaciation fed by the monsoon and the westerly jet is clearly critical in allowing that to happen. Without the strong erosion driven by glaciation those high grade rocks would still be at great depth. Perhaps the high grade rocks at Tso Morari are a good example of purely tectonic exhumation?
As for Cherapungi the exhumation rates are lower there than in these other cases because the crust is not very thickened and rock uplift rates are low, like Scotland for example! It is when we have rapid rock uplift plus rainfall that the fun really begins. The solid Earth provides the opportunity for ranges like the Greater Himalaya to form but I would argue that it is the climate that decides where the faults are located and how fast they move, including the STD.
best wishes
Peter
On Nov 13, 2014, at 5:02 AM, MIke Searle <[log in to unmask]<mailto:[log in to unmask]>> wrote:
Peter,
Bit of a reality check needed here maybe? The Himalayan Main Central
Thrust and South Tibetan detachment are major crustal-scale ductile
shear zones, faults that exhume kyanite and sillimanite grade rocks.
These are entirely tectonic structures and climate cannot possibly have
any control on their initiation at depths 30km. Once mountains are made by Tectonics,
then obviously climate changes follow (increased erosion, monsoon etc)
can take their affect but climate can never induce these type of
structures. Rainfall cannot suck kyanite gneises out for 12 kbar depths!
Tectonics, thrust faults brings these rocks up, mountains are formed and
climate just tags along behind.
Denis is absolutely right to point out the climate differences along the
Himalaya today. The fastest exhuming bit of real estate and fastest
erosion rates anywhere on Earth is Nanga Parbat, the west Himalayan
syntaxis in Pakistan, that exposes Pliocene-Quaternary
sillimanite-cordierite migmatites and tourmaline leucogranites < 1
million year old, and it lies in a desert. Similarly the Karakoram to
the north has little rainfall and has vast area of Miocene - Pliocene
kyanite and sillimanite rocks, migmatites domes and young leucogranites
with fast and active rock uplift-exhumation rates and high erosion. The
wettest place on earth is Cherapungi on the Shillong Plateau in NE India
where exhumation rates are very low.
Mike Searle
On 10/11/2014 14:19, Peter D Clift wrote:
It should also perhaps be kept in mind that without the erosion along the Himalayan front that the amount of exhumation we have observed in those mountains would be relatively small. I don’t think many people would agree that all of the Greater Himalayan exhumation is caused by the South Tibet Detachment and that effectively that this mountain belt is an extensional feature. In any case I’m not entirely convinced that the South Tibet detachment is not climatically modulated anyway. In my opinion that has still yet to be demonstrated. there seem to be good reasons to believe that the Main Central Thrust is climatically controlled so why not South Tibet detachment?
Peter Clift
On Nov 9, 2014, at 10:14 AM, gapais <[log in to unmask]<mailto:[log in to unmask]>> wrote:
Some of you should probably keep in mind that the Himalayas is a belt of about 2500 long and that a comparable zonation of exhumed rocks (UHP, Higher Himalayla Crystallinne, and others, crop out all along the belts, despite drastic variations in climate! It rains a lot in central Himalayas as in Nepal during the Monsoon, but the western Ladakh area is a desert! Nepal, althought one of the most visited section, is not representative of the belt in terms of climate, despite outcroping rocks and tectonic features are quite representative of the Himalayan belt. The only common structure all along the belt, that accounts for the exhumation of deep rocks, is the Zanskar and south Tibet detachment, a major tectonic-induced lithospheric structure that has nothing to do with climate. Keep also in mind that the NS-striking Andes cut across nearlly all possible climate environments, and show rather comparable deformation and structural patterns from South Peru down to Patagonia and Tierra de Fuego.
Best regards
Denis
Le 9 nov. 2014 à 06:12, Michael <[log in to unmask]<mailto:[log in to unmask]>> a écrit :
Dear colleagues,
We would like to draw your attention to the following EGU General Assembly session (TS3.3/CL1.9/GM3.6) to take place between 12th April – 17th April 2015 in Vienna, Austria entitled: “Investigating Tectonism-Erosion-Climate Couplings (iTECC): Himalayan orogenic development and climatic feedbacks from micro- to macro-scale”
Conveners: Guangsheng Zhuang, Michael Kelly, Alessandro Santato, Yani Najman, Jan Wijbrans
This session aims to bring together innovative studies from young and experienced scientists studying the past, present and future implications of the Himalaya-Tibet orogeny from a variety of temporal and special approaches.
Session Description:
A close relationship between tectonics, erosion and climate has been well established over numerous years of research in varying dynamic geological settings and has inspired many rewarding conversations among earth scientists across various fields of research. The Himalaya-Tibet orogen, not only because of its height, extent and rate of uplift, serves as an important natural laboratory for investigating couplings between topographic evolution and climate forces in active mountain belts which are related through surficial processes.
The newly launched Marie Curie Actions Group ‘iTECC’ (Investigating Tectonism-erosion-climate-couplings) program will serve as an excellent podium to inspire discussion and further our understanding of this dynamic geological setting from a wide range of earth science disciplines.
This session aims to bring together innovative studies from young and experienced scientists studying the past, present and future implications of the Himalaya-Tibet orogeny. Himalayan studies from a variety of temporal and special approaches will be presented, ranging from numerical models coupling tectonics, climate and erosion; to quantifying the controls on chemical weathering and physical erosion within the Himalaya by understanding the feedback loops with global atmospheric CO2 levels. Innovative studies utilising compound-level isotopic studies, near- and far-field sedimentary basin records and detrital geochemistry to infer proximal and distal India-Asia collisional tectonics, hinterland exhumation histories of basement rocks and Himalayan evolution are presented. This session will also present studies focusing on Himalayan weathering, erosion and climate through time, as well as, the development and refinement of analytical techniques needed to better interpret the past and present-day records of exhumation, erosion and climate processes within this dynamic orogenic belt.
We welcome oral and poster presentations to this session. Please submit an abstract by following the link: http://meetingorganizer.copernicus.org/EGU2015/session/17980
Abstract deadline is 07 January 2015.
A limited amount of financial support is available through the EGU for students and early career researchers. If you intend to apply for financial support, please submit your abstract by 28th November 2014. More information on financial support can be found at the following webpage
http://www.egu2015.eu/support_and_distinction.html
Please feel free to contact us for any further information (contact details given below). Thank you for your attention.
With best wishes,
Michael Kelly (on behalf of the session organisers)
Guangsheng Zhuang, Lancaster University, UK ([log in to unmask]<mailto:[log in to unmask]>)
Michael Kelly, Cairn India Limited, India ([log in to unmask]<mailto:[log in to unmask]>)
Alessandro Santato, Thermofisher, Germany ([log in to unmask]<mailto:[log in to unmask]>)
Yani Najman, University of Lancaster, UK ([log in to unmask]<mailto:[log in to unmask]>)
Jan Wijbrans, VU University, Amsterdam ([log in to unmask]<mailto:[log in to unmask]>)
Denis Gapais
Géosciences Rennes
UMR 6118 CNRS
Université de Rennes 1
35042 Rennes cedex
France
phone 33 2 23 23 67 36
mobile 33 6 20 01 58 69
fax 33 2 23 23 60 97
======================
Peter D. Clift
Charles T. McCord Chair in Petroleum Geology,
Department of Geology and Geophysics,
E235 Howe-Russell-Kniffen Geoscience Complex
Louisiana State University,
Baton Rouge, LA 70803,
USA
Tel: +1 225-578-2153
Fax: +1 225-578-2302
Email: [log in to unmask]<mailto:[log in to unmask]>
http://www.geol.lsu.edu/pclift/pclift/Home.html
Attend AGU Chapman meeting "Evolution of the Asian monsoon and its impact on landscape, environment and society”, June 15-19th 2015, Hong Kong
http://www.geol.lsu.edu/pclift/Monsoon_AGU_Chapman_Meeting/Welcome.html
--
******************************************
Professor Michael P.Searle
Dept. Earth Sciences
Oxford University,
South Parks Road.,
Oxford, OX1 3AN
England
Professor of Earth Sciences, and
Senior Research Fellow, Worcester College, Oxford.
Tel: +44 1865 272022
Fax: +44 1865 272072
Mike Searle's Home Page: http://www.earth.ox.ac.uk/~mikes<http://www.earth.ox.ac.uk/%7Emikes>
NEW BOOK:-
"Colliding Continents: A Geological Exploration of the Himalaya,
Karakoram and Tibet" [2013] Oxford University Press, £25.00
http://www.amazon.co.uk/Colliding-Continents-geological-exploration-Karakoram/dp/0199653003
*******************************************
======================
Peter D. Clift
Charles T. McCord Chair in Petroleum Geology,
Department of Geology and Geophysics,
E235 Howe-Russell-Kniffen Geoscience Complex
Louisiana State University,
Baton Rouge, LA 70803,
USA
Tel: +1 225-578-2153
Fax: +1 225-578-2302
Email: [log in to unmask]<mailto:[log in to unmask]>
http://www.geol.lsu.edu/pclift/pclift/Home.html
Attend AGU Chapman meeting "Evolution of the Asian monsoon and its impact on landscape, environment and society”, June 15-19th 2015, Hong Kong
http://www.geol.lsu.edu/pclift/Monsoon_AGU_Chapman_Meeting/Welcome.html
--
******************************************
Professor Michael P.Searle
Dept. Earth Sciences
Oxford University,
South Parks Road.,
Oxford, OX1 3AN
England
Professor of Earth Sciences, and
Senior Research Fellow, Worcester College, Oxford.
Tel: +44 1865 272022
Fax: +44 1865 272072
Mike Searle's Home Page: http://www.earth.ox.ac.uk/~mikes
NEW BOOK:-
"Colliding Continents: A Geological Exploration of the Himalaya,
Karakoram and Tibet" [2013] Oxford University Press, £25.00
http://www.amazon.co.uk/Colliding-Continents-geological-exploration-Karakoram/dp/0199653003
*******************************************
