Mark's spot on.
Erosion & uplift/exhumation CAN exist in a positive feedback loop: fast uplift + steep slopes + some form of precipitation = fast erosion ==> fast exhumation - Taiwan & New Zealand obvious examples
The more interesting questions are:
a) where does this positive feedback occur and where not;
b) if not why not (clear for Old Zeeland, not so clear for Andes...);
c) when does it kick in and when (how? Why?) does it cease....
Enjoy!
Maarten Krabbendam
-----Original Message-----
From: Mark Brandon [mailto:[log in to unmask]]
Sent: 14 November 2014 01:49
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"
Mike,
Why the "straw man" arguments... I know of no serious claims that kyanite gneisses were "sucked" to the surface by rain drops. Nor does it make sense to say that thrust faults "push" the gneisses to the surface. Your straw-man argument is built on the notion that erosion rates are related to the local rate of rain fall. Rivers and mountain glacier aggregate upstream precipitation, so the local rate of precipitation is irrelevant. The more relevant "climate" argument is that the local topographic relief in the Himalaya is set by the network of rivers and glacial valleys that surround the mountains.
So, now for a different reality check: The topographic relief in the Nanga Parba area is set by the Indus River and its tributaries. Above these river channels, the mountain slopes are critical, in that they are at the limit of frictional strength. The process of erosion is mass wasting, not rain splash, and the rate of erosion is thought to closely follow the rate of rock uplift. One might be tempted to claim cause and effect, where uplift drives erosion. But, in fact, for a viscous orogen with critical topography, the local rate of rock uplift and the local rate of mass wasting are two interdependent components of the larger orogenic system. If there were no river and mass wasting, then the rate of rock uplift would decrease accordingly.
In my mind, there is little sense in debating whether climate or tectonics is more important. The more important question is how the orogenic system operate in an environment where crustal mass is converging into the orogen and the surface of the orogen is free to erode by fluvial and glacial processes.
One last point: the fastest erosion in the modern world is in Taiwan Alps and the Southern Alps of New Zealand. This fact is well established by simply comparing thermochronologic ages for those areas with those for Nanga Parbat and elsewhere. The maximum pressure for exhumed crustal metamorphic rocks tells us something about the thickness of the crust during metamorphism but it not an indicator of exhumation rate.
Sincerely,
Mark Brandon
> On Nov 13, 2014, at 6:02 AM, MIke Searle <[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]> 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]> 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]) Michael Kelly, Cairn India Limited, India
>>> ([log in to unmask]) Alessandro Santato, Thermofisher,
>>> Germany ([log in to unmask])
>>> Yani Najman, University of Lancaster, UK ([log in to unmask]) Jan
>>> Wijbrans, VU University, Amsterdam ([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]
>>
>> 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.ht
>> ml
>>
>
> --
> ******************************************
> 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-Ka
> rakoram/dp/0199653003
>
> *******************************************
>
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