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Fascinating thread.
Maybe we should be looking at the extent to which the erosion-climate-tectonics linkage is strongest or focussed at Transverse Zones which transect the entire orogeny, or transect /compartmentalise/relay the major orogeny parallel structures
(MCT etc.) - rather than a feedback operating at whole orogen/Himalayas scale. Is there a spatial association between transverse zones and major contemporary topographic features which would act as efficient conduits for the removal of detritus - thus facilitating
mass wastage - thus facilitating uplift and/or differential translation in separate thrust segments? Perhaps the erosion-tectonics link is segmented in the same manner as the finite structural architecture.
Graham Leslie
BGS
-----Original Message-----
From: mike searle [mailto:[log in to unmask]]
Sent: 16 November 2014 07:46
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"
What a wonderful comment Dugald Carmichael! from the Royal Box of Metamorphism (most certainly not the cheap seats!). I agree, a great discussion - its inspired me to continue the debate:
Himalayan Channel Flow/ MCT / STD / Climate links
DATA: data is derived from geological field data, strain data, PT data, multi-system geochronological data. The data from the Himalaya shows a
10-20 km thick Middle crust composed of mostly high-grade gneisses, migmatites and leucogranites. This slab is bounded by top-south MCT ductile shear zone below (inverted metamorphism) and top-north STD low-angle normal fault above (right
way-up isograds). The mid-crust GHS in between these shear zones was moving south relative to the lower crust and upper crust (Tethyan sequence) by >50-100 km. Inter-connected partial melts and leucogranite sills caused it to flow in a wholly ductile manner,
hence Channel Flow. Geochronological data shows that MCT, STD and Channel Flow was all active between ca 24 – 17 Ma.
MODELS:- The Channel Flow model links these Geological constraints to Climate – Erosion driven flow. The link is only a model. We have very little idea what the rainfall or climate was like during this time at that place. Monsoon – rainfall
is very high today yet the MCT-STD are tectonically inactive. A “Climatically-located MCT” is beyond my comprehension; Rainfall does not occur along a line of fault, and rainfall cannot possible be held responsible for initiation of ductile shear zones at
10 kbar Bathograd depths. Linking initiation of the MCT and STD to rainfall, climate to me seems ridiculous.
“It is only focussed erosion that allows the Himalayan slab to flow”.
This is not correct. It is partial melting of the crust that allows the GHS slab to flow. The highest exhumation rates and erosion are along the channel above the MCT and below the STD.
“Erosion – exhumation in a positive feedback loop”
This is plainly obvious! You cannot have 30 km high mountains. The debate is about whether climate, rain etc can cause rock uplift or channel flow. I would say not. Tectonic forces cause mountain – rock uplift; Rain cannot cause rock
uplift. Once topography is formed by tectonics, then rain, ice etc can reduce topography by erosion, and make mountains the shape we see today.
NANGA PARBAT
Nanga Parbat topography is not controlled by the Indus River. The antecedent Indus River rises near Kailas ~800 km east of Nanga Parbat; the river cuts across the axis of high relief (north-south Haramosh – Nanga Parbat trend) at 90
degrees. The river does not control topography or exhumation. Major faults (Raikot-Liachar thrust along west; Rupal shear zone and Stak vertical ‘normal’ fault along east) control the uplift of the mid-crust partially melted gneisses. Once high topography
has been formed then of course you have erosion, rain, rivers etc, but these do not control rock uplift.
Nanga Parbat has the highest rock exhumation rates recorded anywhere (Crowley et al. 2009. EPSL 408-20), Pleistocene migmatites formed 1.7 Ma at 5 kbar, ~15-20 km depth, now at 6500 meters altitude; tourmaline leucogranites crystallised
0.7 Ma now at 6000 meters. The pressures, depth and time of peak metamorphism/melting give us a rate of exhumation. None of this exhumation is controlled by rainfall. Nanga Parbat is in a dry desert climatic zone today.
Neither Taiwan nor New Zealand (mainly greenschist facies rocks at surface today I think) show this amount of rock exhumation/uplift.
... and lastly I have never understood Peter Zeitlers phrase 'Tectonic aneurism'... An aneurism is a blockage. Where's the blockage at Nanga Parbat? Its exhuming, uplifting like crazy.
Namaste!
Mike Searle
On 16/11/2014 01:21, Dugald Carmichael wrote:
> Dear tectonic gladiators, Just a few words from far back in the cheap
> seats: Wow! What a splendid debate! Enthralling and stimulating! A
> prime example of exactly what Tim Berners-Lee was hoping for when he
> envisaged the Internet and set about inventing it!
>
> Dugald Carmichael
>
> *From:*Tectonics & structural geology discussion list
> [mailto:[log in to unmask]] *On Behalf Of *Peter D Clift
> *Sent:* November 15, 2014 4:20 AM
> *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"
>
> I just wanted the chance to reply to some of Mike’s points
>
>
> First the MCT (and STD) are not active faults; they were during the
> Miocene but are they not active now
>
> That’s true but there is evidence that thrust faulting is focused by
> erosion patterns in the recent past and could have done so during the
> Miocene too
>
>
>
> (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).
>
> Indeed, but that does not mean that the MCT was not climatically
> located when it was active, Indeed it is hard to see how it could not
> have been since the Channel Flow only works when erosion removes the
> shallower rocks allowing the channel to flow.
>
>
> Second Rainfall most certainly did not control initiation of the MCT
> (or the STD). Tectonics controlled this, not Rain.
>
> That is not the way that the Channel Flow model is described. The
> channel onlu flows because of the gravitational potential caused by
> the thicker than normal crust but no exhumation would occur without
> the erosion along the mountain front.
>
>
>
> 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!).
>
> I might be. To be precise I am arguing that stresses caused by
> surface processes (erosion) result in rock uplift which has be
> compensated for throughout the crust. At shallow levels this results
> in faulting by these must extend to ductile shear zones at depth. It
> is only the focused erosion that allows the Greater Himalayan slab to
> flow so without that erosion there would be no large thrust like the
> MCT (or STD). This is why we only see these structures on the wet
> south side of Tibet and not on the north side.
>
>
>
> 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.
>
> I agree that tectonic forces are responsible for driving the rock
> uplift but without the erosional influence I don’t see how you would
> explain the deep exhumation unless you think there are large
> extensional faults in the syntaxes too.
>
>
> It rains like hell in the Amazon (and in Oxford sometimes) but I see
> no major MCT type fault induced here.
>
> No of course not because we need rock uplift as well as erosion to
> generate significant exhumation and thus the development of major
> faults. However, rock uplift in the absence of erosion does not cause
> exhumation which is a signature feature of the Greater Himalaya
>
>
>
> Rainfall has nothing to do with mountain building. Tectonics make
> mountains and rain etc erodes them away.
>
> I don’t agree with that at all. Compressional tectonics causes crustal
> thickening and both rock and surface uplift but without erosion,
> modulated by climate there is no exhumation until the orogenic belt
> experiences large scale gravitational collapse.Thus Tibet is the
> product of plate tectonic forces but the Greater Himalayas are only
> possible in the form that we know them because of surface processes
>
>
>
> 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.
>
> Then the exhumation must be caused by other erosional processes unless
> you think that normal faulting is dominating. I would curious to know
> why you think that glaciers only account for 5-10 km of erosion. If
> the rocks were metamorphosed at 12 km then you have to remove that
> overburden to get them exposed. That seems to require either
> extensional faulting or erosion. You choose.
>
>
>
> Greenland, Northern North America and Siberia are covered in huge
> ice sheets and glaciers, theres no active mountain building going on
> there.
>
> No, because there is no significant rock uplift to drive deep
> exhumation
>
>
>
> 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.
>
> Indeed they are
>
>
>
> Rock exhumation rates are pretty similar along the length of the
> Greater Himalaya (and timing too)
>
> This I do not agree with. Ar-Ar cooling ages are generally younger in
> the east than in the western Greater Himalaya. To my mind this is
> because the Channel Flow is faster in the east than the west, driven
> by the heavier rain fall I would suggest.
>
>
>
> but in the east it is wet and rainy and in the west (Ladakh,
> Zanskar, Nanga Parbat) dry.
>
> We are really only talking about Zanskar here, and that has some of
> the oldest cooling ages in the Greater Himalaya because it is drier
> and there exhumes more slowly that the eastern ranges
>
>
>
> Thus in my opinion Tectonics initiates and controls rock uplift
>
> I agree 100%
>
>
>
> and ductile shear zones, faults and mountain building along the
> Himalaya, not Rain, Glaciers or Climate.
>
> This I do not agree with. Tectonics can do all those things but
> surface processes can play a role too in focusing where rock uplift
> occurs and thus where large faults that allow deep exhumation develop.
> Without surface processes, modulated by climate there can be little
> exhumation in compression systems.
>
> best wishes. I am enjoying the debate
>
> Peter
>
>
> ======================
>
> 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.htm
> l
>
--
******************************************
Professor Michael P.Searle
Dept. Earth Sciences
Oxford University,
Parks Road.,
Oxford, OX1 3PR
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
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